Sensitizing soluble guanylyl cyclase to become a highly CO-sensitive enzyme

Organic phosphates as a new class of soluble guanylate cyclase inhibitors

This study aimed to examine effects of varied organic phosphates on activities of soluble guanylate cyclase (sGC). The enzyme was purified from bovine lung. Physiologically relevant concentrations of ATP, 2,3-bisphosphoglyceric acid and inositol hexakisphosphate inhibited its enzyme activities under steady-state conditions as well as those determined under stimulation with S-nitroso-N-acetylpenicillamine, a nitric oxide donor, carbon monoxide or YC-1. Lineweaver-Burk plot analyses revealed that these three organic phosphates act as competitive inhibitors. Other organic phosphates such as cardiolipin and sphingomyelin but not inorganic phosphates exhibited such inhibitory actions. These results suggest that organic phosphates serve as inhibitors for sGC-dependent signaling events.

Rapid nitric oxide-induced desensitization of the cGMP response is caused by increased activity of phosphodiesterase type 5 paralleled by phosphorylation of the enzyme

Most of the effects of the signaling molecule nitric oxide (NO) are mediated by cGMP, which is synthesized by soluble guanylyl cyclase and degraded by phosphodiesterases. Here we show that in platelets and aortic tissue, NO led to a biphasic response characterized by a tremendous increase in cGMP (up to 100-fold) in less than 30 s and a rapid decline, reflecting the tightly controlled balance of guanylyl cyclase and phosphodiesterase activities. Inverse to the reported increase in sensitivity caused by NO shortage, concentrating NO attenuated the cGMP response in a concentration-dependent manner. We found that guanylyl cyclase remained fully activated during the entire course of the cGMP response; thus, desensitization was not due to a switched off guanylyl cyclase. However, when intact platelets were incubated with NO and then lysed, enhanced activity of phosphodiesterase type 5 was detected in the cytosol. Furthermore, this increase in cGMP degradation is paralleled by the phosphorylation of phosphodiesterase type 5 at Ser-92. Thus, our data suggest that NO-induced desensitization of the cGMP response is caused by the phosphorylation and subsequent activity increase of phosphodiesterase type 5.

Neural roles for heme oxygenase: contrasts to nitric oxide synthase

The heme oxygenase (HO) and nitric oxide (NO) synthase (NOS) systems display notable similarities as well as differences. HO and NOS are both oxidative enzymes using NADPH as an electron donor. The constitutive forms of the enzyme are differentially activated, with calcium entry stimulating NOS by binding to calmodulin, whereas calcium entry activates protein kinase C to phosphorylate and activate HO2. Although both NO and carbon monoxide (CO) stimulate soluble guanylyl cyclase to form cGMP, NO also S-nitrosylates selected protein targets. Both involve constitutive and inducible biosynthetic enzymes. However, functions of the inducible forms are virtual opposites. Macrophage-inducible NOS generates NO to kill other cells, whereas HO1 generates bilirubin to exert antioxidant cytoprotective effects and also provides cytoprotection by facilitating iron extrusion from cells. The neuronal form of HO, HO2, is also cytoprotective. Normally, neural NO in the brain seems to exert some sort of behavioral inhibition. However, excess release of NO in response to glutamate's N-methyl-d-aspartate receptor activation leads to stroke damage. On the other hand, massive neuronal firing during a stroke presumably activates HO2, leading to neuroprotective actions of bilirubin. Loss of this neuroprotection after HO inhibition by mutant forms of amyloid precursor protein may mediate neurotoxicity in Familial Alzheimer's Disease. NO and CO both appear to be neurotransmitters in the brain and peripheral autonomic nervous system. They also are physiologic endothelial-derived relaxing factors for blood vessels. In the gastrointestinal pathway, NO and CO appear to function as coneurotransmitters, both stimulating soluble guanylyl cyclase to cause smooth muscle relaxation.

Vascular smooth muscle cell-directed overexpression of heme oxygenase-1 elevates blood pressure through attenuation of nitric oxide-induced vasodilation in mice

To elucidate pathophysiological roles of heme oxygenase (HO)-1 in regulation of vascular tone in vivo, we have developed and characterized transgenic (Tg) mice that overexpress HO-1 site specifically in vascular smooth muscle cells (VSMCs). The Tg mice were generated by use of human HO-1 cDNA under the control of SM22-alpha promoter. The HO-1 gene overexpression was demonstrated by Northern blot analysis and coincided with increases in the protein expression in VSMCs and total HO activities. Tg mice exhibited a significant increase in arterial pressure at various ages and displayed impaired nitrovasodilatory responses in isolated aortic segments versus nontransgenic littermates while enhancing their nitric oxide (NO) production. The pressure of Tg mice was unchanged by systemic administration of either N(omega)-nitro-L-arginine or SNP. Furthermore, the isolated aorta in these mice exhibited lesser extents of NO-elicited cGMP elevation via soluble guanylate cyclase (sGC), while exhibiting no notable downregulation of sGC expression. Such impairment of the NO-elicited cGMP increase was restored significantly by tin protoporphyrin IX, an HO inhibitor. On the other hand, 3-(5'-hydroxymethyl-2' furyl)-1-benzyl-indazol (YC-1), an NO-independent activator of sGC, increased cGMP and relaxed aortas from Tg mice to levels comparable with those from nontransgenic mice, which indicates that contents of functionally intact sGC are unlikely to differ between the two systems. These findings suggest that site-specific overexpression of HO-1 in VSMCs suppresses vasodilatory response to NO and thereby leads to an elevation of arterial pressure.

Synergism between nitric oxide and hydrogen peroxide in the inhibition of platelet function: the roles of soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein

In previous studies, a strong synergism between low concentrations of hydrogen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of cyclic GMP. In this investigation, hydrogen peroxide and NO had no effect on the activity of pure soluble guanylyl cyclase or its activity in platelet lysates and cytosol. H(2)O(2) was found to increase the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-kDa form that results from phosphorylation at serine(157). This occurs both in the presence and in the absence of low concentrations of NO, even at submicromolar concentrations of the peroxide, which alone was not inhibitory to platelets. These actions of H(2)O(2) were inhibited to a large extent by an inhibitor of cyclic AMP-dependent protein kinase, even though H(2)O(2) did not increase cyclic AMP. This inhibitor reversed the inhibition of platelets induced by combinations of NO and H(2)O(2) at low concentrations. The results suggest that the action on VASP may be one site of action of H(2)O(2) but that this event alone does not lead to inhibition of platelets; another unspecified action of NO is required to complete the events required for inhibition.

NO-independent stimulators of soluble guanylate cyclase

SARs around a novel type of guanylate cyclase stimulator which act by a mechanism different from classical NO-donors are described. Several pyrazolopyridinylpyrimidines are shown to relax aortic rings and revealed a long-lasting blood pressure lowering effect in rats after oral application.

NO-independent regulatory site on soluble guanylate cyclase

Nitric oxide (NO) is a widespread, potent, biological mediator that has many physiological and pathophysiological roles. Research in the field of NO appears to have followed a straightforward path, and the findings have been progressive: NO and cyclic GMP are involved in vasodilatation; glycerol trinitrate relaxes vascular smooth muscles by bioconversion to NO; mammalian cells synthesize NO; and last, NO mediates vasodilatation by stimulating the soluble guanylate cyclase (sGC), a heterodimeric (alpha/beta) haem protein that converts GTP to cGMP2-4. Here we report the discovery of a regulatory site on sGC. Using photoaffinity labelling, we have identified the cysteine 238 and cysteine 243 region in the alpha1-subunit of sGC as the target for a new type of sGC stimulator. Moreover, we present a pyrazolopyridine, BAY 41-2272, that potently stimulates sGC through this site by a mechanism that is independent of NO. This results in antiplatelet activity, a strong decrease in blood pressure and an increase in survival in a low-NO rat model of hypertension, and as such may offer an approach for treating cardiovascular diseases.

Molecular mechanisms involved in the synergistic activation of soluble guanylyl cyclase by YC-1 and nitric oxide in endothelial cells

YC-1 is a direct activator of soluble guanylyl cyclase (sGC) and sensitizes the enzyme for activation by nitric oxide (NO) and CO. Because the potentiating effect of YC-1 on NO-induced cGMP formation in platelets and smooth muscle cells has been shown to be substantially higher than observed with the purified enzyme, the synergism between heme ligands and YC-1 is apparently more pronounced in intact cells than in cell-free systems. Here, we investigated the mechanisms underlying the synergistic activation of sGC by YC-1 and NO in endothelial cells. Stimulation of the cells with YC-1 enhanced cGMP accumulation up to approximately 100-fold. The maximal effect of YC-1 was more pronounced than that of the NO donor DEA/NO (approximately 20-fold increase in cGMP accumulation) and markedly diminished in the presence of L-N(G)-nitroarginine, EGTA, or oxyhemoglobin. Because YC-1 did not activate endothelial NO synthase, the pronounced effect of YC-1 on cGMP accumulation was apparently caused by a synergistic activation of sGC by YC-1 and basal NO. The effect of YC-1 was further enhanced by addition of DEA/NO, resulting in a approximately 160-fold stimulation of cGMP accumulation. Thus, YC-1 increased the NO-induced accumulation of cGMP in intact cells by approximately 8-fold. Addition of endothelial cell homogenate increased the stimulatory effect of YC-1 on NO-activated purified sGC from 1.2- to 3.7-fold. This effect was not observed with heat-denatured homogenates, suggesting that a heat-labile factor present in endothelial cells potentiates the effect of YC-1 on NO-activated sGC.

Suppression of the induction of long-term depression by carbon monoxide in rat cerebellar slices

Carbon monoxide (CO) suppresses brain functions at doses lower than that suppressing oxygen (O(2)) supply to the brain, and the cerebellum is one of the sites most susceptible to the neurotoxic effects of CO. We investigated the effects of CO on the induction of cerebellar long-term depression (LTD) in the synapses between parallel fibres (PFs) and Purkinje cells. CO, at concentrations between 8 nM and 5 microM, exhibited almost no effect on synaptic responses in Purkinje cells, O(2) consumption and NO release from PFs in rat cerebellar slices. However, the induction of LTD was significantly suppressed by CO at concentrations between 40 and 200 nM. The suppressive effect of 40 nM CO was antagonized by 10 microM NOR3, an NO donor. In contrast, CO exhibited no clear effect on the induction of LTD at concentrations between 1 and 5 microM. The induction of LTD, suppressed by 10 microM N(G)-nitro-L-arginine, an inhibitor of NO synthase, was not restored by 5 microM CO. CO is not only a neurotoxic substance but also a candidate for an intercellular messenger. delta-Aminolevulinate (30 microM), a substance facilitating endogenous CO production, suppressed the induction of LTD, and the effect of delta-aminolevulinate was antagonized by 10 microM NOR3. These findings suggest that CO may have a suppressive effect on the induction of cerebellar LTD at nanomolar concentrations, probably via its effects on NO/cGMP signalling.

YC-1 enhances the responsiveness of tolerant vascular smooth muscle to glyceryl trinitrate

A major limitation of the use of organic nitrates in cardiovascular medicine is the development of tolerance, which has been attributed, in part, to a decrease in their metabolic activation in the vascular smooth muscle cell. Recently, 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) was shown to potentiate vascular smooth muscle responsiveness to glyceryl trinitrate (GTN), sodium nitroprusside, and the nitric oxide donor NOC 18, in organic nitrate-naïve vascular smooth muscle. We used GTN-tolerant rabbit aortic rings (RARs) to test the hypothesis that a non-vasorelaxant concentration of YC-1 enhances the ability of the prototypical organic nitrate GTN to relax vascular smooth muscle and elevate intravascular cGMP under conditions of GTN tolerance. Treatment with YC-1 (3 µM) produced a left shift of the GTN concentration-response curve and decreased the EC50 value for GTN-induced relaxation in both GTN-tolerant and non-tolerant RARs (P < 0.05). Intravascular cGMP elevation induced by GTN was enhanced in the presence of YC-1 in GTN-tolerant and non-tolerant RARs (P < 0.05). These observations indicate that YC-1, or similarly acting drugs, may be useful in overcoming the tolerance that develops during sustained GTN therapy, and that its mechanism may involve enhanced cGMP formation.Key words: glyceryl trinitrate, 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1), organic nitrate tolerance, cGMP, vasorelaxation.

YC-1, a benzyl indazole derivative, stimulates vascular cGMP and inhibits neointima formation

The pathobiologic process of arterial stenosis following balloon angioplasty continues to be an enigmatic problem in clinical settings. This research project investigates the ability of YC-1, a benzyl indazole derivative that sensitizes sGC/cGMP, to stimulate endogenous cGMP and attenuate balloon injury-induced neointima (NI) formation in the rat carotid artery. Northern and Western blot analyses revealed enhanced acute expression of iNOS and inducible heme oxygenase (HO-1) mRNA and protein in the injured artery. The contralateral uninjured artery also demonstrated acute HO-1 mRNA and protein induction without detectable iNOS expression. Perivascular application of YC-1 immediately following injury significantly stimulated acute vessel wall cGMP compared to untreated controls. YC-1 treated sections demonstrated significant reduction in NI area (-74%), NI area/medial wall area (-72%), and NI thickness (-76%) 2 weeks post-injury. These results directly implicate YC-1 as a potent new therapeutic agent capable of reducing post-angioplasty stenosis through endogenous CO- and/or NO-mediated, cGMP-dependent processes.

Human recombinant soluble guanylyl cyclase: expression, purification, and regulation

The alpha1- and beta1-subunits of human soluble guanylate cyclase (sGC) were coexpressed in the Sf9 cells/baculovirus system. In addition to the native enzyme, constructs with hexahistidine tag at the amino and carboxyl termini of each subunit were coexpressed. This permitted the rapid and efficient purification of active recombinant enzyme on a nickel-affinity column. The enzyme has one heme per heterodimer and was readily activated with the NO donor sodium nitroprusside or 3-(5'-hydroxymethyl-2'furyl)-1-benzyl-indazole (YC-1). Sodium nitroprusside and YC-1 treatment potentiated each other in combination and demonstrated a remarkable 2,200-fold stimulation of the human recombinant sGC. The effects were inhibited with 1H-(1,2, 4)oxadiazole(4,3-a)quinoxalin-1one (ODQ). The kinetics of the recombinant enzyme with respect to GTP was examined. The products of the reaction, cGMP and pyrophosphate, inhibited the enzyme. The extent of inhibition by cGMP depended on the activation state of the enzyme, whereas inhibition by pyrophosphate was not affected by the enzyme state. Both reaction products displayed independent binding and cooperativity with respect to enzyme inhibition. The expression of large quantities of active enzyme will facilitate structural characterization of the protein.

Guanylyl cyclases as a family of putative odorant receptors

Mammals can discriminate among a large number (> 10,000) of unique odorants. The most highly supported explanation for this ability is that olfactory neurons express a large number of seven transmembrane receptors that are not spatially organized at the level of the olfactory epithelium, but whose axonal projections form a distinct pattern within the olfactory bulb. The odor-induced signaling pathway in olfactory neurons includes a Gs-like protein (G(olf)) that activates a specific adenylyl cyclase (type III) isoform, resulting in elevations of cyclic AMP and subsequent activation of a cyclic nucleotide-gated channel. The channel also can be regulated by cyclic GMP. Recently, an olfactory neuron-specific guanylyl cyclase was discovered in rodents, and subsequently a large family of sensory neuronal guanylyl cyclases was identified in nematodes. These guanylyl cyclases are concentrated in the plasma membrane of the dendritic cilia and contain extracellular domains that retain many of the primary sequence characteristics of guanylyl cyclases known to be receptors for various peptides. Thus, the guanylyl cyclases appear to represent a second family of odorant/pheromone receptors.

Generation of cyclic guanosine monophosphate by heme oxygenases in the human testis--a regulatory role for carbon monoxide in Sertoli cells?

Previous studies have demonstrated that cGMP is produced by nitric oxide-mediated activation of soluble guanylyl cyclase (sGC) in seminiferous tubules of the human testis. It is not known, however, whether carbon monoxide (CO), another activator of sGC, is also involved in testicular function. To address this issue, testicular probes from 65- to 75-yr-old men have been examined. The CO-generating enzyme, heme oxygenase-1 (HO-1), could be localized by immunohistochemical and immunoblot analyses to Sertoli cells. In these cells, HO-1 is detectable in adluminal cell compartments, whereas sGC immunoreactivity is distributed exclusively in basal compartments. Treatments of isolated tubules with either sodium arsenite, known to induce HO-1, or hematin, an HO substrate, resulted in 4.4- and 1.8-fold, respectively, increases in cGMP levels. ODQ, a specific sGC inhibitor, inhibited completely the sodium arsenite-stimulated cGMP production. Moreover, the HO inhibitor zinc protoporphyrin-IX and the CO scavenger hemoglobin both significantly reduced (77% or 46% of control, respectively) tubular cGMP generation. These findings, demonstrating for the first time a link between HO-1 activity in Sertoli cells and sGC-dependent cGMP production in seminiferous tubules, suggest a functional role of CO in the human testis.

The mechanism of actions of 3-(5'-(hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1) on Ca(2+)-activated K(+) currents in GH(3) lactotrophs

The effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), an activator of soluble guanylyl cyclase, on ionic currents have been assessed in rat pituitary GH(3) lactotrophs. In GH(3) cells bathed in normal Tyrode's solution, YC-1 (1 microM) reversibly suppressed the amplitude of the Ca(2+)-activated K(+) current (I(K(Ca))). YC-1 at a concentration above 10 microM produced a biphasic response in the amplitude of I(K(Ca)), i.e., an initial decrease followed by a sustained increase. When the pipette solutions were filled with high EGTA (10 mM), the YC-1-induced stimulatory effect on I(K(Ca)) was abolished. Over a similar concentration range, YC-1 also effectively inhibited the voltage-dependent K(+) current (I(K(V))) in GH(3) cells. The IC(50) value required for the inhibition of I(K(V)) by YC-1 was 1 microM. Unlike YC-1, 8-bromo cGMP did not inhibit I(K(Ca)). However, YC-1 (10 microM) did not affect the amplitude of L-type Ca(2+) current. In the cell-attached configuration, application of YC-1 (10 microM) to the bath did not change the single-channel conductance of the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels; however, it did increase the opening probability of BK(Ca) channels. In contrast, in the outside-out configuration, YC-1 (10 microM) significantly suppressed the opening probability of BK(Ca) channels. The present study shows dual effects of YC-1 on I(K(Ca)) in GH(3) cells. The YC-1-mediated stimulation of I(K(Ca)) may result from elevated cytosolic Ca(2+), whereas the inhibition of I(K(Ca)) and I(K(V)) by YC-1 appears to be direct and independent of the activation of soluble guanylyl cyclase. Caution thus needs to be used in attributing the YC-1-mediated response to the activation of soluble guanylyl cyclase.

Acute blood pressure effects of YC-1-induced activation of soluble guanylyl cyclase in normotensive and hypertensive rats

We used YC-1 as a pharmacological tool to investigate the short-term blood pressure effects of NO-independent activation of sGC in normotensive and hypertensive rats. Four groups of normotensive Wistar-Kyoto rats were treated by i.v. injection with vehicle (V), YC-1 (YC-1), sodium nitroprusside (SNP), or YC-1 and SNP (YC-1+SNP). Hypertension was induced in four additional groups of WKY rats by 3 weeks of oral treatment with L-NAME. These animals were investigated with the same protocol as the normotensive animals: L-NAME/V, L-NAME/YC-1, L-NAME/SNP, L-NAME/YC-1+SNP. YC-1 lowered mean arterial blood pressure (MAP) in normotensive and hypertensive animals similarly to SNP alone (P<0.05, respectively). The combination of YC-1 with SNP caused a strong decrease of MAP in both the hypertensive and normotensive animals (P<0.05, respectively). SNP with YC-1 also induced a pronounced cyclic GMP increase in the aorta. This study shows for the first time the blood pressure lowering potential of bimodal targeting of the NO-sGC-system.

Interaction of soluble guanylate cyclase with YC-1: kinetic and resonance Raman studies

The enzyme-soluble guanylate cyclase (sGC), which converts GTP to cGMP, is a receptor for the signaling agent nitric oxide (NO). YC-1, a synthetic benzylindazole derivative, has been shown to activate sGC in an NO-independent fashion. In the presence of carbon monoxide (CO), which by itself activates sGC approximately 5-fold, YC-1 activates sGC to a level comparable to stimulation by NO alone. We have used kinetic analyses and resonance Raman spectroscopy (RR) to investigate the interaction of YC-1 and CO with guanylate cyclase. In the presence of CO and 200 microM YC-1, the V(max)/K(m GTP) increases 226-fold. While YC-1 does not perturb the RR spectrum of the ferrous form of baculovirus/Sf9 cell expressed sGC, it induces a shift in the Fe-CO stretching frequency for the CO-bound form from 474 to 492 cm(-1). Similarly, YC-1 has no effect on the RR spectrum of ferrous beta1(1-385), the isolated sGC heme-binding domain, but shifts the nu(Fe-CO) of CO-beta1(1-385) from 478 to 491 cm(-1), indicating that YC-1 binds in heme-binding region of sGC. In addition, the CO-bound forms of sGC and beta1(1-385) in the presence of YC-1 lie on the nu(Fe-CO) vs nu(C-O) correlation curve for proximal ligands with imidazole character, which suggests that histidine remains the heme proximal ligand in the presence of YC-1. Interestingly, YC-1 does not shift nu(Fe-CO) for the CO-bound form of H105G(Im), the imidazole-rescued heme ligand mutant of beta1(1-385). The data are consistent with binding of CO and YC-1 to the sGC heme-binding domain leading to conformational changes that give rise to an increase in catalytic turnover and a change in the electrostatic environment of the heme pocket.

The vasodilator-stimulated phosphoprotein (VASP): target of YC-1 and nitric oxide effects in human and rat platelets

The effects of the different types of soluble guanylate cyclase (sGC) stimulators on the phosphorylation status of vasodilator-stimulated phosphoprotein (VASP) in both human and rat platelets were studied under in vitro and in vivo conditions. sGC-dependent VASP phosphorylation (at Ser(239) and Ser(157)) both by the new direct sGC stimulator YC-1 and by NO donors was examined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS/PAGE) with different antibodies. One antibody, which recognizes VASP independent of its phosphorylation state, was used to detect the mobility shift of VASP caused by Ser(157) phosphorylation. The other antibody was specifically directed against VASP phosphorylated at Ser(239), the cGMP-dependent protein kinase (PKG) preferred phosphorylation site of VASP. In vitro YC-1 increased both VASP phosphorylation and cyclic guanosine monophosphate (cGMP) levels as did the NO donors 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO) and sodium nitroprusside (SNP). The combination of both types induced a synergistic effect in both VASP phosphorylation and cGMP increase. In rat platelets, similar effects could be shown in vitro. In vivo we observed a significant increase in cGMP and a distinct effect on VASP phosphorylation in rat platelets 1 h after oral administration of YC-1. These biochemical alterations are supported by a significant prolongation in rat-tail bleeding time. Direct stimulators of sGC like YC-1 are on the one hand direct potent stimulators of the cGMP/PKG/VASP pathway in platelets and on the other hand synergize with NO, the physiologic stimulator of sGC. Therefore YC-1-like substances are interesting tools for the development of new cardiovascular drugs with vasodilatory and antithrombotic properties.

Potentiation of carbon monoxide-induced relaxation of rat aorta by YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole]

The hypothesis that endogenous carbon monoxide (CO), produced during the oxidation of heme catalyzed by heme oxygenase (HO), plays a role similar to that of nitric oxide (NO) in the regulation of cardiovascular tone has been criticized because of the low potency of CO compared with NO in relaxing blood vessels and stimulating soluble guanylyl cyclase (sGC). This criticism has been muted by the demonstration that, in the presence of YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole], CO has similar potency to NO in stimulating sGC activity. In this study, we determined that YC-1 potentiated CO-induced relaxation of rat aortic strips (RtAS) by approximately ten-fold. Furthermore, CO-induced relaxation of RtAS was shown to be mediated through stimulation of sGC because vasorelaxation was inhibited by ODQ (1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one), a selective sGC inhibitor, in the absence and presence of YC-1. A gas chromatographic-headspace method was used to measure CO concentration in Krebs' solution following the addition of CO-saturated saline solution to the tissue bath, in order to provide an accurate determination of RtAS exposure to CO. The tissue bath concentration of CO was shown to be approximately one-half of that calculated to be present. We conclude that should an endogenous compound exist with properties similar to that of YC-1, then the potency of CO as a vasorelaxant in the presence of this factor would be increased. As a consequence, CO could play a role in the regulation of cardiovascular tone, comparable to that of NO.Key words: carbon monoxide; YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole]; vasodilation; rat aorta; gas-chromatographic analysis.

Rate of deactivation of nitric oxide-stimulated soluble guanylate cyclase: influence of nitric oxide scavengers and calcium

Soluble guanylate cyclase (sGC) is highly activated by nitric oxide (NO) and is the known mediator of the effects of NO on a variety of physiological processes. The rates at which sGC is activated and deactivated are therefore of wide interest since they determine the duration of a tissue's response to NO. The effect of NO on smooth muscle dissipates in 1-2 min, suggesting that both activation and deactivation are fast. In vitro measurements show that the activation of sGC occurs in less than a second, while the deactivation takes several hours at 20 degrees C. However, recent reports indicate that Mg-GTP, oxyhemoglobin, and reducing and oxidizing agents could deactivate the cyclase in several seconds to minutes, though the effectiveness of each of these agents is in dispute. We investigated the lifetime of NO-sGC in the cytosol of retina by monitoring its enzymatic activity at 20 degrees C. Our results show that Mg-GTP, the substrate of NO-sGC, has no influence on the deactivation. Similarly, reducing agents glutathione and dithiothreitol shortened the half-life of NO-sGC only by about 30%. The greatest effect on the deactivation was caused by scavengers of NO: oxyhemoglobin reduced the half-life of NO-sGC from 106 min to 18 s; another NO scavenger, 2-(4-carboxyphenyl)-4,4,5, 5-tetramethylimidazoline-1-oxyl-3-oxide (CPTIO), reduced it to 42 s (20 degrees C). Similarly rapid deactivation was observed with the enzyme from bovine lung, immunoprecipitated enzyme from bovine retina, and heme-deficient enzyme from bovine retina reconstituted with heme. On the other hand, YC-1, an activator of sGC, stabilized the activated enzyme, preventing NO dissociation, as was evident from the inability of oxyhemoglobin or CPTIO to deactivate NO-sGC. Calcium, which is known to inhibit NO-sGC, also inhibited the effects of oxyhemoglobin and CPTIO, slowing down the deactivation of the enzyme. Lithium, which is also known to inhibit NO-sGC, had no effect on the deactivation rate of the enzyme. These results, taken together, suggest that two factors with major impact on the lifetime of NO-sGC are the proximity to NO scavengers and the calcium concentration in the cell.

Relaxation of rabbit lower urinary tract smooth muscle by nitric oxide and carbon monoxide: modulation by hydrogen peroxide

Recent studies suggest that the body produces two gaseous messengers, nitric oxide (NO) and carbon monoxide (CO), both of which activate soluble guanylyl cyclase and thus modulate the activity of smooth muscle cells. In the present study, the effects of NO and CO on the smooth muscle of the lower urinary tract were compared. In addition, the modulation of tissue NO- and CO-induced relaxation by hydrogen peroxide was examined. NO, produced endogenously by electrical field stimulation (EFS) or applied exogenously as a solution, induced a concentration-dependent relaxation of rabbit cavernosal and urethral smooth muscle strips, but not of bladder tissues. The cavernosal tissue was found to be three times more sensitive to the actions of NO than the urethra. CO also induced relaxation of both tissue types, but with no apparent difference in sensitivity between the tissues. However, CO was much less potent than NO with respect to smooth muscle relaxation. The mechanism of action of the two mediators was cyclic guanosine monophosphate (cGMP)-dependent, as evidenced by enhanced formation of cGMP and inhibition of relaxation by the guanylyl cyclase inhibitor, oxadiazoloquinoxaline-1-one (ODQ.) The data suggests that NO is the dominant messenger in these tissues, but does not exclude a role for CO. In the presence of hydrogen peroxide, the relaxation responses induced by both NO and CO were significantly increased, regardless of tissue type. The mechanism for this effect is unclear, but evidence points to a requirement for the activation of guanylyl cyclase and enhanced formation of cGMP, since potentiation by the peroxide was blocked by a specific guanylyl cyclase inhibitor. We suggest that H(2)O(2) may play a positive role in the amplification or NO and CO-mediated responses.

The heme oxygenase pathway and its interaction with nitric oxide in the control of cellular homeostasis

Heme oxygenase is the rate limiting enzyme in heme degradation to carbon monoxide (CO), iron and bilirubin. The inducible isoform of the protein, heme oxygenase-1 (HO-1), is susceptible to up-regulation by a diverse variety of conditions and agents in mammalian tissue, leading to the common conception that HO-1 is a stress related enzyme. However, as attempts are made to unravel the mechanisms by which HO-1 is induced and as we discover that CO, iron and bilirubin may be important effector molecules, we are learning to appreciate that heme oxygenases may be central to the regulation of many physiological and pathophysiological processes besides their established function in heme catabolism. One such process may be closely linked to nitric oxide (NO). It has been demonstrated that NO and NO donors are capable of inducing HO-1 protein expression, in a mechanism depending on the de novo synthesis of RNA and protein. Thus, it is postulated that NO may serve as a signaling molecule in the modulation of the tissue stress response. This review will highlight the current ideas on the role of CO-heme oxygenase and NO-nitric oxide synthase in cell signaling and discuss how the two systems are interrelated.

Dominant negative mutants of guanylyl cyclase: probes for global functions and intramolecular mechanisms

Dominant negative mutants are unique tools to define functions of a protein, not only within complex cellular and organismal contexts, but also mechanistically within a protein. Guanylyl cyclases are amenable to studies with dominant negative mutants, with their own sets of opportunities for insight and pitfalls to overcome. Membrane and soluble forms of guanylyl cyclase represent self-contained signal transduction modules that recognize, transduce, and amplify an external signal to give a carefully controlled response. Beginning with recognition of peptide hormones versus nitric oxide, membrane and soluble guanylyl cyclases are considerably different, except that their catalytic domains are closely related. Studies on these catalytic domains and their counterparts in adenylyl cyclases have raised an integral question of whether one or two domains form a catalytic site, which remains unresolved. Regardless of which model is correct, guanylyl cyclases appear to require an oligomeric state to function properly. The inferred relationship between protein-protein interaction and function is the basis for developing dominant negative mutants, which can be designed without prior structural information. Soluble guanylyl cyclases exist in a heterodimeric state, whereas membrane guanylyl cyclases are homodimeric, or possibly higher-order oligomers. These properties dictate that dominant negative mutants of membrane and soluble guanylyl cyclases be approached in fundamentally different ways, with regard to their design, their functional consequences, and their limitations. Using dominant negative mutants as specific inhibitors in complex systems, such as transgenic animals, represents a significant advance, and continuing improvements are just an inkling of the extraordinary potential of this approach. For example, the function of a protein can be obscured because it is expressed in multiple cell types; by restricting its pattern of expression, a cell-specific promoter, coupled to a dominant negative mutant, can pinpoint this function. As more sophisticated methods are developed, dominant negative mutants will provide additional opportunities to unveil new regulatory mechanisms, new signaling pathways, or even new therapeutic approaches.

Studying the structure and regulation of soluble guanylyl cyclase

Soluble guanylyl cyclase acts as the receptor for the signaling molecule nitric oxide. The enzyme consists of two different subunits. Each subunit shows the cyclase catalytic domain, which is also conserved in the membrane-bound guanylyl cyclases and the adenylyl cyclases. The N-terminal regions of the subunits are responsible for binding of the prosthetic heme group of the enzyme, which is required for the stimulatory effect of nitric oxide (NO). The five-coordinated ferrous heme displays a histidine as the axial ligand; activation of soluble guanylyl cyclase by NO is initiated by binding of NO to the heme iron and proceeds via breaking of the histidine-to-iron bond. Recently, a novel pharmacological and possibly physiological principle of guanylyl cyclase sensitization was demonstrated. The substance YC-1 has been shown to activate the enzyme independent of NO, to potentiate the effect of submaximally effective NO concentrations, and to turn carbon monoxide into an effective activator of soluble guanylyl cyclase.

A point-mutated guanylyl cyclase with features of the YC-1-stimulated enzyme: implications for the YC-1 binding site?

Guanylyl cyclases (GCs) and adenylyl cyclases (ACs) play key roles in various signaling cascades and are structurally closely related. The crystal structure of a soluble AC revealed one binding site each for the substrate ATP and the activator forskolin. Recently, YC-1, a novel activator of the heterodimeric soluble GC (sGC), has been identified which acts like forskolin on AC. Here, we investigated the respective substrate and potential activator domains of sGC using point-mutated subunits. Whereas substitution of the conserved Cys-541 of the beta(1) subunit with serine led to an almost complete loss of activity, mutation of the respective homologue (Cys-596) in the alpha(1) subunit yielded an enzyme with an increased catalytic rate and higher sensitivity toward NO. This phenotype exhibits characteristics similar to those of the YC-1-treated wild-type enzyme. Conceivably, this domain which corresponds to the forskolin site of the ACs may comprise the binding site for YC-1.

Release of nitric oxide from endothelial cells stimulated by YC-1, an activator of soluble guanylyl cyclase

1 In this study we examined the endothelium-dependent effect of YC-1 - a benzyl indazole derivative which directly activates soluble guanylyl cyclase (sGC) - on vascular relaxation and nitric oxide (NO) and guanosine-3',5'-cyclic monophosphate (cyclic GMP) in endothelial cells. 2 In preconstricted rat aortic rings with intact endothelium, YC-1 produced a concentration-dependent relaxation. However, the concentration response curve was shifted rightward to higher concentrations of YC-1, when (i) the aortas were pre-treated with L-NG-nitroarginine methylester (L-NAME) or (ii) the endothelium was removed. 3 Incubation of bovine aortic endothelial cells (BAEC) with YC-1 produced a concentration-dependent NO synthesis and release as assessed using a porphyrinic microsensor. Pre-incubating cells with L-NAME or with 8-bromo-cyclic GMP decreased this effect indicating that the YC-1 stimulation of NO synthesis is due to an activation of nitric oxide synthase, but not to an elevation of cyclic GMP. No direct effect of YC-1 on recombinant endothelial constitutive NO synthase activity was observed. 4 The YC-1 stimulated NO release was reduced by 90%, when extracellular free calcium was diminished. 5 In human umbilical vein endothelial cells (HUVEC), YC-1 stimulated intracellular cyclic GMP production in a concentration- and time-dependent manner. Stimulation of cyclic GMP was greater with a maximum concentration of YC-1 compared to calcium ionophore A23187. Similar effects were observed in BAEC and rat microvascular coronary endothelial cells (RMCEC). 6 When HUVEC and RMCEC were pre-treated with L-NG-nitroarginine (L-NOARG), the maximum YC-1 stimulated cyclic GMP increase was reduced by >/=50%. 7 These results indicate, that beside being a direct activator of sGC, YC-1 stimulates a NO-synthesis and release in endothelial cells which is independent of elevation of cyclic GMP but strictly dependent on extracellular calcium. The underlying mechanism needs to be determined further.

YC-1 potentiates nitric oxide-induced relaxation in guinea-pig trachea

1. The effects of YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole) on tension, levels of cyclic GMP and cyclic AMP were investigated in guinea-pig trachea. We especially studied the combined effect of YC-1 with exogenous or endogenous nitric oxide on these parameters. 2. YC-1 at the concentration 3 or 10 microM, which caused only minor effect by itself, elicited concentration-dependent potentiation of sodium nitroprusside (SNP)-induced tracheal relaxation. This relaxation of YC-1 with SNP was reversed by ODQ. 3. Relaxant responses to electric field stimulation (EFS) in the presence of indomethacin, atropine, guanethidine, alpha-chymotrypsin and histamine were also markedly increased by YC-1 (10 microM). In the presence of L-NAME or ODQ, the relaxant effects to EFS were attenuated and the following addition of YC-1 did not further enhance relaxation. 4. YC-1 (10 microM) or SNP (0.3 microM) alone did not induce significant elevation of cyclic GMP levels in the presence of IBMX, whereas simultaneous application of both compounds markedly elevated the cyclic GMP accumulation. In contrast, the cyclic AMP levels were not altered even at the combination of YC-1 and SNP. Additionally, YC-1 also affected cyclic GMP metabolism, since it inhibited the activity of phosphodiesterase type V in human platelets. 5. YC-1 (30 microM) did not scavenge superoxide anion and had no effect on the removal of superoxide anion by superoxide dismutase in a xanthine/xanthine oxidase system. 6. In conclusion, these results indicate that although YC-1 elicits negligible relaxation of guinea-pig trachea by itself, it can potentiate the relaxant responses of exogenous or endogenous NO. This synergistic response of YC-1 is via the elevation of cyclic GMP contents.

Cell biology of heme

Heme is a complex of iron with protoporphyrin IX that is essential for the function of all aerobic cells. Heme serves as the prosthetic group of numerous hemoproteins (eg, hemoglobin, myoglobin, cytochromes, guanylate cyclase, and nitric oxide synthase) and plays an important role in controlling protein synthesis and cell differentiation. Cellular heme levels are tightly controlled; this is achieved by a fine balance between heme biosynthesis and catabolism by the enzyme heme oxygenase. On a per-cell basis, the rate of heme synthesis in the developing erythroid cells is at least 1 order of magnitude higher than in the liver, which is in turn the second most active heme producer in the organism. Differences in iron metabolism and in genes for 5-aminolevulinic acid synthase (ALA-S, the first enzyme in heme biosynthesis) are responsible for the differences in regulation and rates of heme synthesis in erythroid and nonerythroid cells. There are 2 different genes for ALA-S, one of which is expressed ubiquitously (ALA-S1), whereas the expression of the other (ALA-S2) is specific to erythroid cells. Because the 5'-untranslated region of the erythroid-specific ALA-S2 mRNA contains the iron-responsive element, a cis-acting sequence responsible for translational induction of erythroid ALA-S2 by iron, the availability of iron controls protoporphyrin IX levels in hemoglobin-synthesizing cells. In nonerythroid cells, the rate-limiting step of heme production is catalyzed by ALA-S1, whose synthesis is feedback-inhibited by heme. On the other hand, in erythroid cells, heme does not inhibit either the activity or the synthesis of ALA-S but does inhibit cellular iron acquisition from transferrin without affecting its utilization for heme synthesis. This negative feedback is likely to explain the mechanism by which the availability of transferrin iron limits heme synthesis rate. Moreover, in erythroid cells heme seems to enhance globin gene transcription, is essential for globin translation, and supplies the prosthetic group for hemoglobin assembly. Heme may also be involved in the expression of other erythroid-specific proteins. Furthermore, heme seems to play a role in regulating either transcription, translation, processing, assembly, or stability of hemoproteins in nonerythroid cells. Heme oxygenase, which catalyzes heme degradation, seems to be an important enzymatic antioxidant system, probably by providing biliverdin, which is an antioxidant agent.

YC-1 potentiates the antiplatelet effect of hydrogen peroxide via sensitization of soluble guanylate cyclase

In the present study, we showed that 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), a nitric oxide (NO)-independent activator of soluble guanylate cyclase, could potentiate H2O2-induced inhibition of platelet aggregation and increase of platelet cGMP levels. The synergistic effect of YC-1 and H2O2 on platelet aggregation and increases of cGMP were almost completely prevented by catalase and a selective soluble guanylate cyclase inhibitor (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), or partially attenuated by the hydroxyl radical scavenger mannitol. In contrast, superoxide dismutase failed to influence H2O2/YC-1-induced inhibition of aggregation. Furthermore, YC-1 could enhance the activation of soluble guanylate cyclase caused by FeSO4/H2O2 and, this effect was prevented markedly by mannitol. These results suggest that YC-1 may enhance the antiaggregatory effect of H2O2 via the sensitization of platelet soluble guanylate cyclase. In addition, this phenomenon is, at least in part, dependent on H2O2-derived hydroxyl radical.

Kinetics and equilibria of soluble guanylate cyclase ligation by CO: effect of YC-1

Previous work has proved that the enzyme-soluble guanylate cyclase, GC, is activated several 100-fold by the combination of carbon monoxide plus a benzylindazole derivative called YC-1. That is about the same as activation by nitric oxide, which has a well-established role both in vivo and in vitro. This report addresses several spectroscopic, equilibrium, and kinetic effects wrought by YC-1 on carboxyl guanylate cyclase, including the following: a shift in the Soret absorption band by 4 nm to shorter wavelength; an increase in CO affinity by an order of magnitude; a dramatic change in the kinetics of CO association. After photolytic dissociation of CO, the majority, but not all, of bimolecular ligand recombination occurs with a time constant about 1000-fold faster than in the absence of YC-1, while a smaller fraction recombines almost, but not quite, the same as usual. This is reminiscent of the kinetics of NO association with GC, which also shows two prominent phases. The results just listed pertain in the presence of GTP/cGMP, which would be present during enzyme catalysis. Qualitatively similar, but smaller, effects occur in the absence of GTP/cGMP. Measurements are reported to characterize other changes in buffer conditions. The results are consistent with a mechanistic model that attributes a crucial role to the proximal bond that connects the heme iron to a histidine side chain in GC but also requires protein control of the distal environment.

Generation and characterization of a stable soluble guanylate cyclase-overexpressing CHO cell line

A stably transfected soluble guanylate cyclase (sGC, alpha1 and beta1 subunits of the rat lung enzyme)-overexpressing CHO cell line was generated for the characterization of different types of activators of the soluble guanylate cyclase. Polyclonal antibodies directed against both subunits of the rat enzyme were used to detect both subunits in the cytosol of the transfected CHO cells. We studied the effects of different nitric oxide (NO) donors like SNP and DEA/NO and, in particular, the direct, NO-independent stimulator of the soluble guanylate cyclase 3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole (YC-1), on intracellular guanosine 3',5'-cyclic monophosphate (cGMP) production. DEA/NO (0.01-3 microM), SNP (1-10 microM), and YC-1 (1-10 microM) induced a concentration-dependent intracellular cGMP increase with maximal effects of 16-fold (3 microM DEA/NO), 8-fold (10 microM SNP), and 6-fold (10 microM YC-1) stimulation compared to controls, respectively. In addition, a synergistic effect of the combination of the NO donor and YC-1 could be observed with a maximal stimulation of 64-fold by SNP (10 microM) and YC-1 (10 microM). 1H-(1,2,4)-Oxadiazolo-(4,3-a)-6-bromo-quinoxazin-1-one (ODQ, 10 microM), a potent and selective inhibitor of sGC, inhibited both the single effects of NO donors [DEA/NO (3 microM), 77%; SNP (3 microM), 83%] and YC-1 [YC-1 (3 microM), 82%], but moreover the synergistic effects between NO donors and YC-1 [DEA/NO (3 microM) + YC-1 (3 microM), 81%; SNP (3 microM) + YC-1 (3 microM),89%] on intracellular cGMP production. In summary,we have generated a simple, sensitive, and useful bioassay method to characterize all types of sGC activators on the cellular level without the need of primary cell culture, several transfections, or purifying enzyme from biological materials.

Comparative effects of several nitric oxide donors on intracellular cyclic GMP levels in bovine chromaffin cells: correlation with nitric oxide production

1. Sodium nitroprusside, S-nitroso-N-acetyl-D,L-penicillamine, Spermine NONOate and DEA NONOate raised cyclic GMP levels in bovine chromaffin cells in a time and concentration dependent manner with different potencies, the most potent being DEA/NO with an EC50 value of 0.38 +/- 0.02 microM. 2. Measurements of NO released from these donors revealed that DEA/NO decomposed with a half-life (t1/2) of 3.9 +/- 0.2 min. The t1/2 for SPER/NO was 37 +/- 3 min. SNAP decomposed more slowly (t1/2 = 37 +/- 4 h) and after 60 min the amount of NO produced corresponded to less than 2% of the total SNAP present. The rate of NO production from SNAP was increased by the presence of glutathione. 3. For DEA/NO and SPER/NO there was a clear correlation between nitric oxide production and cyclic GMP increases. Their threshold concentrations were 0.05 microM and maximal effective concentration between 2.5 and 5 microM. 4. For SNAP, threshold activation was seen at 1 microM, whereas full activation required a higher concentration (500-750 microM). The dose-response for SNAP increases in cyclic GMP was shifted nearly two orders of magnitude lower in the presence of glutathione. At higher concentrations an inhibition of cyclic GMP accumulation was found. This effect was not observed with either the nitric oxide-deficient SNAP analogue or other NO donors. 5. Although NO-donors are likely to be valuable for studying NO functions, their effective concentrations and the amount of NO released by them are very different and should be assessed in each system to ensure that physiological concentrations of NO are used.

Guanylate cyclase and the .NO/cGMP signaling pathway

Signal transduction with the diatomic radical nitric oxide (NO) is involved in a number of important physiological processes, including smooth muscle relaxation and neurotransmission. Soluble guanylate cyclase (sGC), a heterodimeric enzyme that converts guanosine triphosphate to cyclic guanosine monophosphate, is a critical component of this signaling pathway. sGC is a hemoprotein; it is through the specific interaction of NO with the sGC heme that sGC is activated. Over the last decade, much has been learned about the unique heme environment of sGC and its interaction with ligands like NO and carbon monoxide. This review will focus on the role of sGC in signaling, its relationship to the other nucleotide cyclases, and on what is known about sGC genetics, heme environment and catalysis. The latest understanding in regard to sGC will be incorporated to build a model of sGC structure, activation, catalytic mechanism and deactivation.

Effects of the soluble guanylyl cyclase activator, YC-1, on vascular tone, cyclic GMP levels and phosphodiesterase activity

The vasomotor and cyclic GMP-elevating activity of YC-1, a novel NO-independent activator of soluble guanylyl cyclase (sGC), was studied in isolated rabbit aortic rings and compared to that of the NO donor compounds sodium nitroprusside (SNP) and NOC 18. Similarly to SNP and NOC 18, YC-1 (0.3-300 microM) caused a concentration-dependent, endothelium-independent relaxation that was greatly reduced by the sGC inhibitor 1-H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ 10 microM; 59% inhibition of dilation induced by 100 microM YC-1) suggesting the activation of sGC as one mechanism of action. Preincubation with YC-1 (3 and 30 microM) significantly increased the maximal dilator responses mediated by endogenous NO in aortic rings that was released upon exposure to acetylcholine, and enhanced the dilator response to the exogenous NO-donors, SNP and NOC 18, by almost two orders of magnitude. Vasoactivity induced by SNP and YC-1 displayed different kinetics as evidenced by a longlasting inhibition by YC-1 (300 microM) on the phenylephrine (PE)-induced contractile response, which was not fully reversible even after extensive washout (150 min) of YC-1, and was accompanied by a long-lasting elevation of intracellular cyclic GMP content. In contrast, SNP (30 microM) had no effect on the vasoconstrictor potency of PE, and increases in intravascular cyclic GMP levels were readily reversed after washout of this NO donor compound. Surprisingly, YC-1 not only activated sGC, but also affected cyclic GMP metabolism, as it inhibited both cyclic GMP break down in aortic extracts and the activity of phosphodiesterase isoforms 1-5 in vitro. In conclusion, YC-1 caused persistent elevation of intravascular cyclic GMP levels in vivo by activating sGC and inhibiting cyclic GMP break down. Thus, YC-1 is a highly effective vasodilator compound with a prolonged duration of action, and mechanisms that are unprecedented for any previously known sGC activator.

Heme-based sensors in biological systems

The past several years have been witness to a staggering rate of advancement in the understanding of how organisms respond to changes in the availability of diatomic molecules that are toxic and/or crucial to survival. Heme-based sensors presently constitute the majority of the proteins known to sense NO, O2 and CO and to initiate the chemistry required to adapt to changes in their availabilities. Knowledge of the three characterized members of this class, soluble guanylate cyclase, FixL and CooA, has grown substantially during the past year. The major advances have resulted from a broad range of approaches to elucidation of both function and mechanism. They include growth in the understanding of the interplay between the heme and protein in soluble guanylate cyclase, as well as alternate means for its stimulation. Insight into the O2-induced structural changes in FixL has been supplied by the single crystal structure of the heme domain of Bradyrhizobium japonicum. Finally, the ligation environment and ligand interchange that facilitates CO sensing by CooA has been established by spectroscopic and mutagenesis techniques.