Vitamin and Mineral Requirements in Human Nutrition

279 14.1 Role of vitamin B12 in human metabolic processes Although the nutritional literature still uses the term vitamin B12, a more specific name for vitamin B12 is cobalamin. Vitamin B12 is the largest of the B complex vitamins, with a relative molecular mass of over 1000. It consists of a corrin ring made up of four pyrroles with cobalt at the centre of the ring (1, 2). There are several vitamin B12-dependent enzymes in bacteria and algae, but no species of plants have the enzymes necessary for vitamin B12 synthesis. This fact has significant implications for the dietary sources and availability of vitamin B12. In mammalian cells, there are only two vitamin B12-dependent enzymes (3). One of these enzymes, methionine synthase, uses the chemical form of the vitamin which has a methyl group attached to the cobalt and is called methylcobalamin (see Chapter 15, Figure 15.2). The other enzyme, methylmalonyl coenzyme (CoA) mutase, uses a form of vitamin B12 that has a 5¢-adeoxyadenosyl moiety attached to the cobalt and is called 5¢deoxyadenosylcobalamin, or coenzyme B12. In nature, there are two other forms of vitamin B12: hydroxycobalamin and aquacobalamin, where hydroxyl and water groups, respectively, are attached to the cobalt. The synthetic form of vitamin B12 found in supplements and fortified foods is cyanocobalamin, which has cyanide attached to the cobalt. These three forms of vitamin B12 are enzymatically activated to the methylor deoxyadenosylcobalamins in all mammalian cells.