Effects of thiol inhibitors on hepatic guanylate cylase activity

Nitric oxide and thiols: Chemical biology, signalling paradigms and vascular therapeutic potential

Nitric oxide (• NO) interactions with biological thiols play crucial, but incompletely determined, roles in vascular signalling and other biological processes. Here, we highlight two recently proposed signalling paradigms: (1) the formation of a vasodilating labile nitrosyl ferrous haem (NO-ferrohaem) facilitated by thiols via thiyl radical generation and (2) polysulfides/persulfides and their interaction with • NO. We also describe the specific (bio)chemical routes in which • NO and thiols react to form S-nitrosothiols, a broad class of small molecules, and protein post-translational modifications that can influence protein function through catalytic site or allosteric structural changes. S-Nitrosothiol formation depends upon cellular conditions, but critically, an appropriate oxidant for either the thiol (yielding a thiyl radical) or • NO (yielding a nitrosonium [NO+ ]-donating species) is required. We examine the roles of these collective • NO/thiol species in vascular signalling and their cardiovascular therapeutic potential.

Flipping Off and On the Redox Switch in the Microcirculation

Resistance arteries and arterioles evolved as specialized blood vessels serving two important functions: (a) regulating peripheral vascular resistance and blood pressure and (b) matching oxygen and nutrient delivery to metabolic demands of organs. These functions require control of vessel lumen cross-sectional area (vascular tone) via coordinated vascular cell responses governed by precise spatial-temporal communication between intracellular signaling pathways. Herein, we provide a contemporary overview of the significant roles that redox switches play in calcium signaling for orchestrated endothelial, smooth muscle, and red blood cell control of arterial vascular tone. Three interrelated themes are the focus: (a) smooth muscle to endothelial communication for vasoconstriction, (b) endothelial to smooth muscle cell cross talk for vasodilation, and (c) oxygen and red blood cell interregulation of vascular tone and blood flow. We intend for this thematic framework to highlight gaps in our current knowledge and potentially spark interest for cross-disciplinary studies moving forward.

Cellular Factors That Shape the Activity or Function of Nitric Oxide-Stimulated Soluble Guanylyl Cyclase

NO-stimulated guanylyl cyclase (SGC) is a hemoprotein that plays key roles in various physiological functions. SGC is a typical enzyme-linked receptor that combines the functions of a sensor for NO gas and cGMP generator. SGC possesses exclusive selectivity for NO and exhibits a very fast binding of NO, which allows it to function as a sensitive NO receptor. This review describes the effect of various cellular factors, such as additional NO, cell thiols, cell-derived small molecules and proteins on the function of SGC as cellular NO receptor. Due to its vital physiological function SGC is an important drug target. An increasing number of synthetic compounds that affect SGC activity via different mechanisms are discovered and brought to clinical trials and clinics. Cellular factors modifying the activity of SGC constitute an opportunity for improving the effectiveness of existing SGC-directed drugs and/or the creation of new therapeutic strategies.

Cyclic GMP and PKG Signaling in Heart Failure

Cyclic guanosine monophosphate (cGMP), produced by guanylate cyclase (GC), activates protein kinase G (PKG) and regulates cardiac remodeling. cGMP/PKG signal is activated by two intrinsic pathways: nitric oxide (NO)-soluble GC and natriuretic peptide (NP)-particulate GC (pGC) pathways. Activation of these pathways has emerged as a potent therapeutic strategy to treat patients with heart failure, given cGMP-PKG signaling is impaired in heart failure with reduced ejection fraction (HFrEF) and preserved ejection fraction (HFpEF). Large scale clinical trials in patients with HFrEF have shown positive results with agents that activate cGMP-PKG pathways. In patients with HFpEF, however, benefits were observed only in a subgroup of patients. Further investigation for cGMP-PKG pathway is needed to develop better targeting strategies for HFpEF. This review outlines cGMP-PKG pathway and its modulation in heart failure.

Redox Switches Controlling Nitric Oxide Signaling in the Resistance Vasculature and Implications for Blood Pressure Regulation: Mid-Career Award for Research Excellence 2020

The arterial resistance vasculature modulates blood pressure and flow to match oxygen delivery to tissue metabolic demand. As such, resistance arteries and arterioles have evolved a series of highly orchestrated cell-cell communication mechanisms between endothelial cells and vascular smooth muscle cells to regulate vascular tone. In response to neurohormonal agonists, release of several intracellular molecules, including nitric oxide, evokes changes in vascular tone. We and others have uncovered novel redox switches in the walls of resistance arteries that govern nitric oxide compartmentalization and diffusion. In this review, we discuss our current understanding of redox switches controlling nitric oxide signaling in endothelial and vascular smooth muscle cells, focusing on new mechanistic insights, physiological and pathophysiological implications, and advances in therapeutic strategies for hypertension and other diseases.

Selective cysteines oxidation in soluble guanylyl cyclase catalytic domain is involved in NO activation

Nitric oxide (NO) binds to soluble guanylyl cyclase (GC1) and stimulates its catalytic activity to produce cGMP. Despite the key role of the NO-cGMP signaling in cardiovascular physiology, the mechanisms of GC1 activation remain ill-defined. It is believed that conserved cysteines (Cys) in GC1 modulate the enzyme's activity through thiol-redox modifications. We previously showed that GC1 activity is modulated via mixed-disulfide bond by protein disulfide isomerase and thioredoxin 1. Herein we investigated the novel concept that NO-stimulated GC1 activity is mediated by thiol/disulfide switches and aimed to map the specific Cys that are involved. First, we showed that the dithiol reducing agent Tris (2-carboxyethyl)-phosphine reduces GC1 response to NO, indicating the significance of Cys oxidation in NO activation. Second, using dibromobimane, which fluoresces when crosslinking two vicinal Cys thiols, we demonstrated decreased fluorescence in NO-stimulated GC1 compared to unstimulated conditions. This suggested that NO-stimulated GC1 contained more bound Cys, potentially disulfide bonds. Third, to identify NO-regulated Cys oxidation using mass spectrometry, we compared the redox status of all Cys identified in tryptic peptides, among which, ten were oxidized and two were reduced in NO-stimulated GC1. Fourth, we resorted to computational modeling to narrow down the Cys candidates potentially involved in disulfide bond and identified Cys489 and Cys571. Fifth, our mutational studies showed that Cys489 and Cys571 were involved in GC1'response to NO, potentially as a thiol/disulfide switch. These findings imply that specific GC1 Cys sensitivity to redox environment is critical for NO signaling in cardiovascular physiology.

Redox regulation of soluble guanylyl cyclase

The nitric oxide/soluble guanylyl cyclase (NO-sGC) signaling pathway regulates the cardiovascular, neuronal, and gastrointestinal systems. Impaired sGC signaling can result in disease and system-wide organ failure. This review seeks to examine the redox control of sGC through heme and cysteine regulation while discussing therapeutic drugs that target various conditions. Heme regulation involves mechanisms of insertion of the heme moiety into the sGC protein, the molecules and proteins that control switching between the oxidized (Fe³⁺) and reduced states (Fe²⁺), and the activity of heme degradation. Modifications to cysteine residues by S-nitrosation on the α1 and β1 subunits of sGC have been shown to be important in sGC signaling. Moreover, redox balance and localization of sGC is thought to control downstream effects. In response to altered sGC activity due to changes in the redox state, many therapeutic drugs have been developed to target decreased NO-sGC signaling. The importance and relevance of sGC continues to grow as sGC dysregulation leads to numerous disease conditions.

Thiol-Based Redox Modulation of Soluble Guanylyl Cyclase, the Nitric Oxide Receptor

SIGNIFICANCE

Soluble guanylyl cyclase (sGC), which produces the second messenger cyclic guanosine 3', 5'-monophosphate (cGMP), is at the crossroads of nitric oxide (NO) signaling: sGC catalytic activity is both stimulated by NO binding to the heme and inhibited by NO modification of its cysteine (Cys) thiols (S-nitrosation). Modulation of sGC activity by thiol oxidation makes sGC a therapeutic target for pathologies originating from oxidative or nitrosative stress. sGC has an unusually high percentage of Cys for a cytosolic protein, the majority solvent exposed and therefore accessible modulatory targets for biological and pathophysiological signaling. Recent Advances: Thiol oxidation of sGC contributes to the development of cardiovascular diseases by decreasing NO-dependent cGMP production and thereby vascular reactivity. This thiol-based resistance to NO (e.g., increase in peripheral resistance) is observed in hypertension and hyperaldosteronism.

CRITICAL ISSUES

Some roles of specific Cys thiols have been identified in vitro. So far, it has not been possible to pinpoint the roles of specific Cys of sGC in vivo and to investigate the molecular mechanisms in an animal model.

FUTURE DIRECTIONS

The role of Cys as redox sensors, intermediates of activation, and mediators of change in sGC conformation, activity, and dimerization remains largely unexplored. To understand modulation of sGC activity, it is critical to investigate the roles of specific oxidative thiol modifications that are formed during these processes. Where the redox state of sGC thiols contribute to pathologies (vascular resistance and sGC desensitization by NO donors), it becomes crucial to design therapeutic strategies to restore sGC to its normal, physiological thiol redox state. Antioxid. Redox Signal. 26, 137-149.

Identification of novel S-nitrosation sites in soluble guanylyl cyclase, the nitric oxide receptor

Soluble Guanylyl Cyclase (sGC) is the main receptor for nitric oxide (NO). NO activates sGC to synthesize cGMP, triggering a plethora of signals. Recently, we discovered that NO covalently modifies select sGC cysteines via a post-translational modification termed S-nitrosation or S-nitrosylation. Earlier characterization was conducted on a purified sGC treated with S-nitrosoglutathione, and identified three S-nitrosated cysteines (SNO-Cys). Here we describe a more biologically relevant mapping of sGC SNO-Cys in cells to better understand the multi-faceted interactions between SNO and sGC. Since SNO-Cys are labile during LC/MS/MS, MS analysis of nitrosation typically occurs after a biotin switch reaction, in which a SNO-Cys is converted to a biotin-Cys. Here we report the identification of ten sGC SNO-Cys in rat neonatal cardiomyocytes using an Orbitrap MS. A majority of the SNO-Cys identified is located at the solvent-exposed surface of the sGC, and half of them in the conserved catalytic domain, suggesting biological significance. These findings provide a solid basis for future studies of the regulations and functions of diverse sGC S-nitrosation events in cells.

HNO signaling mechanisms

Due to recent discoveries of important and novel biological activity, nitroxyl (HNO) has become a molecule of significant interest. Although it has been used in the past as a treatment for alcoholism, it is currently being touted as a treatment for heart failure. It is becoming increasingly clear that many of the biological actions of HNO can be attributed to its ability to react with specific thiol- and, possibly, heme-proteins. Herein is discussed the chemistry of HNO with likely biological targets. A particular focus is given to targets associated with the pharmacological utility of HNO as a cardiovascular agent and for the treatment of alcoholism.

The effects of nitroxyl (HNO) on soluble guanylate cyclase activity: interactions at ferrous heme and cysteine thiols

It has been previously proposed that nitric oxide (NO) is the only biologically relevant nitrogen oxide capable of activating the enzyme soluble guanylate cyclase (sGC). However, recent reports implicate HNO as another possible activator of sGC. Herein, we examine the affect of HNO donors on the activity of purified bovine lung sGC and find that, indeed, HNO is capable of activating this enzyme. Like NO, HNO activation appears to occur via interaction with the regulatory ferrous heme on sGC. Somewhat unexpectedly, HNO does not activate the ferric form of the enzyme. Finally, HNO-mediated cysteine thiol modification appears to also affect enzyme activity leading to inhibition. Thus, sGC activity can be regulated by HNO via interactions at both the regulatory heme and cysteine thiols.

Thiol oxidation inhibits nitric oxide-mediated pulmonary artery relaxation and guanylate cyclase stimulation

The mechanisms through which thiol oxidation and cellular redox influence the regulation of soluble guanylate cyclase (sGC) are poorly understood. This study investigated whether promoting thiol oxidation via inhibition of NADPH generation by the pentose phosphate pathway (PPP) with 1 mM 6-aminonicotinamide (6-AN) or the thiol oxidant diamide (1 mM) alters sGC activity and cGMP-associated relaxation to nitric oxide (NO) donors [S-nitroso-N-acetylpenicillamine (SNAP) and spermine-NONOate]. Diamide and 6-AN inhibited NO-elicited relaxation of endothelium-denuded bovine pulmonary arteries (BPA) and stimulation of sGC activity in BPA homogenates. Treatment of BPA with the thiol reductant DTT (1 mM) reversed inhibition of NO-mediated relaxation and sGC stimulation by 6-AN. The increase in cGMP protein kinase-associated phosphorylation of vasodilator-stimulated phosphoprotein on Ser239 elicited by 10 microM SNAP was also inhibited by diamide. Activation of sGC by SNAP was attenuated by low micromolar concentrations of GSSG in concentrated, but not dilute, homogenates of BPA, suggesting that an enzymatic process contributes to the actions of GSSG. Relaxation to agents that function through cAMP (forskolin and isoproterenol) was not altered by inhibition of the pentose phosphate pathway or diamide. Thus a thiol oxidation mechanism controlled by the regulation of thiol redox by NADPH generated via the pentose phosphate pathway appears to inhibit sGC activation and cGMP-mediated relaxation by NO in a manner consistent with its function as an important physiological redox-mediated regulator of vascular function.

Zn2+ and Cd2+ increase the cyclic GMP level in PC12 cells by inhibition of the cyclic nucleotide phosphodiesterase

In the present study, the influence of the heavy metal ions Cd2+ and Zn2+ on cGMP metabolism in the neurosecretory rat pheochromocytoma (PC12) cell line has been investigated. Cadmium and zinc ions showed a concentration-dependent increase of intracellular cGMP levels as determined by radioimmunoassay: a 20-fold increase in cGMP concentration was found after 15 min of incubation with 20 microM Cd2+, and a 7-fold increase in cGMP was found after incubation with 50 microM Zn2+ (control: 6.05+/-2.1 pmol cGMP/mg protein). To obtain further mechanistic informations, the effects of Cd2+ and Zn2+ on intracellular 3',5'-cyclic nucleotide phosphodiesterase have been studied by a high performance liquid chromatography-based phosphodiesterase-assay. The cellular cGMP hydrolysis was found to be inhibited by these ions with an IC(50) value of 6+/-0.7 microM for Cd2+ and 13+/-2.5microM for Zn2+ . Hence, dose-dependent increase in cellular cGMP content is due to an inhibition of cGMP hydrolysis and not due to an increase in cGMP synthesis. Cd2+ and Zn2+ were taken up by PC12 cells as determined by atomic absorption spectroscopy, all measurements were performed in a subtoxic concentration range. Our data illustrate that zinc and cadmium ions are efficient inhibitors of the cGMP-stimulated cyclic nucleotide PDEII in PC12 cells resulting in elevated cellular cGMP concentrations. Therefore, subtoxic doses of these metals may disturb intracellular cGMP/cAMP-signalling pathways leading to an impaired or altered gene expression.

Protective effect of N-acetyl-L-cysteine on the renal failure induced by inferior vena cava occlusion

BACKGROUND

Renal ischemia is produced during orthotopic liver transplantation when the inferior vena cava is clamped above the renal veins (inferior vena cava occlusion [IVCO]), and it often leads to postoperative renal failure. Although free radicals and nitric oxide (NO) have been implicated in the pathogenesis of ischemic renal failure, the effect of free radical scavengers in this model is unknown.

METHODS

The effects of N-acetyl-L-cysteine (NAC), a free radical scavenger, on the acute renal failure that follows IVCO were evaluated in pentobarbital-anesthetized dogs. The effect of NO synthesis inhibition with NG-nitro-L-arginine methyl ester (NAME) was also studied. Renal vascular endothelial function was tested by infusing acetylcholine (Ach) into the renal artery before the ischemia and during reperfusion.

RESULTS

Renal failure developed during IVCO and persisted during reperfusion in all groups. However, in NAC-pretreated dogs, the glomerular filtration rate recovered progressively, reaching 31% of basal preischemic values 150 min after reperfusion. During reperfusion, fractional excretion of sodium increased above preischemic values only in the control group, which indicates a beneficial effect of NAC and NAME on the tubular dysfunction observed during reperfusion. The renal response to Ach was abolished in control dogs and in animals given NAME during reperfusion, which indicates endothelial dysfunction. However, in NAC-pretreated dogs, the renal response to Ach was preserved during reperfusion.

CONCLUSIONS

These results demonstrate that NAC ameliorates the renal failure and renal endothelial dysfunction induced by IVCO. This protective effect was abolished by NAME, which suggests that NO is involved in the beneficial effects of NAC. These data also suggest that the use of NAC could be beneficial in ameliorating the acute renal failure observed after orthotopic liver transplantation.

The deactivation of soluble guanylyl cyclase by redox-active agents

Soluble guanylyl cyclase (sGC), an enzyme involved in cGMP signal transduction, is activated by NO binding to the endogenous heme. The mechanism of deactivation is not known. In tissues, cGMP levels decrease within minutes, despite the fact that sGC is activated to levels above the phosphodiesterase activity. Simple dissociation of NO from the heme in sGC has been suggested as a possible deactivation mechanism; however, dissociation rates of NO from ferrous heme proteins are typically very slow. Since oxidants and reductants are known to affect sGC activity, we have tested the effect of a variety of redox-active agents on the activity of NO-activated sGC. All the redox-active compounds tested, covering a wide range of reduction potentials, selectively deactivated the NO-activated sGC while having little or no effect on the basal activity of the enzyme. Among the reagents studied in detail, deactivation of sGC by air occurred slowly, while deactivation by ferricyanide was faster and methylene blue was fastest. The mechanism of deactivation of sGC by dioxygen in the air is straightforward: the heme is oxidized to Fe(III)heme and nitrate is formed. This reaction is similar to that of dioxygen with NOHb and NOMb as occurs in cured meats. Methylene blue and ferricyanide deactivate sGC by a different, as yet undetermined, mechanism.

Sulfhydryl involvement in nitric oxide sequestration and nitric oxide induced guanylyl cyclase activation in vascular smooth muscle

In the present study, the role of vascular smooth muscle sulhydryl groups was investigated with respect to sequestration of nitric oxide (NO) and activation of soluble guanylyl cyclase by NO. Vascular smooth muscle 100,000 x g supernatant (soluble) fraction was prepared in phosphate buffer, using the medial layer of bovine pulmonary artery. The soluble fraction was incubated with 100 pmol NO for 5 min in a sealed flask at 37 degree C under anerobic conditions in the presence or absence of the sulfhydryl reagent, N-ethylmaleimide (NEM, 5 mM). NO sequestration by the soluble fraction was measured as an indicator of NO binding. Total thiol content was measured in the soluble fraction with and without exposure to NEM. Guanylyl cyclase activity was measured in the soluble fraction with and without exposure to NO and a combination of NO and NEM. NEM decreased total thiol content in the soluble fraction from 103.59 nmol/mL to undetectable levels, and decreased guanylyl cyclase activity to below basal levels. The percentage of NO sequestered by the soluble fraction was inhibited by NEM by approximately 25% from a control value of 26.52 +/- 9.39 to 18.72 +/- 8.52, n = 13, p < 0.05. The data indicate that sulfhydryl groups are essential for guanylyl cyclase activation by NO, and are also involved in the sequestration of NO by the vascular smooth muscle soluble fraction.

Peroxynitrite-induced accumulation of cyclic GMP in endothelial cells and stimulation of purified soluble guanylyl cyclase. Dependence on glutathione and possible role of S-nitrosation

Peroxynitrite (ONOO-) is widely recognized as mediator of NO toxicity, but recent studies have indicated that this compound may also have physiological activity and induce vascular relaxation as well as inhibition of platelet aggregation. We found that ONOO- induced a pronounced increase in endothelial cyclic GMP levels, and that this effect was significantly attenuated by pretreatment of the cells with GSH-depleting agents. In the presence of 2 mM GSH, ONOO- stimulated purified soluble guanylyl cyclase with a half-maximally effective concentration of about 20 microM. In contrast to the NO donor 2,2-Diethyl-1-nitroso-oxyhydrazine sodium salt (DEA/NO), ONOO- was completely inactive in the absence of GSH, indicating that thiol-mediated bioactivation of ONOO- is involved in enzyme stimulation. Studies on the reaction between ONOO- and GSH revealed that about 1% of ONOO- was non-enzymatically converted to S-nitrosoglutathione. The authentic nitrosothiol was found to be stable in solution, but slowly decomposed in the presence of GSH. GSH-induced decomposition of S-nitrosoglutathione was apparently catalyzed by trace metals and was accompanied by a sustained release of NO and a 40-100-fold increase in its potency to stimulate purified soluble guanylyl cyclase. Our data suggest that the biologic activity of ONOO- involves S-nitrosation of cellular thiols resulting in NO-mediated cyclic GMP accumulation.

Mechanisms of action of nitrates

Glyceryl trinitrate, isosorbide dinitrate, and isosorbide-5-mononitrate are organic nitrate esters commonly used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Organic nitrate esters have a direct relaxant effect on vascular smooth muscles, and the dilation of coronary vessels improves oxygen supply to the myocardium. The dilation of peripheral veins, and in higher doses peripheral arteries, reduces preload and afterload, and thereby lowers myocardial oxygen consumption. Inhibition of platelet aggregation is another effect that is probably of therapeutic value. Effects on the central nervous system and the myocardium have been shown but not scrutinized for therapeutic importance. Both the relaxing effect on vascular smooth muscle and the effect on platelets are considered to be due to a stimulation of soluble guanylate cyclase by nitric oxide derived from the organic nitrate ester molecule through metabolization catalyzed by enzymes such as glutathione S-transferase, cytochrome P-450, and possibly esterases. The cyclic GMP produced by the guanylate cyclase acts via cGMP-dependent protein kinase. Ultimately, through various processes, the protein kinase lowers intracellular calcium; an increased uptake to and a decreased release from intracellular stores seem to be particularly important.

Regulation of cytosolic guanylyl cyclase by porphyrins and metalloporphyrins

The experimental evidence is convincing that cytosolic guanylate cyclase is a hemoprotein containing stoichiometric amounts of heme, which functions as a prosthetic group for enzyme activation by NO. Nearly all of the studies described in this chapter were conducted before we began to appreciate in 1986 that mammalian vascular endothelial cells could synthesize their own NO. We know now that many different cell types synthesize NO, and that in most instances the NO interacts in a paracrine manner with adjacent target cells to activate cytosolic guanylate cyclase and elevate intracellular levels of cyclic GMP (Ignarro, 1990). The studies on endothelium-derived relaxing factor and authentic NO have shown clearly that heme and hemoproteins have a very high binding affinity for, and inhibit the actions of, these substances (Ignarro, 1989). The interaction between NO and the heme prosthetic group of guanylate cyclase appears to constitute an important signal transduction mechanism whereby NO raises intracellular cyclic GMP levels. This novel signal transduction mechanism is highly conducive to the efficient functioning of NO as a paracrine mediator of cellular function. As a small, lipophilic, and chemically labile molecule, NO diffuses out of its cells of origin and into nearby target cells. The very high binding affinity of enzyme-bound heme for NO ensures interaction of the two to cause guanylate cyclase activation and cyclic GMP formation. Thus, relatively uncomplicated mechanism can account for the paracrine function of endogenous NO in transcellular communication.

Inhibition of nitric oxide synthesis by methylene blue

Methylene blue appears to inhibit nitric oxide-stimulated soluble guanylyl cyclase and has been widely used for inhibition of cGMP-mediated processes. We report here that endothelium-dependent relaxation of isolated blood vessels and NO synthase-dependent cGMP formation in cultured endothelial cells were both markedly more sensitive to inhibition by methylene blue than effects induced by direct activation of soluble guanylyl cyclase. These discrepancies were also observed when superoxide dismutase (SOD) was present to protect NO from inactivation by superoxide anion. Subsequent experiments showed that formation of L-citrulline by purified NO synthase was completely inhibited by 30 microM methylene blue (IC50 = 5.3 and 9.2 microM in the absence and presence of SOD, respectively), whereas guanylyl cyclase stimulated by S-nitrosoglutathione was far less sensitive to the drug (50% inhibition at approximately 60 microM, and maximal inhibition of 72% at 1 mM methylene blue). Experimental evidence indicated that oxidation of NADPH, tetrahydrobiopterin or reduced flavins does not account for the inhibitory effects of methylene blue. Our data suggest that methylene blue acts as a direct inhibitor of NO synthase and is a much less specific and potent inhibitor of guanylyl cyclase than hitherto assumed.

Effects of oxidant stress on endothelium-derived relaxing factor-induced and nitrovasodilator-induced cGMP accumulation in vascular cells in culture

The effects of hydrogen peroxide (H2O2) on the action of basally produced endothelium-derived relaxing factor (EDRF) were investigated by measuring cGMP accumulation in single and cocultures of calf pulmonary artery endothelial cells (CPAEs) and rabbit pulmonary artery smooth muscle cells (RPASMs) as a model for determining the contribution of EDRF dysfunction to altered vascular tone and reactivity frequently associated with oxidant-induced vascular injury. Higher cGMP levels in long-term cocultures (20.4 +/- 1.8 pmol/mg protein/15 min) than in single-cell cultures (CPAE, 9.6 +/- 0.9 pmol/mg protein/15 min; RPASM, 3.7 +/- 0.2 pmol/mg protein/15 min), and CPAE-induced increases (fivefold) in intracellular RPASM cGMP content in short-term cocultures suggest basal release of EDRF. Basal generation and release of an L-arginine-derived endothelial labile factor accounted for the increases in cGMP, since the response was completely blocked by pretreatment of CPAEs with NG-monomethyl L-arginine. Pretreatment of long-term cocultures with H2O2 for 30 minutes resulted in a dose-dependent (0.5-2 mM) decrease in cGMP formation (49-79%). To determine the effects of H2O2 on EDRF synthesis, transport, and RPASM responsiveness, CPAEs or RPASMs were selectively pretreated with H2O2 before establishment of short-term cocultures. In cocultures of H2O2-pretreated CPAEs with untreated RPASMs, RPASM cGMP levels were reduced, suggesting a decrease in EDRF production rather than deterioration of EDRF during transport, because cGMP levels were unaffected by posttreatment with oxygen radical scavengers during coculture. Pretreatment of RPASMs with H2O2 attenuated the untreated CPAE-induced, the putative EDRF S-nitroso-L-cysteine-induced, or the nitroprusside-induced increases in RPASM cGMP levels. This attenuation was prevented by pretreatment with either dimethylthiourea, deferoxamine, or dithiothreitol, suggesting a mechanism of H2O2 action involving iron-catalyzed formation of intracellular hydroxyl radicals and their attack on cellular thiols. H2O2 diminution of cGMP accumulation was not associated with lytic cell injury in the experimental time frame, because morphology and 51Cr release from prelabeled RPASMs and CPAEs were unchanged.

Functional modulation of ANF-sensitive particulate guanylate cyclase by redox mechanisms

We examined the effects of sulfhydryl-reactive redox agents and of the apparent oxidation-reduction (redox) potential of the assay medium on enzyme activity and atrial natriuretic factor (ANF) binding properties of particular guanylate cyclase from bovine adrenal cortex. Redox potential was varied by addition of redox-reactive agents and quantified by electrochemical measurement. The modification of free SH groups by thiol-reactive agents had only a minor effect on ANF binding or on the extent of ANF-dependent enzyme stimulation whereas free thiol groups were essential for basal enzyme activity of ANF-sensitive particulate guanylate cyclase. Basal and ANF-stimulated particulate guanylate cyclase activity was modulated by exposure to different redox potential states. This modulation was different for the substrates Mg.GTP and Mn.GTP. The apparent redox potential had no influence on the extent of guanylate cyclase activation by ANF. Our results suggest that critical free thiol groups, which are sensitive to thiol-reactive redox agents, are involved in the catalytic, but not in the receptor function of ANF-sensitive particulate guanylate cyclase. These thiol groups could be the structural basis for the effects of redox events which modulate basal enzyme activity, but not activation by ANF.

Activation of soluble guanylate cyclase by carbon monoxide and inhibition by superoxide anion

Human platelet soluble guanylate cyclase activity was studied with respect to the function of its heme-containing regulatory subunit. As an enzyme source, the 10,000 x g supernatant was used and, since its specific activity proved to be too low for inhibition studies, also a partially purified preparation was employed. The partially purified enzyme was stimulated about 2.5-fold by carbon monoxide and this effect was abolished by illumination with visible light. Sodium nitroprusside also increased the basal activity about fourfold, which, however, is much less than the greater than 100-fold stimulation seen with the supernatant. Superoxide anions generated by the xanthine/xanthine-oxidase system were strongly inhibitory in the enriched preparation as well as in the CO-stimulated platelet supernatant (median effector concentration = 0.1 mU/ml). Unlike CO and NO, the effect of superoxide cannot be mediated through the heme-containing regulatory subunit, since heme-free enzyme, which could not be activated by NO or CO, was inhibited to the same extent as the heme-containing enzyme. Superoxide dismutase did not influence the basal activity, but resulted in a synergistic stimulation in the presence of CO. When Mn2+ replaced Mg2+ as a cofactor, the basal activity was higher but superoxide could not inhibit the enzyme, possibly due to the superoxide-dismutase-like activity of Mn2+. Superoxide turned out to be a potent and reversible inhibitor of soluble guanylate cyclase which, together with endothelium-derived relaxing factor, recently identified as NO, could form a physiologically relevant regulatory effector system.

Guanylate cyclase in olfactory cilia from rat and pig

A guanylate cyclase was identified in cilia from rat and pig olfactory epithelia. Enzyme activities were 200-250 and 90-100 pmol/min.mg-1, respectively. Activity required the presence of non-ionic detergents, e.g., 0.1% Lubrol PX. MnGTP, not MgGTP was used as a substrate. Furthermore, 0.9 mM free Mn2+ was necessary for optimal activity indicating a regulatory site for a divalent cation. The guanylate cyclase displayed sigmoidal Michaelis-Menten kinetics suggesting cooperativity between MnGTP and enzyme. S0.5 was 160 microM MnGTP. The Hill coefficient of 1.7 indicates that more than one class of substrate-binding sites interact in a positive cooperative manner. ATP inhibited the enzyme and linearized plots of substrate kinetics with MnGTP. SH-Blocking agents reversibly inhibited enzyme activity. Sodium azide and nitroprusside were without effect as were several odorants. A guanylate cyclase activity in cilia from tracheal tissue had properties similar to the olfactory enzyme.

Under anaerobic conditions, soluble guanylate cyclase is specifically stimulated by glutathione

Various thiols exert non-specific effects on the activity of soluble guanylate cyclase under aerobic conditions. We studied the effects of thiols under anaerobic conditions (pO2 less than 6 Torr) on soluble guanylate cyclase, purified from bovine lung. Reduced glutathione stimulated the enzyme concentration-dependently with half-maximal enzyme stimulation at a concentration of about 0.5 mM. The extend of maximal enzyme stimulation (up to 80-fold) was comparable with the activation by NO-containing substances. The activation by glutathione was additive with the effect of sodium nitroprusside. Cysteine and various other thiols increased the enzyme activity 20-fold and 2- to 5-fold, respectively. The stimulatory effect of these thiols was not related to their reducing potency. Activation of soluble guanylate cyclase by glutathione was dose-dependently reduced in the presence of other thiols (cysteine greater than oxidized glutathione greater than S-methyl glutathione). Under aerobic conditions or with Mn-GTP as substrate, the effect of glutathione on soluble guanylate cyclase was suppressed. The results suggest a specific role for glutathione in the regulation of soluble guanylate cyclase activity and a modulation of this effect by redox reactions and other intracellular thiols.

The involvement of catalytic site thiol groups in the activation of soluble guanylate cyclase by sodium nitroprusside

Sodium nitroprusside, a potent activator of soluble guanylate cyclase, potentiated mixed disulfide formation between cystine, a potent inhibitor of the cyclase, and enzyme purified from rat lung. Incubation of soluble guanylate cyclase with nitroprusside and [35S]cystine resulted in a twofold increase in protein-bound radioactivity compared to incubations in the absence of nitroprusside. Purified enzyme preincubated with nitroprusside and then gel filtered (activated enzyme) was activated 10- to 20-fold compared to guanylate cyclase preincubated in the absence of nitroprusside and similarly processed (nonactivated enzyme). This activation was completely reversed by subsequent incubation at 37 degrees C (activation-reversed enzyme). Incorporation of [35S]cystine into guanylate cyclase was increased twofold with activated enzyme, while no difference was observed with activation-reversed enzyme, compared to nonactivated enzyme. Cystine decreased the activity of nonactivated and activation-reversed enzyme about 40% while it completely inhibited activated guanylate cyclase. Mg+2- or Mn+2-GTP inhibited the incorporation of [35S]cystine into nonactivated or activated guanylate cyclase. Also, diamide, a potent thiol oxidant that converts juxtaposed sulfhydryls to disulfides, completely blocked incorporation of [35S]cystine into nonactivated or activated guanylate cyclase. These data indicate that activation of soluble guanylate cyclase by nitroprusside results in an increased availability of protein sulfhydryl groups for mixed disulfide formation with cystine. Protection against mixed disulfide formation with diamide or substrate suggests that these groups exist as two or more juxtaposed sulfhydryl groups at the active site or a site on the enzyme that regulates catalytic activity. Differential inhibition by mixed disulfide formation of nonactivated and activated enzyme suggests a mechanism for amplification of the on-off signal for soluble guanylate cyclase within cells.

Desensitization to nitroglycerin in vascular smooth muscle from rat and human

Guanylate cyclase in high speed supernatant fractions obtained from rat thoracic aorta or human coronary arteries pretreated with nitroglycerin exhibited a marked desensitization to activation by nitroglycerin, nitroprusside, and nitric oxide. However, activation of soluble guanylate cyclase by arachidonic acid was unaffected by pretreatment of vessels with nitroglycerin. Furthermore, activation of soluble guanylate cyclase by protoporphyrin IX was increased 4-fold when vessels were pretreated with nitroglycerin. Soluble guanylate cyclase partially purified from nitroglycerin-pretreated rat thoracic aorta by immunoprecipitation with a specific monoclonal antibody exhibited persistent desensitization to nitrate-induced activation. These data suggest that nitroglycerin-induced desensitization of guanylate cyclase to activation by nitrovasodilators represents a stable alteration of the enzyme. In contrast, activation by protoporphyrin IX of guanylate cyclase immunoprecipitated from nitroglycerin-pretreated or control vessels was not significantly different. This suggests that the mechanism of protoporphyrin activation of guanylate cyclase is different than the mechanism with nitrovasodilators. Activation of particulate guanylate cyclase by Lubrol-PX, hemin, or atrial natriuretic factor was not significantly different with enzyme prepared from nitroglycerin-pretreated or control vessels from rat and human. Thus, nitroglycerin-induced desensitization of rat thoracic aorta or human coronary artery results in a relatively stable molecular alteration of soluble guanylate cyclase such that the enzyme is specifically less sensitive to activation by nitrovasodilators whereas the effects of other activators of the enzyme are either unchanged or increased.

Characterization of D-myo-inositol 1,4,5-trisphosphate phosphatase in rat brain

Rat brain homogenates contain significant amounts of inositol 1,4,5-trisphosphate phosphatase in both 180,000xg (60 min) particulate and supernatant fractions. As other membrane-bound enzymes (e.g. guanylate cyclase), particulate inositol 1,4,5-trisphosphate phosphatase activity is highly sensitive to low concentrations of Triton X-100 (0.03%). Higher concentrations of detergent (1%) partially solubilized the enzyme. Thiol blocking agents (e.g. p-hydroxymercuribenzoate) inactivate inositol 1,4,5-trisphosphate phosphatase activity (an effect reversed with 2-mercaptoethanol). It is thus suggested that enzymatic activity requires the presence of -SH groups.

Effects of retinylacetate on the kinetics of rat liver guanylate cyclase: possible interaction with sulfhydryl groups

The effect of retinylacetate (RA) was investigated on rat liver guanylate cyclase stimulated by nitroprusside (NP). The stimulated enzyme seemed to adhere to Michaëlis-Menten kinetics with an apparent Km for MnGTP of 0.10 mM and a Vmax of 410 pmol cGMP/min. X mg prot. RA (0.1-1mM) dose-dependently inhibited the enzyme stimulated by NP (0.1-10 mM). In the presence of 1 mM RA the activity was only 10% of the control activity. The inhibitory action of RA seemed to be non-competitive since it depressed Vmax without affecting the Km for MnGTP. In contrast, however, RA was instead found to stimulate the enzyme when low concentrations of NP (0.01-0.1 mM) were used. The concentration-activity curve for NP was bell-shaped showing an optimum at 0.5 mM. The inhibition induced by RA could not be surmounted by increasing the NP concentration indicating that RA did not compete with NP. A bell-shaped activity curve was also seen when the enzyme activity was measured in the presence of increasing Mn2+ concentrations and during these conditions RA also caused inhibition. In the presence of the sulphydryl reductant dithiothreitol (DTT), the NP concentration needed for optimal enzyme activation was about 100-fold less than in the absence of DTT. The maximal enzyme activity was also slightly increased. In the presence of DTT, RA was much less effective to induce inhibition of the stimulated enzyme, than when DDT was absent. (In the presence of DTT 1 mM RA caused only 30% inhibition compared to 90% inhibition in its absence).(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization and partial characterization of the intestinal receptor for Escherichia coli heat-stable enterotoxin

Binding of Escherichia coli strain 431 heat-stable enterotoxin (STa) and activation of intestinal particulate guanylate cyclase by E. coli STa were studied with rat intestinal epithelial cells and brush border membranes (BBMs). The rates of guanylate cyclase stimulation by 431 STa in cells and BBMs were rapid, with maximal levels of cyclic GMP observed within 5 min. Specific binding of 125I-labeled STa from E. coli 431 (431 125I-STa) and activation of guanylate cyclase by unlabeled 431 STa were observed with intestinal BBMs; however, neither was detected with membranes from nonintestinal tissues. The STa receptor was solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, a nondenaturing dipolar ionic detergent, in yields of approximately 50%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the detergent-solubilized receptor-431 125I-STa complex, followed by autoradiography, showed that 431 125I-STa bound to a single BBM component with a molecular weight of about 100,000. Binding of 431 STa to its solubilized receptor was saturable, specific, and essentially irreversible. Pretreatment of the soluble receptor with trypsin and pronase but not chymotrypsin decreased binding of 431 125I-STa. The 431 STa-receptor complex was dissociated by boiling in the presence of 1% sodium dodecyl sulfate, incubation with 0.5 M acetic acid, or reduction with dithiothreitol. In contrast to the residual particulate guanylate cyclase activity of detergent-treated membranes, solubilized guanylate cyclase was not stimulated by STa. Membrane structure appears to play an important role in the coordination of STa binding and stimulation of guanylate cyclase activity.

Role of lipoxygenase in the O2-dependent activation of soluble guanylate cyclase from rat lung

Guanylate cyclase activity in rat lung supernatant fractions is stimulated 3-4 fold by aerobic incubation at 30 degrees C for approx. 30 min ('O2-dependent activation'). This stimulation was blocked by 20 microM-eicosa-5,8,11,14-tetraynoic acid (ETYA), an inhibitor of lipoxygenase and cyclo-oxygenase, but not by aspirin or indomethacin, which are cyclo-oxygenase inhibitors. The enzyme activator(s) is presumed to be the fatty acid hydroperoxide(s) formed by lipoxygenase. Removal of lipoxygenase from the supernatant fraction by chromatography on Amberlite XAD-4 also prevented activation, which was restored by the addition of soya-bean lipoxygenase. Bovine serum albumin prevented O2-dependent activation or activation by soya-bean lipoxygenase, through its ability to bind the unsaturated fatty acid substrate of lipoxygenase. The lipoxygenase in the supernatant fraction is inhibited by endogenous glutathione peroxidase plus reduced glutathione (GSH); removal of GSH de-inhibits lipoxygenase and activates guanylate cyclase. This was effected by autoxidation, by cumene hydroperoxide (with GSH peroxidase) and by titration with N-ethylmaleimide (NEM). Activation by NEM was inhibited by serum albumin or ETYA, as was activation by low concentrations (less than 50 microM) of cumene hydroperoxide. Activation by higher concentrations was not so inhibited; therefore, cumene hydroperoxide can also activate by a direct effect on guanylate cyclase. A hypothesis for physiological activation is proposed.

Vascular smooth muscle relaxation by nitro compounds: reduced relaxation and cGMP elevation in tolerant vessels and reversal of tolerance by dithiothreitol

Smooth muscle preparations obtained from bovine mesenteric artery were incubated with high concentrations of nitroglycerin (NG) or nitroprusside (NP) at elevated pH. This treatment was found to make the vessels tolerant towards both the relaxant and cGMP elevating action of a challenging dose of nitrocompounds when tested on the histamine contracted vessels. Also, some cross-tolerance between NP and NG could be found since pretreatment of the vessels with NP caused a reduction in both the relaxant and the cGMP elevating action of NG. The NG induced tolerance could be partly reversed by treatment with the disulfide reducing agent dithiothreitol (DTT). These results are suggested to strengthen the evidence for cGMP as a mediator of vascular smooth muscle relaxation induced by nitrocompounds. The ability of DTT to partly reverse this tolerance indicates the existence of critical tissue sulfhydryl groups with which the nitrocompounds interact. There also seemed to exist certain differences in the mechanism of action between NG and NP since the tolerance induced against NG was much more pronounced than was the case for NP.