Properties of carbamoyl‐phosphate synthetase (ammonia) in rat‐liver mitochondria made permeable with toluene

Some properties of carbamoyl‐phosphate synthetase (ammonia) were studied in rat‐liver mitochondria made selectively permeable by pretreatment with toluene. 1 The Michaelis constants for NH3, MgATP and HCO−3 were 0.7, 1.2 and 2 mM respectively. N‐Acetyl‐glutamate activated the enzyme with a Ka of about 0.1 mM. At sturating concentrations of substrates and effectors the enzyme was inhibited by 50% by carbamoyl phosphate at a concentration of 13 mM. 2 Binding of N‐acetylglutamate to carbamoyl‐phosphate synthetase required the presence of both free Mg2+ ions and MgATP, and was inhibited by Ca2+ ions and by N‐carbamoylglutamate. The known activation of carbamoyl‐phosphate synthetase by free Mg2+ is due to an increased affinity of the enzyme for N‐acetylglutamate. 3 Binding of N‐acetylglutamate to carbamoyl‐phosphate synthetase was a slow process: at N‐acetylglutamate concentrations below 0.5 mM maximal binding was not completed within 30 min. The rate of binding increased with increasing N‐acetylglutamate concentrations. 4 Dissociation of N‐acetylglutamate from the enzyme was relatively fast, with a half‐time of about 5 min. 5 Under all conditions studies there was a close relationship between carbamoyl‐phosphate synthetase activity and the amount of N‐acetylglutamate bound to the enzyme. 6 The data are discussed in relation to the control of carbamoyl‐phosphate synthetase in the intact hepatocyte.