GLUTAMINE CATABOLISM BY HEART-MUSCLE - REGULATION OF PHOSPHATE-ACTIVATED GLUTAMINASE BY ATP, CITRATE, AND CHLORIDE

Production of glutamate from glutamine by rat heart mitochondria was s timulated by citrate and ATP in a dose-dependent manner. Citrate requi red phosphate for manifestation of its stimulatory action, whereas ATP enhanced glutaminase activity with and without phosphate. At low conc entrations (1-50 mM) ATP was more potent than citrate, whereas the opp osite was true at high (50-150 mM) levels of these anions. Both citrat e and ATP decreased the concentration of phosphate required for half-m aximal stimulation of glutamate production (EC(50)) and reduced the va lue of the Hill coefficient. Phosphate lowered the EC(50) for ATP. Chl oride and other anions of chaotropic series inhibited glutamine catabo lism, most likely by causing depolymerization of the enzyme. Rupture o f mitochondrial membranes by freeze-thawing decreased the responsivene ss of glutaminase to phosphate, ATP, and especially to citrate but it did not alter the inhibition of the enzyme by chaotropic anions. It is concluded that phosphate, ATP, and citrate act in concert to modulate glutaminase activity in heart in vivo. Chloride, which exhibits a sma ll inhibitory effect at a concentration normally present in cardiac mu scle, may assume a greater regulatory role in situations accompanied b y a rise in the internal level of this anion. (C) 1994 Academic Press, Inc.