Cobalt-containing compounds (corrinoids) having vitamin B12 activity are referred to as cobalamins. Active forms of vitamin B12 in metabolism are methylcobalamin and 5'-deoxyadenoxylcobalamin. Cyanocobalamin is the co-balamin that is normally used in supplementation and pharmaceutical preparations. Vitamin B12 is best known for its association with Addisonian pernicious anemia.[1,2,14]
Digestion of vitamin B12 begins in the stomach, where gastric acids and enzymes release the vitamin B12 from its peptide bonds in food. As the protein is digested, the B12 is freed and then bound to an intrinsic factor secreted by the gastric parietal cells. The intrinsic factor-bound vitamin B12 is then absorbed in the ileum of the small
Two coenzyme forms of vitamin B12 are known to exist in animals. These are methylcobalamin, which functions as a methyl carrier, and 5'-deoxyadenoxylcobalamin, which serves as a hydrogen carrier. The function of methylcoba-lamin as a methyl carrier is the basis for the interrelationship between vitamin B12 and folate. In one such reaction, an enzyme-bound methylcobalamin is formed as an intermediate in the transfer of the methyl moiety of N5-methyltetrahydrofolate to homocysteine in the resynthesis of methionine. In the form of 5'-deoxyadenox-ylcobalamin, vitamin B12 is a coenzyme for methylma-lonyl-CoA mutase, which catalyzes the conversion of methyl-malonyl CoA to succinyl CoA. This reaction is a step in the catabolism of propionyl CoA, which is derived from the breakdown of valine and isoleucine.[1,2,14]
A vitamin B12 deficiency in animals is evidenced by a reduction in growth rate and feed intake, rough hair coat, dermatitis, enlarged liver, extreme irritability and sensitivity to touch, and unsteadiness of gait. Examination of blood samples from deficient animals has indicated normocytic anemia and high neutrophil counts, with concomitantly low lymphocyte counts. Similar to pernicious anemia in humans, a double deficiency of vitamin B12 and folic acid has been reported to result in the development of macrocytic anemia and bone marrow hyperplasia.[2,14]
Plants and grain materials are completely devoid of naturally occurring vitamin B12, whereas animal products and animal by-products contain varying levels of the vitamin. Vitamin B12 found in nature is made exclusively by microorganisms. Fecal material contains high levels of vitamin B12, but vitamin B12 is not absorbed from the colon.[1A14]
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