Newer aspects of glutamine/glutamate metabolism: the role of acute pH changes.

Abstract

This review focuses on the role of acute pH changes in the regulation of Gln/Glu metabolism in the kidney, liver, and brain. Alterations of proton concentration ([H(+)]) profoundly affect flux through phosphate-dependent glutaminase (PDG) or glutamate dehydrogenase (GDH), the primary enzymes responsible for mitochondrial metabolism of glutamine and glutamate, respectively. In the kidney, acute acidosis stimulates Gln uptake and its metabolism via the PDG pathway. The Glu formed from Gln can be removed via 1) oxidative deamination through the GDH reaction, 2) transamination reactions, and 3) transport of Glu from intracellular to extracellular compartment, thereby diminishing the intramitochondrial pool of glutamate sufficiently to stimulate flux through the PDG pathway. Converse changes may occur with increased pH. In the liver, acidosis diminishes the rate of Gln and Glu metabolism via the PDG and GDH pathways, but stimulates glutamine synthesis (i.e., glutamine recycling). Alkalosis has little effect. Hepatic Gln metabolism via the PDG pathway has a central role in ureagenesis via 1) supplementation of nitrogen for the synthesis of carbamyl phosphate, and 2) providing glutamate for N-acetylglutamate synthesis. In the brain, Gln/Glu metabolism links ammonia detoxification and energy metabolism via 1) detoxification of ammonia and excess glutamate by glutamine synthesis in astrocytes, 2) formation and export of glutamine to neurons where it is metabolized to glutamate and GABA, and 3) production of alpha-ketoglutarate and lactate from Glu and their transport to neurons. Changes in intracellular pH associated with changes in cellular [K(+)] may have a key role in the regulation of these processes of glial-neuronal metabolism of Gln/Glu metabolism.