Decreased enzyme activity and contents of hepatic branched-chain α-keto acid dehydrogenase complex subunits in a rat model for type 2 diabetes mellitus

The mitochondrial branched-chain α-keto acid dehydrogenase complex (BCKDC) is responsible for the committed step in branched-chain amino acid catabolism. In the present study, we examined BCKDC regulation in Otsuka Long-Evans Tokushima Fatty (OLETF) rats both before (8 weeks of age) and after (25 weeks of age) the onset of type 2 diabetes mellitus. Long-Evans Tokushima Otsuka (LETO) rats were used as controls. Plasma branched-chain amino acid and branched-chain α-keto acid concentrations were significantly increased in young and middle-aged OLETF rats. Although the hepatic complex was nearly 100% active in all animals, total BCKDC activity and protein abundance of E1α, E1β, and E2 subunits were markedly lower in OLETF than in LETO rats at 8 and 25 weeks of age. In addition, hepatic BCKDC activity and protein amounts were significantly decreased in LETO rats aged 25 weeks than in LETO rats aged 8 weeks. In skeletal muscle, E1β and E2 proteins were significantly reduced, whereas E1α tended to increase in OLETF rats. Taken together, these results suggest that (1) whole-body branched-chain α-keto acid oxidation capacity is extremely reduced in OLETF rats independently of diabetes development, (2) the aging process decreases BCKDC activity and protein abundance in the liver of normal rats, and (3) differential posttranscriptional regulation for the subunits of BCKDC may exist in skeletal muscle.