Role of mitochondrial glutaminase in rat renal glutamine metabolism.

During normal acid-base balance, the kidney extracts very little of the plasma glutamine. However, during metabolic acidosis, as much as one third of the plasma glutamine is extracted and metabolized in a single pass through this organ. The substantial increase in renal utilization occurs solely within the proximal convoluted tubule and is sustained by compensating adaptations in the intraorgan metabolism of glutamine. The primary pathway for renal glutamine metabolism involves its transport into mitochondria and its deamidation and deamination by glutaminase (GA) and glutamate dehydrogenase (GDH), respectively. The resulting ammonium ions are excreted predominantly in the urine where they function as expendable cations to facilitate the excretion of acids. The resulting alpha-ketoglutarate is further metabolized to phosphoenolpyruvate and subsequently to glucose or CO2. The intermediate steps yield two bicarbonate ions that are selectively transported into the venous blood to partially compensate the metabolic acidosis. In rat kidney, this adaptation is sustained in part by the cell-specific induction of the glutaminase that results primarily from stabilization of the GA mRNA. The 3'-nontranslated region of the GA mRNA contains a direct repeat of an 8-base AU-sequence that functions as a pH-response element. This sequence exhibits a high affinity and specificity for zeta (z)-crystallin. The same protein binds to two separate, but homologous, 8-base AU-sequences within the 3'-nontranslated region of the GDH mRNA. The apparent binding activity of z-crystallin is increased significantly during onset of metabolic acidosis. Thus, increased binding of z-crystallin may initiate the pH-responsive stabilization of the two mRNAs.