The soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one is a nonselective heme protein inhibitor of nitric oxide synthaseand other cytochrome P-450 enzymes involved in nitric oxide donor bioactivation

Soluble guanylyl cyclase (sGC) is an important effector for nitric oxide (N O). it acts by increasing intracellular cyclic GMP (cGMP) levels to mediate numerous biological functions. Recently, 1H-[1,2,4]oxadiazolo[4,3,-a]quino xalin-1-one (ODQ) was identified as a novel and selective inhibitor of this enzyme. Therefore, ODQ may represent an important pharmacological tool for differentiating cGMP-mediated from cGMP-independent effects of NO. In the present study, we examined the inhibitory action of ODQ both functionally a nd biochemically. In phenylephrine-preconstricted, endothelium-intact, isol ated aortic rings from the rat, ODQ, in a concentration-dependent manner, i ncreased contractile tone and inhibited relaxations to authentic NO with ma ximal effects at 3 mu M. Pretreatment of vascular rings with ODQ induced a parallel, 2-log-order shift to the right of the concentration-response curv es (CRCs) to histamine, ATP, NO, the NO-donors S-nitrosoglutathione, S-nitr oso-N-acetyl-D,L-penicillamine, and spermine NONOate [N-[4-[1-(3-amino prop yl)-2-hydroxy-2-nitroso hydrazino]butyl]-1,3-propane diamine], and the dire ct sGC-stimulant [3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole] YC-1 but did not affect relaxations induced by papaverine and atriopeptin II. Moreov er, the rightward shift of the CRCs to Angeli's salt, peroxynitrite, and li nsidomine was similar to that of NO. These results suggested that ODQ is sp ecific for sGC. Furthermore, they indicate that NO can cause vasorelaxation independent of cGMP. Three interesting exceptions were observed to the oth erwise rather uniform inhibitory effect of ODQ: the responses to acetylchol ine, glycerol trinitrate, and sodium nitroprusside. The latter two agents a re known to require metabolic activation, possibly by cytochrome P-450-type proteins. The 3- to 5-log-order rightward shift of their CRCs suggests tha t, in addition to sGC, ODQ may interfere with heme proteins involved in the bioactivation of these NO donors and the mechanism of vasorelaxation media ted by acetylcholine. In support of this notion, ODQ inhibited hepatic micr osomal NO production from both glycerol trinitrate and sodium nitroprusside as well as NO synthase activity in aortic homogenates. The latter effect s eemed to require biotransformation of ODQ. Collectively, these data reveal that ODQ interferes with various heme protein-dependent processes in vascul ar and hepatic tissue and lacks specificity for sGC.