OXIDATION OF COMPOUNDS IN THE KREBS CYCLE BY PULLULARIA PULLULANS

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Abstract
Studies with intact cells and cell-free extracts of Pullularia pullulans showed that the organism has the necessary enzymes for oxidizing most of the compounds of the Krebs cycle. The rates of oxidation of substrates by intact cells were studied in relation to the age of the cells, the concentration of substrate, the concentration of buffer, and the pH of the reaction medium. Cells grown for 36 hours oxidized a greater number of substrates than did those grown in less or greater intervals. Cells grown for 6 and 18 hours had adaptive enzymes for the oxidation of acetate. The respiratory activity of the 36-hour-old cells was generally greater in 0.0143 M concentration of substrate than in lower or higher concentrations; it was not affected by changes in concentration of buffer. With cells at pH 6.8, acetate and ethyl alcohol were oxidized as rapidly as glucose; oxalacetate and pyruvate were oxidized at rates of more than one-half and one-third, respectively, that of glucose; alpha-ketoglutarate, fumarate, cis-aconitate, and succinate were slowly oxidized; and citrate, isocitrate, and malate were not oxidized. At pH 3.0, the cells oxidized most of the substrates more rapidly than they did at pH 6.8, but they did not oxidize citrate, cis-aconitate, nor isocitrate. The organism oxidized malonate, and its enzymatic activity apparently was not inhibited by that compound.Cell extracts oxidized alpha-ketoglutarate, citrate, fumarate, malate, and succinate. TPN stimulated the oxidation of citrate and the activity of isocitrate dehydrogenase. DPN stimulated the oxidation of malate. Moreover, the activity of succinate dehydrogenase was inhibited by malonate. The results suggest that the metabolism of P. pullulans is by means of the reactions of the Krebs cycle.

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