[An endogenous regulator of the guanylate cyclase activity of human platelets].

Chromatography of soluble human platelet guanylate cyclase (105,000 g supernatant) on DEAE-cellulose in a linear gradient of NaCl (0-0.5 M) in 50 mM Tris-HCl buffer pH 7.6 gave two protein peaks, I and II, of which only peak II possessed the guanylate cyclase activity (0.18-0.22 M NaCl). The protein fraction I was found to possess an inhibiting activity; its addition to the partially purified enzyme decreased the guanylate cyclase activity by 60-70% in the presence of Mg2+ with no effect on the enzyme activity in the presence of Mn2+. The isolated enzyme lost (by approximately 80%) its ability to be activated by sodium nitroprusside; the latter was reconstituted after addition of the inhibiting fraction. The data obtained testify to the heme origin of the endogenous inhibitor of human platelet guanylate cyclase.