Action of Cystein on Respiration, Fermentation and Synthesis in Yeast Cells

Abstract

Quastel and Wheatley¹ have demonstrated that cystein and glutathione promote the ærobic fermentation in baker's yeast. Cystein inhibits very strongly the respiration in this material. We have confirmed these results for a Swedish baker's yeast². In our experiments, carried out generally at 30° and pH 5, the concentrations of glucose as well as those of cystein have been greatly varied. In higher concentrations of glucose alone (0·027-0·108 m.mol per 2·4 c.c. suspension containing 20 mgm. fresh yeast) the rate of ærobic fermentation is low at the beginning of the experiment but increases soon considerably. High concentrations of glucose give an effect similar to that following the addition of cystein to lower concentrations of glucose. In presence of cystein, however, the rapid ærobic fermentation starts immediately upon the addition of glucose. In the absence of cystein fermentation comes to an end, giving place to a slow respiration. The amount of carbon dioxide formed through ærobic fermentation is directly proportional to the concentration of glucose. The maximum rates of fermentation are directly proportional to log (glucose), range tested: 0·00455-0·108 m.mol per 2·4 c.c. suspension. During the period of rapid ærobic fermentation the added cystein is partly oxidized. The quotient total CO₂/O₂ decreases fairly rapidly during the period of ærobic fermentation. The mean of the total CO₂/O₂. values during the period of ærobic fermentation is directly proportional to log (cystein), or in other words, to the oxidation reduction potential of the cystein solution (cf. Dixon and Quastel³). The range tested was 0·0002-0·005 m.mol cystein per 2·4 c.c. suspension.