NADPH and heme redox modulate pulmonary artery relaxation and guanylate cyclase activation by NO

The hemoprotein oxidant ferricyanide (FeCN) converts the iran of the heme on soluble guanylate cyclase (sGC) from Fe2+ to Fe3+, which prevents nitric oxide (NO) from binding the heme and stimulating sGC activity. This study uses FeCN to examine whether modulation of the redox status of the heme on sGC influences the relaxation of endothelium-removed bovine pulmonary arteries (BPA) to NO. Pretreatment of the homogenate of BPA with 50 mu M FeCN resulted in a loss of stimulation of sGC activity by the NO donor 10 mu M S-nitroso-N-acetylpenicillamine (SNAP). In the FeCN-treated homogenate reconcentrated to the enzyme levels in BPA, 100 mu M NADPH restored NO stimulation of sGC, and this effect of NADPH was prevented by an inhibitor of flavoprotein electron transport, 1 mu M diphenyliodonium (DPI). In BPA the relaxation to SNAP was not altered by FeCN, inhibitors of NADPH generation by the pentose phosphate pathway [250 mu M 6-aminonicotinamide (6-AN) and 100 mu M epiandrosterone (Epi)], or 1 mu M DPI. However, the combination of FeCN with 6-AN, Epi, or DPI inhibited (P < 0.05) relaxation to SNAP without significantly altering the relaxation of BPA to forskolin. The inhibitory effects of 1 mu M 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (a probe that appears to convert NO-heme of sGC to its Fe3+-heme form) on relaxation to SNAP were also enhanced by DPI. These observations suggest that a flavoprotein containing NADPH oxidoreductase may influence cGMP-mediated relaxation of BPA to NO by maintaining the heme of sGC in its Fe2+ oxidation state.