The hexose monophosphate shunt in glucose catabolism in ascites tumor cells.

Abstract

Summary Examination of the rate-controlling factors in the hexose monophosphate shunt pathway in ascites tumor cells was made by studying the incorporation of C14 of glucose—labeled uniformly or in carbons-1, -2, and -6—into the respiratory CO2 under varied conditions. A study of the effect of glucose concentration on the operation of the alternate pathway in a hyperdiploid Ehrlich ascites tumor revealed that the initial rate of CO2 production by the shunt was independent of substrate concentration in the range of 2.5 × 10-5m to 1 × 10-2m. Artificial electron acceptors such as methylene blue, menadione, and phenazine methosulfate stimulated carbon-1 oxidation from six- to 30-fold with only a slight stimulatory effect on the oxidation of carbon-6, suggesting that one of the ratelimiting factors in the operation of the alternate pathway is the availability of the electron transport system. This stimulation was also observed with homogenates of ascites cells when a source of TPN was supplied. This TPN-dependent oxidative system for carbon-1 was localized in the soluble fraction, which also contained a lactic acid dehydrogenase that catalyzed the oxidation of TPNH by pyruvate. A significant oxidation of carbon-1 of glucose by intact tumor cells was observed under anaerobic conditions. The rate of oxidation of carbon-1 was stimulated by pyruvate to that observed in the presence of oxygen. Oxygen did not increase carbon-1 oxidation by the intact cells in the presence of moderate pyruvate levels, indicating that intermediary metabolites such as pyruvate may serve as the principal electron acceptors for the hexose monophosphate shunt. From the data, it is concluded that the prime function of the hexose monophosphate shunt is to provide reduced triphosphopyridine nucleotide for specific reductive syntheses.