Thiamin

How to Beat Addiction and Quit Alcoholism Easily

Tips On How To Quit Drinking

Get Instant Access

Thiamin (thiamine, vitamin Bl. 3-j4-aniino-2-methyl-5-pynmidmylmethyl]-5-(2-hydroxyethylH-methylthiazolium. aneurine. molecular weight of the hydrochloride

Figure 10.8 Thiamin

337) is a water-soluble vitamin whose heterocyclic rings contain both nitrogen and sulfur.

Abbreviations

RFC-1 reduced folate carrier (SLC19A1 )

TCP thiamin carrier protein ThTr2 thiamin transporter 2 (5LC19A3)

TMP thiamin monophosphate

TPP thiamin pyrophosphate

TTP thiamin triphosphate

AK adenylate kinase

Nutritional summary

Function: Thiamin pyrophosphate is an essential co fact or of live enzymes involved in carbohydrate, amino acid intermediary (tricarboxylic acid cycle), and phvtol metabolism; thiamin triphosphate is important for brain function. Food sources: Good sources are yeast, legumes, enriched grain products, and pork. Requirements Men should get 1.2 nig d women 1.1 mgd: needs are slightly higher during pregnancy and lactation. Use of diuretics increases dietary needs. Deficiency: Early signs may include anorexia, weight loss, muscle weakness, apathy, confusion, and irritability. Later consequences include edema (wet Ben ben), muscle wasting (dry Beriberi), and psychosis (Wernicke Korsakoff syndrome). Onset of cardiac failure in young infants can be very sudden and rapidly lethal. Alcohol abuse is often associated w ith deficiency, possibly because alcohol interferes w ith thiamin uptake and metabolism; a rare genetic disturbance of transporter-mediated cellular uptake results in megaloblastic anemia, diabetes mellilus, and sensorineural deafness (Labay el a!.. 1999). Excessive intake: No adverse elfccts from high oral doses las much as 10 mg d) have been reported. Anaphylactic reactions following parenteral administration have been observed.

Dietary and other sources

Foods may contain thiamin pyrophosphate ( I PPl, thiamin monophosphate (TMP), thiamin triphosphate (TTP). thiamin, thiamin sulfide, thiamin disulfide, thiamin tetrahydrofurfuryl disulfide (added to wine), thiamin hydrochloride: thiamin is a monovalent cation at physiological pll Median intakes of adult men in the US are about 1.9 mg d (Food and Nutrition Board. Institute of Medicine, 1998),

Good sources include yeast, legumes, enriched or whole grain products, bran, and pork. By law. bread in the US is enriched with 1mg/lb, and flour with 2.9 mg lb.

Significant amounts of thiamin are lost when foods arc cooked in water and the vitamin leaches out. Thiamin also is unstable when exposed to high lemperattires, irradiation, alkaline medium (bicarbonate treatment of peas and beans), or metabisul-phite (preservatives in dried fruits and wine): oxidation generates thiochrome and other oxidation products.

Vntithiamin factors are present in betel, tea (caffeic acid tannic acid chlorogeuic acid), and foods (possibly heme and hemin are involved): ascorbate and other organic acids may be protective.

Thiamtnase (EC3.5.99.2) is active in various microorganisms, including saccha-romyces. cerevisiae and Staphyhtcacvm aureus. Thiamin in foods contaminated with thiaminase is cleaved into the inactive breakdown products 4-aniino-5-hydroxy-methyl-2-methylpyrimidine and 5-(2-hydroxyethyI )-4-niethyIthiazole.

Thiamin pyriduiylase (EC2.5.1.2) from viscera of some fish and shellfish diminishes the vitamin content of exposed foods by converting thiamin to heteropyri thiamin.

Large amounts of thiamin are produced in the colon by normal enteric bacteria and secreted in free form, Enterocytes of the colon express specific thiamin transport capacity and thus can take up thiamin synthesized by enteric bacteria (Said et til.. 2001). The quantitative contribution from this source is not known, however.

Digestion and absorption

Dietary thiamin is absorbed throughout the small intestine, maximally in the duodenum. Evidence is mounting that some thiamin from bacterial production can be absorbed in the colon (Said el at, 2001). Absorption of microgram amounts from the small intestine may be nearly complete: it increases with thiamin deficiency (Laforenza et at. 1997). and decreases with folate deficiency. Aging (Rindi and Laforenza. 2000) and alcohol intake (Breen et at. I9K5) decrease fractional absorption: concomitant alcohol ingestion may decrease absorption from a single thiamin dose by nearly .10%. At doses around 5mg d absorption becomes very ineffective (Hayes and Hegsted, 1973). Only about 5% of a 50 mg dose is absorbed (Tallaksen et at., 1993).

The main forms of thiamin in unfortified foods. TPP.TTP, andTMP. are hydrolyzed by nucleotide pyrophosphatase (EC3.6.1.9) from pancreatic exocrine secretions (Beaudoin el at.. 19X3) and brush border alkaline phosphatase (EC3.1.3.1. zinc- and magnesium-dependent). UnphosphoryIated thiamin can then be taken up at low concentration \ia a thiamin-proton antiponer (Dudcja ei at.. 2001; Said et at.. 2001) likely to be the high-aflinily thiamin transporter SLC19A2 (Fleming et at., 2001). I his transporter is present both ill the small and in the large intestine. In addition, a low-aflinity, high-capacity system exists that can meet most of the body's needs in the absence ofSLC19A2 (Stagg etui., 1999). It may be this system that accounts for thiamin uptake by passive diffusion at high concentrations (Rindi and Laforenza. 2(XH)).

TPP. TMP, TTP Nucleotide

Thiamin

Inte si i nal lumen

TPP. TMP, TTP Nucleotide

Thiamin

Inte si i nal lumen

Brush border membrane

Figure 10.9 lntestin.il alnorption of thiamin

TPP

i

1 AMP

Thiamin

,r

PP-kinase

IMg)

ATP

l

Thiamin

TMR

TMP, ,

Thiamin

Capillary lumen

Brush border membrane

Basolaterai membrane

Capillary endothelium

Figure 10.9 lntestin.il alnorption of thiamin

The reduced rotate carrier (RFC-1, SLC19A11 may also play a role, since it can transport phosphorylated thiamin (Zhao, Gao. Wang el id.. 200) I.

Once in the enierocyte, the free thiamin is phosphorylated to TPP by thiamin pyrophosphokinase (EC2.7.6.2) and thereby prevented from returning to the intestinal lumen.

Transport across the basolaterai membrane into portal blood uses an ATP-driven thiamin carrier (Laforenza et al.. 1993) that is not yet well characterized. Some transport of phosphorylated thiamin, in particular TMP. may also be attributable to the reduced folate carrier (SLC19A I: Zhao, Gao. Wang ei a!., 2001 ). Since most of the thiamin in blood is in free or monophosphorylated form. TPP must be clea\ ed prior to export. This step is not well characterized.

Transport and cellular uptake

Blood circulation: Transport in blood plasma occurs mostly as thiamin bound to albumin (about 10 20nmol/l); the concentrations of TMP are slightly lower. Concentrations in whole blood cells are an order of magnitude higher (around 200nmol I). The predominant species in red blood celts is TPP; I MP and thiamin contribute much less (Tallaksen el aL, 1907). Thiamin can be taken up into hepato-cytes. erythrocytes, and other cells by active transport (La bay el a!.. 1999) and from there into mitochondria through the high-affinity thiamin transporter I (SLC19A2). A second closely related, thiamin transporter (ThTr2. SLC19A3) in liver, heart and other tissues also mediates selective high-affinity uptake of tree thiamin (Rajgopal et ai, 2001). The reduced folate carrier I (SLC19A1 i provides an alternative minor access route for TMP (Zhao etal.. 2002), TPP efflux via the reduced folate carrier 1 appears to help regulating cellular thiamin homeostasis (Zhao, Gao. Wang et ai. 2001 ). Blood-brain barrier: Free thiamin and to lesser extent. TMP are transported across the blood -brain barrier (Patrini et ai, I9XN). Thiamin may share a carrier-mediated transport system with choline that mediates uptake (Kang et ai, 1990). The thiamin transporter I (SLC19A2) is expressed in brain, but its exact location and role remains to he elucidated.

Materno-fetal transfer: Transport across the placental membrane is known (Dutta et ai., 1999) to involve both the thiamin transporter I (SLCI9A2) and ThTr2 (SLCT9A3; Rajgopal et ai. 2(K)I), but is not vet completely characterized. In the process, thiamin is concentrated in the placenta, relative to both the maternal and the fetal side. Since transport in the materno-fetal direction is slower than in the feto-matemal direction, the net transfer is from mother to fetus (Sehenker el al.. 1990).

Was this article helpful?

0 0
Losing Weight Natures Way

Losing Weight Natures Way

Feel Healthier And Look Better Without Resorting To Drugs! Would you like to get your hands on a this report that can teach you everything you need to know about losing weight naturally?

Get My Free Ebook


Post a comment