Inhibition of nitric oxide-dependent activation of soluble guanylyl cyclase by the antimalarial drug, artemisinin.

The influence of artemisinin on the activity of human platelet soluble guanylyl cyclase was investigated. Artemisinin (0.1-100 microM) had no effect on the basal activity of the enzyme. Artemisinin inhibited in a concentration-dependent manner the sodium nitroprusside-induced activation of human platelet guanylyl cyclase with an IC(50) value 5.6 microM. Artemisinin (10 microM) also inhibited (by 71+/-4.0%) the activation of the enzyme by the thiol-dependent nitric oxide (NO) donor, the derivative of furoxan, 3,4-dicyano-1,2,5-oxadiazole 2-oxide (10 microM), but did not influence the stimulation of soluble guanylyl cyclase by protoporphyrin 1X. Inhibition of guanylyl cyclase activation by NO donors but not by protoporphyrin 1X represents a possible additional mechanism of the pharmacological action of this drug.

Antitumor antibiotic streptonigrin and its derivatives as inhibitors of nitric oxide-dependent activation of soluble guanylyl cyclase.

The influence of streptonigrin on the activity of human platelet guanylyl cyclase was investigated. Streptonigrin (0.1-5 microM) had no effect on the basal activity of the enzyme, but inhibited in a concentration-dependent manner the sodium nitroprusside-induced activation of human platelet soluble guanylyl cyclase with an IC(50) value of 4.16 microM. Streptonigrin (10 microM) also inhibited (by 28%) the activation of the enzyme by the direct nitric oxide (NO) donor-spermine-NONO (100 microM), but had no influence on the stimulation of soluble guanylyl cyclase by protoporphyrin IX. The absence of a correlation between the inhibition of NO-stimulated guanylyl cyclase activity by streptonigrin (I) and its derivatives (streptonigrone (IV), streptonigrone-2'-imine (V), amide of 1 and 2'-deoxy-2'-amino-D-glucose (VI), amide of 1 and 2'-deoxy-2'-amino-2'-D-galactose (VII), amide of 1 and 1-O-methyl-6-deoxy-6-amino-D-glucose (VIII), diphenylmethyl ester of I (IX), conjugate of I and daunorubicin (X)), and the level of cytotoxic effects of these compounds excludes the involvement of guanylyl cyclase in the mechanism of antitumor action of streptonigrin. Inhibition of guanylyl cyclase activation by NO donors but not by protoporphyrin IX represents a new biochemical effect of streptonigrin, which should be taken into account in addition to its antitumor action.

Activation of soluble guanylate cyclase by NO donors--S-nitrosothiols, and dinitrosyl-iron complexes with thiol-containing ligands.

We studied the capability of dimeric forms of dinitrosyl-iron complexes and S-nitrosothiols to activate soluble guanylate cyclase (sGC) from human platelet cytosol. The dinitrosyl-iron complexes had the ligands glutathione (DNIC-GS) or N-acetylcysteine (DNIC-NAC). The S-nitrosothiols were S-nitrosoglutathione (GS-NO) or S-nitrosoacetylcysteine (SNAC). For both glutathione and N-acetylcysteine, the DNIC and S-nitrosothiol forms are equally effective activators of sGC. The activation mechanism is strongly affected by the presence of intrinsic metal ions. Pretreatment with the potent iron chelator, disodium salt of bathophenanthroline disulfonic acid (BPDS), suppressed sGC activation by GS-NO: the concentration of GS-NO producing maximal sGC activation was increased by two orders of magnitude. In contrast, activation by DNIC-GS is strongly enhanced by BPDS. When BPDS was added 10 min after supplementation of DNIC-GS or GS-NO at 4 degrees C, it exerted a similar effect on sGC activation by either NO donor: BPDS only enhanced the sGC stimulation at low concentrations of the NO donors. Our experiments demonstrated that both Fe(2+) and Cu(2+) ions contribute to the decomposition of GS-NO in the presence of ascorbate. The decomposition of GS-NO induced by Fe(2+) ions was accompanied by formation of DNIC. BPDS protected GS-NO against the destructive action of Fe(2+) but not Cu(2+) ions. Additionally, BPDS is a sufficiently strong chelator to remove the iron from DNIC-GS complexes. Based on our data, we propose that S-nitrosothiols activate sGC via a two-step iron-mediated process: In the first step, intrinsic Fe(2+) ions catalyze the formation of DNICs from S-nitrosothiols. In the secondary step, these newly formed DNICs act as the real NO donors responsible for sGC activation.