Inherited deficiencies of human methylamlonyl CoA mutase activity: Reduced affinity of mutant apoenzyme for adenosylcobalamin

We have studied the affinity of methylmalonyl CoA mutase for its required cofactor, adenosylcobalamin, in extracts of control and mutant human cultured fibroblasts. Control enzyme has an apparent Km for adenosylcobalamin of 6–7 × 10−8 M. Five mutant cell lines from patients with methylmalonicacidemia due to a mutase apoenzyme defect were studied. Three have undetectable mutase activity (<0.15% of control) at all cofactor concentrations. Two others, however, have markedly altered Km's for adenosylcobalamin of 2.8 × 10−4 M and 1.7 × 10−5 M. These mutant lines synthesize adenosylcobalamin normally and, by complementation analysis, are genetically identical to all other mutase apoenzyme mutants tested. We conclude that the mutase deficiency in these two cell lines results from structurally altered mutase apoenzymes with markedly reduced affinities for adenosylcobalamin.