The interconversion of lactate and pyruvate and its effect on the apparent oxidative decarboxylation of these substrates in normal tissue and ascites tumor cells.

when one of the substrates used is labeled, is readily demonstrable in both these tissues, even under aerobic conditions. The rate of interconversion in the actively respiring kidney slices was at least as high as the oxidative decarboxylation of the substrates. It proceeded more rapidly in the tumor cells in which its rate was 4-5 times as high as that of the oxidative decarboxyla tion. The failure of pyruvate to suppress the radioactive COjproduction from lactate-l-14C was observed only in the tumor cells, and only when these were present in excess with regard to the equilibration of the isotopie carbon. This phenomenon has been attributed to the rapid interconversion and high degree of equilibration of the radioactive carbon between the 2 substrates, obtained prior to their oxidative decarboxylation. Pyruvate markedly inhibited the disappearance of lactate in the kidney slices and arrested it completely in the case of the tumor cells even when it failed to suppress the apparent 14C02 production from the carboxyl-labeled substrate. These results suggest that the isotopie CO»produced by the tumor cells from lactate-l-14C in the presence of an equimolar concentration of pyruvate is derived entirely via the interconversion of these substrates. They further suggest that the radioactive CO»production from lactate-l-"C is not a valid criterion for the ability of the mammalian cells to consume and oxidize this substrate, since it may result from the interconversion of the latter with endogenously generated pyruvate.