Abstract: Fermentable sugars, when added to cells of Saccharomyces cerevisiae growing on a non-fermentable carbon source, cause repression of the synthesis of certain enzymes ("catabolite repression") and in addition inactivation of a smaller group of enzymes ("catabolite inactivation"). Enzymes for which "catabolite inactivation" has been observed are listed herein. In five cases, it has been shown that the mechanism of catabolite inactivation is proteolytic in nature. Our present knowledge on the conditions and the mechanisms of initiation of inactivation and the biological significance of the proteolytic inactivation is summarized for these five enzymes: cytoplasmic malate dehydrogenase, aminopeptidase I, fructose-1,6-bisphosphatase (FBPase), phosphoenolpyruvate carboxykinase and isocitrate lyase. With the exception of aminopeptidase I, these enzymes are key enzymes of gluconeogenesis in S. cerevisiae. It is obvious that gluconeogenesis is no longer necessary, if a fermentable carbon source is available.