[1-C-13]GLUCOSE METABOLISM IN BRAIN-CELLS - ISOTOPOMER ANALYSIS OF GLUTAMINE FROM CEREBELLAR ASTROCYTES AND GLUTAMATE FROM GRANULE CELLS

We analyzed the glutamine isotopomers released into the extracellular medium by cerebellar astrocytes incubated with [1-C-13]glucose, We dev eloped a mathematical model of the tricarboxylic acid (TCA) cycle to d etermine the relative flux of molecules through the anaplerotic versus oxidative pathways and the relative pyruvate carboxylase versus pyruv ate dehydrogenase activities, As glutamine C2 and C3 exhibited unequiv alent enrichments, we examined the possibility of: (1) both the entry of the label, into the TCA cycle from pyruvate and the conversion of t he oxaloacetate into citrate before equilibration with fumarate, and ( 2) the occurrence of an orientation-conserved transfer of the symmetri cal 4-carbon intermediates. The best fit required partial cycling betw een oxaloacetate and fumarate, whereas the occurrence of any orientati on-conserved transfer was rejected. On the other hand, the analysis of glutamate isotopomers from perchloric acid extracts of granule cells incubated with [1-C-13]glucose indicated that total cycling of oxaloac etate into fumarate occurred in these cells.