FunctionsThree main enzymatic reactions regarding vitamin B12 have been noted to occur in humans. The first reaction requires methylcobalamin as a coenzyme in the conversion of homocysteine into methionine. This reaction occurs in the cytosol of the cell. The reaction is intrinsic to the production of tetrahydrofolate (THF). The production of THF is irreversible and requires vitamin B12, therefore, a B12 deficiency can trap folate in its methyl form (also known as the folate-methyl trap), which in turn affects purine and thymidylate synthesis. A second set of reactions requiring B12 are those catalyzed by mutases. These reactions occur within the mitochondria. These include the production of succinyl CoA, a Krebs cycle intermediate. Mutase enzymes are also intrinsic in the oxidation of fatty acids. A defect in mutase activity results in impai.red muscle activity and an accumulation of methylmalonyl CoA and methylmalonic acid. The branched-chain amino acid leucine is also catalyzed by a mutase enzyme. Specifically, leucine undergoes isomerization, requiring adenosylcobalamin . SourcesThe only dietary sources of vitamin B12 are animal products. Plants and legumes are not good sources of vitamin B12, but may contain small quantities of B12 due to contamination with microorganisms during harvesting. Cobalamins are found in meat, meat products, poultry, fish, clams, oysters, and eggs. The bioavailability of the cobalamins depends on which form they are found in. Governmental Recommended IntakeTwo micrograms daily is the recommended daily intake for vitamin B12. Doses of vitamin C that are greater than 500 mg and taken within I hour of eating a meal may impair B12 absorption?Deficiency symptomsMore than 95% of the B12 deficiencies diagnosed are related to inadequate absorption rather than inadequate dietary intake. Vitamin B12 deficiency occurs in stages of which changes within the cells and blood are primary. Vitamin B12 deficiency later results in megaloblastic anemia (this can also be caused by folate deficiency, therefore, it is best to treat with a combination of B12 and folate). Signs of progressive B12 deficiency include neuropathy, characterized by demyelination of nerves, and hyperhomocystinemia. The populations at greatest risk for a B12 deficiency do not include athletes, but rather the elderly, alcoholics, gastrectomy patients, and strict vegetarians (vegans).