Glycerylphosphorylcholine Diesterase

Abstract

Summary

Glycerylphosphorylcholine diesterase (glycerolphosphorylcholine glycerophosphohydrolase, EC 3.1.4.2) from rat kidney was inhibited selectively by purine triphosphate nucleotides and inorganic pyrophosphate. Classical competitive inhibition for all compounds tested was observed. Of the purine nucleotides ATP, GTP, and ITP showed the greatest amount of inhibition, producing K _i values of approximately 1.5 mM. The purine diphosphates showed a lesser degree of inhibition, producing K _i values of approximately 20 mM and the corresponding purine monophosphates gave very little inhibition, producing K _i values of approximately 125 mM. In the case of the pyrimidine nucleotides, both the cytidine and uridine triphosphates were not as effective as the purine triphosphates. The K _i values obtained with the pyrimidine triphosphates were approximately seven times greater than for the purine triphosphates. Inhibition by ATP was found to be pH dependent in the range studied, (pH 7.0-10.2) with the greatest amount of inhibition observed at pH 8.6. Michaelis-Menton kinetics were observed with all of the nucleotides studied. However, when considering inorganic pyrophosphate, sigmodial type kinetics were observed. Inorganic phosphate was without effect on the diesterase; 5′-AMP and cyclic AMP did not reverse ATP inhibition; MgCl₂ alleviated the inhibition produced by ATP.