Soluble guanylate cyclase of platelets: function and regulation in normal and pathological states

Chromatography of 105,000 x g supernatants of human and rat platelets on DEAE-cellulose yielded identical elution profiles containing 2 protein fractions (peaks I and II). Only peak II was found to possess guanylate cyclase activity. In the spectrum of the 105,000 x g supernatant of human platelets the absorption maximum was specified at 410 nm (the Soret band) which disappeared from the spectrum of the active protein fraction (peak II) but was detected in the nonactive fraction (peak I). The enzyme preparation was obtained in the heme-deficient form. In the experiments with rat platelets, the Soret band was absent from the corresponding spectra and the enzyme was not activated by sodium nitroprusside; i.e., in soluble guanylate cyclase of rat platelets, unlike the generally accepted notion, the heme is not a prosthetic group of the enzyme. It was shown that carnosine (beta-alanyl-L-histidine), a water-soluble antioxidant, inhibits guanylate cyclase activation by sodium nitroprusside. This inhibitory effect is caused by the interaction of carnosine with the guanylate cyclase heme and can be used for evaluating the degree of saturation of the enzyme with the heme. ADP-induced aggregation of human platelets (donors) is accompanied by a fall in the basal guanylate cyclase activity (with Mg2+) and the enhancement of the enzyme stimulation with sodium nitroprusside, protoporphyrin IX, arachidonic acid and L-arginine with simultaneous cGMP elevation in platelets. A hypothetic scheme of the regulatory role of cGMP in platelet aggregation is proposed. In the experiments with the acute myocardial ischemia of rats, 15 min after the surgery a sharp fall in the platelet guanylate cyclase activity accompanied by a decrease in the enzyme activity in the ischemic zone of the left ventricle of heart took place. The results provided evidence of the high sensitivity of platelet guanylate cyclase to pathological changes occurring in the myocardium at the earliest stages of the development of pathology.