The pathway of oxidation of acetate in baker's yeast

Abstract

The properties of baker''s yeast made permeable to di- and tri-carboxylic acids by exposure to dry ice were studied. The effects of cooling on permeability were found to depend on the rate of cooling. Gradual cooling to the temp. of dry ice had no major effect. Sudden transfer from 20-40[degree] to dry ice almost completely abolished the capacity to oxidize acetate, while it increased that for oxidizing succinate. Transfer from +2[degree] to dry ice partly preserved the capacity to oxidize acetate, ethanol and glucose. Yeast cells heated for 1 hr. to 50[degree] and then transferred to dry ice had a low blank respiration and did not oxidize acetate, ethanol or glucose but attacked succinate and D- and L-lactate. Dry-ice-treated cells were permeable to fumarate, malate and citrate as demonstrated by their high fumarase and aconitase activity. Permeability for succinate was shown by the oxidation of this substance in the cells. The enzymes concerned were all attached to the cells and did not appear in soln. The oxidation of succinate was competitively inhibited by malonate. The inhibition was 50% when the concn. ratio (succinate)/(malonate) was about 60. The oxidation of acetate by dry-ice-treated yeast was not inhibited by malonate under conditions where the oxidation of succinate was inhibited. This result excludes succinate as an intermediate in the oxidation of acetate. Apparent discrepancies between these results and those of Barron et al. (1950) and Lynen (1943) are discussed. Simultaneous oxidation expts. with C14-acetate and non-labelled 2d substrates excluded succinate, fumarate, malate, alpha-ketoglu-tarate, glyoxylate, formate and acetoacetate as intermediates in the oxidation of acetate in dry-ice-treated yeast, the component reactions of the cycle can nevertheless occur in yeast cells. Evidence is presented which suggests that the component reactions of the cycle serve to supply intermediates for organic syntheses. A number of observations are discussed which support the view that in many other micro-organisms the reactions of the tricarboxylic acid cycle are primarily concerned with the supply of intermediates, rather than of energy.