ALPHA-KETOISOCAPROATE STIMULATES BRANCHED-CHAIN AMINO-ACID TRANSAMINASE IN KIDNEY AND MUSCLE

Branched-chain amino acid transaminase (BATase, EC 2.6.1.6) activity of isolated rat kidney was stimulated by perfusion with .alpha.-ketoisocaproate (KL). The mechanism of this effect in kidney was examined and the occurence in intact skeletal muscle stimulation was documented. 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 i.v. 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. Cycloheximide studies indicated a direct effect of KL on muscle BATase. .alpha.-Ketoisocaproate can increase BATase activity and thus may be involved in regulation of its activity.