Requirement for Guanosine Triphosphate in the Prostaglandin Activation of Adenylate Cyclase of Platelet Membranes

Abstract

Abstract Adenylate cyclase in purified platelet membrane was activated by prostaglandin E1 (PGE1) when 2 mm ATP was used as substrate. When the substrate concentration was lowered to 0.1 mm (using an analogue of ATP, 5'-adenylyl imidodiphosphate (AMP-PNP) which has been shown to be a substrate for adenylate cyclase but is not degraded by ATPase), the PGE1 activation of adenylate cyclase was markedly reduced or abolished, without affecting the fluoride stimulation. The PGE1 activation of the enzyme was restored by the addition of GTP to the incubation medium. The effect was evident even at 1 µm GTP, and the optimal effect was obtained at 0.1 mm GTP. The GTP effect was seen at all concentrations of PGE1 used in these experiments. Moreover, GTP effect also was evident at lower substrate concentrations. GTP did not appear to change the apparent affinity of the substrate to the enzyme, but markedly increased the PGE1-stimulated enzyme activity.