ACETIC ACID OXIDATION BY ESCHERICHIA COLI: EVIDENCE FOR THE OCCURRENCE OF A TRICARBOXYLIC ACID CYCLE

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Abstract
-An attempt has been made to clarify some of the controversial points concerning the mechanism of acetate oxidation by Escherichia coli. Acetate-2-C14 was oxidized simultaneously with unlabeled [alpha]-ketoglutarate and succinate by non-proliferating suspensions of cells. A negligible amount of isotope was incorporated into the [alpha]-ketoglutarate, whereas the succinate was highly radioactive. An experiment is described employing acetate-2-C14 as substrate in the absence of added carriers. Citrate, [alpha] -ketoglutarate, succinate, fumarate, and malate were isolated and found to be in isotopic equilibrium with each other and with the respiratory carbon dioxide and in approximate equilibrium with the residual acetate. The isotopic distribution found in these intermediates is presented as evidence for occurrence of a tricarboxylic acid cycle in this organism. It is concluded that the lack of incorporation of isotope into an added carrier cannot be considered con- clusive evidence against participation of this compound as an intermediate in the metabolic pathway under investigation.

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