ROLE OF HEME IN SODIUM NITROPRUSSIDE-INDUCED ACTIVATION OF RAT MYOCARDIAL GUANYLATE-CYCLASE

The basal activity of rat myocardium soluble guanylate cyclase (105,000g supernatant) is 2-fold stimulated by dithiothreitol (2 mM). In the presence of 2 mM dithiothreitol, sodium nitroprusside enhances the enzyme, activity up to 26.5-fold. Addition of heme-containing proteins (either hemoglobin or myoglobin) further increases the enzyme stimulation, by 44 and 69%, respectively. Ion-exchange chromatography of the 105,000g supernatant of rat myocardium using stepwise elution with 50 mM Tris-HCl buffer containing 0.22 M NaCl (pH 7.6) produced two protein peaks (I and II), of which only peak II possessed guanylate cyclase activity. The absorption spectrum of the 105,000g supernatant had a maximum at 415 nm (Soret band) which was not present in the spectrum of peak II, but was detectable in that of the inactive protein-peak I. The guanylate cyclase preparation (peak II) was no longer activated by sodium nitroprusside. Attempts to restore sodium nitroprusside-induced activation by adding either inactive protein peak I or heme-containing proteins (hemoglobin or myoglobin) to peak II failed. A possible mechanism for activation of rat myocardial guanylate cyclase by sodium nitroprusside is discussed.