REGULATION OF VALINE CATABOLISM IN CANINE TISSUES - TISSUE DISTRIBUTIONS OF BRANCHED-CHAIN AMINOTRANSFERASE AND 2-OXO ACID DEHYDROGENASE COMPLEX, METHACRYLYL-COA HYDRATASE AND 3-HYDROXYISOBUTYRYL-COA HYDROLASE

To clarify the valine catabolism, the activities of principal enzymes in its catabolic pathway, branched-chain aminotransferase, branched-ch ain Zero acid dehydrogenase complex, methacrylyl-CoA hydratase and 3-h ydroxyisobutyryl-CoA hydrolase, were measured using canine tissues. Af ter killing of beagle dogs, tissues (liver, pancreas, kidney, heart, s keletal muscle and mucosae of digestive organs such as stomach, small intestine and colon) were removed and immediately frozen. Branched-cha in aminotransferase activity in liver was the lowest among the tissues measured. In contrast, the activities of branched-chain 2-oxo acid de hydrogenase complex in liver as well as in kidney were relatively high and the enzyme complex activities were markedly low in small intestin e and skeletal muscle. The activities of methacrylyl-CoA hydratase and 3-hydroxyisobutyryl-CoA hydrolase were relatively high in all tissues , suggesting that a cytotoxic intermediate, methacrylyl-CoA, is immedi ately degraded to non-toxic compounds, 3-hydroxyisobutyrate and free C oA. These findings suggest that the consumption of branched-chain amin o acids in the absorption site (small intestine) is suppressed in orde r to supply them to the whole body, in particular to skeletal muscle a nd that skeletal muscle might act as a storage of gluconeogenic amino acids. The high capacity to dispose methacrylyl-CoA produced in the va line catabolism is suggested to play an important role in protecting c ells against the toxic effects of methacrylyl-CoA.