Vitamin biochemical function

Naturally occurring quaternary ammonium compounds have been reviewed (179). Many types of aliphatic, heterocycHc, and aromatic derived quaternary ammonium compounds are produced both in plants and invertebrates. Examples include thiamine (vitamin B ) (4) (see Vitamins) choline (qv) [62-49-7] (5) and acetylcholine (6). These have numerous biochemical functions. Several quaternaries are precursors for active metaboUtes. 

Vitamins are a group of organic nutrients required in small quantities for a variety of biochemical functions and which, generally, cannot be synthesized by the body and must therefore be supphed in the diet. 

Padh H Vitamin C newer insights into its biochemical functions. Nutr Rev 1991 49 65. 

Hoekstra WG. 1975. Biochemical function of selenium and its relation to vitamin E. Fed Proc 34 2083-2089. 

Many of the coenzymes are derivatives of vitamins. A coenzyme function is known for each of the water-soluble vitamins except vitamin C. A detailed discussion of the water-soluble vitamins and their conversion to coenzymes is found in Chapter 6. The biochemical functions of fat-soluble vitamins (A, D, E, K) are, with exception, less clearly understood. They are also further considered in Chapter 6. 

Djerassi D, Machlin LJ, Nocka C. Vitamin E biochemical function and its role in cosmetics. Drug Cosmet Ind 1986 3 46-77. 

One could more readily explain the clinical signs of scurvy on the basis of the function of vitamin C in collagen formation, than one could explain the development of wet beriberi for thiamine, or of cheilosis for riboflavin function. For instance, why shouldn t riboflavin cause beriberi, and thiamine cause cheilosis—rather than the reverse We cannot explain this. The situation exemplifies that we often cannot explain the clinical findings on the basis of what is known about the biochemical function of the vitamin. There is still much more to be learned about vitamin nutrition, and this is exemplified by vitamin C. 

Vitamin A performs numenius biochemical functions. It promotes the production nf mucus by the basal cells of the epithelium, whereas in its absence keratin can be fonned. Vitamin A performs a function in the biosynthesis of glycogen and some steroids, and increased quantities nf coen/yme Q are found in the livers nf vitamin-deficicnt rats. Significantly. the best-known action of vitamin A is its function in the chemistry of vision. 

A review by Rase . summarizes. studies on the effects of certain hormones on vitamin B(, nutrition in humans, on the biochemical interrelationship between steroid hormones and pyridoxal phasphate-dependent enzymes, and on the role of vitamin B(, in regulating hypothalamus-pituitary functions. Some of these studies have important clinical implications. The use of estrogen-containing oral contraceptives has been investigated as a factor leading to an abnormality of tiyptophan metaboli.sm. This abnormality resembles dietary vitamin B deficiency and responds favorably to treatment with the vitamin. For some time, there has been clinical interest in the relationship between certain hormones and vitamin B function because abnormal urinary excretion of tiyptophan metabolites was observed during pregnancy and in patients with hyperthyroidism. 

Vitamins Are Required to Maintain Normal Biochemical Functions of the Tissues, 362... 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

A Saturation of Biochemical Function. A reliable biochemical indicator is required. For niacin, which NAD" - or NADP -containing enzyme should be selected Which transaminase will be the indicator for pyridox-ine Which function of vitamin A should be j selected for retinol, vision in the rods or cell differentiation As noted from Table 8.2, many of the assays for vitamin status have significant limitations to estimate reliable doses. 

Early work on this vitamin was confusing because similar outcomes were seen with ingestion of "yellow" vegetables and colorless fish liver oils. It finally was shown that carotene (the yellow pigment) extracts Ifom vegetables were converted to colorless retinal. Because the retinoids are discussed in considerably more detail elsewhere, this chapter presents only a cursory overview of their biochemical functions. 

Two retinoids, retinoic acid and retinal, appear to have most of the biochemical functions attributed to vitamin A Retinoic acid is required for cell differentiation and is the ligand for two families of nuclear receptors, RAR . y and RXR 0 y. These receptors are part of a family of superreceptors that include the steroid hormones and cholecalciferol. Vitamin A deficiency can lead to a variety of symptoms depending on the age of the deficient person. The most serious syndrome is keratomalacia, which results in desiccation, ulceration, and xerophthalmia of the cornea and conjunctiva. It is one of the leading causes of blindness in infants and children. 

Biochemical Function. Calciferol function is complex and, with the exception of calcium transport from the intestinal tract, is poorly understood. Specific vitamin D receptors (VDRs) are found in 30 different tissues including bone, intestine, prostate, hemato-... 

Biochemical Function. The best way to describe tocopherol s role is that of a lipid-soluble antioxidant. It protects unsaturated lipids from o gen-induced peroxide formation. There Is evidence for both free-radical one-electron chemistry (Fig. 8.20) and two-electron qui-none-hydroquinone chemistry (Fig. 8.21) (35). The oxidized/reduced glutathione system may he part of the system that regenerates reduced a-tocopherol. At one time it was thought that the preference for the 2-R stereoisomers indicated that the vitamin was part of a biochemical oaddation/reduction system, possibly as a coenzyme. So far that role for a-tocopherol has not been found. The current evidence points to the... 

Duester, G. Families of retinoid dehydrogenases regulating vitamin A function production of visual pigment and retinoic acid. Eur. J. Biochem. 267 4315—4324, 2000. 

The fat-soluble vitamins share many properties despite their limited chemical similarity. They are absorbed into the intestinal lymphatics, along with other dietary lipids, after emulsification by bile salts. Lipid malabsorption accompanied by steatorrhea usually results in poor uptake of all the fat-soluble vitamins. Deficiency disease (except in the case of vitamin K) is difficult to produce in adults because large amounts of most fat-soluble vitamins are stored in the liver and in adipose tissue. The fat-soluble vitamins are assembled from isoprenoid units this fact is apparent from examination of the structures of vitamins A, E, and K cholesterol, the precursor of vitamin D, is derived from six isoprenoid units (Chapter 18). Specific biochemical functions for vitamins A, D, and K are known, but a role for vitamin E, other than as a relatively nonspecific antioxidant, remains elusive. 

Cobalt is a component of vitamin B, and is thus classed as an essential element, though there is no evidence that the cobalt ion has any other biochemical function. 

The cardiomyopathy is directly related to a reduction in the normal biochemical function of the vitamin thiamine in heart muscle. Inhibition of the a-keto acid dehydrogenase complexes causes accumulation of a-keto acids in heart muscle (and in blood), resulting in a chemically-induced cardiomyopathy. Impairment of two other functions of thiamine may also contribute to the cardiomyopathy. Thiamine pyrophosphate serves as the coenzyme for transketolase in the pentose phosphate pathway, and pentose phosphates accumulate in thiamine deficiency. In addition, thiamine triphosphate (a different coenzyme form) may function in Na conductance channels. 

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. 

Seaweeds are a good source of some water- (Bj, B2, B,2, C) and fat-soluble (P-carotene with vitamin A activity, vitamin E) vitamins. To ensure that the adequate intake of all vitamins is received in the diet, people (especially peopleon specialdiet, strict vegetarians, and vegans) can consume foods enriched with vitamins, for example, in the form of functional foods with vitamins as nutraceuticals, extracted from natural sources such as seaweeds. Seaweed vitamins are important not only due to their biochemical functions and antioxidant activity but also due to other health benefits such as decreasing of blood pressure (vitamin C), prevention of cardiovascular diseases (P-carotene), or reducing the risk of cancer (vitamins E and C, carotenoids). 

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