大鼠慢性缺氧高二氧化碳肺动脉高压模型可溶性鸟苷酸环化酶的表达

目的　观察大鼠慢性缺氧高二氧化碳 (CO2 )肺动脉高压模型肺动脉可溶性鸟苷酸环化酶 (sGC)的表达及其活性。方法　SD大鼠 2 0只 ,随机分为 2组 ,每组 10只。A组 :慢性缺氧高CO2肺动脉高压模型组 ,大鼠置于常压低氧高CO2 饲养舱 ,舱内氧浓度维持在 ( 10 0± 0 5 ) % ,CO2 浓度为 6 %～ 7% ,每天 8h。B组 :健康对照组 ,大鼠室温下常规饲养。免疫组化观察 2组大鼠肺中小动脉sGCα1、β1亚基蛋白的表达 ,原位杂交观察肺中小动脉sGCα1亚基mRNA的表达。酶动力学分析肺组织sGC酶活性。结果　A组大鼠平均肺动脉压、右心室 / (左心室 +室间隔 )比值、右心室 /体重比值明显高于B组 (P值均 <0 0 1) ,A组肺中小动脉sGCα1、β1亚基蛋白及α1亚基mRNA表达与B组相比逐渐减弱 (P值均 <0 0 1) ,硝普钠激活的sGC酶活性及肺组织基础sGC酶活性A组明显低于B组。结论　慢性缺氧高CO2 肺动脉高压时肺中小动脉sGCmRNA、蛋白表达以及酶活性均受抑制     

Dehydroepiandrosterone upregulates soluble guanylate cyclase and inhibits hypoxic pulmonary hypertension

OBJECTIVE: It has been reported that dehydroepiandrosterone is a pulmonary vasodilator and inhibits chronic hypoxia-induced pulmonary hypertension. Additionally, dehydroepiandrosterone has been shown to improve systemic vascular endothelial function. Thus, we hypothesized that chronic treatment with dehydroepiandrosterone would attenuate hypoxic pulmonary hypertension by enhancing pulmonary artery endothelial function. METHODS AND RESULTS: Rats were randomly assigned to five groups. Three groups received food containing 0, 0.3, or 1% dehydroepiandrosterone during a 3-wk-exposure to simulated high altitude (HA). The other 2 groups were kept at Denver's low altitude (LA) and received food containing 0 or 1% dehydroepiandrosterone. Dehydroepiandrosterone dose-dependently inhibited hypoxic pulmonary hypertension (mean pulmonary artery pressures after treatment with 0, 0.3, and 1% dehydroepiandrosterone=45+/-5, 33+/-2*, and 25+/-1*# mmHg, respectively. *P<0.05 vs. 0% and # vs. 0.3%). Dehydroepiandrosterone (1%, 3 wks) treatment started after rats had been exposed to 3-wk hypoxia also effectively reversed established hypoxic pulmonary hypertension. Pulmonary artery rings isolated from both LA and HA rats treated with 1% dehydroepiandrosterone showed enhanced relaxations to acetylcholine and sodium nitroprusside, but not to 8-bromo-cGMP. In the pulmonary artery tissue from dehydroepiandrosterone-treated LA and HA rats, soluble guanylate cyclase, but not endothelial nitric oxide synthase, protein levels were increased. CONCLUSION: These results indicate that the protective effect of dehydroepiandrosterone against hypoxic pulmonary hypertension may involve upregulation of pulmonary artery soluble guanylate cyclase protein expression and augmented pulmonary artery vasodilator responsiveness to nitric oxide.     

可溶型鸟苷酸环化酶(sGC)的激活能够逆转实验性肺动脉高压

背景:严重的肺动脉高压是一种有着很高的死亡率的致残性疾病,其特征是肺部血管重塑以及右心室肥大。利用长期缺氧而导致肺动脉高压的内皮型一氧化氮合酶基因敲除的纯合子(NOS3(-/-))小鼠和野生型小鼠,以及野百合碱性肺动脉高压的大鼠,我们检测了可溶型鸟苷酸环化酶(sGC)的刺激因子。Bay41- 2272或其激活剂Bay58-2667是否会逆转肺部血管的重塑。方法与结果:在肺体外灌注系统中,Bay41-2272和Bay58-2667均能剂量依赖性地抑制急性缺氧导致的血压增高反应。野生型(NOS3( / ))和NOS3基因敲除(NOS3(-/-))小鼠在氧气含量低于10％的环境中饲养21天或35天,外周肺动脉完全肌肉化程度的增高证明:两组小鼠均发生肺动脉高压、右心室肥大以及肺部血管重塑。从第21天到35天,分别用sGC的激活剂Bay58-2667(10 mg/kg/d)或刺激因子Bav41- 2272(10 mg/kg/d)处理野生型小鼠,能够明显减轻肺动脉高压、右心室肥大以及肺动脉的结构重塑。相反的,在NOS3基因敲除小鼠(NOS3(-/-))中长期使用sGC激活剂进行治疗却收效甚微。在注射野百合碱而导致严重肺动脉高压的大鼠中,这两种化合物均能明显逆转其血流动力学和结构上的的改变。结论:sGC的激活能够逆转野百合碱和长期缺氧性肺动脉高压所引起的血流动力学和结构的改变。而这种效果部分的依赖于NOS3产生的内源性一氧化氮。     

NO- activates soluble guanylate cyclase and Kv channels to vasodilate resistance arteries

Nitric oxide (NO) plays an important role in the control of vascular tone. Traditionally, its vasorelaxant activity has been attributed to the free radical form of NO (NO*), yet the reduced form of NO (NO-) is also produced endogenously and is a potent vasodilator of large conduit arteries. The effects of NO- in the resistance vasculature remain unknown. This study examines the activity of NO- in rat small isolated mesenteric resistance-like arteries and characterizes its mechanism(s) of action. With the use of standard myographic techniques, the vasorelaxant properties of NO* (NO gas solution), NO- (Angeli's salt), and the NO donor sodium nitroprusside were compared. Relaxation responses to Angeli's salt (pEC50=7.51+/-0.13, Rmax=95.5+/-1.5%) were unchanged in the presence of carboxy-PTIO (NO* scavenger) but those to NO* and sodium nitroprusside were inhibited. l-Cysteine (NO- scavenger) decreased the sensitivity to Angeli's salt (P<0.01) and sodium nitroprusside (P<0.01) but not to NO*. The soluble guanylate cyclase inhibitor ODQ (3 and 10 micromol/L) concentration-dependently inhibited relaxation responses to Angeli's salt (41.0+/-6.0% versus control 93.4+/-1.9% at 10 micromol/L). The voltage-dependent K+ channel inhibitor 4-aminopyridine (1 mmol/L) caused a 9-fold (P<0.01) decrease in sensitivity to Angeli's salt, whereas glibenclamide, iberiotoxin, charybdotoxin, and apamin were without effect. In combination, ODQ and 4-aminopyridine abolished the response to Angeli's salt. In conclusion, NO- functions as a potent vasodilator of resistance arteries, mediating its response independently of NO* and through the activation of soluble guanylate cyclase and voltage-dependent K+ channels. NO- donors may represent a novel class of nitrovasodilator relevant for the treatment of cardiovascular disorders such as angina.     

Stimulation of soluble guanylate cyclase by an acetylcholine-induced endothelium-derived factor from rabbit and canine arteries

The present study was designed to investigate the hypothesis that, during acetylcholine-induced endothelium-dependent relaxation, a factor(s) is released from endothelial cells which directly activates soluble guanylate cyclase. We attempted to determine what similarities or differences existed between this factor and endothelium-derived relaxing factor. The study was performed on segments of rabbit aorta and canine femoral artery. Purified soluble guanylate cyclase was injected into the lumen of these vascular segments, together with its substrate, for intraluminal incubation of the enzyme. In endothelium-intact vascular segments, the activity of guanylate cyclase was enhanced over control values obtained by incubation in test tubes. The stimulation was further increased by acetylcholine in concentrations which caused relaxation of the vascular segments. The stimulating principle could not be transferred from the vessel lumen to an external solution of guanylate cyclase, indicating a short life-time. Removal of the endothelium prevented formation and release of the guanylate cyclase stimulating factor(s). Atropine, mepacrine, or nordihydroguaiaretic acid, which inhibit acetylcholine-induced endothelium-dependent relaxations, also inhibited acetylcholine-induced endothelium-mediated activation of guanylate cyclase. The results support the hypothesis that acetylcholine-induced endothelium-derived relaxing factor increases cyclic guanosine monophosphate levels of vascular smooth muscle by a stimulation of soluble guanylate cyclase.     

Doxazosin and soluble guanylate cyclase in a rat model of hypertension]

BACKGROUND: Although different studies have evaluated the ability of endothelial cells to produce NO in the setting of the endothelial dysfunction associated with hypertension, less it is known about the soluble guanylate cyclase system. AIM: To analyze the level of expression of sGC in the vascular wall in Stroke-prone spontaneously hypertensive rats (SHRSP). Moreover, the effect of treatment with an alpha1 adrenergic antagonist, doxazosin, on sGC expression was also evaluated. METHODS: The study was performed in 24 untreated 20-week-old SPSHR and 12 SPSHR treated orally with doxazosin (10 mg/Kg bw/day; for 15 days). A group of normotensive Wistar-Kyoto (WKY) rats were used as controls (n = 12). RESULTS: Isolated aortic segments from SHRSP showed impaired response to SNP. Doxazosin treatment prevented impaired vasodilatory response to SNP. Expression of the beta1 sGC in the vascular wall of SHRSP determined by Western blot and immunohistochemistry was markedly reduced with respect to that of WKY. Doxazosin treatment increased of beta1 sGC expression in treated SHRSP particularly at the medium level with respect to that of untreated SHRSP. CONCLUSION: SHRSP showed reduced expression of beta1 sGC in the vascular wall and an impaired vasodilator response to SNP which improved with doxazosin treatment. These results suggest the role the sGC system may play in the global treatment of endothelial dysfunction.     

Estrogens and progesterone increase fetal and maternal guanylate cyclase activity

Since both estrogens and cyclic guanosine 3',5'-monophosphate stimulate protein synthesis, the objective of the present investigation was to determine if estrogens and their precursors might have part of their mechanism of action through stimulation of guanylate cyclase (E.C.4.6.1.2), the enzyme that catalyzes the conversion of guanosine triphosphate to cyclic guanosine 3',5'-monophosphate. The precursors of estrogen synthesis originate from cholesterol. Cholesterol itself had no effect on guanylate cyclase activity. The precursors of estrogen synthesis generated from cholesterol, namely, progesterone, 17 alpha-OH-progesterone, androstenedione, pregnenolone, 17 alpha-OH-pregnenolone, and dehydroepinandrosterone, however, caused a 2- to 3-fold enhancement of fetal and maternal guinea pig hepatic and uterine guaynlate cyclase activity at a concentration of 1 microM. In comparative studies, similar effects were seen on immature female Sprague-Dawley rat hepatic and uterine guanylate cyclase activity. Estrone, estradiol-17 beta, estriol, and the synthetic estrogen, diethylstilbestrol, enhanced guanylate cyclase activity in the same tissues 2- to 3- fold at the 1 microM concentration. Dose-response relationships revealed that these estrogens and their precursors had their maximal effect at 0.001 microM. Estradiol-17 alpha also enhanced uterine guanylate cyclase activity, but a 1000-fold greater concentration compared to the other estrogens was necessary to show any significant effect. The data in this investigation suggest that guanylate cyclase may play a role in the mechanism of action of estrogens and their precursors.     

Purification of soluble guanylate cyclase from rat liver

Soluble guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] has been purified from rat liver and exhibited a single protein band on polyacrylamide gels coincident with activity and indicative of a molecular weight of 150,000. The apparent specific activity of the purified enzyme was 276 nmol of cyclic GMP formed per mg per min with Mn²⁺ as the cation cofactor and 23.8 nmol of cyclic GMP formed per mg per min with Mg²⁺. This represented 9200-fold and 7400-fold purifications of Mn²⁺ and Mg²⁺ activities, respectively. The specific activity of soluble guanylate cyclase was not constant with protein concentration. At all stages of purification, increasing the enzyme concentration in the guanylate cyclase assay increased the apparent specific activity of the preparation. The purified enzyme could be activated by nitroprusside, nitric oxide, arachidonate, linoleate, oleate, and superoxide dismutase. However, the degree of activation was dependent upon the concentration of enzyme protein assayed.     

Increased particulate and decreased soluble guanylate cyclase activity in regenerating liver, fetal liver, and hepatoma.

We determined the activities of soluble and particulate guanylate cyclase [GTP pyrophosphatelyase (cyclizing); ?EC 4.6.1.2] IN REGENERATING RAT LIVER, FETAL AND NEONATAL RAT LIVER, AND HEPATOMA. TIn these tissues we found increased particulate and decreased soluble enzyme activities compared to normal adult rat liver. The particulate activity increased 12 hr after partial hepatectomy, reached maximal activity at 48 hr, and then declined. The soluble enzyme activity decreased within 8 hr and continued to decline. The activity of homogenates did not change. Guanylate cyclase activity was increased in plasma membrane and microsome fractions from regenerating liver. The increase in particulate activity was prevented with cycloheximide. Decreased soluble and increased particulate enzyme activities were found in fetal liver. After birth the soluble activity increased and the particulate activity decreased. Seven to 14 days after birth the activities of soluble and particulate fractions were similar to those of adult rat liver. In hepatoma 3924A, the activity of particulate guanylate cyclase was 9-fold greater and that of the soluble enzyme was 50% that of normal liver. These studies suggest that guanylate cyclase activity and its subcellular distribution may be related to liver growth through some unknown mechanism.     

Antiinflammatory activity of soluble guanylate cyclase: cGMP-dependent down-regulation of P-selectin expression and leukocyte recruitment

Nitric oxide (NO) production by the vascular endothelium maintains an essential antiinflammatory, cytoprotective influence on the blood vessel wall. A key component of this activity is attributed to prevention of leukocyte-endothelial cell interactions, yet the underlying mechanisms remain unclear. The NO receptor, soluble guanylate cyclase (sGC), is expressed in endothelial cells but fulfils an unknown function. Therefore, we used intravital microscopy in mesenteric postcapillary venules from WT and endothelial nitric oxide synthase (eNOS) knockout (eNOS(-/-)) mice, and an sGC activator (BAY 41-2272), to investigate a potential role for sGC in the regulation of adhesion molecule expression and leukocyte recruitment. Leukocyte rolling and adhesion was 6-fold greater in eNOS(-/-) than WT animals. BAY 41-2272 and the NO-donor, diethylamine-NONOate, reduced leukocyte rolling and adhesion in eNOS(-/-) mice to levels observed in WT animals. These effects were blocked by the sGC inhibitor ODQ [1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one], which itself caused a 6-fold increase in leukocyte rolling and adhesion in WT mice. Increased leukocyte rolling and adhesion in IL-1beta-treated mice was also inhibited by BAY 41-2272. Fluorescence-activated cell sorting analysis in vitro and a specific P-selectin neutralizing antibody in vivo revealed that selective down-regulation of P-selectin expression accounted for the antiadhesive effects of sGC activation. These data demonstrate that sGC plays a key antiinflammatory role by inhibiting P-selectin expression and leukocyte recruitment.     

Activity and expression of soluble and particulate guanylate cyclases in myometrium from nonpregnant and pregnant women: down-regulation of soluble guanylate cyclase at term.

The role of cGMP in the regulation of human myometrial smooth muscle contractility is at present unclear. cGMP can be synthesized by a cytoplasmic, soluble guanylate cyclase (sGC), which is stimulated by nitric oxide and carbon monoxide, and by particulate membrane-bound GC, which are activated by natriuretic peptides. The aim of this study was to determine whether sGC or pGC are present in nonpregnant and pregnant human myometrium, and whether the activity and expression of these enzymes and the cGMP content change during pregnancy and with labor. Myometrium was obtained from nonpregnant women (n = 12) and pregnant women who were preterm (25-34 wk gestation; n = 12), term (>38 wk) not in labor (n = 14), or term in active labor (n = 12). The cGMP content in myometrium obtained from preterm deliveries was significantly higher than that in tissue obtained from nonpregnant women and decreased at term, especially in laboring groups. Protein and mRNA for sGC, particulate GC-A, GC-B, and the clearance receptor were detected in human myometrium. cGMP in pregnant human myometrium, however, appears to be produced predominantly by sGC and possibly by GC-B, as GC-A was only weakly expressed. sGC activity was greater in myometrium from preterm (nonlabor) deliveries compared those taken at term (in labor), but was down-regulated compared with activity in nonpregnant myometrium. Neither atrial natriuretic peptide nor C-type natriuretic peptide (agonists for GC-A and GC-B, respectively) altered contractility in vitro of myometrium from women at term (not in labor). We conclude that the cGMP/guanylate cyclase system in human myometrium is gestationally regulated and potentially plays an important role in mediating quiescence during early pregnancy. A reduction in cGMP availability may contribute to the switch to contractile activity at term.     

Alterations in cGMP, soluble guanylate cyclase, phosphodiesterase 5, and B-type natriuretic peptide induced by chronic increased pulmonary blood flow in lambs

OBJECTIVE: The objective of the study was to determine alterations in cGMP, soluble guanylate cyclase (sGC), phosphodiesterase type 5 (PDE5), and B-type natriuretic peptide (BNP), in an animal model of a congenital cardiac defect with increased pulmonary blood flow. DESIGN: Prospective, comparative, experimental study. SUBJECTS: Lambs, from birth until 8 weeks of age. METHODOLOGY: Late gestation fetal lambs underwent in utero placement of an 8 mm aortopulmonary vascular graft (shunt). In shunted and normal age-matched control lambs, at 2, 4, and 8 weeks of age, cGMP and BNP levels were measured, and sGC subunit and PDE5 protein expression were determined by Western blot analysis and immunohistochemistry. RESULTS: In shunted lambs, tissue and plasma cGMP levels were greater than normal throughout the 8-week study period (P < 0.05). sGCalpha protein was greater at 2 and 4 weeks (P < 0.05), and sGCbeta and PDE5 protein were greater at 4 weeks in shunted lambs (P < 0.05). Plasma BNP levels did not change in normal lambs but increased in shunted lambs by 8 weeks of age (P < 0.05). BNP levels were greater in shunted lambs than normal at 4 and 8 weeks (P < 0.05). CONCLUSIONS: Alterations in sGC subunit protein expression during the first post-natal month, and increased BNP levels during the second post-natal month contribute to elevations in plasma and lung tissue cGMP in lambs with increased pulmonary blood flow.     

Targeting heme-oxidized soluble guanylate cyclase in experimental heart failure

Soluble guanylate cyclase is a heterodimeric enzyme with a prosthetic heme group that, on binding of its main ligand, NO, generates the second messenger cGMP. Unlike conventional nitrovasodilators, the novel direct NO- and heme-independent soluble guanylate cyclase activator BAY 58-2667 is devoid of non-cGMP actions, lacks tolerance development, and preferentially activates NO-insensitive heme-free or oxidized soluble guanylate cyclase. BAY 58-2667, therefore, represents a novel therapeutic advance in mediating vasodilation. To date, its cardiorenal actions in congestive heart failure (CHF) are undefined. We, therefore, hypothesized that BAY 58-2667 would have beneficial preload- and afterload-reducing actions in experimental severe CHF together with renal vasodilating properties. We assessed the cardiorenal actions of intravenous administration of 2 doses of BAY 58-2667 (0.1 and 0.3 microg/kg per minute, respectively) in a model of tachypacing-induced severe CHF. In CHF, BAY 58-2667 dose-dependently reduced mean arterial, right atrial, pulmonary artery, and pulmonary capillary wedge pressure (from baseline 19+/-1 to 12+/-2 mm Hg). Cardiac output (2.4+/-0.3 to 3.2+/-0.4 L/min) and renal blood flow increased. Glomerular filtration rate and sodium and water excretion were maintained. Consistent with cardiac unloading, atrial and B-type natriuretic peptide decreased. Plasma renin activity (P=0.31) and aldosterone remained unchanged (P=0.19). In summary, BAY 58-2667 in experimental CHF potently unloaded the heart, increased cardiac output and renal blood flow, and preserved glomerular filtration rate and sodium and water excretion without further neurohumoral activation. These beneficial properties make direct soluble guanylate cyclase stimulation with BAY 58-2667 a promising new therapeutic strategy for cardiovascular diseases, such as heart failure.     

Adenylate cyclase and guanylate cyclase activity in normal and leukemic human lymphocytes

Adenylate cyclase (AC) and guanylate cyclase (GC) activities were studied in normal B-enriched and T-enriched lymphocytes, in lymphocytes of children with acute lymphocytic leukemia (ALL), and in lymphocytes of adults with chronic lymphocytic leukemia (CLL). AC activity was greater in normal B than T lymphocytes (215 pmole/min/mg protein versus 80 pmole in the membrane-enriched fraction) and i both increased greatly after stimulation with isoproterenol and more so with prostaglandins E and F2 alpha. In leukemic lymphocytes, AC showed depressed activity (20 pmole in ALL cells and 55 pmole in CLL cells) and was less sensitive to hormonal stimulation: this loss of sensitivity occurred to a greater extent in ALL than in CLL lymphocytes. GC activity was greater in normal T than B cells (in membrane-enriched fraction: 10.2 pmole versus 5.3 pmole). It increased little with isoproterenol and prostaglandins stimulation, and much more with sodium azide and dehydroascorbic acid stimulation. GC activity was increased in both types of leukemic lymphocytes (23 pmole for ALL cells and 18 pmole for CLL cells) and was insensitive to stimulation. Possible derangement of cyclase and cyclic nucleotide regulation in leukemic cells is suggested.     

Motion of proximal histidine and structural allosteric transition in soluble guanylate cyclase

We investigated the changes of heme coordination in purified soluble guanylate cyclase (sGC) by time-resolved spectroscopy in a time range encompassing 11 orders of magnitude (from 1 ps to 0.2 s). After dissociation, NO either recombines geminately to the 4-coordinate (4c) heme (τ_G1 = 7.5 ps; 97 ± 1% of the population) or exits the heme pocket (3 ± 1%). The proximal His rebinds to the 4c heme with a 70-ps time constant. Then, NO is distributed in two approximately equal populations (1.5%). One geminately rebinds to the 5c heme (τ_G2 = 6.5 ns), whereas the other diffuses out to the solution, from where it rebinds bimolecularly (τ = 50 μs with [NO] = 200 μM) forming a 6c heme with a diffusion-limited rate constant of 2 × 10⁸ M⁻¹⋅s⁻¹. In both cases, the rebinding of NO induces the cleavage of the Fe-His bond that can be observed as an individual reaction step. Saliently, the time constant of bond cleavage differs depending on whether NO binds geminately or from solution (τ_5C1 = 0.66 μs and τ_5C2 = 10 ms, respectively). Because the same event occurs with rates separated by four orders of magnitude, this measurement implies that sGC is in different structural states in both cases, having different strain exerted on the Fe-His bond. We show here that this structural allosteric transition takes place in the range 1–50 μs. In this context, the detection of NO binding to the proximal side of sGC heme is discussed.The soluble guanylate cyclase (sGC), localized in many different cell types, is the receptor of the endogenous messenger nitric oxide (NO) and catalyzes the formation of cGMP from GTP upon activation triggered by NO binding (1, 2). The diatomic messenger NO and sGC play a critical role in several physiological processes: regulation of vascular blood pressure and cardiovascular diseases (3), lung airway relaxation and pulmonary pathologies (4), immune response and inflammatory disorders (5), and tumor progression and apoptosis (6). Thus, sGC is a pharmacological target of very high interest, and several activators have been developed (7, 8), leading to the approval of riociguat for the treatment of pulmonary hypertension (9, 10). Because of its pharmacological interest, the mechanisms of activation, deactivation, and regulation of sGC must be deciphered at the molecular level. Despite numerous efforts, the 3D crystal structure of heterodimeric sGC remains unknown, but the heme domain of the sGC β1-subunit [called heme NO/oxygen-binding (H-NOX)] was modeled from the heme domain of bacterial NO sensors (11, 12) and the sGC catalytic α1-subunit was modeled from the catalytic α1-subunit of adenylate cyclase (13). Recently, the entire quaternary structure of sGC was reconstructed by inserting individual protein domains into the density envelope of entire single-sGC molecules observed by EM (14), revealing a high flexibility of the sGC dimer. Subsequently, the structural perturbations induced by NO binding were mapped at the domain interfaces (15). In the past decade, a diversity of molecular models and regulatory models have been proposed (16–23), with some including structural hypotheses and involving or not involving the hypothetical NO binding to the proximal heme side (vs. distal NO binding).The heterodimeric sGC (∼150 kDa) comprises a regulatory domain in the β-subunit that contains the prosthetic heme group for NO binding, which activates the synthesis of cGMP from GTP, whereas the interface between the C termini of both α- and β-subunits harbors the GTP-binding catalytic site, remote from the heme domain (Fig. 1). The first internal molecular event correlated with sGC activation is the cleavage of the heme-proximal His covalent bond induced by NO binding (24). This event triggers the structural allosteric changes within the protein that induce the increase of catalytic activity. Because the sensing heme domain and the catalytic site are separated by a helical domain (15), there are necessary structural changes for “cross-talk” between both, mediating the allosteric regulation. The NO concentration dependence of NO binding (16) and the modulation of the sGC activity by nucleotides (21), together with the possibility of its activation by artificial compounds (7), illustrate the complexity of sGC regulation when interacting with its messenger NO.Open in a separate windowFig. 1.Sequence domain organization of sGC and focus on the structure of the heme-sensing domain. GC, guanylate cyclase; PAS, period circadian protein-aryl hydrocarbon receptor-single minded protein.One critical aspect for understanding this allosteric mechanism is the identification of structural transitions and intermediate species, which must be addressed by time-resolved techniques. We have previously observed that after NO dissociation from the heme (25, 26), NO recombines geminately to the reactive 4-coordinate (4c) heme with a very high probability (97% of dissociated NO with time constant τ = 7.5 ps). However, the dynamics of the 3% NO population that does not rebind geminately have not been identified so far. Here, we focused on the transitions occurring after NO has left the heme pocket of sGC. We investigated the dynamics of the heme coordination by time-resolved absorption spectroscopy in a time range encompassing 11 orders of magnitude, from 1 ps to 0.2 s. Methodologically, the photodissociation of the ligand NO from the heme with a fast laser pulse allows one to displace the equilibrium and to simulate the thermal release of NO. Then, because NO can either rebind immediately from within the heme pocket or migrate within the protein core and eventually to the solution, from where it may rebind at a longer time, we could monitor each of the intermediate states of the heme during the dynamics of NO from the solution to the heme by exploring 11 time decades. We report the heme iron coordination dynamics triggered by NO release and binding, and we have identified the subsequent transitions, including the time range of the allosteric transition between the activated and resting states of sGC.