Mitochondrial vitamin B12-binding proteins in patients with inborn errors of cobalamin metabolism

Inborn errors of vitamin B12 (cobalamin, Cbl) metabolism are autosomal recessive disorders and have been classified into nine distinct complementation classes (cblA-cblH and mut). Disorders affecting methylcobalamin metabolism cause megaloblastic anemia, which may be accompanied by leukopenia and thrombocytopenia, and a variety of neurological problems. Disorders affecting adenosylcobalamin cause methylmalonic acidemia and metabolic acidosis. Previous studies have shown that cobalamin binds to two enzymes in humans: methylmalonyl-CoA mutase in mitochondria and methionine synthase in the cytosol. In this study, cobalamin binding patterns were analyzed in crude mitochondrial fractions obtained from both control and patient fibroblasts that had been incubated with [57Co]cyanocobalamin. Crude mitochondrial fractions from control fibroblasts confirmed that the majority of [57Co]Cbl eluted with methylmalonyl-CoA mutase. However, in six of the nine disorders, at least one previously unidentified mitochondrial cobalamin binding protein was observed to bind [57Co]Cbl. The proportion of [57Co]Cbl that binds, is increased compared to controls when a deficiency in either adenosylcobalamin synthesis or utilization prevents binding to methylmalonyl-CoA mutase. Furthermore, unique cobalamin binding profiles emerged demonstrating how known mutations in these patients affect cobalamin binding to as yet unidentified proteins.