Quantitative aspects of the tricarboxylic acid cycle in baker's yeast.

The quantitative significance of the tricarboxylic acid cycle in the oxidation of acetate by yeast has remained in doubt despite many investigations concerning this topic. All of the enzymes of the tricarboxylic acid cycle have been demonstrated in either commercial baker's yeast or in pure cultures of Saccharomyces cerevisiae. (Weiland and Sonderhoff, 1932; Lynen and Neciullah, 1939; Novelli and Lipmann, 1950; Foulkes, 1951; Kornberg and Pricer, 1951; Nossal, 1954; Lynen and Kalb, 1955). The assumption that the individual enzymes can function as a group in effecting this organized sequence of reactions is further supported by isotope experiments. The results of experiments by Sonderhoff and Thomas (1937) and by Lynen (1948) employing deutero-acetate are in agreement with the concept that succinate arises from acetate in yeast via the currently accepted reactions of the tricarboxylic acid cycle. Similarly, Weinhouse and Millington (1947) found that the isotope distribution in citrate produced by yeast in the presence of magnesium and barium acetate-C14 was in agreement with the mechanism of the cycle. Such qualitative observations leave little doubt as to the occurrence of the tricarboxylic acid cycle in yeast but provide little information concerning the quantitative significance of the cycle as a terminal oxidative pathway. In experiments designed to evaluate the quantitative role of the tricarboxylic acid cycle in acetate oxidation, Krebs et al. (1952) employed yeast preparations which had been made permeable to diand tricarboxylic acids by treatment with dry ice. The results obtained from studies on the rate of substrate utilization, from isotope carrier experiments, and from additional criteria of the type frequently applied to mammalian systems, were