Inhibition of nitric oxide formation by neuronal nitric oxide synthase by quinones: nitric oxide synthase as a quinone reductase

Fetal growth in rats treated with lapachol

Lapachol is a naphthoquinone well known for its therapeutic potential. Previous studies have shown that lapachol does not interfere with embryonic development during the pre-implantation period. However, when administered during the organogenic period at the same dose level, it induces a high fetal death incidence. To evaluate the effect of lapachol during fetogenesis, 20 pregnant Wistar rats were randomly divided into two groups: vehicle (10 mL of a 50% aqueous ethanol solution/kg body weight) and treated (100 mg of lapachol/kg body weight). Lapachol was administered from the 17th to 20th day of pregnancy. The following variables were analyzed: maternal body weight from 16th to 21st day of pregnancy, food intake from 17th to 21st day of pregnancy, clinical signs of physical discomfort, ovarian weights, implantations, resorptions and mortality indices, fetal and placenta weights, external malformations, and fetal organ weights. Results indicated that lapachol was not toxic to mothers, although it was fetotoxic leading to fetal growth retardation.

hsp90 is required for heme binding and activation of apo-neuronal nitric-oxide synthase: geldanamycin-mediated oxidant generation is unrelated to any action of hsp90

It is established that neuronal NO synthase (nNOS) is associated with the chaperone hsp90, although the functional role for this interaction has not been defined. We have discovered that inhibition of hsp90 by radicicol or geldanamycin nearly prevents the heme-mediated activation and assembly of heme-deficient apo-nNOS in insect cells. This effect is concentration-dependent with over 75% inhibition achieved at 20 microm radicicol. The ferrous carbonyl complex of nNOS is not formed when hsp90 is inhibited, indicating that functional heme insertion is prevented. We propose that the hsp90-based chaperone machinery facilitates functional heme entry into apo-nNOS by the opening of the hydrophobic heme-binding cleft in the protein. Previously, it has been reported that the hsp90 inhibitor geldanamycin uncouples endothelial NOS activity and increases endothelial NOS-dependent O(2)() production. Geldanamycin is an ansamycin benzoquinone, and we show here that it causes oxidant production from nNOS in insect cells as well as with the purified protein. At a concentration of 20 microm, geldanamycin causes a 3-fold increase in NADPH oxidation and hydrogen peroxide formation from purified nNOS, whereas the non-quinone hsp90 inhibitor radicicol had no effect. Thus, consistent with the known propensity of other quinones, geldanamycin directly redox cycles with nNOS by a process independent of any action on hsp90, cautioning against the use of geldanamycin as a specific inhibitor of hsp90 in redox-active systems.

6-Arylamino-5,8-quinazolinediones as potent inhibitors of endothelium-dependent vasorelaxation

6-(Substituted-phenyl)amino-5,8-quinazolinediones (3) were synthesised by regioselective substitution of 5,8-quinazolinedione (5) with appropriate arylamines in the presence of Ce(III) ions. All synthesised 5,8-quinazolinediones 3 showed a potent and efficacious inhibitory effect on the acetylcholine (ACh)-induced vasorelaxation of rat aorta with the endothelium. The quinones 3, at a low concentration of 0.1 microM, reduced the maximal response with increase of EC(50) values for ACh. The results indicate that quinones 3 are potent inhibitors of endothelium-dependent vasorelaxation.

Menadione induces endothelial dysfunction mediated by oxidative stress and arylation

Our previous studies showed that menadione causes endothelial dysfunction which results in decreased relaxation and increased contraction of blood vessels. This investigation examined the role of two possible mechanisms (oxidative stress and arylation) in menadione-induced endothelial dysfunction. Menadione increased superoxide anion generation in aortic rings in a dose-dependent manner. Superoxide dismutase (SOD), reversed the inhibitory effects of menadione on vascular relaxation. The relaxation induced by the NO donor, sodium nitroprusside, was inhibited by menadione pretreatment in a dose-dependent manner. Endothelial nitric oxide synthase activity (eNOS) was suppressed by menadione. Menadione resulted in a dose-dependent reduction of cGMP levels accumulated by acetylcholine. This reduction of cGMP levels was blocked by SOD treatment, suggesting that superoxide anion generated by menadione could play a role in the inhibition of the nitric oxide pathway. Evidence supporting a possible role for arylation in impaired vascular relaxation was suggested by the observation that benzoquinone, which does not induce oxidative stress in aortic rings, inhibited acetylcholine-induced vascular relaxation to the same extent as menadione. Collectively, these results suggest that menadione can cause endothelial dysfunction in blood vessels by the inhibition of the nitric oxide pathway via superoxide anion generation and that arylation activity may also be another important mechanism.

Phenanthrenequinone disrupts progesterone production in rat luteal cells

The ability of the environmental contaminant phenanthrene (PH) and its photooxidized product phenanthrenequinone (PHQ) to disrupt progesterone secretion was examined in a model system of in vitro suspensions of luteal cells from the rat. Treatment with PHQ dramatically inhibited luteinizing hormone (LH) stimulated progesterone secretion. PHQ also generated a significant increase in reactive oxygen species (ROS). In the absence of LH, however, PHQ stimulated a small increase in basal progesterone secretion. The parent compound, PH, did not alter progesterone or ROS release. Since there is evidence that PHQ lowers the activity of nitric oxide synthase (NOS) and that nitric oxide (NO) affects progesterone production, we examined the response to the NOS inhibitors N-monomethyl-L-arginine, Zn protoporphyrin-9, and aminoguanidine in luteal cells. However, there was no effect of these agents on LH stimulated progesterone secretion. These results indicated that PHQ is a potent disrupter of progesterone secretion and should perhaps be considered in assessing the risk of PH to humans.

Phenanthraquinone inhibits eNOS activity and suppresses vasorelaxation

Diesel exhaust particles cause an impairment of endothelium-dependent vasorelaxation and are associated with cardiopulmonary-related diseases and mortality, but the mechanistic details are poorly understood. Since we reported previously that phenanthraquinone, an environmental chemical contained in diesel exhaust particles, suppresses neuronal nitric oxide synthase (nNOS) activity by shunting electrons away from the normal catalytic pathway, it was hypothesized that phenanthraquinone inhibits endothelial NOS (eNOS) activity and affects vascular tone. Therefore, the effects of phenanthraquinone on eNOS activity, endothelium-dependent relaxation, and blood pressure were examined in the present study. Phenanthraquinone inhibited NO formation evaluated by citrulline formed by total membrane fraction of bovine aortic endothelial cells with an IC(50) value of 0.6 microM. A kinetic study revealed that phenanthraquinone is a competitive inhibitor with respect to NADPH and a noncompetitive inhibitor with respect to L-arginine. Endothelium-dependent relaxation of rat aortic rings by ACh was significantly inhibited by phenanthraquinone (5 microM), whereas the endothelium-independent relaxation by nitroglycerin was not. Furthermore, an intraperitoneal injection of phenanthraquinone (0.36 mmol/kg) to rats resulted in an elevation of blood pressure (1.4-fold, P < 0.01); under this condition, plasma levels of stable NO metabolites, nitrite/nitrate, in phenanthraquinone-treated rats was reduced to 68% of control levels. The present findings suggest that phenanthraquinone has a potent inhibitory action on eNOS activity via a similar mechanism reported for nNOS, thereby causing the suppression of NO-mediated vasorelaxation and elevation of blood pressure.

In vitro and in vivo Leishmanicidal activity of 2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone (lapachol)

This study aims to evaluate the in vitro and in vivo leishmanicidal activity of lapachol, a naphthoquinone found in the seeds and heartwood of certain tropical plants, and to compare its efficacy with a reference drug, sodium stibogluconate (Pentostam(R)). These compounds (0.0125-4.0 mg/mL) were evaluated in vitro against intracellular amastigotes of Leishmania (Viannia) braziliensis (LVb), then tested in an animal model (hamster) to try to reproduce the leishmanicidal activity. In vitro, lapachol exhibited an anti-amastigote effect, whereas in vivo it did not prevent the development of LVb-induced lesions at an oral dose of 300 mg/kg/day for 42 days. Pentostam(R) demonstrated a significant anti-amastigote effect in vitro for LVb and apparent clinical cure in vivo (60 mg/kg/day). However, it could not completely eradicate parasites from the tissues of infected animals. The observation that lapachol exerts leishmanicidal activity in vitro without offering significant protection against LVb-infected lesions in hamsters suggests that lapachol in vivo might possibly inhibit the microbicidal functioning of macrophages. Alternatively, it might be transformed into an inactive metabolite(s) or neutralized, losing its leishmanicidal activity. It is also possible that an optimal and sustained plasma level of the drug could not be achieved at the dose used in this study.

One-electron reduction of quinones by the neuronal nitric-oxide synthase reductase domain

Flavin electron transferases can catalyze one- or two-electron reduction of quinones including bioreductive antitumor quinones. The recombinant neuronal nitric oxide synthase (nNOS) reductase domain, which contains the FAD-FMN prosthetic group pair and calmodulin-binding site, catalyzed aerobic NADPH-oxidation in the presence of the model quinone compound menadione (MD), including antitumor mitomycin C (Mit C) and adriamycin (Adr). Calcium/calmodulin (Ca2+/CaM) stimulated the NADPH oxidation of these quinones. The MD-mediated NADPH oxidation was inhibited in the presence of NAD(P)H:quinone oxidoreductase (QR), but Mit C- and Adr-mediated NADPH oxidations were not. In anaerobic conditions, cytochrome b5 as a scavenger for the menasemiquinone radical (MD*-) was stoichiometrically reduced by the nNOS reductase domain in the presence of MD, but not of QR. These results indicate that the nNOS reductase domain can catalyze a only one-electron reduction of bivalent quinones. In the presence or absence of Ca2+/CaM, the semiquinone radical species were major intermediates observed during the oxidation of the reduced enzyme by MD, but the fully reduced flavin species did not significantly accumulate under these conditions. Air-stable semiquinone did not react rapidly with MD, but the fully reduced species of both flavins, FAD and FMN, could donate one electron to MD. The intramolecular electron transfer between the two flavins is the rate-limiting step in the catalytic cycle [H. Matsuda, T. Iyanagi, Biochim. Biophys. Acta 1473 (1999) 345-355). These data suggest that the enzyme functions between the 1e- <==> 3e- level during one-electron reduction of MD, and that the rates of quinone reductions are stimulated by a rapid electron exchange between the two flavins in the presence of Ca2+/CaM.

Pharmacological effects of novel quinone compounds, 6-(fluorinated-phenyl)amino-5,8-quinolinediones, on inhibition of drug-induced relaxation of rat aorta and their putative action mechanism

Two 6-(fluorinated-phenyl)amino-5,8-quinolinedione derivatives, OQ21 and OQ1, were newly synthesized as potent inhibitors of endothelial-dependent vasorelaxation. The purpose of the present study was to investigate the effects of OQ21 and OQ1 on different types of vasorelaxation and to pursue their action mechanisms. For acetylcholine both compounds, at a low concentration (0.1 microM), reduced the maximal response with increase of EC(50) values. OQ21 is a novel quinone compound and showed a more potent and efficacious inhibitory effect on acetylcholine-induced relaxation of rat aorta than that of LY83583 (6-anilino-5,8-quinolinedione). Relatively high concentrations (1 microM) of OQ21 and OQ1 inhibited the sodium nitroprusside-induced relaxation of endothelium-denuded ring, producing rightward shifts of the curve for sodium nitroprusside without altering the maximal response. They also prevented acetylcholine and sodium nitroprusside-induced elevations of cyclic GMP. In addition, OQ21 and OQ1 (1 microM) significantly decreased (52-72%) the sensitivity of L-arginine-induced relaxation of precontracted endothelium-denuded aortic rings from lipopolysaccaride-treated (20 mg/kg, i.p.) rats. The inhibitory effect of OQ21 on endothelium-dependent vasodilation was enhanced by N(G)-nitro-L-arginine, which inhibits nitric oxide synthase (NOS) by binding the oxygenase domain of the enzyme, but not by diphenylendiodonium, which inhibits NOS by binding to the reductase domain of the enzyme. Treatment of blood vessels with OQ21 or OQ1 showed a significant increase in chemiluminescence output, which was prevented by adding superoxide dismutase, suggesting that superoxide generation is involved in the action mechanism for OQ21. Present results indicate that a novel naphthoquinone compound, OQ21, potently inhibits endothelial NOS, possibly by interacting with the reductase domain of the enzyme, which leads to induce superoxide formation. The new benzoquinone compounds, OQ21 and OQ1, inhibit not only endothelium-dependent vasorelaxation but also endothelium-independent relaxation induced by exogenous NO generated from a nitrovasodilator via the reduction of cyclic GMP. They also reduced L-arginine-induced vasorelaxation in endotoxin-treated rats, indicating their possession of inhibitory effect on inducible NOS.

6-arylamino-5,8-quinolinediones and 7-arylamino-5,8-isoquinolinediones as inhibitors of endothelium-dependent vasorelaxation

6-Arylamino-5,8-quinolinediones 3 and 7-arylamino-5,8-isoquinolinediones 4 were synthesized as inhibitors of endothelium-dependent vasorelaxation. The quinones inhibited the vasorelaxation of rat aorta with the endothelium. Among them, the quinones 3a, 3b, 3f, 4b, 4d and 4g strongly inhibited the vasorelaxation.

Inhibition of nitric oxide formation and superoxide generation during reduction of LY83583 by neuronal nitric oxide synthase

6-Anilino-5,8-quinolinedione (LY83583) has been widely used as an agent to reduce levels of nitric oxide (NO)-dependent cGMP in tissues. We report here that suppression of NO formation and production of superoxide during enzymatic reduction of LY83583 by neuronal NO synthase appeared to be potentially involved in the pharmacological action caused by LY83583. LY83583 suppressed neuronal NO synthase activity of 20,000 x g rat cerebellar supernatant preparation in a concentration-dependent manner (IC50 value = 12.9 microM). A kinetic study revealed that LY83583 is a competitive inhibitor with respect to NADPH, with a Ki value of 2.57 microM. With purified neuronal NO synthase it was found that LY83583 was a potent inhibitor of NO formation by the enzyme and served as efficient substrate for reduction with a specific activity of 173 nmol of NADPH oxidized per mg of protein per minute. The reductase activity was stimulated about 19.8-fold by addition of CaCl2/calmodulin, indicating that the presence of CaCl2/calmodulin is essential to express maximal activity of LY83583 reduction. Although LY83583 was a good substrate for both NADPH-cytochrome P450 reductase (P450 reductase) and DT-diaphorase, these flavin enzymes-catalyzed reductions of LY83583 were less than the neuronal NO synthase-mediated reduction in the presence of CaCl2/calmodulin. Enzymatic generation of superoxide during reduction of LY83583 by neuronal NO synthase, P450 reductase or DT-diaphorase was confirmed by electron spin resonance (ESR) experiments. Thus the present results indicate that a benzoquinone derivative LY83583 appears to interact with the P450 reductase domain on neuronal NO synthase, resulting in inhibition of NO formation and superoxide generation, which is involved in suppression of intracellular cGMP content.