Vitamin vitamers

Vitamins are minor components of foods that play an essential role in human nutrition. Many vitamins are unstable under certain conditions of processing and storage (Table 9-1), and their levels in processed foods, therefore, may be considerably reduced. Synthetic vitamins are used extensively to compensate for these losses and to restore vitamin levels in foods. The vitamins are usually divided into two main groups, the water-soluble and the fat-soluble vitamins. The occurrence of the vitamins in the various food groups is related to their water-or fat-solubility. The relative importance of certain types of foods in supplying some of the important vitamins is shown in Table 9-2. Some vitamins function as part of a coenzyme, without which the enzyme would be ineffective as a biocatalyst. Frequently, such coenzymes are phosphorylated forms of vitamins and play a role in the metabolism of fats, proteins, and carbohydrates. Some vitamins occur in foods as provitamins—compounds that are not vitamins but can be changed by the body into vitamins. Vitamers are members of the same vitamin family. 

Vitamin Vitamer Light Heaf o2 Acid Base Metals Most Stable... 

Vitamer. The term vitamer is used to signify one of two or more different chemical forms of a particular vitamin. Vitamers are structurally related and exhibit biological activity related to the vitamin but have some difference in their chemical structure. 

Salts and Derivatives. Generally the vitamers are high melting crystalline soHds that are very soluble in water and insoluble in most other solvents. Properties of the common forms are Hsted in Table 1. The only commercially important form of vitamin B is pytidoxine hydrochloride (7). This odorless crystalline soHd is composed of colorless platelets melting at 204—206°C (with decomposition). In bulk, it appears white and has a density of - 0.4 kg/L. It is very soluble in water (ca 0.22 kg/L at 20°C), soluble in propylene glycol, slightly soluble in acetone and alcohol (ca 0.014 kg/L), and insoluble in most lipophilic solvents. A 10% water solution shows a pH of 3.2. Both the hydrochloride and corresponding free base sublime without decomposition (16). 

Over 250 analogues of the B vitamers have been reported (11,100). Nearly all have low vitamin B activity and some show antagonism. Among these are the 4-deshydroxy analogue, pyridoxine 4-ethers, and 4-amino-5-hydroxymeth5i-2-methyipyrimidine, a biosynthetic precursor to thiamine. StmcturaHy unrelated antagonists include dmgs such as isoniazid, cycloserine, and penicillamine, which are known to bind to pyridoxal enzyme active sites (4). 

In general, pyridoxamine and pyridoxin are more stable than pyridoxal. All vitamers are relatively heat-stable in acid media, but heat labile in alkaline media. All forms of vitamin B6 are destroyed by UV light in both neutral and alkaline solution. The majority of vitamin B6 in the human body is stored in the form of pyridoxal phosphate in the muscle, bound to glycogen phos-phorylase. 

Vitamin B12 (Fig. 1) is defined as a group of cobalt-containing conoids known as cobalamins. The common features of the vitamers are a corrin ting (four reduced pyrrole rings) with cobalt as the central atom, a nucleotide-like compound and a variable ligand. Vitamin B12 is exceptional in as far as it is the only vitamin containing a metal-ion. The vitamers present in biological systems are hydroxo-, aquo-, methyl-, and 5 -deoxyadenosylcobalamin. 

Vitamin B12. Figure 1 Structure of the different vitamin B12 vitamers. 

Boshtam M, Rafiei M, Golshadi ID et al (2005) Long-term effects of oral vitamin E supplement in type II diabetic patients. Int J Vitam Nutr Res 75 341—346... 

Figure 45-1. P-Carotene and the major vitamin A vitamers. Shows the site of cleavage of P-carotene into two molecules of retinaldehyde by carotene dioxygenase.

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. 

Figure 45-7. The vitamin K vitamers. Menadiol (or menadione) and menadiol diacetate are synthetic compounds that are converted to menaquinone in the liver and have vitamin Kactivity.

BARTH T J, ZOLER J, KUBELER A, BORN A I and OSSWALD H (1997) Redifferentiation of oral dysplasic mucosa by the application of the anti-oxidants beta-carotene, a-tocopherol and vitamin C. Int J Vitam Nutr Res 67(5) 368-76. 

Tanumihardjo, S. A., H. C. Furr, O. Amedee-Manesme, and J. A. Olson. 1990. Retinyl ester (vitamin A ester) and carotenoid composition in human liver. Int. J. Vitam. Nutr. Res. 60 307-313. 

Furusho T, Kataoka E, Yasuhara T, Wada M and Masushige S. 2000. Retinol equivalence of carotenoids can be evaluated by hepatic vitamin A content. Int J Vitam Nutr Res 70 43-47. 

Sklan D, Yosefov T and Friedman A. 1989. The effects of vitamin A, betacarotene and canthaxanthin on vitamin A metabolism and immune responses in the chick. Int J Vitam Nutr Res 59 245-250. 

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