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 14C-butyrate, -octanoate, and -leucine normally after culture in riboflavin-supplemented medium (2 mg/L). After culture in riboflavin-depleted medium (≤1.4 μg/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-[2-14C]riboflavin concentrations (3, 31.4, and 100 μg/L) synthesized 14C-flavin mononucleotide and 14C-flavin adenine dinucleotide (FAD) normally and had normal cytosolic 14C-flavin mononucleotide and 14C-FAD contents, which argues against defects in cellular riboflavin uptake and conversion to flavin mononucleotide and FAD. After culture in 31.4 μg 14C-riboflavin /L for 2 wk, 14C-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 14C-FAD content to only 23% of control levels. Similarly, after incubation in low 14C-riboflavin concentrations (4.4 μ g/L), the patient's mitochondrial 14C-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 14C-riboflavin (31.4 μ g/L), which raised the patient's cellular 14C-FAD levels 3- to 4-fold, his mitochondrial 14C-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.