Function

High enerQ- phosphate esters: Most transfers of chemical energy in the body involve phosphate ester bonds. This is particularly true for ATP as the main metabolic energy currency, but also applies to more specialized forms, including GTP CTP, creatine phosphate. and argimne phosphate. These and additional nucleotides are also the essential building blocks for the synthesis of DNA and RNA. Several nucleotides, such ascAMP, arc also critical for intracellular signaling.

Parathyroid gland

Parathyroid gland

Small intestine (+) A

Figurr 11.15 A web ollmrmoncs maintains phosphate balance

Small intestine (+) A

Figurr 11.15 A web ollmrmoncs maintains phosphate balance

Organophosphates: A large spectrum of endogenous!} synthesized compounds, aside from the ones mentioned above, contain one or more phosphate groups as an integral part of their structure. Large amounts of various types of phospholipids are needed for the construction of cell membranes, myclm-sheathing of neurons, transport of lipids with lipoproteins, facilitation of intestinal lipid absorption, among other functions, I he phosphate-containing cholesterol precursors should also be mentioned. Activating phosphate esters: I he metabolism of many nutrients proceeds via phosphate esters at some point to provide the necessary reaction energy. Examples include the phosphates of glucose, fructose, galactose, glycerol, etc. The list of vitamins that are active only as phosphate esters includes thiamin, riboflavin, pyridoxine. niacin, and pantothenate. Phosphorylation can also serve to prevent efflux of such nutrients from cells (trapping) and aid uptake by passive diffusion. A typical example of such trapping is the intestinal absorption of vitamin B6. Free vitamin B6 diffuses across the intestinal brush border membrane via as yet unknown earners and is then immediately phosphorylated by pyridoxal kinase (EC2.7.1.35). Since the phosphorvlated form cannot return and intracellular concentration of the free forms is very low. diffusion into the enterocytes continues as long as there arc significant amounts in the intestinal lumen.

Protein phosphorylation The activity of many proteins is regulated through phosphorylation and dephosphorylation. The effect of an added phosphate group depends on the particular protein. Phosphorylation inactivates glycogen synthase (Et'2.4. 111), for instance, and dephosphorylation reactivates the enzyme.

Buffering: Aqueous solutions at physiological pH (around 7,4) contain about four-fifths of the inorganic phosphate as hydrogen phosphate ion (HPOi ). and nearly one-fifth as dihydrogen phosphate ion (H>P04 ). This means that the significant amount of inorganic phosphate in cells, extracellular fluid, and blood acts as an effective buffer and stabilizes pi L

Polyphosphates: Some types of osteoblasts contain significant amounts of polyphosphates (Leyhausen et ai. 1998). These polyphosphates can contain up to several thousand phosphates linked into a chain. The function of these structures is not well understood. Suggested roles include phosphate storage, inhibition of bone mineralization. divalent ion and basic amino acid chelation, apoptosis, pi I regulation, and protection against osmotic stress. Incompletely characterized cxopolyphosphatases, and in some eases pyrophosphatase (EC3.6.1.1) release phosphate ions from the polyphosphate chains.

References

ADA, Pediatric Manual of Clinical Dietetics. Williams CP, editor. The American Dietetic

Association, 1998, pp.385-7 Aloia JF, Vaswani ANM, Ych JK, F.llis K. Colin SI 1. Total body phosphorus in postmenopausal women. Miner Electrolyte Metah 1984; 10:73 6 Arnnabh S, Feuerman M, Ma R, Aloia JF. Total body phosphorus in healthy women and ethnic variations. Metah Clin Exp 2002:51:180 3 Barsotti (i. Cupisti A. Morel 111. Meola M, Coz/a V. Barsotti M, Giovannetti S Secondary hyperparathyroidism in severe chronic renal failure is corrected by very-low dietary phosphate intake and calcium carbonate supplementation. Nephron 1998:79: 137-41

Calvo MS. Park YK. Changing phosphorus content of the US diet: potential for adv erse effects on bone JNutr 1996; 126:116XS I tXOS Dallatre 1.. Bcliveau R. Phosphate transport by capillaries of the blood brain barrier.

J Biol Chem 1992:267:22323 7 Econs M.t New insights into the pathogenesis of inherited phosphate wasting disorders. Bone 1999;25:131-5

Food and Nutrition Board Institute of Medicine. Dietary Reference Intakes for calcium, phosphorus, magnesium, vitamin D. and fluoride. National Academy Press. Washington. DC, 1997 Giachelli CM. Jono S, Shioi A. NIshizawaY, Mori K. Morii H, Vascular calcification and inorganic phosphate. Am J Kidney Dis 20Ol:38:S34 7 Guu R. l.iu S. Spurney RF. Quarles l.D. Analysis of recombinant Pliex; an endopeptidase in search of a substrate, t m.l Physiol Endocrinol Metah 200I;281:E837 47 Lajeuncsse D. Brunette MG. Sodium gradient-dependent phosphate transport in placental brush border membrane vesicles. Placenta 1988:9:117 28 Leyhausen G, Lorenz B. Zhu 11, Geurtsen W. Bohnensaek R. Muller WF. Schroder HC. Inorganic polyphosphate in human osteoblast-like cells. J Bone Min Res 1998:13: 803-12

Moz Y, Silver J. Naveh-Many T. Protein-RNA interactions determine the stability of the renal NaPi-2 cotransporter mRNA and its translation in hypophosphatemic rats. ./ Biol Clwm 1999:274:25266-72

Murer II. Hernando N. Forster 1. Biber J, Molecular mechanisms in proximal tubular and small iniestinal phosphate reabsorption Vfol Membr Biol 2001.18:3-11 Palmer O, /hau J, Bonjour J. Hofstettcr W, Caverzasio J. In vivo expression of transcripts encoding the Glvr-I phosphate transporier/retrovirus receptor during bone development. Bone 1999;24:1-7 Tencnhouse HS, Roy S, Martel J, Gauthier C. Differential expression, abundance, and regulation of Na -phosphate cot rail s port er genes in murine kidney. Am J Physiol IW8:275:F527-34

Walton R.I, BijvoetOL. Nomogram for derivation of renal threshold phosphate concentration.

Lancet 1975;2( 7929):305-10 Voshida I, Yoshida N. Monkawa 1. Hayashi M. Saruta I Dietary phosphorus deprivation induces 25-hydrOxy vitamin D(3) lalpha-hydroxylasc gene expression. Endocrinol 2001:142:1720 6

Magnesium (atomic weight 24.305) is an alkaline earth metal with valence 2.

Abbreviation

Mg magnesium

Nutritional summary

Function: Magnesium (Mg) is an essential cofactor for a large number of reactions, including all of those involving ATP and GT P. participates in muscle and nerve depolarization. stabilizes DNA and RNA. and is a component of the mineral in bone. Food sources: Whole grains, nuts and seeds, spinach, legumes, potatoes, and bananas are good sources, 1 lard tap water and some bottled mineral waters also can be significant sources.

Requirements Men should get at least 400 mg (420 mg over age 30) from food women at least 310mg (320mg overage 30, and 360mg when pregnant). Deficiency: Signs include confusion, disorientation, personality changes, loss of appetite, depression, muscle contractions and cramps, tingling, numbness, hypertension, abnormal heart rhythms, coronary spasm, and seizures. Deficiency is often induced by diarrhea, malabsorption, or vomiting, overuse of laxatives or diuretics, medication (cyclosporin, amphotericin, gentamycin, cisplatin). alcohol abuse, diabetes or hyperparathyroidism.

ExccssiVe intake: Intakes of more than 350 mg from supplements and other nonfood sources may cause diarrhea, nausea, appetite loss, muscle weakness, mental impairment. difficulty breathing, extremely low blood pressure, and irregular heartbeat. Risk of toxicity is greater with impaired kidney function.

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