Ch

Figure 9.21 Synthetic all-rac alpha-tocopherol contains about equal amounts of eight isomers characteristic vitE activity have been identified: alpha-tocopherol. beta-tocophcrol, gamma-tocopherol, delta-tocopherol, alpha-tocotnenol, beta-tocotrienol. gamma-tocotrienol, delta-tocotrienol. the tocotrienols d-P(21 )-T3 and d-P(25)-T3 (Qureshi etul.. 2001), alpha-tocomonoenol. and marine-derived tocopherol (Yamamoto et at.. 20011,

The food constituent most closely identified with vitF properties is RRR-alpha-tocopherol v\ ith methyl groups in the side chain at positions 5. 7, and H, The side chain of RRR-beta-tocopherol is methylated at positions 5 and S, in RRR-gamma-tocopherol at positions 7 and fi. RRR-de!ta-tocopherol has only one methyl group in the side chain at position K. The members of the analogous series of tocotrienols contain three double bonds in the side chain. All four members of the tocopherol series and the four members of the toeolrienol series are naturally present, though in varying amounts, in a w ide range of foods.

Synthetic production of vitE usually yields about equal amounts of the eight possible isomers. RRR. RSS, RRS. RSR. SRR. SRS. SSR. and SSS. The first three in this list of isomers are often called the 2R isomers because they are R-isomeric at position 2. The metabolic fate of the various iso forms differs and needs to be determined in every case.

The biological potency of vitE doses is often expressed as USP vitamin E units or International Units (IU). One such unit corresponds to I.Omg raccmic < synthetic, all-rac) alpha-tocopheryl acetate (this is the original reference standard), or 1.1 mg all-rac alpha-tocophcrol.or 1.36mg RRR-alpha-tocopheryl acetate, or 1,49mg RRR-alpha-locopherol, or 0.89 mg al I-rac-alpha-tocophery 1 succinate, or 1.2! mg RRR-aIpha-tocophery I succinate. In the following villi content will be expressed as alpha-tocopherol equivalents (ATE), which is the amount ofRRR alpha-tocopherol (hat is expected to have the same potency as all v itE forms in a food combined.

t here are only a tew good sources that provide one serv ing with at least 2.5 mg (one-sixth of the recommended intake). Since gamma-tocopherol may differ in its action prolile from gamma-tocopherol, the exact composition (which is often not reliably known) may be as important as the ATE figure. Vegetable oils with high to moderate content include wheat germ oil (1.9mg ATE'g. more than half as alpha-tocopherol) and sunflower oil (0.5 mgg. most as alpha-tocopherol). Most other commonly consumed oils have a much lower content, such as corn and soybean oil (0.2 mg g, most as gamma-tocopherol), canola oil (0.2 mg g. most as alpha-tocopherol), or olive oil (0.1 mg g. most as alpha-tocopherol). Sunflower seeds (0.5mgg) are also a good source. Walnuts (0.03 mg ATE/g) contain nearly equal amounts of alpha-tocopherol, gamma-tocopherol. and dclta-tocophcrol.

American men have a daily vitE intake of about Smg, women get close to ftmg (Phillipset al. 2000). American food consumption data indicate that only about 10% of men and virtually none of the women reach the recommended intake level 115 nig. day) with food alone (Food and Nutrition Board Institute of Medicine, 2000: Appendix D),

Digestion and absorption

Most forms of vitE are absorbed with similarly high efficiency from the proximal small intestine. VitE esters are cleaved by esterases from pancreas.

Uptake into enteroevtes depends on the prior incorporation of free vitE into mixed micelles (Borcl el al.. 2001). This means that a small amount of fat has to be absorbed along with v itE. A very low-fat meal or poor fat digestion effectively minimize vitE absorption. The exact mechanism of transfer from micelle to enteroevte is not well understood but is likely to involve al least some of the actors involved in fatty acid uptake. VitE is exported with chylomicrons into intestinal lymph and eventually blood circulation.

Transport and cellular uptake

Blood circulation; About 27 p.mol'1 alpha-tocopherol, 1.7 p.mol 1 gamma-tocopherol (Ruiz Rejon et al.. 2002). and much smaller quantities of other \ itE species are present in plasma. Virtually all plasma vitE is associated with low-density lipoprotein (LDL; about ft molecules of alpha-tocopherol and 0.5 gamma-tocopherol molecules per particle in vitE-replete people; Esterbauer ei al.. 1992) and high-density lipoprotein (HDL). Liver cells take up chylomicron remnants w ith recently absorbed vitE via receptor-mediated endocytosis. The highly lipophilic vitE then redistributes rapidly to the plasma membrane and intracellular membranes. VitE, like all other plasma membrane constituents, is internalized into endoeytic compartments (sorting endosomes) several times per hour (Hao and Maxfield 2000). It seems that at this point alpha-tocopherol transfer protein (TIP) plays a critical role by preferentially redirecting RRRalpha-tocophcrol back towards newly forming plasma and other membranes (Blatt et al.. 2001).

Two additional proteins are likely to be involved in directing the intracellular recycling ofvitE, tocopherol-associated protein (TAP) and tocopherol-binding protein (TBP). Due to their much lower affinity toTTP a smaller percentage of2R-alpha-tocophero!s and RRR-beta-tocopherol is recycled to the membranes, and an even smaller percentage of other forms ofvitE. The portion left behind tends to be eliminated into bile. Newly secreted very-low-density lipoproteins (VLDL) preferentially carry RRR-alpha-tocopberol. with lower preference 2R-a!pha-tocopherol and beta-tocopherol. and very little of the other Ibrms. The vilE may come mostly from plasma and endosomal membrane, hence the preference for the RRR-alpha-form (Blatt et at. 2001). A typical nascent VLDL contains about six molecules of vilE (Mertens and Holvoet, 2001), but only every other particle contains a gamma-toeopherol molecule (Esterbauer et a!.. 1992). Delivery ofvitE to some tissues (lung, ovaries, testes) involves the scavenger receptor class B type 1 (SR-BI: Mardones et aI., 2002).

Some \ itE from extrahepatic tissues appears to be mobilized by the ATP-dependcnt transporter ABC AI (Oram el til.. 2001). shuttled by phospholipid transfer protein to HDL (I luuskonen et at., 2001) and returned with this vehicle of reverse cholesterol transport back to the liver (Mardones et at.. 2002).

Blood-bram barrier. Very little v iiamin E readies the brain, which keeps the concentration in cerebrospinal fluid to about one-hundredth of the concentration in plasma (Pappert et al., 1996). The mechanisms whereby even those low concentrations are maintained remain uncertain. High-density lipoprotein (HDL) is live times more effective than L.DL in delivering tocopherol to the blood brain barrier (Goti ei al.. 2001 ).The transfer of vitE from HDL into the capillary epithelial cells uses the SR-BI (Mardones et at., 2002). TTP in glial cells helps to direct vitE into compartments that ultimately supply it to Purkinje cells and other neurons (Blatt el al.. 2(H) 1). Matemo fetal transfer: Understanding of the exact mechanism of vitE transport across the syntrop ho blast is still incomplete. HDL (Christiansen-Weber et ui. 2000)

Venous blood

Venous blood

Arterial blood

VLDL

Figure 9.22 Alpha tocopherol transfer protein preferentially recovers KRK-alpha-tocophero) from endosomei

Arterial blood

VLDL

Figure 9.22 Alpha tocopherol transfer protein preferentially recovers KRK-alpha-tocophero) from endosomei and other lipoproteins are known to deliver lipophilic compounds through receptor-mediated endoeytosis. TTP plays an important role, possibly through a sequence of events like in liver (redirecting of vitE in sorting endosomes).

Metabolism

Side-chain breakdown A combination of microsomal w-oxidation. peroxisomal betaoxidation and less well-characterized reactions generate a series of hydroxy-chroman

2R 4'R, 8'R-i-Tocopheroi rvp,. red llavoproloin t O;

[putative intermediate]

2R 4'R, 8'R-i-Tocopheroi rvp,. red llavoproloin t O;

[putative intermediate]

CH] CM, [putative intermediate]

CH] CM, [putative intermediate]

one round ol jJ-OKidation one round ol jJ-OKidation

«^-[e -carboxy-d -methylhexyllhydroxychroman (u-CMHHC)

fj-omdalton fj-omdalton n -2-16' -ca rboxy-4 - me Ihy lb u ty I ¡hydroxy c hroma n (<i-CMBHC)

One round el li-oxtdaüon '

One round el li-oxtdaüon '

Figure 9.23 Metabolism of vilamm E ".irfr chains ¿eneraies polar compounds

<i-2-[6'-cartxMy-4'-e(hyl]hydroxychroman (u-CEHC)

Figure 9.23 Metabolism of vilamm E ".irfr chains ¿eneraies polar compounds metabolites (Parker ei ul.. 2000; Birringer et til.. 2001), TTP in liver shields RRR-alpha-tocopherol to a large extent from this type of breakdown. Cytochrome p450 3A4 and/or CYP4F2 initiates breakdown by oxidizing the methyl group at the terminat end of the side chain (it)-oxidation). A I ter another oxidation step the molecule now has a carboxyl group at the end of the side chain. Successive rounds of beta-oxidation can then shorten the side chain. Since the methyl groups of the side chain are in a configuration relative to the carboxyl group they do not block the initial step of betaoxidation, catalyzed by long-chain acyl-CoA dehydrogenase (FCU.99.t3). Fhe final metabolite is the 2(2' -carboxyethy I >-6-hydrox y e h roman (C'EI IC) of the original vitl

C». cm. 2R. 4 R, 8'Rn-Tocopherol Two-electron One-electron radical (R") / \ radical (R )

Tocopheroxylium cation

One-electron

One-electron

Tocopheroxylium cation

Tocopheroxyl radical

Tocopheroxyl radical

Tocopherone

Lipid peroxide radical

Tocopheryl quinone

Lipid peroxide radical

' Pv v V V -V- V iB| »OylVTv CH.OOM CH, CH.OOH CM,

8< I -Hyd roxypero xy-5.6-e poxy tocopherone 8«-H yd roxyperoxy-8.9-epoxy tocopherone

Otj CH,

Tocop h eiy Iq u i non e-8.9-epoxyide

Figure 9.2-1 Reaction with free radicals initiates metabolism of the vitamin E ring compound. Since the ring system is not a fleeted, distinctive CEHCs result from side-chain metabolism. This means thai all isomers of alpha-tocopherol and alpha-tocotrienol result in alpha-CHI 1C, the beta-tocopherols and beta-tocotrienols generate beta-CEl IC. and so on (Lodge et ul., 2(H) I ).

Ringmodificadott: Reaction with two-electron oxidants, such as liypochlorous acid t from the myeloperoxidase-generated oxidative burst of leukocytes) and peroxynitrite (a reactive nitrogen species formed from the reaction of superoxide with nitric oxide) generate tocopheryl quinone in one step without the chance of regeneration (Terentis et aL, 2(H)2), Reaction of vitE with one-electron oxidants, such as superoxide anion, produces a toco-pheroxyl radical, which may he converted into tocopheryl quinone by another reaction with an oxidant. Reaction of a lipid peroxyl radical w ith the tocopheroxyl radical converts this to 5.b-cpoxy-tocopherol or 2.3-epoxy-tocopherol. NAD(P)H:quinone oxidore-dueta.se I (EC1.6.99.2) may lie able to revert some of the tocopheroxyl radical to tocopherol (Ross vt ol.. 2000). More important may be the reduction of tocopheryl quinone to tocopheryl hydroquinone by this enzyme, because the reduced metabolite has antioxidant activity. It should be noted that quercetin, the most abundant ilavonoid in food induces NAD(P)H:quinone oxidoreductase I (Valeno et ul.. 2001).

Excretion

Intact villi and some of its apolar metabolites are excreted with bile. Various polar metabolites appear in urine.

The scavenger receptor class B type 1 (SR-BI) plays an imponant rote in the transfer ofvitE from HDL through the liver cells into bile (Mardones et ui., 2002). This pathway is in some ways analogous to the reverse cholesterol transport from peripheral tissues into bile. The linal step of translocation into the bile canaltculi uses the multidrug resistance P-glycoprotein 2 (MDR2, ABCBI. Mustacich et ul., 1998). Biliary excretion of unchanged alpha-tocopherol increases with the presence of excess.

The four different 2(2'-carboxyeihyl)-6-hydroxyehromans (CEHCs) from alpha-, beta-, gamma-, and delta-tocopherols and tocotrienols are excreted with urine (Lodge et ul., 2001). It may be of note that all isomers of alpha-iocopherol generate the same alpha-CEHC, and provide no indication whether R R R-al pha- tocopherol orall-rac-alpha-tocopherol has been ingested.

Regulation

The limited capacity of alpha-tocopherol transfer protein may be the most important protection against vitl excess. A modulating effect of alpha-tocopherol on the expression of alpha-tocopherol transfer protein is likely (Azzi et til.. 2001), 1 lowcv er. experience clearly show s that high consumption levels can at least partially overwhelm such limitations, presumably by using unspecific pathways for transport.

Function

Antioxidant protection: Molecules with vitE biological activity can abstract free electrons from an oxygen free radical and thereby render it much less reactive. It is presumed ascorbate dehydro ascorbate semi dehydro asrorbala a seo reate ascorbate dehydro ascorbate semi dehydro asrorbala a seo reate

Figure 9J5 Vitamin E quenches free radicals

that lack of this protective role underlies the progressive cerebellar ataxia, nerve damage, retinal atrophy and other symptoms related to severe vitE deficiency, such as seen with defective alpha-tocopherol transfer protein or abetalipoproteinemia. Increased breath exhalation of ethane and pentane due to increased lipid peroxidation retleeis the inadequate antioxidant action even v. ith more moderate depletion.

There is a downside to quenching one-electron oxidants, however, since this type of reaction turns the vitE molecule itself into a free radical. Recent observations on atherosclerotic lesions indicate, however, that the tocopheroxyl radical promotes lipid oxidation to a greater extent than providing protection against it (Terentis el a!., 2002) The presence of co-ant i oxidants, such as ascorbate, becomes critical therefore, to quench and reactivate the tocopheroxyl radical, before it can attack a lipid double bond.

VitE is also suspected of lowering 11 HI cholesterol and may promote the activity of chotcsicryl ester transfer protein (Kuller. 20011.

Another concern is about the suppression of vitally important signaling events that use free radical reactions (Dröge, 2002), High vitE concentration may interfere with important functions such as programmed cell death (apoptosis). cell adherence, and immune response (Dröge. 2002),

Fertility: Inadequate vitE status interferes with normal oocyte production and placenta function in rodent animal models. This role was one of the earliest postulated functions that led to the discovery of vitE. However, the relevance of vitE status for human reproduction is not known,

Nonantioxidant functions. It has become increasingly clear that the needs for at least some \ itE species extend beyond interactions with free radicals (Azzi et al., 2001: Ricciarelli el ul., 2001), It was thus demonstrated that alpha-tocopherol, but not other vitE species, induces smooth muscle cell growth arrest by inhibiting Phosphokinase C.

Alpha-tocopherol also inhibits cell adhesion, platelet aggregation, and the production of oxygen Tree radicals by neutrophils and monocytes. Expression of several genes is inhibited by aipha-tocopherol, including liver collagen al, a-iropomyosin, collagenase MMP1, ICAM-1, VCAM-1. Other effects are activation of PP-A, diacy I glycerol kinase, and inhibition of 5-lipoxygenase. scavenger receptor SR-A and scavenger receptor CD36,

Pathogen mutations: An otherwise benign strain of Coxsackie virus B3 was shown to cause myocarditis in \ itamin E-deficient mice (Heck and Lev ander. 20(H)). This sheds a new light on the role of nutrient deficiency in host -pathogen interactions and could pose a significant health risk on its own.

Hemorrhage. High levels of tocopherol quinonc. which can arise from excessive vitE intakes, competitively inhibit regeneration of vitamin K epoxide by NAP(P)H:quinone oxidoreductase 1 (EC'l .6.99.2). A daily dose of 50 mg a 11-rac-alpha-tocopherol in the largest long-term study conducted so far increased mortality from hemorrhagic stroke (ATBC Cancer Prevention Study Group, 1994). People with low v itamin K intake are likely to be at particularly high risk. Inhibiting effects on platelet aggregation (Cul/ada et a I., 1997) may contribute slightly to a bleeding tendency, but may be beneficial for the prevention of coronary thrombosis.

References

ATBC Cancer Prevention Study Group. The effect of \ itamin F. and beta carotene on the incidence of lung cancer and other cancers in male smokers. .V Engl J Med I994;330: 1029-35

A/zi A. Breyer I. Feher M. Rieeiarelli R, Stocker A. Zimmer S. Zingg J. Nonantioxidant functions of alpha-tocopherol in smooth muscle cells. J Nutr 2001.131:378S 381S Beek MA, Lc\ander OA, Host nutritional status and its effect on a viral pathogen. J InfDis 2000; 182:S93-6

Birnnger VI. Drogan D, Brigelius-Flohi R. Tocopherols are metabolized in HepG2 cells by side chain omega-oxidaiion and consecutive beta-oxidation, Free Rail Hiol Med 2001: 31:226-32

Blatt DH, Leonard SW, Traber MG. Vitamin 1 kinetics and the function of tocopherol regulatory proteins, Nutr 2001;17:799-805 Borel I'. Pasquier B. Armand M,Tyssandier V Grolicr P, Alexandre-Gotiahau MC, Andre M, Senfi M. Peyrot J, Jaussan V Lairon D, Azais-Braesco V Processing of vitamin A and E in the human gastrointestinal tract. Am J Physio! Gastmtnt Liver Physiol 2001:280: G95 103

Calzada C, Bruckdorfer KR. Rice-Evans CA. The influence of antioxidant nutrients on platelet function in healthy volunteers. Atherosclerosis )997;128:97 105 Christiansen-Weber TA. Voland JR. Wu V. Ngo K. Roland BL. Nguyen S. Peterson PA, Fung-Leung WP. Functional loss of ABCAl in mice causes severe placental malformation. aberrant lipid distribution, and kidney glomerulonephritis as well as high-density lipoprotein cholesterol deficiency. Am J Pathol 2000; 157:1017 29 Dröge W. Free Radicals in the Physiological Control of Cell Function. Phvs Rev 2002:82: 47-95

Esterbauer 11, Gebicki J. Puhl 11. Jürgens G. The role of lipid peroxidation and antioxidants in oxidative modification or LDL. Five Rad Bio! Med 1992:13:341-90 Food and Nutrition Hoard. Institute of Medicine. Dietary reference intakes for \ itamin C, vitamin E. selenium, and c arytenoids. National Academy Press. Washington, DC. 2000

Ciott D, Hr/enjak A. Lcvak-Frank S, Frank S. van der VVesterhuysen DR. Malle E, Sattler W, Scavenger receptor class B, type I is expressed in porcine brain capillary endothelial cells and contributes to selective uptake of MDL-associated vitamin L. J Neumchem 200l;76:498 50X Mao M. Maxfield I R. Characterization of rapid membrane internalization and recycling.

J BioI Chem 2000;275:15279 87 Huuskonen J, Olkkonen VM. Jauhiaincn M. Ehnhotm C. The impact of phospholipid transfer protein tPLTP)on HDL metabolism. Atherosclerosis2001:155:269 SI Kuller LH, A time to stop prescribing antioxidant vitamins to prevent and treat heart disease'.' Arteriosclerosis 2001 ;21:1253 LodgeJK. Ridlington J, Leonard S. Vaulc H,Traber MG. Alpha- and gamma-tocotrienols are metabolized to earboxyethyl-hydroxychroman derivatives and excreted in human urine. Lipids 2001:36:43 8

Mardones P. Strobel P. Miranda S, Leighton F. Quinones V Amigo L, Rozowski J. Krieger M. Rigoin A. Alpha-tocopherol metabolism is abnormal in scavenger receptor class B type I (SR-131 ¡-deficient mice.J Nutr 2002:132:443 9 Mertens A. Molvoet P Oxidized LDL and HDL: antagonists in atherothrombosis. FASEBJ 2001:15:2073-84

Mustaeieh DJ. Shields J. Morton RA, Bnnvn MK, Reed DJ Biliary secretion of alpha-tocopherol and the role of the nulr2 P-glycoprotcin in rats and mice. 4nh Biochem Biophys 1998:350:183-92 Oram JF, Vaughan AM, Stocker It ATP-binding cassette transporter Al mediates cellular secretion of alpha-tocopherol,./ Riol Chem 2001 ;276:39898 402 Pappert LJ, Tangney CC, Goetz CG. Ltng ZD, I ipton JW. Stebbins GT, Carvey PM. Alpha-tocopherol in the ventricular cerebrospinal fluid of Parkinson's disease patients; dose response study and correlations with plasma levels, Neurol 1996; 47:1037-12

Parker RS. Sontag TJ. Swanson JE. Cytochrome P4503A -dependent metabolism of tocopherols and inhibition by sesamin. Biochem Biophys Res Comm 2000:277:531 4 Phillips EL, Amen DR. Mimes .III. McGovern PG. Blackburn H, Luepker RV. Differences and trends in antioxidant dietary intake in smokers and non-smokers. 1980 1942: the Minnesota Heart Survey. Inn Epidemiol 2000;10:417-23 Qureshi AA, Peterson DM. Haslcr-Rapacz JO, Rapacz J. Novel tocotrienols of rice bran suppress cholesterogenesis in hereditary hypercholesterolemie swine. J Nutr 2001: 131:223-30

Ricciarelli R. Zingg JM. Azzi A. Vitamin li: protective role of a Janus molecule. FASEBJ 2001;15:2314-25

Ross D. Repa JK. Winski SL. Bcall HD. Anwar A. Siegel D. NAD(P)H:quinone oxidore-ductase 1 (NQOl): chemoprotection, bioactivation. gene regulation and genetic polymorphisms. t'hemico-Biol Interact 2000:124:77 97

Ruiz Rejon F, Marttn-Pena G, Granado F, Ruiz-Ualiana J. Blanco t, Olmcdilla Ii. Plasma status of retinol. alpha- and gamma-tocophcrols, and main carotcnoids to lirst myocardial infarction: ease control and follow-up study. 'VWr 2002;I 8:26 31 Te rent is AC, Thomas SR. Burr JA. liebler IX, Stocker R. Vitamin E oxidation in human atherosclerotic lesions. Cin Res 2002;90:333-9 Valeno LG Jr. Kepa JK. Pickwell GY Quattrochi EC. Induction of human NADtP)H:quinone oxidoreduetase (NQOI > gene expression by the Havonol quercetin. Toxicol Lett 2001;119:49-57

Yamamoto Y. Fujisawa A. Kara A. Dunlap WC. An unusual vitamin E constituent (alphas tocomonoenol) provides enhanced antioxidant protection in marine organisms adapted to cold-water environments, Proc Natl Acad Sei USA 2001:98:13144 X

Vitamin K designates a group of compounds characterized by a 2-methylnaphthalene-1.4-dione ring system with aiuihemorrhagtc properties, The natural form in plants is

Vitamin K

Vitarmr K, (pflylloquinone)

Vitarmr K, (pflylloquinone)

VI lam in K j menadione]

VI lam in K j menadione]

Menalelfenone

Menalelfenone

Monaqu>none(7)

Monaqu>none(7)

Menaqumone i8i

Figure 9.26 (.timmiin Turms ol Vir.imui K

Menaqumone i8i

Figure 9.26 (.timmiin Turms ol Vir.imui K

phylloquinone <2-methyl-3-| 3.7,11,15-tctramethyl-2-hexadecenylJ-1,4-naphthalene-dione, phytonadione, 3-phyty I menadione; molecular weight 450). Bacteria produce menaquinones whose side chain consists of 7 13 isoprenyl units. The mixture of" produced menaquinones is highly specific lor individual species. Additional natural and synthetic compounds are covered under the definition above.

Abbreviations

IL-6 mterleukin-6

K1 phylloquinone

MK4 menaquinone-4 (menatetrenone)

Nutritional summary

Function: Vitamin K activates proteins needed for blood coagulation, counteracting calcification of arteries and other soft tissues, promoting mineralization ofbone, and regulation cell division and differentiation.

Requirements: Newborn infants should get at leasl one supplemental dose to prevent cerebral hemorrhage, several during the first few months may be desirable. Adequate daily intakes for women are 9(1 (¿g. Men need 120 p.g day.

Sources: Intestinal bacteria pro\ ide enough under optimal circumstances to prevent bleeding, but not enough to optimize other functions. Many antibiotics suppress this important baseline contribution. Cooked spinach, kale, and other green leafy vegetables provide much more than enough to achieve adequate intakes with a single serving. The small amounts in other sources make it difficult to meet requirements without eating greens. Human milk contains very little.

Deficiency The greatest risk is for new born breastfed infants, especially those with disorders (e.g.. biliary atresia) interfering wuh absorption. Brain hemorrhage in very young children is a rare, but extremely serious affliction due to low vitamin K status. Oral antibiotics can cause Weeding in some adults who have persistently minimal intakes. Suboptiinal intakes are likely to increase the risk of artery calcification, osteoporosis. and possibly accelerated cognitive decline.

Excessive intake: No unfavorable health effects have been observed following consumption of45mg day for a year. Patients takingantivitamin K {Coumadin type) anticoagulants have to maintain a constant intake, which is likely to be easier at a high than al a low level.

D ihydrophy I loqu mo ne f if;«™ lndusin.il hydrngenarton of fats generates dihydrqvitamtn K

D ihydrophy I loqu mo ne f if;«™ lndusin.il hydrngenarton of fats generates dihydrqvitamtn K

Endogenous sources

Several normal intestinal bacteria produce considerable quantities ol'menaquinones. some oF which is likely to be absorbed (C'only and Stein. 1992). How much is actually available from the distal intestines remains uncertain (Lipsky, 1994).

Dietary sources

Dark-green vegetables contain large amounts of phylloquinone (K1). Vitamin K-rich foods are chard (8.3p,gg), kale (7|jtg(g), green asparagus (4|ig/g). spinach (3.X fig g). broccoli (2.7 n-g g) and Brussels sprouts (1.5 p.gg). Soya oil (1.5 p.g g). canola oil (1.1 )ig g). and virgin olive oil (0.8 )ig'g) also can contribute significantly. Natto, a fermented bean curd commonly consumed in some regions of Japan (Kaneki el al.. 2001), is a very rich source of menaquinones (9 fig/g). Some cheeses (about 0.3p.gg) are minor, but significant menaquinone sources. Most other oils and foods do not provide enough for adequate intakes in the absence of vitamin K-rich foods (Booth el ul„ 2001; Shearer el al.. 1996). Hydrogenation also saturates Kl in oils and appears to render it largely inactive (Booth el al.. 2(K)1). Vitamin K is very sensitive to light and alkali, which induce its decomposition to inactive chromenol compounds.

Typical daily intakes in the US and in the UK (Thane et al.. 2002) are commonly below what are considered adequate intake levels, but may be much higher in some Asian countries.

membrane membrane

Figure y.ZH Intestinal absorption ofvitamm K

membrane membrane

Figure y.ZH Intestinal absorption ofvitamm K

Digestion and absorption

The absorption of vitamin K (nam the small intestine is tightly coupled to fat absorption. Most of a ntieellar K.1 emulsion is absorbed (Gijsbcrs et al.. 1996). KI in lealy vegetables and lettuces, on the other hand, is firmly embedded in the chloroplast cotyledons and not easily released. Less than 20% is absorbed from cooked spinach served with fat. and almost nothing from uncooked spinach (tiijsbcrs et al., 19%; Garber et at.. 1999; Schurgers and Vermeer, 2000). Vitamin K. which is highly lipophilic, becomes part of mixed micelles formed during fat digestion in the small intestine. Micelles are taken up by small intestinal enterocyles through a poorly understood process, and the vitamin K \\ ith it. During the formation of chylomicrons vitamin k is incorporated into these triglyceride-rich lipoprotein panicles by an unknown mechanism. Chylomicrons with vitamin K are secreted into intestinal lymph ducts and reach blood circulation from there. There may also be some interference from other fat-soluble micronutrients. High intakes of lutein and vitamin E in combination were found to reduce bioavailability of k 1 and menaquinone-4 in rats (Mitchell et at., 2001).

Transport and cellular uptake

Blood circulation: Vitamin k concentrations typically are around 0.t-2p.g I and increase considerably after a vitamin k-rich meal. Vitamin k is transported in blood primarily with chylomicrons, to a much smaller extent with other lipoproteins (Kohlmeier et at.. 1995). Upon exposure to lipoprotein lipase chylomicrons release most of their lipid load, hut retain vitamin k. This enters cells in liver, bone, and other tissues during receptor-media ted uptake of chylomicron remnants. Uptake requires the binding of apolipoprotein II at the surface of chylomicron remnants to specific receptors including the LDI rcceptor anil the LDL-reccptor-related protein I ILRP. apolipoprotein E receptor). Common variant forms of apolipoprotein E (E2. E3, E4) mediate uptake with differing efficacy and are associated with different residual vitamin k plasma concentrations after an overnight fast. Individuals with the 114 variant lend to have lower concentrations than those who have only the E3 isoform {Kohlmeier et al., 1995). People with the E2 isoform tend to have the highest concentrations.

There is no indication of significant transport of intact vitamin K between cells or organs,

Bload brain barrier. The transport of vitamin k into brain is not well understood. Materno-fetal transfer: The fetal blood concentration (around 0.01 [tg/l without maternal supplementation) is held to much lower levels than tn the maternal blood. While some transfers undoubtedly take place, possibly with a delay and slow release from placenta to the fetus (lioka et al.. 19911, the precise mechanisms arc not known. Uptake of vitamin k-containing lipoproteins from maternal blood is certain to involve one or more of the many lipoprotein receptors at the microvillous side of the syntro-phoblast.

mcoitnamide mcoitnamide

Figure 9.29 The vitamin K cycle

Metabolism

Vitamin K cycle; The best-characterized function of vitamin K is the gamma-carboxylation of specific glutamyl residues in a handful of proteins. Vitamin K-dependent carboxylase (no EC number assigned for the carboxylase activity; phyl-loquinone monooxygenase, EC1.14.99.20) uses the large redox potential of hvdro-quinone to drive the reaction. One mole of vitamin k 2.3-cpoxidc is generated for each carboxylated glutamyl residue, A two-step system of reductases regenerates vitamin K hydroquinone for the next round ofcarboxytation. Warfarin-sensitive vitamin-k-epoxide reductase (EC 1.1.4.1) uses thioredoxin or another reduced thiol for reduction to the vitamin K form. A Warfarin-insensitive form of vitamin-K-epoxidc reductase (EC 1.1.4.2) has lower activity. The oxidized thioredoxin. in turn, is restored to its reduced form again by NADPH-dependent thioredoxin reductase (ECl.6.4.51. The sceond step is catalyzed alternatively by the vitamin-«.-epoxide reductases or by NAD(P)H:quinone oxidoreductases 1 and 2 (diaphorases, EC 1.6.99.2). Both NA1)( P)H> quinone oxidoreductases 1 and 2 contain EAD. Coenzyme I is inhibited by warfarin, whereas coenzyme 2 (a zinc mctalloenzyme) is not. The latter uses dihydronieotinamide riboside (Mill) instead of NADPH as the electron donor. Both NAD(P)H:quinone oxidoreductases also convert dietary v itamin K into the active hydroquinone form. Side-cham resynthesis: Menaquinone-4 (menatetrcnone. MK4) is the main vitamer of vitamin K in the brain and is present in most other tissues. Since normally very little

Fijjurti y . Ji Mrn,iquinone-4 is an important vttamin K metabolite

two slut» omegnuJdanon multiple rpuntls ol beta oxKlalion one round of beta-oiKfcitiori

Figur« '>,31 The vitamin K suit cham is roetaboli«d by omega-oxidation

Figur« '>,31 The vitamin K suit cham is roetaboli«d by omega-oxidation

MK4 is ingested and intestinal bacteria are not necessary for its appearance (Ronden et ul.. 1998). endogenous activities must exist that convert pbylloquinone and other forms of vitamin into \1K4. The exact activities involv ed in this conversion have not been identified yet.

Catabalism: The side chain of vitamin k is shortened by omega-oxidation to the acidic catabolitcs 2-met hy 1-3-(5'-carboxy-3'-methy I -2'pentenyI)-1,4-naphtoquinone and 2-mcthy l-3-( 3 '-carboxy-3 '-methy Ipropyl)-1.4-naphthoquinone (Shearer et at.. 1972).

Storage

Liver contains the largest amount (typically M2nmol) of vitamin K in the body tThijssen and Drittij-Reinders, 1996), mostly as menaquinones (64nmolkg), and much less as phylloquinone (10.6 nmol/kg), Bone also contains significant amounts (about 53 nmol kg), as does the fat tissue (about 22 nntol kg) in bone (Hodges et ah, 1993). 1 lowever, since no mechanism seems to provide for the mobilization and transport of stored vitamin K, each cell or tissue appears to be on its own in times of need.

Excretion

About 60% of vitamin k losses are into bile, the remainder goes into urine (Shearer el ul„ 1972), The bile contains conjugated inactive metabolites, but little, if any. intact vitamin K. Since most circulating vitamin K is contained in lipoproteins, very little gels into glomerular ultraliltratc in the kidney, and those small amounts are recovered again from the proximal tubulus. The main cataboliics in urine are glueuronides of 2-methyl-3-( 5'-carboxy-3'-methv 1-2'penteny I)-1.4-naphtoquinone and 2-methvl-3-(3'-carboxy-3'-methylprppyl)-l^naphthoquinone (Shearer et a!., 1972).

Regulation

It is not clear whether there is any homeostatic regulation of tissue concentration or stores in the body.

Function

Blood coagulation: "flic first property assigned to \ itamin K was the ability of compounds to prevent bleeding. Indeed the k from the German word for coagulation ("koagulation") became the vitamin's moniker. Four clotting factors (11. VI1. IX. and Xiare known to require vitamin K-dcpendcnt modifications. In addition, and of equal importance, are another three v itamin K-dependent proteins (proteins C, S. and Z), which counteract and regulate the action of the clotting factors.

Thrombinogen (coagulation factor 111 and the blood coagulation factors VII, IX. and X have 10 be post-translationally modified to lie active. Membrane-bound microsomal gamma-carboxylase uses the v ery high activ ation energy of vitamin k hydro-quinone to attach an additional carboxvl group in gamma-position to a few specific glutamyl residues in the target proteins.

Regulation of tissue mineralization: The v itamin K-dependcnt matrix Cjla protein and other calcium-complexing proteins suppress the calcification of soil tissues, and in particular of the arterial wall (Schinke el al„ 1998). Genetic deficiency of matrix Gla protein (in animal models) causes fatal calcification in a very short time. Low vitamin k intake of human adults promotes aortal calcification much more slowly (J ie et at., 1996).

Bone mineralization: Adequate vitamin K intake is critical For bone mineral retention and prevents fractures, and high intakes often are beneficial in this respect (Shiraki et ul.. 2000). The role of the vitamin K-dependent proteins osteocalcin, matrix Ola protein, and protein S in bone remains unclear. The attenuation of PT1 ¡-stimulated IL-6 production by vitamin K (which is not abolished by warfarin) or other mechanisms may he of importance (Kohlmeier et al., 1998). Indeed, the vitamin K catabolite 2-methyl. 3-(2'methyl)-hexanoic acid-1,4-naphthoquinone. which does not support gamma-carboxylation, is a more potent suppressor of stimulated IL-6 secretion than vitamin K (Reddi etal, 1995),

Signaling: At least four of the v itamin K-dependent proteins (thrombin, factor Xa. protein S. and gas6) bind to specific cell surface receptors and elicit typical responses. Gas6 (growth arrest-specific protein ii) binds to the tyrosine kinase receptors Axl (Ufo, Ark). Dtk (Sky Rse/Tyro3/Brt Tif). and Mer (Eyk). Protein S also binds to Dtk. but may not be the native ligand. Binding to Axl affects proliferation and differentiation through its effects on cell cycle progression (Goruppi et al., 1999), and possibly through protection against apoptosis (Uellosta et al.. 1997; Fee el al., 2002). 1 he gas6-induced effects are at least partially mediated through different signaling pathways that involve phosphatidylinositol 3-hydroxy kinase i P13K), the oncogen sre, p3J> MAPK (Goruppi et al., 1999), and Akt (Lee et al.. 2002). Some tissues, such as the renal mesangium (Yanagila et ah, 1999). are critically dependent on adequate v itamin K supplies for normal cell growth in adulthood. Gas6 is also essential for the initiation of phagocytosis of photoreceptor outer segments in the human retina (Hall c/ at.. 2001). The retina degenerates vv ithout this activity. Other tissues may have critical needs during fetal or childhood development. Anuvitamin K medication during early pregnancy is known to cause typical craniofacial malformations (Howe and Webster, 1994).

Thrombin binds to specific G-protein-coupled receptors and induces the synthesis of several humoral messengers, including endothelin, vasopressin, nerve growth factor, and platelet-derived growth factor.

Gla-protems with unknown function: Proline-rich Gla proteins I (PRGP I) and 2 arc another two newly recognized v itamin K-dependent proteins, but their function is not yet known. Recently the transmembrane Gla proteins 3 (TY1G 3) and 4 (TMG 4) have been identified.

Sulfur metabolism: The acliv ities of galactocerehroside sulfotransferasc (tt 2.8.2,111 and arylsulfata.se (EC3.1.6.1) in brain are vitamin K-dependent (Sundaram and Lev. 1992), though the precise nature of this requirement is not known. Vitamin K deficiency (warfarin treatment) induces typical changes in the profiles of complex lipids m brain. Aryl.su I fat ase E (EC3.1.6.I) in bone and cartilage is another fully characterized v itamin K-dependent enzyme (Panicle et a/.. 1998) whose inhibition during early pregnancy is mostly likely related to the typical carniofacial defects in children of warfarin-treated mothers.

Prostaglandin metabolism: Vitamin K inhibits the activity of prostaglandin II synthase (COX-2, KC 1.14.99.1) in bone and thereby decreases the production of prostaglandin E2 (Koshthara et al., 1993). This may explain the inhibiting effect of v itamin K on stimulated interleukin-ii (II -6) production in fibroblasts (Reddi et al., 1995) and bone cells (Kohlmeicr ei a!., 1998), since prostaglandin E2 is a potent activator of IL-6 synthesis.

The reported antinocivc effect ofvitamin K (Onodera et uL, 2000) may be due to a similar mechanism.

Mitochondria: Vitamin K is oxidized very actively in mitochondria (Inyangetor and Thierry-Palmer, 1988). Since all currently known post-translational protein carbox-ylation reactions lake place at the endoplasmic reticulum, the target of the oxidative activity in mitochondria remains unclear.

References

Bellqsta P. /hang Q, Goff SI'. Basilico C. Signaling through the ARK tyrosine kinase receptor protects from apoptosis in the absence of growth stimulation. Oncogene 1997:15:23X7 97

Booth SL. Lichtenstein AH, O'Brien-Morse M. Mckeown NM. Wood RJ. Saltzman E, Gundberg CM. ¡fleets of a hydrogenated form ofvitamin K on bone formation and resorption. Am J Clin Nutr 2001:74:783 90 Conlj JM. Stein K. The production of menaquinones (vitamin K2| by intestinal bacteria and their role m maintaining coagulation homeostasis. Prvgr Food t\ulr Sci 1992: 16:307 43

Daniele A. Parenti Ci, d'Addio M. Andria G. Ballabio A. Meroni G. Biochemical characterization of arylsulfatase E and functional analysis of mutations found in patients w ith X-linked chondrodysplasia punctata. Am J Hum (tenet 1998;62:562-72 (iarber AK. B ink ley Ni'. krueger IK'. Suttie JW. Comparison ofphylloquinonc bioavailability from food sources or a supplement in human subjects../ Nutr 1999:129: 1201-3

(iijsbers BL. Jie kS. Vernieer C. Effect of food composition on vitamin k absorption in human volunteers. Br J Nutr 1996:76:223-9 (ioruppi S. Ruaro E, Vamum B. Schneider ('. (iasO-mcdiatcd surv ival in N1H3T3 cells activates stress signalling cascade and is independent of Ras. Oncogene 1999;18: 4224-36

Hall MO. Pricto AL. Obm MS, Abrams TA, Burgess BL, Ileeb Ml. Agnew HJ Outer segment phagocytosis by cultured retinal pigment epithelial cells requires tiasft. Exp Eye Res 2001;73:509 20 Hodges SJ, Bejui J, Leclcrcq M. Delmas PD. Detection and measurement of vitamins kl and k2 in human cortical anil trabecular bone. J Bone I tin Res 1993:8:1005-8 Howe AM. Webster WS, Vitamin k-its essential role in craniofacial development. A review of the literature regarding vitamin k and craniofacial development. Austral Dent J 1994:39:88-92 lioka II, Moriyama IS. Morimoto k, Akada S, Hisanaga 11. Ishihara V. Ichijo M. Pharmacokinetics of vitamin k in mothers and children in the perinatal period: transplacental transport of vitamin k2 (Mk-41, Asia Oceania J Ohstet Gynaecol 1991:17:97- 100

Inyangetor PT, Thierry-Palmer M. Synthesis ofvitamin k 1,2,3-epoxide in rat liver mitochondria. Anli Biochem Biophys 1988:262:389 96

Jie KG. Hots ML. VermeerC. Witteman JC, Urobbce DL. Vitamin K statu* and bone mass in women with and without aortic atherosclerosis: a population-based study. Calc Tissue int 1996;59:352-6 Kaneki M. Hedges SJ. Hosot T. Fujiwara S. Lyons A, Crean SJ, Ishida N. Nakagawa M, Takechi M, SanoY. MbrunoY, Hoshino S. Miyao M, Inoue S, Horiki K, Shiraki M. Ouchi Y. Orimo H. Japanese fermented soybean food as the major determinant of the large geographic difference in circulating levels of vitamin K2: possible implications for hip-fracture risk. Nutr 2001; 17:315 21 Kohlmeier M. Saupe J, Drossel HJ. Shearer MJ. Variation of phylloquinone (vitamin K,)

concentrations in hemodialysis patients. Thromb Haemostas 1995;74:1252-4 Kohltneier M. Chen XVV. Anderson JJB. Vitamin K reduces the II .-6 stimulating effect of

PTH in murine osteoblasl-like cells. Bone l99}i;23:S564 KoshiharaY, Hosht K, Shiraki M. Vitamin K2 (menatetrenone I inhibits prostaglandin synthesis in cultured human osteoblast-like periosteal cells by inhibiting prostaglandin H synthase activity. Biochem Pharmacol 1993;46:1355-62 Lee WP, Wen Y. Varnum B. Flung MC. Akt is required for Axl-Gas6 signaling to protect cells front F.l A-inedialed apoptosis. Oncogene 2002;21:329-36 Lipsky JJ. Nutritional sources of vitamin K. Mayo Clinic Pmc 1994:64:462-6 Mitchell GV, Cook KK. Jenkins MY. Grunde! E. Supplementation of rats with a lutein mixture preserved with vitamin E reduces tissue phylloquinone and menaquinone-4. Int J Vit Nutr Res 2001:71:30-5 Onodera K. Shinoda 11. Zushida K, Taki K. Kamei J. Antinociceptive elleci induced by intraperitoneal administration of vitamin K2 (menatetrenone) in ICR mice. Life Sei 2000:68:91 -7

Reddi K. Henderson B. Meghji S. Wilson M. Poole S. Hopper C, Harris M. Hodges S.l. Interlcukin 6 production by lipopolysaccharide-stimulated human fibroblasts is potently Inhibited by naphthoquinone (vitamin Kl compounds. Cytokine 1945:7:2X7 90 Ronden JE. Drittij-Reijnders MJ. Vermecr C.Thijssen HH. Intestinal flora is not an intermediate in the phylloquinone-menaquinonc-4 conversion in the rat, Btochim Biophys Acta 1998;1379:69-75 Schinke T. McKee MD. Kiviranta R. Karsenty (i. Molecular determinants of arterial calcification, Ann Med 1998:30:538- 41 Schurgers LJ. Venneer C. Determination of phylloquinone and menaquinones in food. Effect of food matrix on circulating vitamin K concentrations. Haemostasis 2000;30:298-307 Shearer MJ. Mallinson CN, Webster GR, Barkhan P Clearance from plasma and excretion in urine, faeces and bile of an intravenous dose of tritiated vitamin k 1 in man. Br J Haematol 1972:22:579-88 Shearer MJ. Bach A. Kohlmeier M. Chemistry, nutritional sources, tissue distribution and metabolism of vitamin K with special reference to bone health../ S'utr 1996;126: II8IS-I186S

Shiraki M. Shiraki Y. Aoki C. Miura M. Vitamin K2 (menatetrenone) effectively prevents fractures and sustains lumbar bone mineral density in osteoporosis. ■> Bone Min Re\ 2000; 15:515-21

Sundaram KS. Lev M, Purification and activation of brain sulfotransferase../ Biol Chem 1992; 267:24041-4

Thane CW, Paul AA, liuies CJ. Bolton-Smith C. Prentice A. Shearer MJ. Iniake and sources of phylloqumone (vitamin K1): variation with socio-de mug rapine and lifestyle factors in a national sample of British elderly people. BrJNutr 2002;87:605-13 Thijssen HHW, Drittij-Rcinders MJ. Vitamin k status in human tissues: tissue specific accumulation of phylloquinone and menaquinone-t. Br J N'tttr 1996;75:121-7 Yanagita M, Ishii K. Qzaki H. Ami II. NakanoT. Ohashi k. Mi/uno k. Kita T. Doi T. Mechanism of inhibitory effect of warfarin on mcsangial cell proliferation. J Am Sue Nephro! 1999:10:2503 9

Cholesterol

Cholesterol (molecular weight 387) is the principal neutral sterol in mammals.

Note: The mechanisms involved in the synthesis, intestinal absorption, transport and maintenance of cellular and whole-body homeostasis of cholesterol are likely to involve well in excess of a hundred distinct genes and arc by far the most complex for any nutrient.

Abbreviations

ASCA1 ATP-bmding cassette A1

A8CC5 ATP-binding cassette G5

ABCC8 ATP-binding cassette G8

ApoB apolipoprotein B

ApoB48 apolipoprotein B48

ApoBlOO apolipoprotein B100

ApoE apolipoprotein E

Choi cholesterol chylos chylomicrons

CYP cytochrome p450

HDL high-density lipoproteins

LDL low-density lipoproteins

LDL-R LDL receptor

Lp lipoprotein

Fiyur* 9.32 Cholesterol

LRP LDL receptor-related protein

SR-B1 Scavenger receptor class B type 1

SREBP sterol regulatory element-binding protein

VLDL very-low-density lipoproteins

Nutritional summary

Function: Cholesterol (Choi) is an essential component of membranes and serves as a precursor for the synthesis ofbile acids and steroid hormones. Intermediates of Choi synthesis are also used for protein modification and for the s\ nthesis of ubiquinone, dolichol. and vitamin D.

Food sources: Most foods of animal origin contain Choi, the largest amounts come from fatty foods and from liver. Plant-derived foods contain no significant amounts of Choi, regardless of their fat content.

Requirements: More than adequate amounts can be produced by the body, and there is no evidence that intake at any level improves health.

Deficiency: No adverse effects of low or absent intake are known.

Excessive intake: I ligh Choi intake increases blood Choi concentrations, and possibly cardiovascular risk, in some people, but not in others.

Endogenous sources

Daily Choi synthesis in healthy people is about 15mgkg body weight (Rajaratnam ei ul., 2001). Choi intake decreases endogenous synthesis moderately iJones et ul.. 1996), but this effect is seen more clearly in some people (responders) than in others. High fat intake (Ltnazasoro et id.. 1958) and many other factors increase Choi synthesis. Consumption of the Choi precursor squalene at levels comparable to those in normal foods also promotes cholesterol synthesis (Relas et id.. 2(111(1)

The main sites of production arc liver and intestines, smaller amounts come from many Other tissues. Choi synthesis proceeds in cytosol with aeety 1-CoA as a precursor and requires adequate availability of riboflavin, niacin, pantothenate, magnesium, and iron. The rate-limiting step of Choi synthesis is the production of mevalomite, which is then converted to the 5-carbon isoprcnoid isopentenyl pyrophosphate Addition of six isoprcnoid units generates squalene. which is eyclized and then convened in two more steps 10 Choi. The first reactions serve the synthesis of both Choi and several important isoprcnoid compounds. A parallel system for the synthesis of isoprenoids. which operates in peroxisomes, uses some genetically distinct isoenzymes and is likely to be regulated differently. Farnesyl pyrophosphate provides a lipophilic membrane anchor for specific proteins, and is the precursor for the synthesis of ubiquinone (oxidative phosphorylation) and dolichol (synthesis of N-linked glycoproteins). Isopentenyl phosphate might also be important as a precursor for the side-chain re-synthesis ofvitamin K.

The acetate precursor for Choi synthesis is shuttled from mitochondria (where it is generated by the catabolism of fatty acids and some amino acids) to thecytosol as citrate bv the citrate transport system. Acetyl-Co A is then released from citrate in cytosol by ATP-citratc (pro-S-(-lyase (EC4-1.3.8). a magnesium-dependent enzyme. The activity of this critical branch-point enzyme is regulated in part through phosphorylation by cAMP-dependent protein kinase.

Hydroxymethylglutaryl-CoA synthase (HMG-CoA synthase: EC4.1.3.5) condenses aeetyl-CoA and acctoacetyl-CoA. Acctyl-CoA C-acety [transferase uhiolase; EC2.3.I.9) produces the latter by linking two acetyl-I oA molecules. 1 he cytoplasmic forms of both enzymes for sterol synthesis are genetically distinct from the mitochondrial forms for ketone body synthesis.

The next step. NADPH-dependent reduction ofhydroxymethylglutaryl-CoA to meva-lonate by hydroxymethylglutaryl-CnA reductase(HMCi CoA reductase; EC]. 1.1.34), is a tightly controlled reaction that sets the pace of Choi production. Phosphorylation by hydroxymethylglutaryl-CoA reductase(NAPPH) kinase (EC2.7.I.109) inactivates HMG CoA reductase: dephosphorylation by hydroxymethylglutaryl-CoA reductase (NADPH )-phosphatase (EC3.1.3.47) reactivates the phosphorylated enzyme. Mevalonate is then diphosphorylated in two ATP-consuming steps (catalyzed by mevalonate kinase. EC2.7,1.36; and phosphomevalonate kinase, EC2.7.4.2) and converted to isopentenyl phosphate by diphosphomevalonate decarboxylase (EC4.I.1.33). The magnesium-dependent enzyme isopentcnyl-diphosphatc delta-isomerase (EC5.3.3.2) produces dimethylallyl phosphate, the second live-carbon precursor used in isoprenoid and Choi synthesis.

A protein expressed in liver and many other tissues first generates gcranyl diphosphate by linking isopentenyl phosphate and dimethylallyl phosphate (dtmethylallyl-transferase; EC2.5.1.11 and then produces trans.trans-faniesyl diphosphate by adding another isopentenyl diphosphate moiety (geranyltranstratisferuse; EC2.5.1.10). A genetically distinct protein, which is strongly expressed in testis, at lower levels in most other tissues, has the additional capacity to synthesize by adding yet another isopentenyl diphosphate to trans, trans-farnesyl diphosphate (famesy I trans-trans fe rase; EC2.5.I.29). The further fate and ultimate purpose of this 20-carbon intermediate is not completely understood.

Two larnesyl pyrophosphates are joined by farncsy [-diphosphate famesy I transferase (Squalene synthase; EC2.5.I.21, cofactor magnesium) and then reduced by the same NADPIl-dependent enzyme. The linear 30-carbon precursor squalene is oxidized in an NADPH-consuming reaction by the FAD-enzyme squalene monooxygenase (squalene epoxida.se; EC 1.14.99.7) and cyclized to the first sterol intermediate lanostero! by lanosterol synthase (EC5.4.99.7).

To complete Choi synthesis, the methyl groups at positions 4-alpha. 4-bcta and 14 have to be removed, the double bond al position saturated, and the double bond at position 8 moved to position 5. These reactions are catalyzed by lanosterol demethy-lase (uses NADPH for the decarboxylation reaction, no EC number assigned), lathos-terol oxidase (EC 1.3.3.2), and 7-dehydro-cholesterol reductase (EC 1.3.1,2) I. Several pathways are possible, depending on relative activities of the involved enzymes, but the preferred sequence may be lanosterol > 14-alpha desmethyllanosterol

Irtxn mlochoodna to mrtochofxlna

COOH g ho-c-cooh

COOH

COOH

COOH Oaaioacetste

HydrcxyiTOtnvtgfutsiyi CoA

CoA-S—C Acsfyl-CoA COOH

HydrcxyiTOtnvtgfutsiyi CoA

Acetyl-CcA Cacetyi

Acetyl-CcA Cacetyi

COOH

Acetoatwyt-Co»

vCOOM

COOH

Meiralonale

Mcvakmjilc U" MP tiirAAo [ t<nignt>lum]N»ADP . p

,COOH

S-PtKHPhomiVBIOnste Pfwiimo- (, ATP

mevakdialfi f mevakdialfi f

.COOH

.COOH

Dtp* »ouphfiiTMjv «loi ta 1 e rtacartioxyta&fl

Dtp* »ouphfiiTMjv «loi ta 1 e rtacartioxyta&fl t&cpcnto nyliJ'p-IvîsphrttP

IsopGntorvyl diptiowhaie o-isomerase

OvPaHtO DmwttiyJolJyfdiplKKphate

t&cpcnto nyliJ'p-IvîsphrttP

GcranyidiphMpiwle

EjipMOùpfallG

Farni^ttyhotphate

NAOPH NADPH

Fame»yl'<iipN»OMitevx^' 2 P, tnmonyilmrtfiloras« {magn&siumi

Squale no rnooooxygcnasfl

Squatenu

Squale no rnooooxygcnasfl

S'Squa&onc 2.3-opoJtKj®

S'Squa&onc 2.3-opoJtKj®

NAOPH

umosleroi f-igur* 9.33 Endogenous cholesterol synthesis starts from acetyl-CoA

figure 9.54 Sleroli in foods

> zymosterol > 5-alpha-cholest-8-en-3-bcta-ol > lathosterol > 7 dehydrocholes-terol > cholesterol. Desmosterol (delta-3,24-eholestadien-3-beta-ol I is one of several alternative intermediates. 7-dchydrocbolcstcrol is important because it is the precursor for ultraviolet (UV-I3) light-induced synthesis of vitamin f) in skin (Obi-Tabot ft aL 2000).

Dietary sources

Membranes and fat deposits of animal-derived foods contain cholesterol, plant-derived foods contain plant sterols instead, A large portion of cholesterol in animals is linked to fatty acids, predominantly the long-chain mono- and polyunsaturated ones. The single large si source of Choi in the American diet is eggs (4.3 mg/g) providing about 215 nig per serving (50 g). Other Choi-rich include organ meats, such as liver (3,9mg g), and animal fats, such as tallow (1.1 mg/g), lard (l.Omg'g), and butter (2.2 mg/g) and all meats and lish. The fat-free pan of meats still contains between 0.7 0.9mg/g. Typical daily intakes in the US are around 300-500 mg. but are much lower in vegans and other groups with minimal intake of animal-derived products. Signilicant amounts of squalen and other Choi precursors are also be present in some foods. Plant-derived foods contain a wide range of neutral sterols thai resemble Choi structurally, but cannot substitute for most of its functions and have different metabolic fates. Several hundred milligrams of such phytosterols are consumed daily with typical mixed diets.

Digestion and absorption

Dietary Clio! is absorbed best from mixed micelles that contain bile acid, phospholipids, and monoglycerides. Bile acids and phospholipids are from bile, the monoglycerides are generated by the action of gastric and pancreatic lipases (EC3.1.1.3) on dietary triglyceride. Bile-salt-activated lipase from pancreas and mammary gland has car-boxvlcster lipase (EC3.1.1.13) activity and cleaves Choi esters in concert with pancreatic lipase (EC3.1.1.3)andcolipase. Most people absorb 40 45% of a moderate Choi dose ingested with triglycerides (Rajaratnam ei ul.. 20011. Absorption occurs mainly in the proximal small intestine, is selective (high compared to closely related plant sterols), and responsive to whole-body Choi status (Lu et a/„ 2001).

The mechanism whereby Choi moves from the mixed micelles across the cnteroeyte brush border membrane is still under dispute (Mardones ef at., 2001). It has been suggested that the scavenger receptor CD36 is involved in uptake of free Choi, and that the scavenger receptor class B type 1 (SR-B1) plays a parallel role in the uptake of Choi esters (Werderet ul.. 2001). The interaction takes place at cholesterol and sphingomyel in-enriched domains of the plasma membrane called caveolae, which facilitate lipid exchange between cells and Lp (Grafel a!., 1999). The caveolae contain caveolin, a protein with as yet incompletely understood function.

Once in the cell, most ( hoi is estertlied by at least two genetically distinct forms of sterol O-acyltransfcrase (EC2.3.1.26), AC AT, and AC AT2 (Buhman el at., 2000). Both forms specifically csterify neutral sterols with omega-9 fatty acids, such a> oleic acid.

membrane membrane

Figurr 9.JS Intestinal absorption ofcholKtrrol membrane membrane

Figurr 9.JS Intestinal absorption ofcholKtrrol

Some of the Choi is incorporated into chylomicrons (Chylos) and leaves the entero-cyte as Chylos are secreted into adjacent lymph ducts. Some of the intracellular Choi is actively pumped back into the intestinal lumen (Repa et at.. 2(HHI) by the ATP-binding cassette transporter AI (ABCAI), Absorption efficiency is modulated by a heterocom-plex containing ABCG5 and ABCG8 (Berge el at.. 2000) in an as yet unclear manner.

Choi is exported from enierocytes into adjacent lymph ducts with chylomicrons (Chylos). very triglyceride-rich lipoproteins. Each chylomicron contains a single copy of apolipoprotein B48 (apoB48). A highly specific cytidine deaminase (apolipopro-tein B editing catalytic suhunit I. APOBEC-I) is part of a multiprotein complex, which modifies cytidine 666b of the apoB inRNA to uridine and thereby introduces a stop eodon into the sequence. Due to this modification, the intestinal protein transcripts arc shortened to about 48"» of the full-length version (hence B48). In humans this enzyme is expressed only in small intestine (Chen el at.. 200J). which means that all intestinal apoB is of the apoB4X variety. Rodents and many other mammals, in contrast, produce in their eutcrocyics both apoB48 and the full-length version, apoBIOO.

Transport and cellular uptake

Blood circulation: All cholesterol in blood is contained in lipoproteins (Lp), and all Lp contain Choi. Lipoproteins arc classified based on their buovant density and additional pathogenetic characteristics. The major lipoproteins in blood are low-density lipoprotein (LDL), high-density lipoprotein (HDL), very-low-density lipoprotein (VLDL), Lp(a). Chylos. VLDL remnants, and Chylo remnants.

Chylos. which carry triglycerides from small intestinal absorption, release much of their triglycerides as they pass through small arterioles and arterial capillaries, due lo triglyceride hydrolysis by lipoprotein lipase (EC3.1.1.31. The resulting Chylo remnants are taken up mainly into liver and bone marrow (Cooper, 1997). VLDL. which transport

Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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