MULTIPLE ACYL-COENZYME A DEHYDROGENATION DISORDER RESPONSIVE TO RIBOFLAVIN - SUBSTRATE OXIDATION, FLAVIN METABOLISM, AND FLAVOENZYME ACTIVITIES IN FIBROBLASTS

Multiple acyl-CoA dehydrogenation disorders result from generalized defects in intramitochondrial acyl-CoA dehydrogenation. Fibroblasts from a riboflavin-responsive multiple acyl-CoA dehydrogenation disorder patient catabolized C-14-butyrate, -octanoate, and -leucine normally after culture in riboflavin-supplemented medium (2 mg/L). After culture in riboflavin-depleted medium (less-than-or-equal-to 1.4 mug/L), his cells oxidized the same substrates poorly at 20 to 33% of control (p < 0.05). Patient cells incubated in a wide range Of D-[12-C-14]riboflavin concentrations (3, 31.4, and 100 mug/L) synthesized C-14-flavin mononucleotide and C-14-flavin adenine dinucleotide (FAD) normally and had normal cytosolic C-14-flavin mononucleotide and C-14-FAD contents, which argues against defects in cellular riboflavin uptake and conversion to flavin mononucleotide and FAD. After culture in 31.4 mug C-14-riboflavin/L for 2 wk, C-14-FAD specific radioactivities plateaued and were similar in patient and control cells. However, culturing these uniformly labeled cells in riboflavin-depleted medium for 2 wk lowered the patient's cellular C-14-FAD content to only 23% of control levels. Similarly, after incubation in low C-14-riboflavin concentrations (4.4 mug/L), the patient's mitochondrial C-14-FAD content was only 51% of control after 1 h and 29% of control at 4 h. After a 4-h incubation in a high physiologic concentration of C-14-riboflavin (31.4 mug/L), which raised the patient's cellular C-14-FAD levels 3- to 4-fold, his mitochondrial C-14-FAD content rose to normal; control values did not change. We also investigated possible defective FAD binding to flavoenzymes essential for acyl-CoA dehydrogenation. Medium-chain acyl-CoA dehydrogenase activities did not fall significantly in either patient or control mitochondria from cells cultured in riboflavin-depleted medium. However, after culture in riboflavin-depleted medium, the patient's electron transfer flavoprotein activity fell to 59% of control in mitochondrial preparations, which is compatible with decreased matrix FAD content. We postulate that defective maintenance of mitochondrial FAD levels explains this patient's riboflavin-responsive multiple acyl-CoA dehydrogenation disorder phenotype.