Direct physical evidence for stabilization of branched-chain alpha-ketoacid dehydrogenase by thiamin pyrophosphate.

Branched-chain alpha-ketoacid dehydrogenase is a multienzyme complex composed of four subunits. The 46,500-dalton protein is a subunit of the decarboxylase component, which is selectively digested by chymotrypsin. Two peptides of apparent mol. wts. of 36,000 and 15,000 result with loss of enzyme activity. When the complex is saturated with thiamin pyrophosphate and ketoacid substrate, digestion by chymotrypsin does not occur. These data provide direct physical evidence for the stabilization of the complex by the presence of the vitamin B1-derived cofactor.