EFFECTS ON PLASMA AMINO-ACID-CONCENTRATIONS AND HEPATIC BRANCHED-CHAIN ALPHA-KETO ACID DEHYDROGENASE-ACTIVITY OF FEEDING RATS DIETS CONTAINING 9 PERCENT OR 50 PERCENT CASEIN

Plasma concentrations of amino acids and .alpha.-ketoisocaproate and .alpha.-keto-.gamma.-methiolbutyrate decarboxylation activities in livers of rats trained to eat 9 or 50% casein diets for 5 h/day, were measured 1/2 h before and 1/2 and 3 h after the start of the feeding period. Decarboxylation of both .alpha.-ketoisocaproate and .alpha.-keto-.gamma.-methiolbutyrate by liver increased significantly within one-half after rats had ingested either the 9 or the 50% casein diet. Liver decarboxylation activity of rats fed the 50% casein diet was from 2-5-fold higher than that of rats fed the 9% casein diet. The greatest difference was observed when Ca, NAD and CoA were included in the decarboxylation assay medium. Although the activity of the branched-chain .alpha.-keto acid dehydrogenase increased in response to food ingestion, plasma concentrations of branched-chain amino acids also increased greatly after the ingestion of food. The similarity in the responses of .alpha.-ketoisocaproate and .alpha.-keto-.gamma.-methiolbutyrate decarboxylation in rats fed diets differing in protein content and subjected to different feeding regimens allows for the suggestion that the branched-chain .alpha.-keto acid dehydrogenase is responsible, in part, for the oxidative decarboxylation of the .alpha.-keto acid analog of methionine.