Autocatalytic degradation of proteins in extracts of a brewing strain of Saccharomyces cerevisiae. The role of endoproteinases and exopeptidases

Cells of a brewing strain of Saccharomyces cerevisiae NCYC 1108 were harvested after the end of fermentation, but before decline in viability had commenced, and extracted by passage through an Eaton press. Immediately after preparation the proteolytic enzymes PrA and CpY, but not PrB, could be detected in the extracts. After 24 h incubation at 25°C all three enzymes were found. PrA activity was unaffected by incubation whereas CpY activity increased up to threefold. Measurement of the release of acid-soluble material in the extracts themselves over an 18-h period at 25°C and a range of pH values using the Lowry and trinitrobenzoyl sulphonic acid methods, suggested that the endoprotease, PrB, was a major contributor to autocatalysis. Further studies using pepstatin, phenylmethylsulphonyl fluoride (PMSF), HgCl₂, EDTA, bestatin and ZnCl₂ confirmed the importance of PrB but also indicated that about 30% of the initial proteolytic degradation was due to another enzyme. This enzyme was inhibited by Zn²⁺ and Hg²⁺ but not by PMSF or pepstatin, and could be the recently characterised enzyme, PrE.