Catabolite inactivation of bakers'-yeast uridine nucleosidase. Isolation and partial purification of a specific proteolytic inactivase.

When bakers’ yeast cells are grown in culture media containing ethanol instead of glucose, uridine nucleosidase specific activity increased up to 10 times its original value. Addition of glucose to ethanol- grown cells caused a rapid drop of uridine nucleosidase activity. This inactivation could be prevented by the presence of phenylmethylsulfonyl fluoride. Experiments with cycloheximide indicated that both the inactivation and the reappearance of the enzyme activity require protein synthesis. In addition a fraction has been isolated and partially purified from bakers’ yeast that is able to inactivate uridine nucleosidase in vitro. The inactivating fraction is not dialysable, and can be purified using conventional protein purification procedure, suggesting its protein character. The pH optimum of the inactivating activity is around 5 and the factor shows a molecular weight of 75000. Glucose-6- phosphate dehydrogenase, hexokinase, glyceraldehyde-3-phosphate dehydrogenase, cytidine de- aminase, cytosine deaminase, 3-phosphoglycerate kinase and glutathione reductase were not inac- tivated by the preparation. Bakers’ yeast boiled crude extract supernatant contained an inhibitor of the inactivating factor. The inactivating protein is endowed also with proteolytic activity and the inhibition exerted by phenylmethylsulfonyl fluoride and by the endogeneous inhibitor indicate that the factor is a protease.