alpha-Ketoisocaproate stimulates branched-chain amino acid transaminase in kidney and muscle.

We have reported that branched-chain amino acid transaminase (BATase) activity of isolated rat kidney is stimulated by perfusion with alpha-ketoisocaproate (KL). This study examines the mechanism of this effect in kidney and documents that stimulation occurs in intact skeletal muscle. Increased activity was not attributable to synthesis of enzyme because it occurred in the presence of cycloheximide. The in vivo degradation rate of BATase estimated from sequential measurements of activity following intravenous cycloheximide was longer than 90 min, whereas during in vitro perfusion stimulation could be detected within 5 min. Incubation of supernatant from kidney homogenate with KL stimulated BATase; incubation with alpha-keto-beta-methylvalerate (KI), alpha-ketoisovalerate (KV), leucine (leu), or isovaleryl CoA did not. Perfusion of rat hindquarter with KL increased muscle BATase activity; perfusion with acetoacetate, KI, KV, or leu did not. Again, cycloheximide studies indicated a direct effect of KL on muscle BATase. These findings suggest that alpha-ketoisocaproate can increase BATase activity and thus may be involved in regulation of its activity.