Catalytic defect of medium-chain acyl-coenzyme A dehydrogenase deficiency. Lack of both cofactor responsiveness and biochemical heterogeneity in eight patients.

Medium-chain acyl-coenzyme A (CoA) dehydrogenase (MCADH; EC 1.3.99.3) deficiency (MCD) is an inborn error of beta-oxidation. We measured 3H2O formed by the dehydrogenation of [2,3-3H]acyl-CoAs in a 3H-release assay. Short-chain acyl-CoA dehydrogenase (SCADH; EC 1.3.99.2), MCADH, and isovaleryl-CoA dehydrogenase (IVDH; EC 1.3.99.10) activities were assayed with 100 microM [2,3-3H]butyryl-, -octanoyl-, and -isovaleryl-CoAs, respectively, in fibroblasts cultured from normal controls and MCD patients. Without the artificial electron acceptor phenazine methosulfate (PMS), MCADH activity in fibroblast mitochondrial sonic supernatants (MS) was 54% of control in two MCD cell lines (P less than 0.05). Addition of 10 mM PMS raised control acyl-CoA dehydrogenase activities 16-fold and revealed MCADH and SCADH activities to be 5 (P less than 0.01) and 73% (P greater than 0.1) of control, respectively. Thus, the catalytic defect in MCD involves substrate binding and/or dehydrogenation by MCADH and not the subsequent reoxidation of reduced MCADH by electron acceptors. 20 microM flavin adenine dinucleotide (FAD) did not stimulate MCD MCADH activity in either the 3H-release or electron-transfer(ring) flavoprotein-linked dye-reduction assays. Mixing experiments revealed no MCADH inhibitor in MCD MS; IVDH activities were identical in both control and MCD MS. In postmortem liver MS from another MCD patient, 3H2O formation from [2,3-3H]octanoyl-CoA was 15% of control. When 3H2O formation was assayed with 200 microM [2,3-3H]acyl-CoAs, 15 mM PMS, and 20 microM FAD in fibroblast sonic supernatants from seven MCD cell lines, SCADH, MCADH, and IVDH activities were 72-112% (P greater than 0.1), 4-9% (P less than 0.01), and 86-135% (P greater than 0.1) of control, respectively, revealing no significant biochemical heterogeneity among these patients.