Figure 28.45 • Biologically active forms of cobalamin.
the vitamin. The structure of cyanocobalamin was announced simultaneously by Hodgkin et al.220 and Bonnett et al.221 in 1955 and the complete synthesis completed in 1972.222
De novo synthesis of cobalamin is only found in microorganisms. Therefore, cobalmin is found naturally in commercial fermentation processes of antibiotics, such as those of Streptomyces griseus, Streptomyces olivaceus, and Streptomyces aureofaciens as well as sewage, Milorganite and others. Some of these fermentations furnish a commercial source of vitamin B12. Animals depend on synthesis by the intestinal flora or, as in humans, foods from animals that have already consumed cobalamin. Excellent dietary sources are meats, eggs, dairy, fish, and shellfish. The only dietary plant products containing the vitamin are legumes, because of their symbiosis with microorganisms, or those that have been fortified with the vitamin.
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