Lineage-Specific Domain Fusion in the Evolution of Purine Nucleotide Cyclases in Cyanobacteria

Cyclic nucleotides (both cAMP and cGMP) play extremely important roles in cyanobacteria, such as regulating heterocyst formation, respiration, or gliding. Catalyzing the formation of cAMP and cGMP from ATP and GTP is a group of functionally important enzymes named adenylate cyclases and guanylate cyclases, respectively. To understand their evolutionary patterns, in this study, we presented a systematic analysis of all the cyclases in cyanobacterial genomes. We found that different cyanobacteria had various numbers of cyclases in view of their remarkable diversities in genome size and physiology. Most of these cyclases exhibited distinct domain architectures, which implies the versatile functions of cyanobacterial cyclases. Mapping the whole set of cyclase domain architectures from diverse prokaryotic organisms to their phylogenetic tree and detailed phylogenetic analysis of cyclase catalytic domains revealed that lineage-specific domain recruitment appeared to be the most prevailing pattern contributing to the great variability of cyanobacterial cyclase domain architectures. However, other scenarios, such as gene duplication, also occurred during the evolution of cyanobacterial cyclases. Sequence divergence seemed to contribute to the origin of putative guanylate cyclases which were found only in cyanobacteria. In conclusion, the comprehensive survey of cyclases in cyanobacteria provides novel insight into their potential evolutionary mechanisms and further functional implications.     

Activation of particulate guanylate cyclase by adrenomedullin in cultured SV-40 transformed cat iris sphincter smooth muscle (SV-CISM-2) cells

We investigated the effects of adrenomedullin (ADM) on cGMP production in cultured SV-40 transformed cat iris sphincter smooth muscle (SV-CISM-2) cells. ADM increased cGMP accumulation in a time- and concentration- dependent manner. The peptide increased cGMP formation in the transformed cells by 405-fold as compared to 1. 6-fold in primary cultured CISM cells. The basal cGMP concentrations in both cell types were comparable. In addition, ADM increased cAMP accumulation in SV-CISM-2 cells and in primary cultured cells by 18. 9- and 5.8-fold, respectively. The ADM receptor antagonist, ADM(26-52), but not the atrial natriuretic peptide (ANP) receptor antagonist, anantin, inhibited ADM-induced cGMP formation. The phorbol ester, phorbol 12, 13-dibutyrate (PDBu), which inhibits particulate guanylate cyclases in smooth muscle, blocked ADM-stimulated cGMP accumulation. In contrast, inhibitors of the soluble guanylate cyclases, such as LY83583 and ODQ, and inhibitors of the nitric oxide cascade had little effect on ADM-stimulated cGMP production. The stimulatory effect of ADM on cGMP formation is due to activation of the guanylate cyclase system and not to a much reduced phosphodiesterase activity. ADM stimulated guanylate cyclase activity in membrane fractions isolated from SV-CISM-2 cells in a concentration-dependent manner with EC(50) value of 72 nM. Pertussis toxin, an activator of the G-protein, Gi, inhibited ADM-stimulated cGMP accumulation, whereas cholera toxin, a stimulator of the Gs G-protein and subsequently cAMP accumulation, had little effect. Pretreatment of the plasma membrane fraction with Gialpha antibody attenuated ADM-stimulated guanylate cyclase activity by 75%. We conclude that ADM increases intracellular cGMP levels in SV-CISM-2 cells through activation of the ADM receptor and subsequent stimulation of a Gi-mediated membrane-bound guanylate cyclase.     

Phosphodiesterase type 2 and the homeostasis of cyclic GMP in living thalamic neurons

The ubiquitous second messenger cyclic GMP (cGMP) is synthesized by soluble guanylate cyclases in response to nitric oxide (NO) and degraded by phosphodiesterases (PDE). We studied the homeostasis of cGMP in living thalamic neurons by using the genetically encoded fluorescence resonance energy transfer sensor Cygnet, expressed in brain slices through viral gene transfer. Natriuretic peptides had no effect on cGMP. Basal cGMP levels decreased upon inhibition of NO synthases or soluble guanylate cyclases and increased when PDEs were inhibited. Single cell RT-PCR analysis showed that thalamic neurons express PDE1, PDE2, PDE9, and PDE10. Basal cGMP levels were increased by the PDE2 inhibitors erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) and BAY60-7550 but were unaffected by PDE1 or PDE10 inhibitors. We conclude that PDE2 regulates the basal cGMP concentration in thalamic neurons. In addition, in the presence of 3-isobutyl-1-methylxanthine (IBMX), cGMP still decreased after application of a NO donor. Probenecid, a blocker of cGMP transporters, had no effect on this decrease, leaving PDE9 as a possible candidate for decreasing cGMP concentration. Basal cGMP level is poised at an intermediate level from which it can be up or down-regulated according to the cyclase and PDE activities.     

Identification of bacterial guanylate cyclases

The ability of bacteria to use cGMP as a second messenger has been controversial for decades. Recently, nucleotide cyclases from Rhodospirillum centenum, GcyA, and Xanthomonas campestris, GuaX, have been shown to possess guanylate cyclase activities. Enzymatic activities of these guanylate cyclases measured in vitro were low, which makes interpretation of the assays ambiguous. Protein sequence analysis at present is insufficient to distinguish between bacterial adenylate and guanylate cyclases, both of which belong to nucleotide cyclases of type III. We developed a simple method for discriminating between guanylate and adenylate cyclase activities in a physiologically relevant bacterial system. The method relies on the use of a mutant cAMP receptor protein, CRP_G, constructed here. While wild‐type CRP is activated exclusively by cAMP, CRP_G can be activated by either cAMP or cGMP. Using CRP‐ and CRP_G‐dependent lacZ expression in two E. coli strains, we verified that R. centenum GcyA and X. campestris GuaX have primarily guanylate cyclase activities. Among two other bacterial nucleotide cyclases tested, one, GuaA from Azospillrillum sp. B510, proved to have guanylate cyclase activity, while the other one, Bradyrhizobium japonicum CyaA, turned out to function as an adenylate cyclase. The results obtained with this reporter system were in excellent agreement with direct measurements of cyclic nucleotides secreted by E. coli expressing nucleotide cyclase genes. The simple genetic screen developed here is expected to facilitate identification of bacterial guanylate cyclases and engineering of guanylate cyclases with desired properties. Proteins 2015; 83:799–804. © 2015 Wiley Periodicals, Inc.     

Conversion of a guanylyl cyclase to an adenylyl cyclase

Guanylyl cyclases catalyze the formation of cGMP from GTP, but display extensive identity at the catalytic domain primary amino acid level with the adenylyl cyclases. The recent solving of the crystal structures of soluble forms of adenylyl cyclase has resulted in predictions of those amino acids important for substrate specificity. Modeling of a membrane-bound homodimeric guanylyl cyclase predicted the comparable amino acids that would interact with the guanine ring. Based on these structural data, the replacement of three key residues in the heterodimeric form of soluble guanylyl cyclase has led to a complete conversion in substrate specificity. Furthermore, the mutant enzyme remained fully sensitive to sodium nitroprusside, a nitric oxide donor.     

GUANYLATE CYCLASES IN CNS: ENZYMATIC CHARACTERISTICS OF SOLUBLE AND PARTICULATE ENZYMES FROM MOUSE CEREBELLUM AND RETINA

Guanylate cyclase activity is present in both soluble and particulate fractions of homogenates of mouse cerebellum and retina. Soluble guanylate cyclases in cerebellum and retina have an apparent K_m for GTP of approx 40 and 70 μM, respectively; are stimulated by Ca²⁺ and Mg²⁺ in the presence of low Mn²⁺; and do not respond to NaN₃, NH₂OH or detergent. The particulate guanylate cyclase found in brain has an apparent K_m GTP of 237 7mu;M, is not stimulated by Ca²⁺ or Mg²⁺ in the presence of low Mn²⁺, but is stimulated by NaN₃, NH₂OH, and detergent. In particulate fractions of normal retina, guanylate cyclase has two apparent K_m GTP values (42 and 225 μM); has higher activity at low concentrations of Mn²⁺ (0.5 mM) than at high concentrations (5.0 mM); is inhibited by Ca²⁺; and does not respond to NaN₃, NH₂OH, or detergent. Retinas essentially devoid of photoreceptor cells (from mice with photoreceptor dystrophy) have soluble guanylate cyclase activity which is similar to that in normal retina, but have only 4% as much particulate guanylate cyclase activity. This residual particulate guanylate cyclase has an apparent K_m GTP value of 392 μM and other properties similar to particulate guanylate cyclase from brain. These data indicate the presence of three distinguishable guanylate cyclases in CNS: (1) a soluble enzyme present in both brain and retina: (2) a particulate enzyme which is also present in brain and in the inner or neural retina: and (3) another particulate enzyme which is apparently unique and confined to retinal photoreceptor cells.     

Guanylate cyclase activity in permeabilized Dictyostelium discoideum cells

Dictyostelium discoideum cells respond to chemoattractants by transient activation of guanylate cyclase. Cyclic GMP is a second messenger that transduces the chemotactic signal. We used an electropermeabilized cell system to investigate the regulation of guanylate cyclase. Enzyme activity in permeabilized cells was dependent on the presence of a nonhydrolysable GTP analogue (e.g., GTPγS), which could not be replaced by GTP, GDP, or GMP. After the initiation of the guanylate cyclase reaction in permeabilized cells only a short burst of activity is observed, because the enzyme is inactivated with a t_1.2 of about 15 s. We show that inactivation is not due to lack of substrate, resealing of the pores in the cell membrane, product inhibition by cGMP, or intrinsic instability of the enzyme. Physiological concentrations of Ca²⁺ ions inhibited the enzyme (half-maximal effect at 0.3 μM), whereas InsP₃ had no effect. Once inactivated, the enzyme could only be reactivated after homogenization of the permeabilized cells and removal of the soluble cell fraction. This suggests that a soluble factor is involved in an autonomous process that inactivates guanylate cyclase and is triggered only after the enzyme is activated. The initial rate of guanylate cyclase activity in permeabilized cells is similar to that in intact, chemotactically activated cells. Moreover, the rate of inactivation of the enzyme in permeabilized cells and that due to adaptation in vivo are about equal. This suggests that the activation and inactivation of guanylate cyclase observed in this permeabilized cell system is related to that of chemotactic activation and adaptation in intact cells. © 1996 Wiley-Liss, Inc.     

The crystal structure of the catalytic domain of a eukaryotic guanylate cyclase

Background

Soluble guanylate cyclases generate cyclic GMP when bound to nitric oxide, thereby linking nitric oxide levels to the control of processes such as vascular homeostasis and neurotransmission. The guanylate cyclase catalytic module, for which no structure has been determined at present, is a class III nucleotide cyclase domain that is also found in mammalian membrane-bound guanylate and adenylate cyclases.

Results

We have determined the crystal structure of the catalytic domain of a soluble guanylate cyclase from the green algae Chlamydomonas reinhardtii at 2.55 Å resolution, and show that it is a dimeric molecule.

Conclusion

Comparison of the structure of the guanylate cyclase domain with the known structures of adenylate cyclases confirms the close similarity in architecture between these two enzymes, as expected from their sequence similarity. The comparison also suggests that the crystallized guanylate cyclase is in an inactive conformation, and the structure provides indications as to how activation might occur. We demonstrate that the two active sites in the dimer exhibit positive cooperativity, with a Hill coefficient of ~1.5. Positive cooperativity has also been observed in the homodimeric mammalian membrane-bound guanylate cyclases. The structure described here provides a reliable model for functional analysis of mammalian guanylate cyclases, which are closely related in sequence.     

Characterization of particulate and soluble guanylate cyclases from rat lung.

Rat lung homogenates contained significant amounts of guanylate cyclase activity in both 100,000 times g (60 min) particulate and supernatant fractions. In the presence of detergent, the particulate fraction contained 40% as much activity as did the supernatant fraction. Detergent-dispersed particulate and partially purified soluble guanylate cyclase preparations were characterized with respect to divalent cation requirements, divalent cation interactions, kinetic behavior, and gel filtration profiles. Both soluble and particulate guanylate cyclases required divalent cation for activity. The soluble preparation was 10 times more active in the presence of Mn-2plus than in the presence of Mg-2plus or Ca-2plus and no detectable activity was seen with Ba-2plus or Sr-2plus. Particulate guanylate cyclase activity was detectable only in the presence of Mn-2plus. Both enzyme preparations required Mn-2plus in excess of GTP for optimal activity at subsaturating amounts of GTP. At near-saturating GTP, the soluble enzyme required excess Mn-2plus, but the particulate enzyme did not. For kinetic analyses the enzymes were considered to require two substrates: metal-GTP and Me-2plus. Apparent negative cooperative behavior was seen with the soluble enzyme when excess Mn-2plus (in excess of GTP) was varied from 0.01 to 0.2 mM; above 0.2 mM excess Mn-2plus classical kinetic behavior was seen with an apparent KMn-2plus of 0.2 mM at near-saturating MnGTP. Similar studies using the particulate preparation yielded only classical kinetic behavior, but the apparent KMn-2plus decreased to near zero when MnGTP was near-saturating. Kinetic patterns for the particulate and soluble enzymes also differed when reciprocal initial velocities were plotted as a function of reciprocal MnGTP concentrations; classical kinetic behavior was seen with the soluble enzyme with an apparent KMnGTP of about 12 muM (at near-saturating excess Mn-2plus), whereas apparent positive cooperative behavior was seen with the particulate preparation (Hill coefficient equals 1.6, S0.5 EQUALS 70 MUM. Ca-2plus "activation" of soluble guanylate cyclase was related to the Mn-2plus:GTP ratio. Activation was most apparent when saturating amounts of Mn-2plus and MnGTP. At relatively high concentrations of Ca-2plus (0.1 to 4 mM), the addition of 10 muM Mn-2plus resulted in a 3- to 5-fold increase in soluble guanylate cyclase activity. In contrast, Ca-2plus sharply inhibited particulate guanylate cyclase activity. Gel filtration profiles of particulate and soluble preparations indicated differences in physical properties of the enzymes. As estimated by gel filtration, particulate (detergent-dispersed)evels. Here, removal of renal tissue is contraindicated. In all renal hy     

A functional chimera of mammalian guanylyl and adenylyl cyclases

Adenylyl and guanylyl cyclases synthesize second messenger molecules by intramolecular esterification of purine nucleotides, i.e., cAMP from ATP and cGMP from GTP, respectively. Despite their sequence homology, both families of mammalian cyclases show remarkably different regulatory patterns. In an attempt to define the functional domains in adenylyl cyclase responsible for their isotypic-common activation by Galphas or forskolin, dimeric chimeras were constructed from soluble guanylyl cyclase alpha1 subunit and the C-terminal halves of adenylyl cyclases type I, II, or V. The cyclase-hybrid generated cAMP and was inhibited by P-site ligands. The data establish structural equivalence and the ability of functional complement at the catalytic sites in both cyclases. Detailed enzymatic characterization of the chimeric cyclase revealed a crucial role of the N-terminal adenylyl cyclase half for stimulatory actions, and a major importance of the C-terminal part for nucleotide specificity.     

Soluble guanylate cyclases in the retina

Soluble guanylate cyclase catalyzes the formation of cyclic GMP using GTP as substrate. It is now well established that soluble guanylate cyclase is highly activated by nitric oxide, and that many of the effects of nitric oxide on various cells and tissues are mediated through increased production of cyclic GMP. This review discusses the evidence for the presence of soluble guanylate cyclases in different classes of cells in vertebrate retina and the role of these enzymes in retinal physiology. It is concluded that the enzyme is present in nearly every class of cells in the retina and that it may be involved in signal transmission between some cells and in the modulation of signal transmission between others.     

Mechanisms involved in the relaxation of bovine aortic endothelial cells

The importance of endothelial cell contraction in the regulation of vascular biology is being increasingly recognized. Our group has demonstrated that reactive oxygen species, particularly hydrogen peroxide, which are released in pathological conditions such as ischemia-reperfusion, are able to induce contraction in bovine aortic endothelial cells (BAEC). The cGMP-dependent relaxation of contractile cells depends on the ability of the cyclic nucleotide to interfere with intracellular calcium; however, this is not the only mechanism involved. The present experiments were designed to analyse the mechanism by which cGMP induces relaxation in BAEC. Sodium nitroprusside (SNP), an activator of soluble guanylate cyclase, as well as atrial natriuretic (ANP) and C-type natriuretic (CNP) peptides, activators of particulate guanylate cyclase, blunted the hydrogen peroxide-induced contraction of BAEC and myosin light chain phosphorylation. The inhibitory effect was more marked with SNP and CNP than with ANP, and the action of SNP and CNP were partially reversed by blocking soluble and particulate guanylate cyclases, respectively. Dibutyryl cGMP (db-cGMP), a cGMP analogue, mimicked the effect of SNP and CNP. Cyclic GMP-dependent protein kinase (cGK) protein levels and activity were measured. Hydrogen peroxide induced a significant reduction in cGK activity without any change in protein level. This effect was completely reversed by preincubation with db-cGMP. Calyculin A, a myosin light chain phosphatase inhibitor, prevented the cGMP-induced relaxation of BAEC. SNP, CNP and db-cGMP also partially prevented the hydrogen peroxide-induced increase in intracellular calcium levels. Catalase completely blocked this effect. In summary, the present results support a role for those metabolites which activate guanylate cyclases in the relaxation of BAEC, and suggest that the cGMP-induced BAEC relaxation could be due, at least partially, to the stimulation of cGK and/or myosin light chain phosphatase activity, and to calcium blockade.     

Changes in the system of cyclic nucleotides in irradiated body tissues

It has been shown that the content of cAMP in the rat thymus, spleen, and liver 1 and 3 days after gamma-radiation with 7.5 Gy decreases, and that of cGMP increases. Analogous dynamics has been revealed when studying adenylate cyclase and guanylate cyclase activities. The activity of cAMP and cGMP phosphodiesterases increased during the first period of study but subsequently it showed no distinction from the initial data level. The revealed postradiation changes in the content of cyclic nucleotides seem to be basically caused by the cyclases activity alterations.     

The guanylate cyclase/receptor family of proteins

Guanylate cyclase, which catalyzes the formation of cGMP from GTP, exists in both the soluble and particulate fractions of cells. At least two different cellular compartments for the particulate enzyme exist: the plasma membrane and cytoskeleton. The enzyme form found in the soluble fraction is a heterodimer that can be regulated by free radicals and nitrovasodilators, whereas the membrane form exists as a single-chain polypeptide that can be regulated by various peptides. These peptides include resact and speract obtained from eggs and atrial natriuretic peptides (ANP). The species of guanylate cyclase present in cytoskeletal fractions resists solubilization with non-ionic detergents; its structural properties are not yet known. cDNAs encoding the membrane form of guanylate cyclase have been isolated from different tissues and species, and in all cases the DNA sequences predict a protein containing a single transmembrane domain. The carboxyl (intracellular) domain is highly conserved from sea urchins through mammals, whereas the extracellular domain (amino terminus) varies considerably. The predicted amino acid sequences demonstrate that the membrane form of guanylate cyclase is a member of a diverse and complex family of proteins that includes a low molecular weight ANP receptor, protein kinases, and the cytoplasmic form of guanylate cyclase. cDNA encoding a membrane form of the enzyme from mammalian tissues has been expressed in cultured cells, and the expressed guanylate cyclase specifically binds ANP and is activated by ANP. The membrane form of guanylate cyclase, then, serves as a cell surface receptor, representing the first recognized protein to directly catalyze formation of a low molecular weight second messenger in response to ligand binding.     

Localization of particulate guanylate cyclase in plasma membranes and microsomes of rat liver.

The subcellular localization of guanylate cyclase was examined in rat liver. About 80% of the enzyme activity of homogenates was found in the soluble fraction. Particulate guanylate cyclase was localized in plasma membranes and microsomes. Crude nuclear and microsomal fractions were applied to discontinuous sucrose gradients, and the resulting fractions were examined for guanylate cyclase, various enzyme markers of cell components, and electron microscopy. Purified plasma membrane fractions obtained from either preparation had the highest specific activity of guanylate cyclase, 30 to 80 pmol/min/mg of protein, and the recovery and relative specific activity of guanylate cyclase paralleled that of 5'-nucleotidase and adenylate cyclase in these fractions. Significant amounts of guanylate cyclase, adenylate cyclase, 5'-nucleotidase, and glucose-6-phosphatase were recovered in purified preparation of microsomes. We cannot exclude the presence of guanylate cyclase in other cell components such as Golgi. The electron microscopic studies of fractions supported the biochemical studies with enzyme markers. Soluble guanylate cyclase had typical Michaelis-Menten kinetics with respect to GTP and had an apparent Km for GTP of 35 muM. Ca-2+ stimulated the soluble activity in the presence of low concentrations of Mn-2+. The properties of guanylate cyclase in plasma membranes and microsomes were similar except that Ca-2+ inhibited the activity associated with plasma membranes and had no effect on that of microsomes. Both particulate enzymes were allosteric in nature; double reciprocal plots of velocity versus GTP were not linear, and Hill coefficients for preparations of plasma membranes and microsomes were calculated to be 1.60 and 1.58, respectively. The soluble and particulate enzymes were inhibited by ATP, and inhibition of the soluble enzyme was slightly greater. While Mg-2+ was less effective than Mn-2+ as a sole cation, all enzyme fractions were markedly stimulated with Mg-2+ in the presence of a low concentration of Mn-2+. Triton X-100 increased the activity of particulate fractions about 3- to 10-fold and increased the soluble activity 50 to 100%.     

Cyclic GMP controls Rhodospirillum centenum cyst development

Adenylyl cyclases are widely distributed across all kingdoms whereas guanylyl cyclases are generally thought to be restricted to eukaryotes. Here we report that the α-proteobacterium Rhodospirillum centenum secretes cGMP when developing cysts and that a guanylyl cyclase deletion strain fails to synthesize cGMP and is defective in cyst formation. The R. centenum cyclase was purified and shown to effectively synthesize cGMP from GTP in vitro, demonstrating that it is a functional guanylyl cyclase. A homologue of the Escherichia coli cAMP receptor protein (CRP) is linked to the guanylyl cyclase and when deleted is deficient in cyst development. Isothermal calorimetry (ITC) and differential scanning fluorimetry (DSF) analyses demonstrate that the recombinant CRP homologue preferentially binds to, and is stabilized by cGMP, but not cAMP. This study thus provides evidence that cGMP has a crucial role in regulating prokaryotic development. The involvement of cGMP in regulating bacterial development has broader implications as several plant-interacting bacteria contain a similar cyclase coupled by the observation that Azospirillum brasilense also synthesizes cGMP when inducing cysts.     

Highly purified particulate guanylate cyclase from rat lung: characterization and comparison with soluble guanylate cyclase

Guanylate cyclase was purified 1000-fold from washed rat lung particulate fractions to a final specific activity of 500 nmoles cyclic GMP produced/min/mg protein by a combination of detergent extraction and chromatography on concanavalin A-Sepharose, GTP-agarose, and blue agarose. Particulate guanylate cyclase has a molecular weight of 200 000 daltons, a Stokes radius of 48 A and a sedimentation coefficient of 9.4 while the soluble form has a molecular weight of 150 000 daltons, a Stokes radius of 44 A, and a sedimentation coefficient of 7.0. Whereas the particulate enzyme is a glycoprotein with a specific affinity for concanavalin A and wheat germ agglutinin, the soluble form of guanylate cyclase did not bind to these lectins. Purified particulate guanylate cyclase did not cross-react with a number of monoclonal antibodies generated to the soluble enzyme. While both forms of the enzyme could be regulated by the formation of mixed disulfides, the particulate enzyme was relatively insensitive to inhibition by cystine. With GTP as substrate both forms of the enzyme demonstrated typical kinetics, and with GTP analogues negative cooperativity was observed with both enzyme forms. These data support the suggestion that the two forms of guanylate cyclase possess similar catalytic sites, although their remaining structure is divergent, resulting in differences in subcellular distribution, physical characteristics, and antigenicity.     

Properties of the guanylate cyclase-guanosine 3':5'-monophosphate system of rat renal cortex. Activation of guanylate cyclase and calcium-independent modulation of tissue guanosine 3':5'-monophosphate by sodium azide.

The effects of sodium azide on guanylate cyclase activity of homogenates of rat renal cortex and on the guanosine 3':5'-monophosphate (cGMP) content of cortical slices were examined and compared to those of carbamylcholine and NaF. In complete Krebs-Ringer bicarbonate buffer containing 10 mM theophylline, tissue cGMP content was increased 5- to 6-fold by 0.05 mM carbamylcholine or 10 mM NaN3, and 3-fold by 10 mM NaF. Increases in cGMP were maximal in response to these concentrations of the agonists and occurred within 2 min. Exclusion of Ca2+ from the incubation media reduced basal cGMP by 50% in 20 min and abolished responses to carbamylcholine and NaF, while exclusion of Mg2+ was without effect. Analogous reductions in cGMP were observed in complete buffer containing 1 mM tetracaine, an agent which blocks movement of Ca2+ across and binding to biologic membranes. By contrast, exclusion of Ca2+ or addition of tetracaine did not alter relative cGMP responses to NaN3 (6-fold increase over basal), although levels were reduced in slices exposed to these buffers for 20 min. When slices were incubated without Ca2+ or with tetracaine for only 2 min prior to addition of agonists, basal cGMP did not decline. Under these conditions, both absolute and relative increases in cGMP in response to NaN3 were comparable to those of slices incubated throughout in complete buffer, while carbamylcholine and NaF effects on cGMP were abolished. NaN3 increased guanylate cyclase activity of whole homogenates (10- to 20-fold), and of the 100,000 X g soluble (20-fold) and particulate (4-fold) fractions of cortex. Prior incubation of slices with NaN3 in the presence or absence of Ca2+ or with Ca2+ plus tetracaine also markedly enhanced enzyme activity in homogenates and subcellular fractions subsequently prepared from these slices. In the presence of 3 mM excess MnCl2, NaN3 raised the apparent Km for MnGTP of soluble guanylate cyclase from 0.11 mM to 0.20 mM, and reduced enzyme dependence on Mn2+. Thus, when Mg2+ was employed as the sole divalent cation in the enzyme reaction mixture basal and NaN3-responsive activities were 7% and 30% of those seen with optimal concentrations of Mn2+, respectively. Under a variety of assay conditions where responses to NaN3 were readily detectable, alterations in guanylate cyclase activities could not be demonstrated in response to carbamylcholine or NaF. By contrast Ca2+ increased the guanylate cyclase activity 6- to 7-fold over basal under conditions of reduced Mn2+ (0.75 mM Mn2+/1 mM GTP). This latter effect of Ca2+ was shared by Mg2+ and not blocked by tetracaine. Carbamylcholine, NaF, Ca2+, and NaN3 all failed to alter cGMP phosphodiesterase activity in cortex. Thus, while carbamylcholine and NaF enhance renal cortical cGMP accumulation through actions which are dependent upon the presence of extracellular Ca2+, NaN3 stimulates cGMP generation in this tissue through an apparently distinct Ca2+-independent mechanism.     

Nucleotidyl cyclase activity of soluble guanylyl cyclase α1β1

Soluble guanylyl cyclase (sGC) regulates several important physiological processes by converting GTP into the second-messenger cGMP. sGC has several structural and functional properties in common with adenylyl cyclases (ACs). Recently, we reported that membranous ACs and sGC are potently inhibited by 2',3'-O-(2,4,6-trinitrophenyl)-substituted purine and pyrimidine nucleoside 5'-triphosphates. Using a highly sensitive high-performance liquid chromatography-tandem mass spectrometry method, we report that highly purified recombinant sGC of rat possesses nucleotidyl cyclase activity. As opposed to GTP, ITP, XTP and ATP, the pyrimidine nucleotides UTP and CTP were found to be sGC substrates in the presence of Mn(2+). When Mg(2+) is used, sGC generates cGMP, cAMP, cIMP, and cXMP. In conclusion, soluble "guanylyl" cyclase possesses much broader substrate specificity than previously assumed. Our data have important implications for cyclic nucleotide-mediated signal transduction.     

Inhibition of soluble guanylate cyclase activity by citrate

Soluble guanylate cyclase activity from guinea pig heart is inhibited by increasing concentrations of sodium citrate. The Ki value was found to be 2.83 +/- 0.05 mM in the presence of 3 mM Mn2+ and 0.6 mM GTP. Citrate acts by lowering Vmax and increasing the apparent values of Km for GTP and K0.5 for Mn2+ and Mg2+. The soluble guanylate cyclase, activated by sodium nitroprusside, was also inhibited by citrate. This inhibitory action of citrate was not restricted to soluble guanylate cyclase activity of the heart and has been demonstrated also in the supernatant of lung, liver, diencephalon and in the homogenate of blood platelets. Since citrate is known to be an important intermediate of metabolism, its intracellular concentration may be also of relevance for guanylate cyclase activity.