Effect of aeration intensity on the biochemical composition of baker's yeast. II. Activities of the oxidative enzymes

When the effect of catabolite repression is eliminated Saccharomyces cerevisiae prefers an aerobic metabolism. The potential for completely aerobic catabolism exists even in circumstances where its action is limited by the oxygen available. When the oxygen absorption in the medium is adequate, yeast uses a solely oxidative metabolism for energy-yielding reactions. The changes observed in the activity of malate dehydrogenase can be described as a function of two isoenzymes, both of which are affected by oxygen; the isoenzyme participating in the glyoxylate cycle shows variations in activity similar to that observed in isocitrate lyase. NAD-linked glutamate dehydrogenase activity roughly follows that of malate dehydrogenase and isocitrate lyase; in cultivations with the same growth rate the NADP-linked dehydrogenase is insensitive to the oxygen level. The cytochromes aa₃, b, and c have a clear maximum at low oxygen tension, the most sensitive being cytochrome aa₃. The imbalance between cytochrome c:oxygen oxidoreductase activity and the amount of cytochrome aa₃, and the correlation observed between respiration rate and the activities of cytochrome c oxidase and NADH₂:cytochroine c oxidoreductase are discussed. Methods used for estimation of cytochromes are compared.