Another activation switch for endothelial nitric oxide synthase: why does it have to be so complicated?

Regulation of the endothelial isoform of nitric oxide synthase (eNOS) appears to be much more complex in comparison to that of other NOS isoforms. A recent paper has expanded the regulation of the enzyme to the realm of sphingolipid signaling, specifically implicating that sphingosine 1-phosphate, endothelial differentiation gene (Edg) receptors and Akt kinase induce a signal transduction pathway via phosphorylation of a serine residue in eNOS. Bradykinin, a nonapeptide formed by enzymatic cleavage of a plasma protein precursor, activates eNOS by an independent pathway that does not require serine phosphorylation, suggesting a complex interplay of signals in the control of endothelial formation of nitric oxide.

Slow-binding inhibition of human prostaglandin endoperoxide synthase-2 with darbufelone, an isoform-selective antiinflammatory di-tert-butyl phenol.

The antiinflammatory agent darbufelone, ((Z)-5-[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl] methylene]-2-imino-4-thiazolidinone, methanesulfonate salt), was discovered as a dual inhibitor of cellular prostaglandin and leukotriene production. To study the mechanism of action of this drug, we expressed human prostaglandin endoperoxide synthase-1 (PGHS-1) and PGHS-2 and purified the recombinant enzymes using buffers that contain octylglucoside. In cyclooxygenase assays following a 15-min incubation of enzyme with inhibitor, darbufelone potently inhibits PGHS-2 (IC(50) = 0.19 microM) but is much less potent with PGHS-1 (IC(50) = 20 microM). Interestingly, when the assay buffer contains traces of Tween 20 (0.0001%), darbufelone appears inactive with PGHS-2 due to a detergent interaction that is detectable by absorption spectroscopy. We therefore used octylglucoside, which does not affect darbufelone in this way, in place of Tween 20 in our PGHS buffers. Inhibition of PGHS-2 with darbufelone is time dependent: with no preincubation, darbufelone is a weak inhibitor (IC(50) = 14 microM), but after a 30-min incubation it is 20-fold more potent. Plots of PGHS-2 activity vs preincubation time at various darbufelone concentrations reach a plateau. This finding is inconsistent with irreversible or one-step slow-binding inhibition. A two-step slow-binding inhibition model is proposed in which the E.I complex (K(i) = 6.2 +/- 1.9 to 14 +/- 1 microM) slowly transforms (k(5) = 0.015-0.030 s(-)(1)) to a tightly bound E.I form with K(i) = 0.63 +/- 0.07 microM and k(6) = 0.0034 s(-)(1). In steady-state kinetics inhibition experiments performed with no preincubation, we find that darbufelone is a noncompetitive inhibitor of PGHS-2 (K(i) = 10 +/- 5 microM). Darbufelone quenches the fluorescence of PGHS-2 at 325 nm (lambda(ex) = 280 nm) with K(d) = 0.98 +/- 0.03 microM. The PGHS substrate, arachidonate, and various cyclooxygenase inhibitors do not alter this binding affinity of darbufelone but a structural analogue of darbufelone competes directly for binding to PGHS-2. Di-tert-butyl phenols such as darbufelone may inhibit PGHS-2 by exploiting a previously unrecognized binding site on the enzyme.

Reconstitution of pterin-free inducible nitric-oxide synthase.

Inducible nitric-oxide synthase (NOS) was expressed and purified in the absence of 6(R)-tetrahydro-l-biopterin (H(4)B). Pterin-free NOS exhibits a Soret band (416-420 nm) characteristic of predominantly low spin heme and does not catalyze the formation of nitric oxide (. NO) (Rusche, K. M., Spiering, M. M., and Marletta, M. A. (1998) Biochemistry 37, 15503-15512). Reconstitution of pterin-free NOS with H(4)B was monitored by a shift in the Soret band to 396-400 nm, the recovery of.NO-forming activity, and the measurement of H(4)B bound to the enzyme. As assessed by these properties, H(4)B binding was not rapid and required the presence of a reduced thiol. Spectral changes and recovery of activity were incomplete in the absence of reduced thiol. Full reconstitution of holoenzyme activity and stoichiometric H(4)B binding was achieved in the presence of 5 mm glutathione (GSH). Preincubation with GSH before the addition of H(4)B decreased, whereas lower concentrations of GSH extended, the time required for reconstitution. Six protected cysteine residues in pterin-free NOS were identified by labeling of NOS with cysteine-directed reagents before and after reduction with GSH. Heme and metal content of pterin-free and H(4)B-reconstituted NOS were also measured and were found to be independent of H(4)B content. Additionally, pterin-free NOS was reconstituted with 6-methylpterin analogs, including redox-stable deazapterins. Reconstitution with the redox-stable pterin analogs was neither time- nor thiol-dependent. Apparent binding constants were determined for the 6-methyl- (50 microm) and 6-ethoxymethyl (200 microm) deazapterins. The redox-stable pterin analogs appear to bind to NOS in a different manner than H(4)B.

Escherichia coli LipA is a lipoyl synthase: in vitro biosynthesis of lipoylated pyruvate dehydrogenase complex from octanoyl-acyl carrier protein.

The Escherichia coli lipA gene product has been genetically linked to carbon-sulfur bond formation in lipoic acid biosynthesis [Vanden Boom, T. J., Reed, K. E., and Cronan, J. E., Jr. (1991) J. Bacteriol. 173, 6411-6420], although in vitro lipoate biosynthesis with LipA has never been observed. In this study, the lipA gene and a hexahistidine tagged lipA construct (LipA-His) were overexpressed in E. coli as soluble proteins. The proteins were purified as a mixture of monomeric and dimeric species that contain approximately four iron atoms per LipA polypeptide and a similar amount of acid-labile sulfide. Electron paramagnetic resonance and electronic absorbance spectroscopy indicate that the proteins contain a mixture of [3Fe-4S] and [4Fe-4S] cluster states. Reduction with sodium dithionite results in small quantities of an S = 1/2 [4Fe-4S](1+) cluster with the majority of the protein containing a species consistent with an S = 0 [4Fe-4S](2+) cluster. LipA was assayed for lipoate or lipoyl-ACP formation using E. coli lipoate-protein ligase A (LplA) or lipoyl-[acyl-carrier-protein]-protein-N-lipoyltransferase (LipB), respectively, to lipoylate apo-pyruvate dehydrogenase complex (apo-PDC) [Jordan, S. W., and Cronan, J. E. (1997) Methods Enzymol. 279, 176-183]. When sodium dithionite-reduced LipA was incubated with octanoyl-ACP, LipB, apo-PDC, and S-adenosyl methionine (AdoMet), lipoylated PDC was formed. As shown by this assay, octanoic acid is not a substrate for LipA. Confirmation that LipA catalyzes formation of lipoyl groups from octanoyl-ACP was obtained by MALDI mass spectrometry of a recombinant PDC lipoyl-binding domain that had been lipoylated in a LipA reaction. These results provide information about the mechanism of LipA catalysis and place LipA within the family of iron-sulfur proteins that utilize AdoMet for radical-based chemistry.

Inhibition of soluble guanylate cyclase by ODQ.

The heme in soluble guanylate cyclases (sGC) as isolated is ferrous, high-spin, and 5-coordinate. [1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one] (ODQ) has been used extensively as a specific inhibitor for sGC and as a diagnostic tool for identifying a role for sGC in signal transduction events. Addition of ODQ to ferrous sGC leads to a Soret shift from 431 to 392 nm and a decrease in nitric oxide (NO)-stimulated sGC activity. This Soret shift is consistent with oxidation of the ferrous heme to ferric heme. The results reported here further define the molecular mechanism of inhibition of sGC by ODQ. Addition of ODQ to the isolated sGC heme domain [beta1(1-385)] gave the same spectral changes as when sGC was treated with ODQ. EPR and resonance Raman spectroscopy was used to show that the heme in ODQ-treated beta1(1-385) is indeed ferric. Inhibition of the NO-stimulated sGC activity by ODQ is due to oxidation of the sGC heme and not to perturbation of the catalytic site, since the ODQ-treated sGC has the same basal activity as untreated sGC (68 +/- 12 nmol min(-)(1) mg(-)(1)). In addition, ODQ-oxidized sGC can be re-reduced by dithionite, and this re-reduced sGC has identical NO-stimulated activity as the original ferrous sGC. Oxidation of the sGC heme by ODQ is fast with a second-order rate constant of 8.5 x 10(3) M(-)(1) s(-)(1). ODQ can also oxidize hemoglobin, indicating that the reaction is not specific for the heme in sGC versus that in other hemoproteins.

Cu2+ and Zn2+ inhibit nitric-oxide synthase through an interaction with the reductase domain.

Cu(2+) and Zn(2+) inhibit all of the NADPH-dependent reactions catalyzed by neuronal nitric-oxide synthase (nNOS) including ferricytochrome c reduction, NADPH oxidation, and citrulline formation. Cu(2+) and Zn(2+) also inhibit ferricytochrome c reduction by the independent reductase domain. Zn(2+) affects all activities of the full-length nNOS and the reductase domain to the same extent (estimated IC(50) values from 9 to 31 microm), suggesting Zn(2+) occupation of a single site in the reductase domain. Citrulline formation and NADPH oxidation by the full-length nNOS and ferricytochrome c reduction by the reductase domain are affected similarly by Cu(2+), with estimated IC(50) values ranging from 6 to 33 microm. However, Cu(2+) inhibits ferricytochrome c reduction by the full-length nNOS 2 orders of magnitude more potently, with an estimated IC(50) value of 0.12 microm. These data suggest the possibility that Cu(2+) may interact with nNOS at two sites, one composed exclusively of the reductase domain (which is perhaps also involved in Zn(2+)-mediated inhibition), and another that includes components of both domains. Occupation of the second (higher affinity) site could then promote the selective inhibition of ferricytochrome c reduction in full-length nNOS. Neither the inhibition by Cu(2+) nor that by Zn(2+) is dependent on calmodulin.

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.

A molecular basis for nitric oxide sensing by soluble guanylate cyclase.

Nitric oxide (NO) functions as a signaling agent by activation of the soluble isoform of guanylate cyclase (sGC), a heterodimeric hemoprotein. NO binds to the heme of sGC and triggers formation of cGMP from GTP. Here we report direct kinetic measurements of the multistep binding of NO to sGC and correlate these presteady state events with activation of enzyme catalysis. NO binds to sGC to form a six-coordinate, nonactivated, intermediate (k(on) > 1.4 x 10(8) M(-1).s(-1) at 4 degrees C). Subsequent release of the axial histidine heme ligand is shown to be the molecular step responsible for activation of the enzyme. The rate at which this step proceeds also depends on NO concentration (k = 2.4 x 10(5) M(-1).s(-1) at 4 degrees C), thus identifying a novel mode of regulation by NO. NO binding to the isolated heme domain of sGC was also rapid (k = 7.1 +/- 2 x 10(8) M(-1).s(-1) at 4 degrees C); however, no intermediate was observed. The data show that sGC acts as an extremely fast, specific, and highly efficient trap for NO and that cleavage of the iron-histidine bond provides the driving force for activation of sGC. In addition, the kinetic data indicate that transport or stabilization of NO is not necessary for effective signal transmission.

Spectroscopic characterization of the heme-binding sites in Plasmodium falciparum histidine-rich protein 2.

Proteolysis of hemoglobin provides an essential nutrient source for the malaria parasite Plasmodium falciparum during the intraerythrocytic stage of the parasite's lifecycle. Detoxification of the liberated heme occurs through a unique heme polymerization pathway, leading to the formation of hemozoin. Heme polymerization has been demonstrated in the presence of P. falciparum histidine-rich protein 2 (PfHRP2) [Sullivan, D. J., Gluzman, I. Y., and Goldberg, D. E. (1996) Science 271, 219-221]; however, the molecular role that PfHRP2 plays in this polymerization is currently unknown. PfHRP2 is a 30 kDa protein composed of several His-His-Ala-His-His-Ala-Ala-Asp repeats and is present in the parasite food vacuole, the site of hemoglobin degradation and heme polymerization. We found that, at pH 7.0, PfHRP2 forms a saturable complex with heme, with a PfHRP2 to heme stoichiometry of 1:50. Spectroscopic characterization of heme binding by electronic absorption, resonance Raman, and EPR has shown that bound hemes share remarkably similar heme environments as >95% of all bound hemes are six-coordinate, low-spin, and bis-histidyl ligated. The PfHRP2-ferric heme complex at pH 5.5 (pH of the food vacuole) has the same heme spin state and coordination as observed at pH 7.0; however, polymerization occurs as heme saturation is approached. Therefore, formation of a PfHRP2-heme complex appears to be a requisite step in the formation of hemozoin.

Conformationally-restricted arginine analogues as alternative substrates and inhibitors of nitric oxide synthases.

Conformationally restricted arginine analogues (1-5) were synthesized and found to be alternative substrates or inhibitors of the three isozymes of nitric oxide synthase (NOS). A comparison of k(cat)/Km values shows that (E)-3,4-didehydro-D,L-arginine (1) is a much better substrate than the corresponding (Z)-isomer (2) and 3-guanidino-D,L-phenylglycine (3), although none is as good a substrate as is arginine; 5-keto-D,L-arginine (4) is not a substrate, but is an inhibitor of the three isozymes. Therefore, it appears that arginine binds to all of the NOS isozymes in an extended (E-like) conformation. None of the compounds exhibits time-dependent inhibition of NOS, but they are competitive reversible inhibitors. Based on the earlier report that N(omega)-propyl-L-arginine is a highly selective nNOS inhibitor (Zhang, H. Q.; Fast, W.; Marletta, M.; Martasek, P.; Silverman, R. B. J. Med. Chem. 1997, 40, 3869), (E)-N(omega)-propyl-3,4-didehydro-D,L-arginine (5) was synthesized, but it was shown to be weakly potent and only a mildly selective inhibitor of NOS. Imposing conformational rigidity on an arginine backbone does not appear to be a favorable approach for selective NOS inhibition.

Cellular applications of a sensitive and selective fiber-optic nitric oxide biosensor based on a dye-labeled heme domain of soluble guanylate cyclase.

Nitric oxide-selective sensors have been prepared with the heme domain of soluble guanylate cyclase (sGC), the only known receptor for signal transduction involving nitric oxide. Expressed in and purified from E. coli, the heme domain contains a stoichiometric amount of heme that has electronic and resonance Raman spectra almost identical to those of heterodimeric (native) sGC purified from bovine lung. The small size of the heme domain, its inability to bind oxygen, and its high affinity for nitric oxide make it well-suited for sensor applications. The heme domain has been labeled with a fluorescent reporter dye and changes in this dye's intensity are observed based on the sGC heme domain's characteristic binding of nitric oxide. The current sensors are prepared with 100-microns optical fiber but could also be prepared using submicrometer fiber tips. These sensors have fast, linear, and reversible responses to nitric oxide and are unaffected by numerous common interferents, such as oxygen, nitrite and nitrate. The sensor limit of detection is 1 microM nitric oxide. Glutathione has been shown to decrease the sensitivity of the sensor; however, the sensor response remains linear and can be calibrated on the basis of the glutathione concentration present in the biological environment of interest. The sensors have been used to measure extracellular nitric oxide production by BALB/c mouse macrophages. Minimal nitric oxide was produced by untreated cells, while high levels of nitric oxide were released from activated cells, e.g., 111 +/- 2 microM in a given cell culture.

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.

A new decoration for nitric oxide synthase - a Zn(Cys)4 site.

Intense interest in the action and synthesis of nitric oxide has fueled structural studies of nitric oxide synthase (NOS). The monomeric and dimeric heme domains of inducible NOS were the first NOS structures to be described. A recent independent analysis of the corresponding heme domains from endothelial NOS confirms most of the features found earlier and also reveals a novel Zn(Cys)4 center - a new feature for NOS.

Formation of a pterin radical in the reaction of the heme domain of inducible nitric oxide synthase with oxygen.

The heme domain (iNOS(heme)) of inducible nitric oxide synthase (NOS) was expressed in Escherichia coli and purified to homogeneity. Rapid freeze-quench (RFQ) EPR was used to monitor the reaction of the reduced iNOS(heme) with oxygen in the presence and absence of substrate. In these reactions, heme oxidation occurs at a rate of approximately 15 s(-)(1) at 4 degrees C. A transient species with a g = 2.0 EPR signal is also observed under these conditions. The spectral properties of the g = 2.0 signal are those of an anisotropic organic radical with S = (1)/(2). Comparison of the EPR spectra obtained when iNOS(heme) is reconstituted with N5-(14)N- and (15)N-substituted tetrahydrobiopterin (H(4)B) shows a hyperfine interaction with the pterin N5 nitrogen and identifies the radical as the one-electron oxidized form (H(3)B.) of the bound H(4)B. Substitution of D(2)O for H(2)O reveals the presence of hyperfine-coupled exchangeable protons in the H(4)B radical. This radical forms at a rate of 15-20 s(-)(1), with a slower decay rate that varies (0.12-0.7 s(-)(1)) depending on the substrate. At 127 ms, H(3)B. accumulates to a maximum of 80% of the total iNOS(heme) concentration in the presence of arginine but only to approximately 2.8% in the presence of NHA. Double-mixing RFQ experiments, where NHA is added after the formation of H(3)B., show that NHA does not react rapidly with H(3)B. and suggest that NHA instead prevents the formation of the H(4)B radical. These data constitute the first direct evidence for an NOS-bound H(3)B. and are most consistent with a role for H(4)B in electron transfer in the NOS reaction.

Regeneration of the ferrous heme of soluble guanylate cyclase from the nitric oxide complex: acceleration by thiols and oxyhemoglobin.

Soluble guanylate cyclase (sGC) catalyzes the conversion of GTP to cGMP and is activated several hundred-fold by binding of nitric oxide (*NO) to the heme prosthetic group. We have examined the stability of the nitrosyl-heme complex of sGC (*NO-sGC) at 37 degreesC in order to determine whether simple dissociation of *NO from sGC could account for the observed in vivo deactivation time. Recombinant sGC was purified from Sf9 cells coinfected with baculoviruses containing the cDNAs for the alpha1 and beta1 subunits of rat lung sGC. The purified protein contained a stoichiometric equivalent of ferrous high-spin heme. Characterization of the purified protein found it to be essentially identical to that purified from bovine lung. Ferrous-nitrosyl sGC prepared anaerobically and exchanged into aerobic buffer containing no reducing agents was essentially stable on ice and had a half-life of approximately 90 min at 37 degreesC. In the presence of thiols [DTT, glutathione (GSH), or L-cysteine], *NO was rapidly lost from sGC regenerating the ferrous high-spin form of the heme. The half-life of *NO-sGC in the presence of 1 mM GSH at 37 degreesC was 6.3 min. In the presence of oxyhemoglobin, the half-life was further reduced to 2.9 min. Although these rates are not fast enough to account for that observed in vivo, and thus probably involve additional agent(s), these data do imply a role for low molecular weight thiols, such as GSH, and oxyferrohemoproteins, such as oxymyoglobin, in the deactivation of sGC.

Resonance raman characterization of the heme domain of soluble guanylate cyclase.

We report the resonance Raman characterization of the heme domain of rat lung soluble guanylate cyclase (sGC) expressed in Escherichia coli. Like heterodimeric sGC isolated from bovine lung, the sGC heme domain [beta1(1-385)] and its heme ligand mutant H105G(Im) contain a stoichiometric amount of heme, which is five-coordinate, high-spin ferrous in both beta1(1-385) and chemically reduced H105G(Im). In the presence of NO, both beta1(1-385) and H105G(Im) form a five-coordinate nitrosyl heme complex with a nu(Fe-NO) value of 525 cm-1 and a nu(NO) value of 1676 cm-1. For the first time, the Fe-N-O bending mode near 400 cm-1 has been identified in a five-coordinate nitrosyl heme complex. Both beta1(1-385) and H105G(Im) form a six-coordinate, low-spin complex with CO. We find evidence for two binding conformations of the Fe-CO unit. The conformation that is more prevalent in beta1(1-385) has a nu(Fe-CO) value of 478 cm-1 and a delta(Fe-C-O) value of 567 cm-1, whereas the dominant conformation in H105G(Im) is characterized by a nu(Fe-CO) value of 495 cm-1 and a delta(Fe-C-O) value of 572 cm-1. We propose that in the dominant conformation of H105G(Im)-CO the Fe-CO unit is hydrogen bonded to a distal residue, while this is not the case in beta1(1-385). Reexamination of sGC isolated from bovine lung tissue indicates that it also has two binding conformations for CO; the more populated form is not hydrogen-bonded. We propose that the absence of hydrogen-bond formation between a distal residue and exogenous ligands is physiologically relevant in lowering the oxygen affinity of heterodimeric sGC and, therefore, stabilizing the ferrous, active form of the enzyme under aerobic conditions.

Catalysis by nitric oxide synthase.

The enzyme nitric oxide synthase catalyzes the oxidation of the amino acid L-arginine to L-citrulline and nitric oxide in an NADPH-dependent reaction. Nitric oxide plays a critical role in signal transduction pathways in the cardiovascular and nervous systems and is a key component of the cytostatic/cytotoxic function of the immune system. Characterization of nitric oxide synthase substrates and cofactors has outlined the broad details of the overall reaction and suggested possibilities for chemical steps in the reaction; however, the molecular details of the reaction mechanism are still poorly understood. Recent evidence suggests a role for the reduced bound pterin in the first step of the reaction--the hydroxylation of L-arginine.

Reactions catalyzed by tetrahydrobiopterin-free nitric oxide synthase.

Murine macrophage nitric oxide synthase (NOS) was expressed in E. coli and purified in the presence (holoNOS) or absence (H4B-free NOS) of (6R)-tetrahydro-L-biopterin (H4B). Isolation of active enzyme required the coexpression of calmodulin. Recombinant holoNOS displayed similar spectral characteristics and activity as the enzyme isolated from murine macrophages. H4B-free NOS exhibited a Soret band at approximately 420 nm and, by analytical gel filtration, consisted of a mixture of monomers and dimers. H4B-free NOS catalyzed the oxidation of NG-hydroxy-L-arginine (NHA) with either hydrogen peroxide (H2O2) or NADPH and O2 as substrates. No product formation from arginine was observed under either condition. The amino acid products of NHA oxidation in both the H2O2 and NADPH/O2 reactions were determined to be citrulline and Ndelta-cyanoornithine (CN-orn). Nitrite and nitrate were also formed. Chemiluminescent analysis did not detect the formation of nitric oxide (*NO) in the NADPH/O2 reaction. The initial inorganic product of the NADPH/O2 reaction is proposed to be the nitroxyl anion (NO-) based on the formation of a ferrous nitrosyl complex using the heme domain of soluble guanylate cyclase as a trap, and the formation of a ferrous nitrosyl complex of H4B-free NOS during turnover of NHA and NADPH. NO- is unstable and, under the conditions of the reaction, is oxidized to nitrite and nitrate. At 25 degreesC, the H2O2-supported reaction had a specific activity of 120 +/- 14 nmol min-1 mg-1 and the NADPH-supported reaction had a specific activity of 31 +/- 6 nmol min-1 mg-1 with a KM,app for NHA of 129 +/- 9 microM. HoloNOS catalyzed the H2O2-supported reaction with a specific activity of 815 +/- 30 nmol min-1 mg-1 and the NADPH-dependent reaction to produce *NO and citrulline at 171 +/- 20 nmol min-1 mg-1 with a KM, app for NHA in the NADPH reaction of 36.9 +/- 0.3 microM.

Structural changes in the heme proximal pocket induced by nitric oxide binding to soluble guanylate cyclase.

When expressed in Escherichia coli, the heme domain [beta1(1-385)] of rat lung soluble guanylate cyclase (sGC) is isolated with a stoichiometric amount of bound heme [Zhao, Y., and Marletta, M. A. (1997) Biochemistry 36, 15959-15964]. Nitric oxide (NO) binding to the heme in beta1(1-385) leads to cleavage of the Fe-His bond and formation of a five-coordinate NO-heme complex. Addition of imidazole to the five-coordinate NO complex shifts the Soret peak from 399 to 420 nm, which appears to result from the formation of a six-coordinate NO complex. Removal of the added imidazole by gel filtration results in formation of the five-coordinate NO complex once again. The EPR spectrum of the putative six-coordinate NO complex has nine distinct derivative-shaped lines (a triplet of triplets), which is the signature spectrum of a six-coordinate NO complex with two nitrogen atoms as the axial ligands. [15N]Imidazole simplifies the six-coordinate NO complex EPR spectrum to six distinct derivative-shaped lines (a triplet of doublets), indicating that the other axial ligand in the six-coordinate NO complex is an imidazole molecule. These results show that NO binding to sGC not only leads to the cleavage of the Fe-His bond but also induces a conformational change which opens the heme proximal pocket large enough to accommodate an exogenous imidazole molecule. These observations have important implications for determining the NO activation mechanism of sGC.