DEFECT IN 5′-DEOXYADENOSYLCOBALAMIN SYNTHESIZING ENZYME IN METHYLMALONICACIDEMIA

Abstract

Inherited methylmalonicacidemia (MM-emia) due to deficient activity of methylmalonyl-CoA mutase may be caused by an abnormal mutase apoenzyme or by reduced holoenzyme activity secondary to impaired synthesis of the mutase-requiring coenzyme, 5'-deoxyadenosylcobalamin (Ado-Cbl). We studied Ado-Cbl synthesis in 5 fibroblast lines from unrelated patients who have MM-emia due to defective cobalamin (vitamin B12) metabolism. Their whole cells in culture fail to synthesize Ado-Cbl but make normal amounts of the other cobalamin coenzyme, methylcobalamin.After breaking the cells to define the steps in Ado-Cbl synthesis, we assayed the final step which combines the reduction of cob(II)alamin to cob(I)alamin and the adenosylation of cob(I)alamin. Subcellular fractionation studies localized this activity to mitochondria. In crude broken cell extracts, enzyme activity was deficient or absent in 3 of the 5 mutant cell lines (4,0,0 pg Ado-Cbl/mg protein/30 min;control lines, 23 ± 8). The other two MM-emia lines had normal activities (23 and 17). These results establish deficiency of Ado-Cbl synthesizing enzyme as one cause of abnormal cobalamin metabolism and MM-emia. The two MM-emia lines that had normal enzyme activity suggest there is further heterogeneity within the disorders of cobalamin metabolism as well as in the MM-emia phenotype.