Glutamine metabolism in endothelial cells: ornithine synthesis from glutamine via pyrroline-5-carboxylate synthase

L-Glutamine (the most abundant free amino acid in plasma and the body) is a potent inhibitor of endothelial NO synthesis. However, little is known abo ut glutamine metabolism in endothelial cells (EC). As an initial step towar d understanding the role of glutamine in endothelial physiology, the presen t study was conducted to quantify glutamine catabolism in microvascular, ao rtic and venous EC. For metabolic studies, EC were incubated for 1 h in Kre bs bicarbonate buffer containing 5 mM glucose and 0.5-4 mM L-[U-C-14]-gluta mine. For enzymological studies, cell extracts and mitochondrial fractions were prepared to determine the activities of glutamine-degrading enzymes. O ur results reveal extensive hydrolysis of glutamine to glutamate and ammoni a in a concentration-dependent manner via phosphate-dependent glutaminase i n all EC studied. In addition, both metabolic and enzymological evidence in dicate a novel pathway for endothelial synthesis of ornithine from glutamin e via pyrroline-5-carboxylate synthase. This new knowledge of glutamine met abolism may pave a new path for understanding the physiological role of glu tamine in vascular function. (C) 2000 Elsevier Science Inc. All rights rese rved.