TERMINAL OXIDATION SYSTEM IN RESPIRING YEAST

The role of action of the cytochrome b₂ contained in baker's yeast lactic dehydrogenase was re-evaluated by using the cytochrome b2 separated from the dehydrogenase as the final electron acceptor.<br> 1. Cytochrome b₂ is reduced anaerobically by lactate and by malate in the presence of the yeast lactic dehydrogenase preparation. The reduction by malate is much slow in rate compared with that by lactate as well as in the case where yeast cytochrome c is used as the final electron acceptor.<br> 2. Cytochrome b₂ is notably autoxidizable.<br> 3. The reductions of cytochrome b2 by lactate and by malate in the presence of the enzyme preparation are different in optimum pH from those of cytochrome c by both substrates. In the latter cases, the optima pH are at 6.5 and 8.0, while in the former cases, the reductions by lactate and by malate hardly decrease in rate at pH 4.0 and 9.0, respectively.<br> 4. The reduction of cytochrome bg by lactate in the presence of the enzyme preparation is partially inhibited in the presence of FMN or FAD, when it is measured spectrophotometrically.<br> 5. The reduction of cytochrome b₂ by lactate in the presence of a test solution of yeast lactic dehydrogenase is almost same in rate as the reduction of cytochrome c in the presence of the test solution diluted forty two times.<br> Based upon these facts, the role of action of cytochrome b₂ in the dehydrogenations of lactate and of malate in the respiring yeast cells was discussed with stress on an importance of a conjugate form between the cytochrome b₂ moiety and the dehydrogenase-FMN moiety in yeast lactic dehydrogenase.<br> We are very grateful to Mr. K. Fujii of Oriental Yeast Co., Ltd., Osaka for supplies of baker's yeast. We should like to thank our colleagues Messrs. M. Nozaki, T. Higashi, H. Matsubara, T. Yamanaka and H. Mizushima for helpful discussions during the course of this work.