Role of heme in the regulation of human and rat platelet soluble guanylate cyclases.

The chromatography of soluble human and rat platelet guanylate cyclases (105000 g supernatants) on DEAE-cellulose in 50 mM Tris HCl buffer, containing 0.22 M NaCl, has yielded virtually identical elution profiles, each with two protein peaks (I and II). Only peak II was found to have guanylate cyclase activity. Experiments with human platelets showed that inactive protein peak I inhibited the activity of guanylate cyclase preparation (peak II) and restored the already lost ability of the enzyme to be activated by sodium nitroprusside. In experiments with rat platelets, inactive fraction I had no effect on guanylate cyclase activity (peak II), and the enzyme was not activated by sodium nitroprusside either before or after DEAE-cellulose. 105000g supernatant of human platelets had an absorbance maximum at 415 nm (Soret band), which disappeared from the spectrum of the active fraction (II) but was found in the spectrum of the inactive (inhibitory) fraction I. Experiments with rat platelets demonstrated the absence of Soret band in the corresponding spectra. It was concluded that, contrary to the generally accepted notion, heme is not a prosthetic group of the soluble rat platelet guanylate cyclase.