The mechanism of Ca2+ stimulation of citrulline and N-acetylglutamate synthesis by mitochondria.

ATP-driven citrulline synthesis by mitochondria treated with oligomycin, uncoupler and Ca2+ ionophore is stimulated more than 2-fold by an increase in extramitochondrial free [Ca2+] in the range 1-5 microM. Stimulation increases with the length of preincubation of mitochondria with Ca2+. EGTA prevents stimulation if added at the start of the preincubation period, but is without effect if added at the end, suggesting that Ca2+ acts indirectly on citrulline synthesis via the accumulation of an intermediate. Neither carbamoyl-phosphate synthase (ammonia) nor ornithine carbamoyltransferase are stimulated by up to 50 microM free Ca2+ in mitochondrial extracts, but N-acetylglutamate synthase is stimulated about 30% by 10 microM free Ca2+. We propose that an increase in the activity of N-acetylglutamate synthase in response to an increase in free [Ca2+] in the mitochondrial matrix may contribute to hormonal stimulation of the urea cycle.