MOLECULAR-MODEL OF HUMAN BRANCHED-CHAIN AMINO-ACID-METABOLISM

To establish an accurate molecular model of human branched-chain amino acid (BCAA) metabolism, the distribution, activity, and expression of the first 2 enzymes in the catabolic pathway-branched-chain-amino-aci d aminotransferase (BCAT) and branched-chain alpha-keto acid dehydroge nase (BCKD) complex-were determined in human tissues. The same enzyme activities were measured in rat and African green monkey tissues. Over all, the activities of BCAT and BCKD were higher in rat than in human and monkey tissues; nevertheless, the ratio of the 2 activities was si milar in most tissues in the 3 species. Total oxidative capacity was c oncentrated in skeletal muscle and liver (> 70%) with muscle having a higher proportion of the total in humans and monkeys. In humans, brain (10-20%) and kidney (8-13%) may contribute significantly to whole-bod y BCAA metabolism. Furthermore, in primates the high ratio of transami nase to oxidative capacity in the entire gastrointestinal tract serves to prevent loss of essential BCAA carbon and raises the possibility t hat the gastrointestinal tract contributes to the plasma branched-chai n cr-keto acid pool. Quantitative polymerase chain reaction was used t o examine expression of human branched-chain cr-keto acid dehydrogenas e kinase (BCKDK), the key enzyme that regulates the activity state of the human BCKD complex and human BCAT isoenzymes. To design the primer s for the polymerase chain reaction, human BCKDK was cloned. BCKDK mes sage was found in all human tissues tested, with the highest amount in human muscle. As in rats, there was ubiquitous expression of mitochon drial BCAT, whereas mRNA for the cytosolic enzyme was at or below the limit of detection outside the brain. Finally, the role of BCAA in bod y nitrogen metabolism is discussed.