ACETIC ACID OXIDATION BY ESCHERICHIA COLI: QUANTITATIVE SIGNIFICANCE OF THE TRICARBOXYLIC ACID CYCLE

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Abstract
The anaerobic metabolism of acetate by E. coli, in the presence of fumarate as an oxidant, was studied with the aid of labeled acetate. Experiments with acetate -1-C14 showed that succinate was an end product of the oxidation and that the respiratory carbon dioxide was not labeled. The latter was derived from the carboxyl carbons of fumarate and was equivalent quantitatively to the acetate utilized. By mass analysis of the succinate formed from acetate-2-C13, it was possible to distinguish between succinate synthesized from acetate via the tricarboxylic acid cycle and that formed by oxidative condensation (Thunberg condensation). To accomplish this, the succinate was degraded in such a manner as to obtain the methylene carbons as ethylene. The ethylene was subjected to mass spectrographic analysis and found to contain the mass types corresponding to singly- and non-labeled succinate. Double labeled ethylene (succinate) was not present in excess of the normal value. These data are presented as evidence that the tricarboxylic acid cycle accounts quantitatively for acetate oxidation by nonproliferating suspensions of this organism and that a methyl to methyl condensation of acetate to yield succinate does not occur under these conditions.