The metabolism of C2-compounds in micro-organisms. 8. A dicarboxylic acid cycle as a route for the oxidation of glycollate by Escherichia coli

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Abstract
Escherichia coli, strain w, grew readily in a medium containing 50 mM-glycollate as C source. The mutant M 22-64 (Gilvarg and Davis, 1957), devoid of the citrate-forming condensing enzyme, failed to grow in this medium unless small quantities (2 mM) of glutamate were also added. The mutant and the wild-type organisms, grown on 50 mM-glycollate plus 2 mM-glutamate, oxidized glycollate or glyoxylate at the same rate and to the same extent. These oxidations were not affected by 2.5 mM-hypophosphite, which abolished formate oxidation. Isotope from [1-14C]- or [2-14C]-glycollate was incorporated at early times, by organisms oxidizing unlabelled glycollate, into glyoxylate, malate, oxaloacetate and pyruvate. Under these conditions, isotope from [2-14c]acetate appeared in malate, oxaloacetate and pyruvate. Although the mutant did not oxidize acetate, acetate stimulated the rate and extent of oxygen uptake by cells oxidizing glyoxylate. Washed suspensions of the mutant evolved, as 14CO2, 0.5% of the isotope from [1-14C]acetate. This was increased 70-80 fold when unlabelled glycollate or glyoxylate was also added. Pyruvate was ineffective in promoting this oxidation of [1-14C]acetate. Similar results were obtained with [2-14c]acetate. Addition of unlabelled glycollate or glyoxylate stimulated the release of isotope as 14CO2 from [1-14C]malate, [2,3-14C2]malate and [3-14C]pyruvate. These results suggest the operation of a dicarboxylic acid cycle, in which glyoxylate is totally oxidized with the catalytic participation of malate, oxaloacetate, pyruvate and acetyl-coenzyme A. Measurement of the absolute levels of enzymes in glycollate-grown cells, and alterations in their relative proportions when cells adapt to grow on glycollate, support the occurrence of this postulated cycle.