The defect in the cblB class of human methylmalonic acidemia: Deficiency of cob(I)alamin adenosyltransferase activity in extracts of cultured fibroblasts

We show that the reductants present in the $\text{in}\text{vitro}$ assay used to measure the formation of adenosylcobalamin from cob(III)alamin by cell-free extracts of human fibroblasts result in the non-enzymatic reduction of cob(III)alamin to cob(I)alamin. Hence, the $\text{in}\text{vitro}$ assay uniquely estimates the activity of ATP:cob(I)alamin adenosyltransferase (EC 2.5.1.17). Based on additional studies with extracts of fibroblasts from patients in the $\text{cbl}\text{B}$ class of human methylmalonic acidemia and from their parents, we conclude that this mutant class results from a specific deficiency of adenosyltransferase activity which is inherited as an autosomal recessive trait.