Vitamin activity

The related compounds pyridoxamine and pyridoxal, in which the CH2OH group in the 4-position is replaced by CH2NH2 and CHO respectively, also possess vitamin activity and for certain bacteria are much more active than pyridoxine. 

This synthesis was the first step toward industrial vitamin production, which began in 1936. The synthetic product was shown to have the same biological activity as the natural substance. It is reversibly oxidized in the body to dehydro-L-ascorbic acid (3) (L-// fi (9-2,3-hexodiulosonic acid y-lactone), a potent antiscorbutic agent with hiU vitamin activity. In 1937, Haworth and Szent-Gyn rgyi received the Nobel Prize for their work on vitamin C. 

No unequivocal unique function for vitamin E has been defined. However, it does act as a hpid-soluble antioxidant in cell membranes, where many of its functions can be provided by synthetic antioxidants. Vitamin E is the generic descriptor for two famihes of compounds, the tocopherols and the tocotrienols (Figure 45—5). The different vitamers (compounds having similar vitamin activity) have different biologic potencies the most active is D-a-tocopherol, and it is usual to express vitamin E intake in milhgrams of D-a-tocoph-erol equivalents. Synthetic DL-a-tocopherol does not have the same biologic potency as the namrally occurring compound. 

Vitamin C (Figure 45-19) is a vitamin for human beings and other primates, the guinea pig, bats, passerine birds, and most fishes and invertebrates other animals synthesize it as an intermediate in the uronic acid pathway of glucose metabohsm (Chapter 20). In those species for which it is a vitamin, there is a block in that pathway due to absence of gulonolactone oxidase. Both ascorbic acid and dehydroascorbic acid have vitamin activity. 

Mammals lack the ability to synthesize astaxanthin or convert dietary astaxanthin into vitamin A. Unlike p-carotene, astaxanthin has no pro-vitamin activity in these animals. Astaxanthin has been shown in both in vitro and in a study with human subjects to be effective for the prevention of the oxidation of low-density protein, suggesting that it can be used to prevent arteriosclerosis, coronary artery disease, and ischemic brain development. A number of astaxanthin health products are under study. 

Biotin is solnble in water and insolnble in organic solvents. It is stable at pH 5-8 and to heating in strong acid. However it can be oxidized in the sulfur atom and the shortening of the valeric acid side chain results in the loss of vitamin activity. 

Vitamin Active compound Sources Recommended daily allowances... 

Niacin is relatively stable to most food-processing operations. It is stable to exposure to air and resistant to autoclaving (and is therefore stable to pasteurization and UHT treatments). The amide linkage of nicotinamide can be hydrolysed to the free carboxylic acid (nicotinic acid) by treament with acid but the vitamin activity is unaffected. Like other water-soluble vitamins, niacin can be lost by leaching. 

Folate is a relatively unstable nutrient processing and storage conditions that promote oxidation are of particular concern since some of the forms of folate found in foods are easily oxidized. The reduced forms of folate (dihydro- and tetrahydrofolate) are oxidized to p-aminobenzoylglutamic acid and pterin-6-carboxylic acid, with a concomitant loss in vitamin activity. 5-Methyl-H4 folate can also be oxidized. Antioxidants (particularly ascorbic acid in the context of milk) can protect folate against destruction. The rate of the oxidative degradation of folate in foods depends on the derivative present and the food itself, particularly its pH, buffering capacity and concentration of catalytic trace elements and antioxidants. 

Thiamin 0.23 mg or more per 1000 kcal of food consumed and a minimum total of 0.8 mg/day. Replacement of the methyl group on the pyrimidine ring by ethyl, propyl, or isopropyl gives compounds with some vitamin activity, but replacement by hydrogen cuts activity to 5% of the original. The butyl analog is a competitive inhibitor. 

Radiation is used in medicine in the form of X-rays and radioactive isotopes. It is used in industry in many ways, e.g., as vitamin activator, sterilizing agents, and polymerization initiator it is also the basis of all types of spectroscopic analysis.)... 

Vitamin F, substances are necessary for the normal growth of animals. Without vitamin E, the animals develop infertility, abnormalities of the central nervous system, and myopathies involving both skeletal and cardiac muscle. The antioxidant activity of the tocopherols is in reverse order to that of their vitamin activity. Muscular tissue taken from a deficient... 

The principal components in vegetable fats with vitamin activity are the tocopherols. Vitamin A is found in butterfat and in fish oils. The carotenes (provitamin A) are found at significant levels in palm oil and butterfat and as traces in other fats. Vitamin D is found primarily in some fish oils. 

From an analytical perspective, the single most important physicochemical characteristic of riboflavin is its photosensitivity (80-82). Exposure of this vitamin to ultraviolet and visible light results in irreversible photoreduction to lumiflavin and lumichrome and loss of vitamin activity. In addition, the coenzymes are subject to hydrolysis by endogenous phosphatases that are present in a number of foods. Since these enzymes are generally inactivated by thermal processing, they are a concern only in the analysis of fresh products. 

Cyanocobalamin appears to be the most stable of the B12 vitamers (167,168). It can be autoclaved at 120°C in aqueous solution at pH 4-7. It is susceptible to degradation and loss of vitamin activity under alkaline conditions. Short exposure to UV or visible light causes conversion to HOCbl prolonged exposure results in irreversible decomposition. CNCbl is soluble in water, short-chain alcohols, and phenol, but it is insoluble in acetone, chloroform, and ether. 

D-Ascorbic acid was prepared by Gatzi and Reichstein28 by oxidation of diacetone-D-sorbose to 2-keto-D-gulonic acid, and a desoxy derivative, 6-desoxy-L-a8corbic acid, with some vitamin activity, was synthesized from L-sorbomethylose.116... 

B vitamin activation, muscle relaxation, and adenosin triphosphate (ATP) formation - the energy the body runs on. Concentrations of Ca and Mg in human milk range between 250-300 and 25-35 mgl-1, respectively, but they remain constant along the entire lactating period [11],... 

The molecule consists of a d-ribitol unit attached to an isoalloxazine ring (Figure 9-15). Anything more than a minor change in the molecule results in a loss of vitamin activity. Aqueous solutions of riboflavin are yellow with a yellowish-green fluorescence. The vitamin is a constituent of two coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). FMN is... 

There is a third plasma vitamin B12 binding protein, transcobalamin III, which is rapidly cleared by the liver, with a plasma half-life of the order of 5 minutes. This seems to provide a mechanism for returning vitamin B12 and its metabolites from peripheral tissues to the liver, as well as for clearance of other corrinoids without vitamin activity, which may arise either from foods or the products of intestinal bacterial action and be absorbed passively from the lower gut. These corrinoids are then secreted into the bile, bound to cobalophUins. 

D-lso-ascorbic acid (erythorbic acid see Figure 13.1) also has vitamin activity. in vivo and in cell culmre, it has only about 5% of the biological activity of ascorbate, but this seems to be from poor intestinal absorption and tissue uptake. In vitro with purified enzymes, it has the same cofactor activity as ascorbate. Although it is not a namrally occurring compound, erythorbic acid is widely used interchangeably with ascorbic acid, in cured meats and as an antioxidant in a variety of foods. 

Because it is a potent reducing agent, vitamin C is commonly determined by titrimetric orpotentiometric redoxmethods. Such methods underestimate the amount of the vitamin present because dehydroascorbate - which has vitamin activity - is formed by atmospheric oxidation of ascorbate in the sample, especially under neutral conditions, and is not detected by redox assay methods. 

Vitamin C can also be determined colorimetrically, after oxidation to dehydroascorbate, by reaction with dinitrophenylhydrazine. Under appropriate conditions, neither ascorbic acid itself nor potentially interfering sugars react with dinitrophenylhydrazine. However, diketogulonate, which has no vitamin activity, also reacts with dinitrophenylhydrazine under the same conditions. Unless diketogulonate is determined separately after reduction of dehydroascorbate to ascorbate, this method overestimates the vitamin. 

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