[The role of heme in activating rat myocardial guanylate cyclase by sodium nitroprusside].
The basal activity of soluble rat myocardial guanylate cyclase (105000 x g supernatant) is stimulated by 2 mM dithiothreitol (2-fold). In the presence of 2 mM dithiothreitol, sodium nitroprusside enhances the enzyme activation up to 26.5-fold. Addition of heme-containing proteins (hemoglobin or myoglobin) produces further stimulation of the enzyme--by 44% and 69%, respectively. Ion-exchange chromatography of rat myocardial 105000 x g supernatant by stepwise elution with 50 mM Tris-HCl buffer pH 7.6 containing 0.22 M NaCl revealed two protein peaks (I and II), of which only peak II possessed the guanylate cyclase activity. The spectrum of the 105000 x g supernatant had an absorption maximum at 415 nm (Soret band) which disappeared from the spectrum of the protein peak II but was detected in the inactive protein peak I. The guanylate cyclase preparation (peak II) lost its ability to be activated by sodium nitroprusside. All the attempts to reconstitute the nitroprusside-induced activation of the enzyme by adding the inactive protein peak I or the heme-containing proteins (hemoglobin or myoglobin) to peak II were unsuccessful. The possible mechanism of rat myocardial guanylate cyclase activation by sodium nitroprusside is discussed.