GLUTAMINE SYNTHESIS FROM GLUCOSE AND AMMONIUM-CHLORIDE BY GUINEA-PIG KIDNEY-TUBULES

1. At a physiological concentration (5 mM), glucose was found to be me tabolized by isolated kidney cortex tubules prepared from fed guinea p igs. 2. The release of (CO2)-C-14 from [U-C-14]glucose indicated that oxidation of the glucose carbon skeleton represented about 50% of the glucose removed; significant amounts of lactate and glutamine also acc umulated. 3. Addition of 0.1-10 mM NH4Cl led to a dose-dependent stimu lation of glucose metabolism which was accompanied by a large increase in lactate and glutamine accumulation and, to a lesser extent, in glu cose oxidation. 4. Comparison of the release of (CO2)-C-14 from [1-C-1 4]- and [6-C-14]glucose indicates that, in both the absence and the pr esence of NH4Cl, the pentose phosphate shunt was only a minor pathway of glucose metabolism. 5. The central role of pyruvate carboxylase in the conversion of glucose carbon into glutamine carbon was demonstrate d by using a bicarbonate-free medium and measuring the fixation of (CO 2)-C-14 from [C-14]bicarbonate, which was recovered mostly at C-1 of g lutamine plus glutamate. 6. The NH4Cl-induced stimulation of glucose r emoval was secondary not only to increased glutamine synthesis, as sho wn by the effect of methionine sulphoximine, an inhibitor of glutamine synthetase, but also to the stimulation of phosphofructokinase activi ty by NH4Cl. 7. Renal arterio-venous difference measurements revealed that, in vivo, the guinea-pig kidney removed glucose from the circulat ing blood, which suggests that glucose carbon may contribute to the ca rbon skeleton of the glutamine released by this organ.