Studies of new intracellular proteases in various organs of rats. Participation of proteases in degradation of ornithine aminotransferase in vitro and in vivo.

Homogenates of the muscle layer of rat small intestine irreversibly inactivated endogenous ornithine aminotransferase at 37 degrees C. Addition to the homogenate of coenzymes and the various keto-acids which act as substrate inhibited conversion of the holoenzyme to the apoenzyme and its subsequent degradation. Addition of protease inhibitors including soybean trypsin inhibitor, chymostatin and phenylmethylsulfonyl fluoride almost completely prevented inactivation of he enzyme. Immunological activity decreased during inactivation of the enzyme, but its rate of decrease was much slower than that of loss of enzyme activity. Antigen-antibody precipitates from homogenates containing inactivated enzyme, were separated by electrophoresis on sodium dodecylsulfate-polyacrylamide gels. In this way breakdown products of the enzyme were found, indicating that the enzyme in homogenates was inactivated by limited proteolysis. These results obtained in vitro support our previous suggestion (1975) of a stepwise mechanism for degradation of pyridoxal enzymes.