Inhibition of Ns‐Protein‐Stimulated Human‐Platelet Adenylate Cyclase by Epinephrine and Stable GTP Analogs

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Abstract
The influence of purified, preactivated Ns protein, mediating adenylate cyclase stimulation by hormones and guanine nucleotides, was studied on adenylate cyclase activity in membranes of human platelets. The preactivated coupling protein caused a large increase in platelet enzyme activity. The activation occurred after a short lag phase and exhibited saturation. Ns protein increased the V of the platelet adenylate cyclase without change in the enzyme's affinity for the substrate, MgATP, whereas the apparent affinity for Mg2+ was increased by more than one order of magnitude. The Ns‐protein‐activated human platelet adenylate cyclase was inhibited by the hormone, epinephrine, and by the stable GTP analogs, guanosine 5′‐[β,γ‐imido]triphosphate and guanosine 5′‐[γ‐thio]triphosphate. The stable GTP analog or hormone‐induced adenylate cyclase inhibition was not competitive with regard to the concentration of Ns protein. Inhibition of Ns‐protein‐stimulated platelet adenylate cyclase caused by stable GTP analogs appeared to be persistent, was amplified by epinephrine and was accompanied by a large decrease in the enzyme's apparent affinity for Mg2+. The data suggest that the hormone‐sensitive adenylate cyclase is regulated by two guanine‐nucleotide‐binding sites, Ns and Ni, mediating enzyme stimulation and inhibition, respectively, by hormones and guanine nucleotides, and that the two coupling components interact in a non‐competitive manner with the adenylate cyclase, exhibiting high and low affinity for Mg2+ when affected by Ns and Ni, respectively.