Pyruvate carboxylation in glutamine synthesis from alanine by isolated guinea-pig renal cortical tubules.

Isolated guinea-pig kidney cortex tubules were incubated in Krebs-Henseleit buffer containing NaH14CO3 (25 mM) and L-alanine (5 mM). A high rate of alanine metabolism was found to be accompanied by a high rate of both 14CO2 fixation and glutamine synthesis. The fixation of 14CO2 was virtually abolished in the presence of oxalate, a known inhibitor of pyruvate carboxylase, indicating that, in guinea-pig renal cortex, this enzyme is responsible for the synthesis of oxaloacetate in the conversion of alanine into glutamine. More than 90% of the label fixed was found in carbon 1 mainly of glutamine and to a lesser extent of glutamate. In the presence of alanine + NaH14CO3 + MSO, an inhibitor of glutamine synthetase, most of the 14CO2 fixed by pyruvate carboxylase was subsequently released and carbon 1 of glutamate was the only site of labelling. In the presence of alanine + NaH14CO3, the fact that not all the glutamine found was labelled in carbon 1 could be explained by glutamine synthesis from endogenous substrates as well as by glutamine synthesis from alanine after prior equilibration of [4-14C]-oxaloacetate with fumarate; that such equilibration occurred was demonstrated by the observation that [1-14C]-glutamine and [1-14C]-glutamate were synthesized from [1-14C]-alanine.