THE RABBIT KIDNEY TUBULE UTILIZES GLUCOSE FOR GLUTAMINE SYNTHESIS - AC-13 NMR-STUDY

The metabolism of variously labeled [C-13]- and [C-14]glucoses, used a t a physiological concentration (5 mM), has been studied in isolated r abbit kidney tubules both in the absence and the presence of NH4Cl. Wh en present as sole exogenous substrate, glucose was metabolized at hig h rates and converted not only into CO2 and lactate but also, in contr ast to a previous conclusion of Krebs (Krebs, H. A. (1935) Biochem. J. 29, 1951-1969), into glutamine. Absolute fluxes through enzymes of gl ycolysis and gluconeogenesis and of enzymes of three different cycles operating simultaneously were assessed by using a novel model describi ng reactions of glucose metabolism in conjunction with the C-13 MMR an d, to a lesser extent, the radioactive data obtained. The presence of NH4Cl (5 mM) caused a large stimulation of glucose removal and a large increase in lactate, glutamine, and glycerol 3-phosphate accumulation . Under this condition, the stimulation of glutamine synthesis was acc ompanied not by an activation of citrate synthesis but by an inhibitio n of flux through alpha-ketoglutarate dehydrogenase. The resulting dep letion of citric acid cycle intermediates was compensated by anapleros is at the level of pyruvate carboxylase. The ''futile'' cycle involvin g oxaloacetate, phosphoenolpyruvate and pyruvate, which was intense in the presence of glucose alone, was greatly stimulated by the addition of NH4Cl.