Nitroxyl (HNO) may be formed endogenously by uncoupled nitric-oxide (NO) synthases, enzymatic reduction of NO or as product of vascular nitroglycerin bioactivation. The established HNO donor Angeli's salt (trioxodinitrate, AS) causes cGMP-dependent vasodilation through activation of soluble guanylate cyclase (sGC). We investigated the mechanisms underlying this effect using purified sGC and cultured endothelial cells. AS (up to 0.1 mM) had no significant effect on sGC activity in the absence of superoxide dismutase (SOD) or dithiothreitol (DTT). In the presence of SOD, AS caused biphasic sGC activation (apparent EC(50) approximately 10 nM, maximum at 1 microM) that was accompanied by the formation of NO. DTT (2 mM) inhibited the effects of <10 microM AS but led to sGC activation and NO release at 0.1 mM AS even without SOD. AS had no effect on ferric sGC, excluding activation of the oxidized enzyme by HNO. The NO scavenger carboxy-PTIO inhibited endothelial cGMP accumulation induced by AS in the presence but not in the absence of SOD (EC(50) approximately 50 nM and approximately 16 microM, respectively). Carboxy-PTIO (0.1 mM) inhibited the effect of <or=10 microM AS in the presence of SOD but caused NO release from 0.1 mM AS in the absence of SOD. These data indicate that AS activates sGC exclusively via NO, formed either via SOD-catalyzed oxidation of HNO or through a minor AS decomposition pathway that is unmasked in the presence of HNO scavenging thiols.