Inactivation of carbamoyl phosphate synthetase (ammonia) by elastase as a probe to investigate binding of the substrates.

Rat liver carbamoyl phosphate synthetase I is inactivated by elastase. Addition of ATP, Mg2+, K+ and N-acetyl-L-glutamate (the physiological allosteric activator) protects entirely, whereas acetylglutamate alone speeds inactivation. We have exploited these properties to investigate binding of these ligands. Acetylglutamate binds with low affinity (KD 0.25 mM) in the absence of other ligands, and with higher affinity (KD much less than 0.1 mM) when ATP, Mg2+ and K+ are present. The apparent KD for ATP in the presence of acetylglutamate is intermediate between the KD values for the two ATP binding sites present in the enzyme; thus, binding of ATP to both sites is involved in protecting the synthetase. The data also indicate binding of MgATP and Mg2+ in the absence of acetylglutamate. The results provide further evidence for conformational changes associated with allosteric activation of the enzyme.