Vitamins biological activity, metabolic

The discovery that vitamin was metabolized to biologically active derivatives led to a significant effort to prepare 25-hydroxy vitamin and, subsequendy, the 1 a-hydroxy and 1,25 dihydroxy derivatives. Initial attempts centered around modification of steroidal precursors, which were then converted to the D derivatives by conventional means. 

Niacin was discovered as a nutrient during studies of pellagra. It is not strictly a vitamin since it can be synthesized in the body from the essential amino acid tryptophan. Two compounds, nicotinic acid and nicotinamide, have the biologic activity of niacin its metabolic function is as the nicotinamide ring of the coenzymes NAD and NADP in oxidation-reduction reactions (Figure 45-11). About 60 mg of tryptophan is equivalent to 1 mg of dietary niacin. The niacin content of foods is expressed as mg niacin equivalents = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable, this is discounted. 

Enhanced biorestoration is a means by which naturally occurring processes are deliberately manipulated to increase or enhance the rate of cleanup. Biological activity in the subsurface is controlled by the availability of one or more of the necessary metabolic requirements such as an electron acceptor or nutrient. Although electron acceptors are most often the limiting factor, inadequate availability of nitrogen, phosphorus, or micronutrients (such as potassium, copper, or even vitamins) can restrict optimum restoration. When the proper balance of these factors is established, the rate of chemical degradation is maximized. 

Vitamin Be is again a small family of related compounds having the same biological activity. These include pyridoxine, pyridoxai, and pyridoxamine. In humans, these molecules are readily interconverted, accounting for their equivalence as vitamins. The stuff in your vitamin pill is likely to be pyridoxine. The actual molecule that functions as a coenzyme in metabolism is pyridoxai phosphate, in which a phosphate group has been added to pyridoxai in an ATP-dependent reaction. 

A vitamin has been defined [399] as a biologically active organic compound that is essential for the organism s normal metabolic and physiologic functions. Vitamins are not produced by human body itself, thereby they have to be obtained from the diet. As micronutrients, in contrast to nonessential chemical substances, they present both a minimum and a maximum level of intake beyond which arise risks of deficiency conditions or adverse effects [400]. 

Niacin is also known as vitamin PP or vitamin Bj. The term niacin describes two related compounds, nicotinic acid and nicotinamide (Figure 19.18), both with biological activity. Niacin is formed from the metabolism of tryptophan, and therefore it is not strictly a vitamin. It is a precursor of two cofactors nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), which are essential for the functioning of a wide range of enzymes involved in redox reactions. 

The primary supply of vitamin in humans is not obtained from the diet but rather is derived from the ultraviolet photoconversion of 7-dehydrocholesterol to vitamin Ds in skin. Thus, vitamin Dj synthesis varies with the seasons. D3 is a prohormone and requires further metabolic conversion to exert biological activity in its target organs (Fig. 66.2). The liver and the kidney are the major sites of metabolic activation of this endogenous sterol hormone. The initial transformation of D3 occurs in the liver and is catalyzed by the enzyme 25-OH-D3-hydroxylase... 

Vitamin D3 is transported to liver where it undergoes a hydroxylation at C-25 into 1a,25-dihydroxyvitamin D3 (calcitriol) (Fig. 64). In the kidney, it undergoes further hydroxylations at different sites, depending on the serum Ca + concentration. The most biologically active metabolite of vitamin D3 is calcitriol, which plays important roles in the regulation of calcium and phosphorus metabolism. It is used for treating bone diseases, but is also involved in the cell proliferation and the inducement of cell differentiation [151]. 

Fluorination of the vitamin D3 side chain was anticipated to have an impact on its metabolism pathway. 26,27-Hexafluorocalcitriol, faiecaicitrioi, was found to be several times more potent than caicitrioi in the regulation of Ca metabolism and of the immune system. The reason for this higher biological activity has been attributed to several mechanisms a higher activity of its 23(5)-hydroxylated metabolite [26,27-hexafluoro-l,23(5),25(OH)3D3], a lower affinity of faiecaicitrioi for the vitamin D binding protein and a higher affinity of falecalcitriol-... 

Three hormones serve as the principal regulators of calcium and phosphate homeostasis parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and the steroid vitamin D (Figure 42-2). Vitamin D is a prohormone rather than a true hormone, because it must be further metabolized to gain biologic activity. PTH stimulates the production of the active metabolite of vitamin D, l,25(OH)2D. l,25(OH)2D, on the other hand, suppresses the production of PTH. l,25(OH)2D stimulates the intestinal absorption of calcium and phosphate. l,25(OH)2D and PTH promote both bone formation and resorption in part by stimulating the proliferation and differentiation of osteoblasts and osteoclasts. Both... 

Since ascorbate reduces photooxidation of lipid emulsions and multivitamin preparations (see Figure 4) [19], Lavoie et al. [34] studied the formation of oxidative by-products of vitamin C in multivitamins exposed to light. They found that the loss of ascorbic acid in photoexposed multivitamin preparations was associated with the generation of products other than dehydroascorbate and 2,3-diketogulonic acid, which are the usual products of vitamin C oxidation. The authors showed that hydrogen peroxide at concentrations found in TPN solutions induced the transformation of dehydroascorbate into new, biologically active compounds that had the potential to affect lipid metabolism. They believe that these species have peroxide and aldehyde functions [35]. 

Doxercalciferol, the active ingredient in Hectorol, is a synthetic vitamin D analog that undergoes metabolic activation in vivo to form 1 (alpha),25-dihydroxyvitamin D2 (l(alpha),25-(OH)2D2), anaturally occurring, biologically active form of vitamin D2. Hectorol is available as soft... 

Sea animals are rich in soluble dietary fibers, proteins, minerals, vitamins, antioxidants, phytochemicals, and polyunsaturated fatty acids, with low caloric value. Polysaccharides from marine animals have been reported to possess biological activities with potential medicinal values in addition to their current status as a source of dietary fibers and prebiotics. Moreover, they have a lot of dietary fiber, which lowers blood cholesterol, and iodine, which improves metabolism, vascular and cardiac action, body temperature, and perspiration regulation, and are effective in... 

The leveisal of the oxidative pathway of vitamin A (retinol —r retinal —>-retinoic add) does not occur in the body, When retinoic acid is feci to animals, even in relatively large doses, there is no storage and, in fact, die molecule is rapidly metabolized and cannot be found several hours after administration. The metabolic products have not been fully identified. Several fractions from liver or intestine, isolated after administering retinoic add marked with carbon-14, have been shown to have biological activity. 

Ascorbic acid and dehydroascorbic acid have equivalent biological activities, whereas isoascor-bic acid has only 5% of the biological activity of AA (15,17,19). The absorption and metabolism of vitamin C were recently reviewed (17,20,21). 

FIGURE 66.1 Vitamins D3 and D2 are produced by ultraviolet irradiation of animal skin and plants, respectively. The precursor of vitamin D3 in skin is 7-dehydrocholesterol, or provitamin D. In humans, the storage, transport, metabolism, and potency of vitamins D2 and D3 are identical, and the net biologic activity of vitamin D in vivo results from the combined effects of the hydroxylated derivatives of vitamins D2 and D3. 

Because several of the metabolites of vitamin D3 are biologically active, the molecular species of vitamin D3 which passes into the yolk cannot be determined just from measurement of antirachitic activity. Consequently, a reliable and sensitive method for determining the amount of the unchanged form of vitamin D3 would be extremely beneficial to those interested in the metabolism and other factors that influence the chicken to deposit vitamin D3 in... 

Lund and DeLuca (101) administered [ H] vitamin D3 to rats and found biologically active metabolites in the bone, liver and serum. The aqueous-soluble metabolites from the tissues and the feces did not have vitamin D activity. At least three biologically active metabolites were isolated from the chloroform-soluble portion of the extract. One of these was found in large amounts in the liver, blood and bone. In 1968, Blunt et. al. (102) proved convincingly that this major metabolite is 25-hydroxyvitamin D3 (25-OH-D3). Two other groups of investigators (103,104) independently found clues to the metabolic hydroxylation of vitamin Do. It was soon established that 25-hydroxylation of vitamin D3 takes place primarily in the liver (105,106) and that 25-0H-D3 is the major form of circulating vitamin D3 in human plasma (107). 

Vitamins are essential nutrients, which must be supplied exogenously. They are organic compounds with indispensable biological activities as coenzymes in a multitude of cellular metabolic processes. Vitamin A, retinoids (vitamin A-derivatives), carotenoids, vitamin D, vitamin E, and vitamin K are fat-soluble, vitamin C and vitamins of the B-complex are water-soluble. This is of importance for gastrointestinal absorption in oral supplementation as well as the transdermal penetration for topical applications. 

Antiepileptic Metabolic bone disease Vitamin D3 biologically active for chronic venours, for vascular disorders... 

Vitamin D3 can be obtained either through the diet or by the conversion of 7-de-hydrocholesterol in the skin by the action of ultraviolet light. Extensive metabolism of vitamin D3 can occur (no less than 30 metabolites of vitamin D have been isolated and chemically characterized). To obtain the most biologically active metabolite, vitamin D3 is first hydroxylated in the liver to form 25-hydroxyvitamin D3, which is then converted to l,25(OH)2D3 by a cytochrome P-450 mitochondrial en-... 

The skin has a well-documented role in vitamin D metabolism. 7-Dehydrocho-lesterol (provitamin D3) is activated by exposure to ultraviolet radiation in the skin to previtamin D3, which isomerizes to vitamin D3. Recently, further metabolism of 24,25-dihydroxyvitamin D to biologically active 1,25-dihydroxy vitamin D has also been demonstrated in skin, a conversion previously assumed to occur only in the kidney. 

Vitamin B12 (1) was first isolated in 1948. But it was not until 1958 that the biologically active form of this coenzyme was identified by H.A. Barker and his colleagues [4], who at that time were studying the metabolism of Clostridium tetanomorphum growing on glutamate. 

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