Kinetic studies of allosteric catabolic ornithine carbamoyltransferase from Pseudomonas aeruginosa.

The pseudo-reverse reaction of Pseudomonas aeruginosa catabolic ornithine carbamoyltransferase in which arsenate is first coupled to citrulline followed by elimination of carbamylarsenate has been studied. Arsenate and citrulline saturation curves are sigmoidal. The different responsiveness of the transcarbamoylase to isosteric and allosteric ligands was examined both in the forward reaction, the carbamoylation of ornithine, and in the pseudo-reverse reaction, the arsenolytic cleavage of citrulline. Nucleoside monophosphates and polyamines that act as allosteric activators and inhibitors, respectively, on the carbamoylation reaction have the same effect on the rate of the arsenolytic cleavage of citrulline. ATP and other nucleoside triphosphates were found to stimulate enzyme activity at low carbamoylphosphate concentration with little influence on the carbamoylphosphate concentration at half-maximum velocity as well as on the cooperative index. When measuring the initial rate of the reverse reaction, the arsenolytic cleavage of citrulline, ATP was found to be a weak inhibitor, whereas CTP still stimulates the reaction and UTP was without influence. This unidirectional inhibition or activation phenomenon is likely apparent since initial studies were conducted and no consideration was given to equilibrium conditions. Regulation of catabolic OTCase by nucleoside triphosphates is without physiological meaning. In contrast, stimulation by nucleoside monophosphates may indicate that energy limitation could promote the synthesis and activity of the catabolic enzyme.