Rat tissue concentrations of branched-chain 2-oxo acid dehydrogenase complex. Re-evaluation by immunoassay and bioassay.

A rabbit polyclonal antibody to purified ox kidney branched-chain oxo acid dehydrogenase complex was shown by a variety of techniques to be an antibody to the E2 (acyltransferase) component. Rocket immunoelectrophoresis showed that the antibody does not discriminate between phosphorylated (inactive) or dephosphorylated (active) complex, and the same technique is used to assay total branched-chain complex (sum of active and inactive forms) in rat liver and heart mitochondrial extracts. The values obtained in normal rats fed on normal diet were comparable with those obtained by spectrophotometric assay of the holocomplex reaction after conversion of inactive complex into active complex. The values obtained in liver mitochondria from rats fed on 0%-casein diet or starved for 48 h were comparable with those in rats fed on normal diet, whereas earlier studies using spectrophotometric assay had shown substantial decreases in rats fed on 0%-casein diet or starved for 48 h. It has been shown that conversion of inactive complex into active complex requires prolonged incubation (120 min) in the presence of ketoleucine (4-methyl-2-oxopentanoate; to inhibit branched-chain oxo acid dehydrogenase kinase) to effect complete conversion in mitochondria from rats fed on 0%-casein diet, or starved for 48 h, or made diabetic with alloxan. By this technique, total activity of the complex in rat liver mitochondria was unaffected by diet or diabetes. The effects of diet and diabetes to decrease the activity of branched-chain complex in rat liver are therefore apparently mediated wholly through inactivation of the complex by phosphorylation.