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This paper reports the isolation of a proteinase and a proteinase inhibitor from Saccharornyces cerevisiae S288C (a). The purified proteinase catalyzes the hydrolysis of casein a t neutral pH. It inactivates the holo-tryptophan synthase from Neurospora c r a m but does not inactivate the holo-ornithine aminotransferase from pig kidney. This proteinase can be inhibited either by phenylmethanesulfonyI fluoride (a specific inhibitor for serine proteases), the sulfhydryl reagent, p-mercuribenzoate, or the endogenous proteinase inhibitor present in yeast. The yeast proteinase inhibitor is a heat-stable material with a molecular weight of approximately 13000. This proteinase inhibitor can protect both yeast and Neurospora tryptophan synthase against proteolytic inactivation. The proteinase and its inhibitor coexist in the yeast extracts. The cellular level of the proteinase and the inhibitor activities varies with the composition of the medium in which the cells are grown. Since some yeast proteinases have been shown to form inactive complexes with their respective inhibitors, it is proposed that proteinase-inhibitor interaction is a possible mechanism for regulation of intracellular proteolytic activity.Previous work on Neurospora tryptophan synthase [l] has shown that inactivation of partially purified enzyme can be prevented by phenylmethanesulfonyl fluoride, a known inhibitor of serine protease [2,3]. From this observation, together with the fact that the highly purified enzyme is stable, i t was cancluded that the inactivation in vitro of the Neurospora synthase is due to proteolysis [l]. More recent studies by us have demonstrated that the protease which causes the inactivation of the Neurospora synthase is a serine endopeptidase [4]. Furthermore, we demonstrated that the purified endopeptidase can be strongly inhibited by a proteinaceous inhibitor present in Neurospora [4].The activity of the tryptophan synthase of Saccharomyces cerevisiae is also known to be influenced by macromolecular factors, inactivating and protecting, present in yeast [5]. Recently a partial purification of both factors was reported [6,7]; however, no information was presented regarding the mechanism of inactivation.

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Abteilung MolekulareBiologie, Max‐Planck‐Institut Experimentelle Medizin, Gottingen, Germany