Reversible inactivation of soluble liver guanylate cyclase by disulfides

Redox regulation of guanylate cyclase and protein kinase G in vascular responses to hypoxia

The production of cGMP by the soluble form of guanylate cyclase (sGC) in bovine pulmonary arteries (BPA) is controlled by cytosolic NADPH maintaining reduced thiol and heme sites on sGC needed for activation by NO, and the levels of Nox oxidase-derived superoxide and peroxide that influence pathways regulating sGC activity. Our recent studies in BPA suggest that the activities of peroxide metabolizing pathways in vascular smooth muscle potentially determine the balance between sGC stimulation by peroxide and a cGMP-independent activation of cGMP-dependent protein kinase (PKG) by a disulfide-mediated subunit dimerization. Cytosolic NADPH oxidation also appears to function in BPA through its influence on protein thiol redox control as an additional mechanism promoting vascular relaxation through PKG activation. These processes regulating PKG may participate in decreases in peroxide and increases in NADPH associated with contraction of BPA to hypoxia and in cytosolic NADPH oxidation potentially mediating bovine coronary artery relaxation to hypoxia.

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

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

Sulfur K-edge x-ray absorption spectroscopy: a spectroscopic tool to examine the redox state of S-containing metabolites in vivo

The sulfur K-edge x-ray absorption spectra for the amino acids cysteine and methionine and their corresponding oxidized forms cystine and methionine sulfoxide are presented. Distinct differences in the shape of the edge and the inflection point energy for cysteine and cystine are observed. For methionine sulfoxide the inflection point energy is 2.8 eV higher compared with methionine. Glutathione, the most abundant thiol in animal cells, also has been investigated. The x-ray absorption near-edge structure spectrum of reduced glutathione resembles that of cysteine, whereas the spectrum of oxidized glutathione resembles that of cystine. The characteristic differences between the thiol and disulfide spectra enable one to determine the redox status (thiol to disulfide ratio) in intact biological systems, such as unbroken cells, where glutathione and cyst(e)ine are the two major sulfur-containing components. The sulfur K-edge spectra for whole human blood, plasma, and erythrocytes are shown. The erythrocyte sulfur K-edge spectrum is similar to that of fully reduced glutathione. Simulation of the plasma spectrum indicated 32% thiol and 68% disulfide sulfur. The whole blood spectrum can be simulated by a combination of 46% disulfide and 54% thiol sulfur.

Reversible activation of soluble guanylate cyclase by oxidizing agents

Soluble guanylate cyclase of human platelets was stimulated by thiol oxidizing compounds like diamide and the reactive disulfide 4, 4'-dithiodipyridine. Activation followed a bell-shaped curve, revealing somewhat different optimum concentrations for each compound, although in both cases, higher concentrations were inhibitory. Diamide at a concentration of 100 microM transiently activated the enzyme. In the presence of moderate concentrations of diamide and 4,4'-dithiodipyridine, causing a two- to fourfold activation by themselves, the stimulatory activity of NO-releasing compounds like sodium nitroprusside was potentiated. In contrast, higher concentrations of thiol oxidizing compounds inhibited the NO-stimulated activation of soluble guanylate cyclase. Activation of guanylate cyclase was accompanied by a reduction in reduced glutathione and a concomitant formation of protein-bound glutathione (protein-SSG). Both compounds showed an activating potency as long as reduced glutathione remained, leading to inhibition of the enzyme just when all reduced glutathione was oxidized. Activation was reversible while reduced glutathione recovered and protein-SSG disappeared. We propose that diamide or reactive disulfides and other thiol oxidizing compounds inducing thiol-disulfide exchange activate soluble guanylate cyclase. In this respect partial oxidation is associated with enzyme activation, whereas massive oxidation results in loss of enzymatic activity. Physiologically, partial disulfide formation may amplify the signal toward NO as the endogenous activator of soluble guanylate cyclase.

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

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

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

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

Regulation of activity of purified guanylate cyclase from liver that is unresponsive to nitric oxide

Guanylate cyclase was purified from rat liver supernatant. Electrophoresis under denaturing conditions revealed one major peptide of Mr approx. 69 000. On the basis of the Stokes radius (4.7 nm) and S20,w (6.4S), the calculated Mr value of the native enzyme was 133 000, i.e. it is apparently a homodimer. Kinetics of inactivation by diamide (which was reversible with dithiothreitol) suggested that oxidation of a single class of thiol sites was involved. In the absence of other additions, cyclase activity assayed with Mn2+ was over 7 times that assayed with Mg2+; maximal effects were observed with approx. 5 mM of each (with 1 mM-GTP). The purified enzyme was markedly activated by nitrosylhaemoglobin. Relative activation was much greater in assays with Mg2+ than with Mn2+, although maximal activities were similar. When assayed with Mg2+, the enzyme exhibited a single Km (0.35 mM) for GTP; with Mn2+, plots of 1/v versus 1/[S] were non-linear. Activator or nitrosylhaemoglobin increased Vmax, but did not alter Km in the presence of either Mg2+ or Mn2+. The enzyme was inhibited by Na3VO4, Na2WO4 and Na2B4O7. Reduction from VV to VIV abolished the inhibitory effect of vanadate. Na2B4O7 (2 mM) inhibited activity with Mn2+, but not with Mg2+. In assays with Mg2+, but not with Mn2+, FMN, NAD+ and NADH (each 0.5 mM) inhibited activation by protoporphyrin IX and nitrosylhaemoglobin. Rotenone (0.6 mM) inhibited activity with protoporphyrin IX to a greater extent than with nitrosylhaemoglobin. Methylene Blue (1 mM) inhibited activation by nitrosylhaemoglobin, protoporphyrin IX and activator. It appears that this enzyme purified from rat liver lacks haem (and perhaps other components) required for activation by NO, and it should be particularly useful for elucidating the mechanism of action of NO, protoporphyrin IX and other activators.

Soluble guanylate cyclase activation by nitric oxide and its reversal. Involvement of sulfhydryl group oxidation and reduction

Pre-incubation of either crude or purified nitric oxide-stimulated soluble lung guanylate cyclase resulted in a temperature-dependent decay of enzyme activity. The decay of nitric oxide-stimulated activity during pre-incubation was associated with a reduced responsiveness of the enzyme to reactivation by a second exposure to nitric oxide. This loss of enzyme responsiveness to reactivation by nitric oxide was greater with purified guanylate cyclase than with the crude enzyme and was highly dependent upon the nitric oxide dose. The addition of dithiothreitol or other thiols to nitric oxide-stimulated enzyme markedly accelerated the decay of activity in a dose-dependent manner. In addition, thiols prevented the loss of responsiveness of guanylate cyclase to reactivation by nitric oxide. Nitric oxide-stimulated enzyme activity was, therefore, reversed by the addition of thiol reducing agents. The addition of the thiol oxidizing agents, diamide or oxidized glutathione, to nitric oxide-stimulated guanylate cyclase caused a rapid and irreversible loss of activity. The effects of diamide or oxidized glutathione on the crude enzyme were prevented by excess dithiothreitol. Dithiothreitol did not prevent the destruction of purified nitric oxide-stimulated guanylate cyclase activity by diamide or oxidized glutathione, however. The results suggest that nitric oxide activation and its reversal are linked to the reversible oxidation and reduction, respectively, of sulfhydryl groups on guanylate cyclase which are involved in enzyme activation. The results further suggest the existence of a second class of sulfhydryl groups involved in the maintenance of enzyme activity.

Characterization of a Ca2+-dependent guanylate cyclase in the excitable ciliary membrane from Paramecium

The membraneous guanylate cyclase of cilia from Paramecium tetraurelia used MgGTP and MnGTP as substrate with Michaelis constants for GTP of 71.5 microM and 36 microM, respectively. A linear Arrhenius plot indicated that a single enzyme entity exists not sensitive to possible phase transitions of membrane lipids. Guanylate cyclase is activated by low concentrations (less than 100 microM) and inhibited by high concentrations (greater than 100 microM) of calcium, half-maximal effects were obtained with 8 microM and 500 microM Ca2+, respectively. Only strontium ions displayed partial activating and inhibiting potency, all other divalent cations tested, Ba2+, Fe2+, Co2+, Mn2+, Sn2+ and Ni2+ had no effect on guanylate cyclase activity. Ca2+ activation increased V; Km remained identical. The Ca2+ stimulated activity was not inhibited by trifluoperazine, tentatively suggesting that the stimulation may not be mediated by calmodulin. Ca2 inhibition was due to a single binding site of Ca2+ at the guanylate cyclase as evidence by a Hill coefficient h = -1 and was noncompetitive. The lanthanides La3+, Ce3+ and Tb3+ were powerful inhibitors of guanylate cyclase, with La3+ the half-maximal effect was obtained with 0.6 microM, it was kinetically a mixed-type inhibition. La3+ and CA2+ competed for the same binding site on the guanylate cyclase as determined by detailed kinetic analysis. Addition of EDTA reversed the activation and inhibition by Ca2+ and the inhibition by La3+. It is discussed that guanylate cyclase may be the initial target enzyme in the cilia for the calcium transient of the calcium-potassium action potential of Paramecium.