Cobalamin and inborn errors of cobalamin absorption and metabolism

Cobalamin (Cbl), or vitamin Bit, is required for activity of the mitochondr ial enzyme, methyl-malonyl-CoA mutase, and the cytoplasmic enzyme, methioni ne synthase. A number of inborn errors affecting ability to absorb dietary cobalamin or to convert exogenous cobalamin to its active coenzyme derivati ves have been recognized. Intrinsic factor deficiency and Imerslund-Grasbec k syndrome result in inability to absorb dietary cobalamin from the gut. Tr anscobalamin II deficiency results in dcreased levels of the blood cobalami n binding protein that facilitates cellular cobalamin uptake. The inborn er rors of cobalamin metabolism, the CblA-CblH disorders, result in inability to synthesize one or both of the cobalamin coenzyme derivatives, adenosylco balamin and methylcobalamin, resulting in methylmalonic aciduria or hyperho mocystinemia and hypomethioninemia, or both.