Inhibition of Ca2+ of carbamoylphosphate synthetase (ammonia).

The effect of Ca2+ ions on carbamoylphosphate synthetase (ammonia) (EC 6.3.4.16) of rat liver mitochondria was studied. In lysed mitochondria, carbamoylphosphate synthetase (ammonia) was inhibited by Ca2+ ions. In intact rat liver mitochondria, the rate of citrulline production from added ammonia, ornithine, and bicarbonate was inversely correlated with the magnitude of the intramitochondrial concentration of Ca2+. This effect was found at concentrations of Ca2+ which are within the physiological range. The inhibition was reversed by added Mg2+ and was not due to effects of Ca2+ on mitochondrial energy production. It is proposed that the activity of carbamoylphosphate synthetase (ammonia) may be modulated in vivo by changes in the concentration of Ca2+ ions in the mitochondrial matrix.