Myelin proteins: degradation in rat brain initiated by metabolites causative of maple syrup urine disease.

Maple syrup urine disease (MSUD), an inborn error of metabolism in humans, is expressed as an inability to oxidatively decarboxylate the branched-chain alpha-keto acids derived from leucine, isoleucine and valine. Rats 14 days old were injected intracranially with a solution containing leucine, alpha-ketoisocaproate, and tracer amounts of 3H-lysine. Myelin isolated from these rat brains at 28 days of age had a washed dry weight 85 per cent of controls. The protein content of the myelin prepared from treated and control rats was identical, as were the specific activities of the individual proteins separated by polyacrylamide gel electrophoresis. Myelin protein from treated rats was deficient in myelin high molecular weight proteins including glycoproteins, and degradation products of these proteins were observed in myelin of treated rats. MSUD associated metabolites in man may initiate a process leading to the proteolytic degradation of myelin proteins, thereby producing abnormal myelin sheaths.