The metabolism of C2 compounds in micro-organisms. 4. Synthesis of cell materials from acetate by Aspergillus niger

Abstract

Isotope from [2-C14] acetate added to A. niger growing on acetate as the sole C source was rapidly incorporated into cellular components. At the earliest times, only intermediates of the tricarboxylic acid cycle and amino acids derived therefrom became labelled. Isotope from acetate entered the tricarboxylic acid cycle at 2 sites, to form citrate at one and malate at the other. At the earliest times, labelled malate could not have been solely derived from labelled succinate. These results are consistent with the simultaneous operation of the tricarboxylic acid and glyoxylate cycles. These findings also exclude the formation of succinate from the oxidative condensation of acetate as being of quantitative importance in the growth of A. niger on acetate. Cell-free extracts of the acetate-grown mold contained the 2 key enzymes of the glyoxylate cycle, isocitratase and malate synthe-tase, in quantities adequate to account for the observed rates of growth of the organism. When supplemented with cofactors, such extracts also catalyzed the overall formation of malate and succinate from acetate and isocitrate. Cell-free extracts of the mold grown on glucose or succinate contained malate synthetase but only traces of isocitratase. This indicates that the operation of the glyoxylate cycle is controlled by factors influencing the intracellular activity of isocitratase.