The action of streptomycin.

Previous papers (Umbreit, 1949; Oginsky, Smith, and Umbreit, 1949) have presented data showing that one of the actions of streptomycin on a sensitive Escherichia coli strain was to inhibit the oxalacetate-pyruvate reaction. It was admitted that the exact nature of this reaction was not known and that methods for its measurement were somewhat indirect. At about the time these papers were published, three developments altered the general concepts of the nature of this reaction. Stern and Ochoa (1949) demonstrated that in cell-free extracts of pigeon liver a condensation occurred between acetate and oxalacetate to yield citrate. The hypothesis of the "three-point landing" was proposed (Ogston, 1948) to account for asymmetry in the citrate formed. There was the demonstration by Potter and Heidelberger (1948) that the citrate formed was indeed asymmetrical. These three factors emphasized the position of citric acid in the "tricarboxy acid cycle." It is, therefore, now generally agreed that the product of condensation of acetate and oxalacetate is citrate. It has been known for years that pyruvate can be oxidized to acetate (or "active acetate") so that the pathway pyruvate -÷ active acetate -+>citrate was indeed a probable one, particularly since, in the presence of trace quantities of oxalacetate, pyruvate would tend to be oxidized to completion. By way of a wide variety of studies there was conclusive evidence that pyruvate could be oxidized by way of the citric acid cycle (cf. Krebs, 1943; Martius and Lymen, 1950). Since, therefore, pyruvate could enter the citric acid cycle via acetate, there was no longer any need to assume a "direct" pyruvate-oxalacetate condensation. Since previous data have shown that streptomycin acts somewhere in this complex of enzymes, its effect should be evident on the citrate-forming system. Drs. Stern and Ochoa very kindly tested streptomycin on the isolated system from liver carrying out this reaction, but found no appreciable inhibition.1 Nevertheless, it was still possible that either (1) the acetate-oxalacetate condensation in the susceptible bacteria differed in some subtle way from that in the animal or (2) that streptomycin acted upon this reaction by inhibiting another step, for example, the formation of some cofactor that was added in the experiments of Stern and Ochoa. The first possibility is quite unlikely since, although the features of the reaction susceptible to streptomycin inhibition may differ from the animal to the bacteria, they have in common the streptomycin-inhibited step, since animal tissues as well as E. coli show streptomycin inhibition of this reaction (Umbreit and Tonhazy, 1949).