Vitamins metabolic role

The first two of these roles appear to focus on the conformal chemistry associated with the external profile of the molecules. The latter role focuses on the internal, quantum-mechanical structure of the molecules. While retinol, in its metabolic role as a vitamin participates in the manufacture of components of the disks of the Outer Segment of the Photoreceptor cells, it primary role is the last one. It acts as the critical chromogen, independent of any vitamin or hormonal role, leading to the production of chromophores in the retinal pigment epithelium (RPE) cells of the eye. Morton noted this role specifically In the retina, retinol is indubitably a precursor. ... 

It is not strictly correct to regard niacin as a vitamin. Its metabolic role is as the precursor of the nicotinamide moiety of the nicotinamide nucleotide coenzymes, nicotinamide adenine dinucleotide (NAD) and NADP, and this can also be synthesized in vivo from the essential amino acid tryptophan. At least in developed countries, average intakes of protein provide more than enough tryptophan to meet requirements for NAD synthesis without any need for preformed niacin. It is only when tryptophan metabolism is disturbed, or intake of the amino acid is inadequate, that niacin becomes a dietary essential. 

The Methyl Folate Trap Hypothesis The reduction of meth-ylene-tetrahydrofolate to methyl-tetrahydrofolate is irreversible (Section 10.3.2.1), and the major source of folate for tissues is methyl-tetrahydrofolate. The only metabolic role of methyl-tetrahydrofolate is the methylation of homocysteine to methionine, and this is the only way in which methyl-tetrahydrofolate can be demethylated to yield free tetrahydrofolate in tissues. Methionine synthetase thus provides the link between the physiological functions of folate and vitamin B12. 

Transferring results of studies of the metabolic role and fate of ascorbate in animal experiments to humans is limited because most animals are able to synthesize their need of ascorbate endogenously and in those animals where ascorbate is a vitamin (e.g., the guinea pig), the metabolism of ascorbate differs from that in humans. Estimation of human needs will therefore have to be derived from experiments with humans. 

Figs are a convenient single-food source broad in nutrient content, having exceptional amounts of insoluble and prebiotic dietary fiber, essential dietary minerals, and an unsaturated omega-6 fat, linoleic acid. Essential vitamins A (from carotenoids), B, and K are also present in high densities in the fig. These vitamins have an array of uses in the body—from antioxidant and metabolic roles to participation in blood coagulation and vascular function—that together support cardiovascular health. 

Dutta-Roy, A.K., Gordon, M.J., Campbell, F.M., Duthie, G.G., and James, W.P. 1994. Vitamin E Requirements, Transport and Metabolism Role of CX-Tocopherol Binding Proteins. J. Nutr. Biochem. 5 562-570. 

Metabolic Role. Riboflavin coenzymes are required for most oxidations of carbon-carbon bonds (Fig. 8.29). Examples include the oxidation of succinyl CoA to fumarate in the Krebs cycle and introduction of a,jS-unsaturation in /3-oxidation of fatty acids. Riboflavin is also required for the metabolism of other vitamins, including the reduction of 5,10-methylene tetrahydrofolate to 5-methyl tetrahydrofolate (Fig. 8.49), and interconversion of pyridoxine-pyridoxal phos-phate-pyridoxamine (Fig. 8.33). Because oxi-dation/reductions that use FAD or FMN as the coenzyme constitute a two-step process, some flavin coenzyme systems contain more than one FAD or FMN. 

Chemistry, Uptake, and Metabolic Role. This vitamin, which can be considered a derivative of j3-alanine, is asymmetric (Fig. 8.38). The natural form has the T>(+) configuration. TheL(-) stereoisomer is inactive. The reduced alcohol form, pantothenol, is considered as equally active as the parent acid. Many of the multiple vitamin products use a synthetic, racemic mixture. This means that double the amount of synthetic vitamin must be used to obtain equivalent active vitamin. 

Table 8.5 Metabolic Roles of Ascorbic Acid (Vitamin C)...

Although many of these patients had either a diet poor in folate or malab-sorbed the vitamin, a number appeared to have an increased demand suggesting an abnormality in the metabolic pathway of the vitamin. The role of folate in neurological function is not clear folate is important in DNA synthesis, but little DNA synthesis takes place in nervous tissue. There is a requirement for folate in RNA synthesis and it may be here that a deficiency of folate is able to exert its neurological efiect. 

FIGURE 53-6 Interrelationships and metabolic roles of vitamin and folic acid. See text for explanation and Figure 53-9 for structures of the various folate coenzymes. FIGLU, formiminoglutamic acid, which arises from die catabolism of histidine Tell, transcobalamin II CH3H4PteGlUj, mediyltetrahydrofolate. 

The age pigments (lipofiiscin), which accumulate with age, are largely made up of these precipitated Hpid-protein complexes resulting from such cross-linking. Vitamin E may function to help prevent formation of these complexes. The metabolic role of antioxidants (qv) such as vitamin E in animal tissues, however, remains quite controversial. 

The water-soluble vitamins with hsted DVs are vitamin G, which is necessary for the prevention of scurvy (Section 4.3), and the B vitamins—niacin, pantothenic acid, vitamin Bg, riboflavin, thiamine, fohc acid, biotin, and vitamin Bj2. The B vitamins are the precursors of the metabohcally important coenzymes listed in Table 7.1, where references to the reactions in which the coenzymes play a role are given. We have seen many pathways in which NADH, NADPH, FAD, TPP, biotin, pyridoxal phosphate, and coenzyme A were found, all of which came from vitamins. A summary of vitamins and their metabolic roles is given in Table 24.2. Frequently, the actual biochemical role is played by a metabolite of the vitamin rather than by the vitamin itself, but this point does not affect the dietary requirement. 

Other vitamins are known as growth faetors because it can be shown that test animals will not grow if certain substances aside from carbohydrates, fats, and proteins are omitted from their diets. At present, vitamins have been chemically identified, and in many cases their metabolic roles as enzyme cofactors identified. In other cases, their precise biochemical function has not yet been made clear. 

Vitamin Coenzyme Form Metabolic Role and/or Associated Deficiency Disease RDA... 

The fat-soluble vitamins are stored in body fat, and therefore need not be ingested daily. However, the water-soluble vitamins are excreted or destroyed during metabolic turnover, and must be replaced by regular ingestion. A listing of the vitamins, their coenzyme forms and their metabolic roles where known, and their recommended daily allowances for males 23 to 50 years old is found in Table 23-6. 

Middleton, H. M. (1985). Uptake of pyridoxine by in vivo perfused segments of rat small intestine A possible role for intracellular vitamin metabolism. /. Nutr. 115,1079-1088. 

Coates, M.E (1984). Metabolic role of the vitamins. In Physiology and Biochemistry of the Domestic Fowl, vol. 5, ed. B.M. Freeman, pp. 27-37. London Academic Press. 

It is well known that at the present time there is still a remarkable difference between the water-soluble and the fat-soluble vitamins in regard to knowledge of their metabolic function. Whereas we know the ways in which many of the water-soluble vitamins participate in metabolic processes and what functions they fulfill in the cells (even vitamin C was recently shown to be a coenzyme of an enzyme that hydrolyzes mustard oil glycosides of the sinigrin type), the metabolic role of the fat-soluble vitamins is at the most a matter of speculation. Any progress in this direction would be of exceptional value. It would be a great achievement if our vitamin E symposium co ild contribute to the elucidation of this problem. 

Vitamins (Latin vita + amine) substances present in the animal diet in only small amounts, and indispensable for growth and maintenance of the organism. A dietary requirement is implicit in the definition of a V. Most of the substances that are V. in animals are essential for the metabolism of all living organisms, but plants and microorganisms can synthesize them (some fat-soluble V., however, may have metabolic roles unique to animals). The dietary re quirement in the animal results from the evolutionary loss of this biosynthetic ability. Animals differ in their ability to synthesize certain V., and they therefore display difierent dietary requirements for V. For example, ascorbic acid (V.C) is a V. only for primates and a few other animals (e.g. guinea pig) most animals can synthesize it, and for them it is therefore not a V. Some V. can be synthesized from provitamins in the diet. In addition some of the V. requirement of humans and higher animals is supplied by the intestinal flora, e.g. most of the V.K required by humans is supplied in this way. 

Although it is possible to describe the clinicopatho-logical manifestations of pyridoxine deficiency and the metabolic role of pyridoxal phosphate, each pathological alteration cannot be explained by a specific metabolic alteration. Deficiency of a vitamin involved in several steps of the intermediary metabolism of amino acids is bound to be associated with severe clinicopath-ological changes, but the specific metabolic alterations responsible for the anemia and convulsions in pyridoxine deficiency have not been identified. y-Amino butyric acid, cystathione, sphingosine, and 5-hydroxy-tryptamine are compounds abundant in the brain. Pyridoxal phosphate is involved in their metabolic formation. Is there any correlation between the role of pyridoxal phosphate in the metabolism of these compounds and the development of convulsions and ataxia in pyridoxine deficiency Is the role of pyridoxine phosphate in the intermediary metabolism of sulfur amino acid related to the development of seborrheic dermatitis ... 

Debier, C and Larondelle, Y 2005 Vitamins A and E metabolism, roles and transfer to offspring. British Journal of Nutrition 93 153-74. 

The metabolic role of many minerals and vitamins is as prosthetic groups or coenzymes in different enzyme systems. Consequently, mineral and vitamin deficiencies can cause a breakdown of the processing system and precipitate metabolic disease. For example, methylmalonyl-CoA isomerase (see p. 203) is an important vitamin Bi2-dependent enzyme in the gluconeogenic pathway. A deficiency of vitamin B12 (or cobalt) may reduce enzyme activity, decrease the efficiency of glucose synthesis and predispose the animal to ketosis. Similarly, ceruloplasmin is a copper-dependent enzyme responsible for releasing iron from cells into blood plasma. A copper deficiency may reduce ceruloplasmin activity, decrease the efficiency of iron utilisation for haemoglobin synthesis and predispose the animal to anaemia. 

As the kidney has a major role in B vitamin metabolism, it is plausible that chronic kidney disease may affect vitamin status to a clinically significant extent. This holds especially true in end-stage renal disease, when the dialysis process may cause additionally vitamin losses (Heinz et al. 2008). Although vitamin supplementation among patients with end-stage renal disease is widely practised, the scientific evidence for doing so was, until recently, very vague. And contrary to common beliefs, supplementation with B vitamins in patients... 

Riboflavin in its coenzyme forms (FMN and FAD) plays key metabolic roles in biological oxidation-reduction reactions involving carbohydrates, amino acids and lipids, and in energy production via the respiratory chain. These coenzymes also act in cellular metabolism of other water-soluble vitamins through the production and activation of folate and pyridoxine (vitamin Bg) to their respective coenzyme forms and in the synthesis of niacin (vitamin B3) from tryptophan. In addition, some neurotransmitters and other amines require FAD for their metabolism. Recently, Chocano-Bedoya et al. (2011) suggested a possible benefit of high intakes of riboflavin (about 2.5 mg/ day) from food sources on the reduction of incidence of premenstrual syndrome. 

Robson, L. C., and "Schwarz, M. R. (1980). The effects of vitamin Bg deficiency on the lymphoid system and immune responses, in Tryfiates, G.P. (ed.) Vitamin Bfi Metabolism Role in Growth (Westport, CT Food and Nutr. Press, Inc.) 20S-222. 

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