NO is the physiological activator of soluble guanylyl cyclase (sGC) thereby acting as a signaling molecule in the nervous and cardiovascular systems. Despite its poor sGC-activating ability, CO, produced by the enzyme heme oxygenase (HO), has also been implicated as a physiological stimulator of sGC in neurotransmission and vasorelaxation. Zinc protoporphyrin IX (ZnPPIX) and tin protoporphyrin IX (SnPPIX) are competitive HO inhibitors and have been used in studies implicating a messenger role for CO in the brain and periphery; however, little is known about the specificity of these metalloporphyrins. In the present study, the effects of ZnPPIX and SnPPIX on sGC activity have been investigatedin vitro.Interestingly, purified sGC is markedly activated by SnPPIX (20- to 30-fold) but has a very low affinity for this metalloporphyrin (K_a= 4.9 μM); high concentrations of SnPPIX (25 μM) still activated the enzyme. On the other hand, sGC has a high affinity for ZnPPIX (K_a= 16.1 nM). ZnPPIX activates heme-containing sGC weakly at low (nM) concentrations (3- to 4-fold) but at higher concentrations, ZnPPIX is a potent inhibitor; at 2.5 μM, it inhibits the basal activity of sGC by about 80%. These results imply that HO inhibitors may affect cGMP levels independently of HO activity.