GDP does not mediate but rather inhibits hormonal signal to adenylate cyclase

This study was aimed to elucidate whether GDP can mediate hormonal signal to adenylate cyclase in hepatic glucagon sensitive adenylate cyclase with ATP as substrate. Conversion of added GDP to GTP catalyzed by nucleoside diphosphate kinase was suppressed to less than 0.3% of added GDP by including UDP. Inhibition of this enzyme activity by UDP was accompanied by a preferential loss of the stimulatory effect of glucagon plus GDP on cyclase activity without changes in effects of glucagon plus GTP, glucagon plus guanosine 5'-(beta, gamma-imino)triphosphate, and NaF. Under this condition, i.e. in the presence of UDP, GDP competitively inhibited the actions of GTP (Ki for GDP, 1 microM) and guanosine 5'-(beta, gamma-imino)triphosphate in the presence of glucagon, the inhibition being complete at high GDP concentrations. GDP also inhibited cyclase activity stimulated by NaF with UDP but did only slightly without UDP. It was demonstrated that nucleoside diphosphate kinase is located in membranes in addition to cytosol fraction. However, the activity of membrane-associated enzyme was not affected by the addition of glucagon. Based on these observations, it is concluded that GDP is unable to mediate hormonal signal to adenylate cyclase and that it acts as an inhibitor of cyclase activity stimulated by GTP or its analog along with hormone. The results suggest a possible role of membrane-associated nucleoside diphosphate kinase in determining GTP and GDP levels at or near their binding site so as to replenish GTP and, thereby, decrease the inhibitory action of GDP when hormone is present.     

Adenosine 5'-(beta, gamma-imino)triphosphate and guanosine 5'-O-(2-thiodiphosphate) do not necessarily provide non-phosphorylating conditions in adenylate cyclase studies

Commercial preparations of adenosine 5'-(beta, gamma-imino)triphosphate (App(NH)p) were found to be contaminated with a GTP-like substance(s) as well as a phosphate donor(s) for GDP. Thus, when these preparations were used as substrate with no purification, GDP was as effective as GTP in promoting PGE1 stimulation of human platelet adenylate cyclase. With purified App(NH)p as substrate, the effect of PGE1 with GDP was reduced but still observable, while that with GTP was unaltered. PGE1 also caused a stimulation in the presence of guanosine 5'-o-(2-thiodiphosphate)(GDP beta S) with ATP as substrate. Both of the PGE1-stimulated activities observed with GDP and its analog were completely lost by the addition of UDP, thereby, inhibiting GTP formation catalyzed by membrane-associated nucleoside diphosphate kinase. The results demonstrate that the stimulatory effects of PGE1 observed with GDP and App(NH)p, and with GDP beta S and ATP were transphosphorylation dependent and, therefore, the analogs must be used with special caution in adenylate cyclase studies.     

Antibodies directed against transducin beta subunits interfere with the regulation of adenylate cyclase activity in brain membranes

In an attempt to study the mechanisms of action of membrane-bound adenylate cyclase, we have applied to rat brain synaptosomal membranes antibodies raised against purified bovine transducin (T) beta gamma subunits. The antibodies recognized one 36-kDa protein in Western blots of the membranes. Adenylate cyclase activation by GTP non-hydrolyzable analogues was greatly decreased in immune, as compared to preimmune, antibody-treated membranes, whereas the enzyme basal activity was unaffected by both types of antibodies. The inhibition of forskolin-stimulated adenylate cyclase by guanine 5'-(beta, gamma-imino)triphosphate (Gpp-(NH)p) was decreased in membranes preincubated with immune, but not preimmune, antibodies. Anti-T beta antibodies moderately decreased the extent of subsequent adenylate cyclase activation by forskolin, while not affecting activation by Al3+/F-. The enzyme activation by Gpp(NH)p in untreated membranes remained the same upon further incubation in the presence of either type of antibodies. Such results were consistent with the decreased exchange of guanine nucleotides which occurred in membrane treated with immune, but not preimmune antibodies, upon addition of GTP. The blockade of the regulation of adenylate cyclase by Gpp(NH)p observed in membranes pretreated by anti-T beta antibodies thus appears to be caused by the impairment of the guanine nucleotide exchange occurring on Gs alpha subunits. The G beta subunits in the adenylate cyclase complex seem to be instrumental in the guanine nucleotide exchange on G alpha subunits, just as T beta subunits are in the transducin complex.     

Functional homology between signal-coupling proteins. Cholera toxin inactivates the GTPase activity of transducin

Both the light-stimulated cGMP phosphodiesterase of retinal rod outer segments (ROS) and hormone-stimulated adenylate cyclase are regulated by guanine nucleotide-binding regulatory proteins (N). Transducin serves as the signal-carrying regulatory protein in ROS, and the N protein (also called G or G/F) performs this role in the adenylate cyclase system. The GTP form of these regulatory proteins activates the corresponding enzyme, whereas the GDP form does not. Both transducin and the N protein possess a GTPase activity that restores the regulatory protein to the unstimulated state. Cholera enterotoxin catalyzes the transfer of ADP-ribose from NAD+ to the N protein, which inhibits its GTPase activity and activates adenylate cyclase. We report here that the toxin also catalyzes ADP-ribosylation of the alpha-subunit of transducin in ROS membranes. This modification of the guanine nucleotide-binding subunit of transducin is markedly enhanced by the bleaching of rhodopsin and by the addition of guanosine-5'-(beta, gamma-imino)triphosphate. In contrast, GDP, GTP, and guanosine-5'-(3-O)thiotriphosphate inhibit the reaction, while GMP and ATP have no effect. Under optimal conditions, toxin catalyzes labeling of 0.7 mol of the alpha-subunit of transducin/mol of bound [3H]guanosine-5'-(beta, gamma-imido)triphosphate and causes 70% inhibition of the light-dependent GTPase activity of transducin in ROS. These results indicate close functional homology between transducin of ROS and the N protein of adenylate cyclase.     

Dissociation of guanosine 5'-[gamma-thio]triphosphate from guanine-nucleotide-binding regulatory proteins in native cardiac membranes. Regulation by nucleotides and muscarinic acetylcholine receptors.

Binding of the poorly hydrolyzable GTP analog, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), to purified guanine-nucleotide-binding regulatory proteins (G proteins) has been shown to be nonreversible in the presence of millimolar concentrations of Mg2+. In porcine atrial membranes, binding of [35S]GTP[S] to G proteins was stable in the presence of 1 mM Mg2+. However, either large dilution or, even more strongly, addition of unlabelled guanine nucleotides, in the potency order, GTP[S] greater than GTP greater than or equal to guanosine 5'-[beta,gamma-imino]triphosphate greater than GDP greater than or equal to guanosine 5'-[beta-thio]diphosphate greater than GMP, markedly enhanced the observed dissociation, with 20-30% of bound [35S]GTP[S] being released by unlabelled guanine nucleotide within 20 min at 25 degrees C. Most interestingly, dissociation of [35S]GTP[S] was rapidly and markedly stimulated by agonist (carbachol) activation of cardiac muscarinic acetylcholine receptors. Carbachol-stimulated release of [35S]GTP[S] was strictly dependent on the presence of Mg2+ and an unlabelled guanine nucleotide. Although having different potency and efficiency in releasing [35S]GTP[S] from the membranes by themselves, the guanine nucleoside triphosphates and diphosphates studied, at maximally effective concentrations, promoted the carbachol-induced dissociation to the same extent, while GMP and ATP were ineffective. GTP[S]-binding-saturation experiments indicated that one agonist-activated muscarinic acetylcholine receptor can cause release of bound GTP[S] from three to four G proteins. The data presented indicate that binding of GTP[S] to G proteins in intact membranes, in contrast to purified G proteins, is reversible, and that agonist-activated receptors can even, either directly or indirectly, interact with GTP[S]-bound G proteins, resulting in release of bound guanine nucleoside triphosphate.     

Specificity for guanine nucleotide activation and stabilization of rabbit cardiac adenylate cyclase

These studies examined the structural specificity for guanine nucleotide-facilitated hormonal activation and guanine nucleotide stabilization of cardiac adenylate cyclase. 1. The phosphonate analogues of GTP, p[CH(2)]ppG (guanosine 5'-[betagamma-methylene]-triphosphate) and pp[CH(2)]pG (guanosine 5'-[alphabeta-methylene]triphosphate), were the most effective activators of adenylate cyclase. Other nucleotides producing significant activation (P<0.01) were, in decreasing order of activation: ITP, GDP, GMP, GTP, XTP, CTP, p[NH]ppG (guanosine 5'-[betagamma-imido]triphosphate), dGTP and 2'-O-methyl-GTP. Guanosine, cyclic GMP, UTP and ppppG (guanosine tetraphosphate) had no effect, and 7-methyl-GTP caused a decrease in the activity. 2. Preincubation of membranes at 37 degrees C for 15min before assay at 24 degrees C produced an 80% decrease in adenylate cyclase activity, and preincubation with p[CH(2)]ppG and pp[CH(2)]pG protected and resulted in a net increase in activity. Other nucleotides that completely or partially preserved activity in decreasing order of effectiveness were p[NH]ppG, GDP, GTP, dGTP, ITP, ppppG, 2'-O-methyl-GTP, GMP, CTP and XTP. Several compounds had no effect, including guanosine, cyclic GMP and UTP, whereas preincubation with 7-methyl-GTP produced a further decrease (P<0.05) in activity. 3. The concentration-dependence for activation and stabilization by the naturally occurring guanine nucleotides was examined in the absence of a regenerating system and revealed GMP to have no stabilizing effect and to be less potent than either GDP or GTP in activating adenylate cyclase. 4. A significant correlation (r=0.90) was found between the properties of activation and stabilization for the compounds examined. These findings are consistent with there being a single nucleotide site through which both the activation and stabilization of adenylate cyclase are mediated.     

Guanine nucleotides stimulate soluble phosphoinositide-specific phospholipase C in the absence of membranes

The effect of guanine nucleotides on platelet and calf brain cytosolic phospholipase C was examined in the absence of membranes or detergents in an assay using labeled lipid vesicles. Guanine nucleotides stimulate hydrolysis of [3H]phosphatidylinositol 4,5-bisphosphate [( 3H]PtdIns-4,5-P2) catalyzed both by enzyme from human platelets and by partially purified enzyme from calf brain. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) was the most potent guanine nucleotide with a half-maximal stimulation at 1-10 microM, followed by guanosine 5'-(beta, gamma-imido)triphosphate greater than GTP greater than GDP = guanosine 5'-O-(2-thiodiphosphate). Guanosine 5'-O-(2-thiodiphosphate) was able to reverse the GTP gamma S-mediated stimulation. NaF also stimulated phospholipase C activity, further implying a role for a guanine nucleotide-binding protein. In the presence of GTP gamma S, the enzyme cleaved PtdIns-4,5-P2 at higher pH values, and the need for calcium ions was reduced 100-fold. The stimulation of PtdIns-4,5-P2 hydrolysis by GTP gamma S ranged from 2 to 25-fold under various conditions, whereas hydrolysis of [3H]phosphatidylinositol was only slightly affected by guanine nucleotides. We propose that a soluble guanine nucleotide-dependent protein activates phospholipase C to hydrolyze its initial substrate in the sequence of phosphoinositide-derived messenger generation.     

Surface receptor-mediated activation of adenylate cyclase in Dictyostelium. Regulation by guanine nucleotides in wild-type cells and aggregation deficient mutants

GTP and GTP analogs produced significant (up to 17-fold) and persistent activation of adenylate cyclase in lysates of Dictyostelium discoideum amoeba. The activation was enhanced 2- to 4-fold by cAMP (the agonist for receptor-mediated adenylate cyclase activation), was specific for guanine nucleoside triphosphates, and was inhibited by guanosine 5'-(O-2-thio)diphosphate. The order of potency of guanine nucleotides was guanosine 5'-(O-3-thio)triphosphate greater than guanyl-5'-yl imidodiphosphate greater than GTP; half-maximal activation was observed with 1-10 microM guanine nucleotide. Maximal activation occurred when the guanine nucleotide was added within seconds after cell lysis and the lysate was preincubated for 5 min prior to assay. Under these optimal in vitro conditions, the capacity of guanine nucleotides to activate decreased, closely correlating with adaptation or desensitization induced by exposure of intact cells to cAMP during a period of 10 min. These data strongly support that regulation of adenylate cyclase in Dictyostelium occurs via a receptor-linked GTP/GDP exchange protein. Two mutants, designated synag 7 and 49 were isolated in which cAMP and/or guanine nucleotides were not sufficient to activate adenylate cyclase. The wild-type pattern of guanine nucleotide regulation was restored to synag 7 lysates by the addition of a high-speed supernatant from wild-type cells. Characterization of these mutants demonstrates that activation of adenylate cyclase is not required for growth or cell-type specific differentiation but is essential for cellular aggregation and influences morphogenesis and pattern formation. This suggests that Dictyostelium may provide a model suitable for detailed genetic analysis of surface receptor-guanine nucleotide-binding regulatory protein linked adenylate cyclase systems and for determining the role of these systems in development.     

The effect of carbacyclin, a prostaglandin analogue, on adenylate cyclase activity in platelet membranes

The effect of carbacyclin, a chemically stable analogue of prostacyclin, on the activity of adenylate cyclase in platelet membranes was measured, and compared with the effect of PGE1. When GTP was added in concentrations up to 10 microM the activation of adenylate cyclase by carbacyclin was increased, whereas higher concentrations of GTP were inhibitory. The addition of a non-hydrolysable analogue of GDP, guanosine 5'-[beta-thio]diphosphate (GDP[beta S] ) resulted in a dose-dependent inhibition of adenylate cyclase activation by carbacyclin; this inhibition was relieved by adding increased amounts of GTP.     

Detergent-induced distinctions between fluoride- and vanadate-stimulated adenylate cyclases and their responses to guanine nucleotides

The influence of detergents on fluoride- and vanadate-stimulated adenylate cyclases was investigated with enzyme from liver and adipocyte plasma membranes. Stimulation of the adipocyte cyclase by Na3VO4 was maximal (sixfold) at 3 mM, was not additive with fluoride stimulation, and was readily reversed by washing of the membranes. Vanadate stimulation of the hepatic cyclase was specifically blocked by catechol, which had no effect on basal activity or on fluoride- or glucagon-stimulated activities. The hepatic enzyme, stimulated by fluoride ion, guanyl-5'-yl-(beta,gamma-imino)diphosphate (GPP(NH)P), or GPP(NH)P and glucagon, was inhibited by vanadate with 50% inhibition seen with 2 to 6 mM vanadate. The fluoride-activated adipocyte adenylate cyclase was inhibited by guanosine 5'-O-(3-thio-triphosphate) (GTP gamma S) more potently than by GPP(NH)P, with 50% inhibition being seen with 10 nM GTP gamma S or 100 nM GPP(NH)P. These nucleotides also inhibited the vanadate-stimulated enzyme, but with one-third the potency seen with the fluoride-activated cyclase. Dispersion of the adipocyte cyclase by Lubrol-PX into a 30,000g supernatant fraction caused no change in activation of the enzyme by fluoride, but reduced vanadate-stimulated activity 80%. By comparison, this treatment enhanced stimulation by GPP(NH)P twofold and by GTP gamma S threefold. More importantly, perhaps, the treatment with detergent blocked inhibition of the basal enzyme by GTP, blocked inhibition of fluoride- and vanadate-stimulated cyclases by GTP, GPP(NH)P, or GTP gamma S, and rendered vanadate-stimulated activity sensitive to enhancement by guanine nucleotides. The data indicate differences in the actions of vanadate and fluoride, made evident by the influence of guanine nucleotides and detergent treatment. The observations would be consistent with the idea that the effects of vandate may be due to the formation of GDP X V on the enzyme. The data strongly suggest that treatment of adenylate cyclase with Lubrol-PX causes a functional blockade in the guanine nucleotide-dependent inhibitory regulation (mediated by Ni), thereby allowing activation by the stimulatory guanine nucleotide-dependent regulatory component (Ns).     

Effect of guanine nucleotides on phospholipase C activity in permeabilized pituitary cells: possible involvement of an inhibitory GTP-binding protein

Cultured pituitary cells prelabeled with myo-[2-3H] inositol were permeabilized by ATP4-, exposed to guanine nucleotides and resealed by Mg2+. Addition of guanosine 5'-0-(3-thio triphosphate) (GTP gamma S) to permeabilized cells, or gonadotropin releasing hormone (GnRH) to resealed cells, resulted in enhanced phospholipase C activity as determined by [3H] inositol phosphate (Ins-P) production. The effect was not additive, but the combined effect was partially inhibited by guanosine 5'-0-(2-thiodiphosphate) (GDP beta S) or by neomycin. Surprisingly, addition of GDP beta S (100-600 microM) on its own resulted in a dose-related increase in [3H]Ins-P accumulation. Several nucleoside triphosphates stimulated phospholipase C activity in permeabilized pituitary cells with the following order: UTP greater than GTP gamma S greater than ATP greater than CTP. The stimulatory effect of UTP, ATP and CTP, but not GTP gamma S or GDP beta S, could also be demonstrated in normal pituitary cells suggesting a receptor-activated mechanism. GTP and GTP gamma S decreased the affinity of GnRH binding to pituitary membranes and stimulated LH secretion in permeabilized cells. These results suggest the existence of at least two G-proteins (stimulatory and inhibitory) which are involved in phospholipase C activation and GnRH action in pituitary cells.     

Evidence for specific binding sites for guanine nucleotides in adipocyte and hepatocyte plasma membranes. A difference in fate of GTP and guanosine 5'-(beta, gamma-imino) triphosphate.

Binding and degradation of GTP and guanosine 5'-(beta, gamma-imino)triphosphate (Gpp(NH)p by plasma membranes from rat liver and fat cells were investigated. Gpp(NH)p is hydrolyzed predominantly by nucleotide pyrophosphohydrolases in the membranes, whereas GTP is hydrolyzed primarily by nucleotide phosphohydrolases. These enzymes are not specific for the guanine nucleotides since co-addition of the analogous adenine nucleotides spares their hydrolysis. Both Gpp(NH)p and GTP are taken up by the membranes at sites which, to the extent that high concentrations of the corresponding adenine nucleotides fail to inhibit uptake, appear to be specific for guanine nucleotides. Gpp(NH)p taken up at these sites remains essentially intact irrespective of the degree of hydrolysis of unbound Gpp(NH)p by nucleotide pyrophosphohydrolases, indicating that the binding siteis incapable of degrading Gpp(NH)p. GTP and GDP inhibit competitively the binding of Gpp(NH)p; the binding constants for the three nucleotides are similar (0.1 to 0.4 muM) and are in the same range required for their effects on adenylate cyclase activity. Binding of the nucleotides is inhibited by sulfhydryl agents, suggesting that a sulfhydryl group is involved in the binding process. In contrast to binding of Gpp(NH)p, uptake of GTP is accompanied by substantial hydrolysis, primarily to GDP, under incubation conditions (high [ATP] plus ATP regenerating system) in which [GTP] in the medium remains essentially constant. GDP bound to the membranes is progressively hydrolyzed to 5'-GMP. Thus, GTP and Gpp(NH)p, although binding to the same specific sites, are differentially susceptible to hydrolysis at their terminal phosphates when bound to these sites. These findings are discussed in terms of the markedly different potencies of GTP and Gpp(NH)p as activators of adenylate cyclase systems.     

The role of a guanine nucleotide-binding protein in the activation of rat liver plasma-membrane adenylate cyclase by forskolin.

The effects of the photoreactive GTP analogue GTP-gamma-azidoanilide on rat liver plasma-membrane adenylate cyclase are described. U.v. irradiation in the presence of the analogue abolished activation by any effector or combination of effectors that function via the activatory G protein. Partial protection against this inhibition was given by F- and guanosine 5'-[gamma-thio]triphosphate. It is concluded that GTP-gamma-azidoanilide acts by a light-induced covalent reaction with the G protein. In the dark the effects of the analogue were similar to those of GTP. Irradiation in the presence of GTP-gamma-azidoanilide was found to reduce but not to abolish activation of rat liver plasma membrane adenylate cyclase by forskolin. The activation by forskolin and GTP together were greater than the sum of the individual activations. Forskolin doubled adenylate cyclase activity in the presence of glucagon and guanosine 5'-[beta, gamma-imido]triphosphate, which might be expected to activate to the maximum possible extent via the G protein. It is concluded that there are two components to the forskolin activation, a guanine nucleotide-dependent and a guanine nucleotide-independent component.     

Differential Sensitivity of Basal and Opioid-Stimulated Low Km GTPase to Guanine Nucleotide Analogs

In membranes derived from NG108-15 cells, the opioid peptide [D-Ala2,D-Leu5]enkephalin (DADLE) stimulates a low Km GTPase. The nucleotide analogs guanosine 5'-O-(2-thio)diphosphate (GDP beta S), guanosine 5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] and guanosine 5'-O-(3-thio)-triphosphate (GTP gamma S) inhibit the basal enzymatic activity with the order of potency GTP gamma S greater than Gpp (NH)p greater than GDP beta S. In the presence of DADLE, the inhibition isotherms of GDP beta S and Gpp(NH)p are shifted to the right five- and fourfold, respectively, compared to the inhibition observed in the absence of DADLE. In contrast, the IC50 of GTP gamma S for inhibiting the enzyme is reduced by 55% in the presence of the opioid. Both Gpp(NH)p and GTP gamma S produce a concentration-dependent increase in the Km(app) of GTPase, without affecting its Vmax, indicating a competitive inhibition. However, the replots of Km(app) versus inhibitor concentration are hyperbolic, suggesting a partial type of inhibition. Both Gpp(NH)p and GTP gamma S, but not GTP, induce an increase in the EC50 of DADLE for stimulating GTPase. These findings indicate that the basal and the opioid-stimulated low Km GTPase differ in their respective sensitivities to inhibition by guanine nucleotide analogs.     

Synthesis of 2',3'-O-(2,4,6-trinitrocyclohexadienylidine)guanosine 5'-triphosphate and a study of its inhibitory properties with adenylate cyclase

A fluorescent GTP analog 2',3'-O-(2,4,6-trinitrocyclohexadienylidine) guanosine 5'-triphosphate (TNP-GTP) has been prepared and some of its physical properties characterized. TNP-GTP was found to be a potent inhibitor of chick embryo heart adenylate cyclase as activated by guanyl 5'-(beta,gamma-imido)triphosphate (GppNHp), F-, and forskolin with Ki values in the 8-15 microM range. It also appeared to inhibit substantially basal adenylate cyclase in this system. TNP-GTP demonstrated an effective competition with [3H]GppNHp, binding to membranes equivalently to GppNHp and about three times better than GTP. 8-Azidoguanosine 5'-triphosphate (8N3GTP) mimics GTP activation of chick embryo heart adenylate cyclase and [gamma-32P]8N3GTP is effectively photoincorporated into a 42,000- to 44,000-Mr doublet when proteins are separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. TNP-GTP effectively prevents this photoincorporation, as does GTP, at concentrations that agree with their respective apparent inhibition and activation binding constants. The data suggest that TNP-GTP could prove to be a valuable tool for studying the mechanisms of GTP regulation of adenylate cyclase and other GTP-regulated systems.     

Effects of guanyl nucleotides and rhodopsin on ADP-ribosylation of the inhibitory GTP-binding component of adenylate cyclase by pertussis toxin

Hormonal inhibition of adenylate cyclase is mediated by a guanyl nucleotide binding protein, Gi, which is composed of alpha, beta, and gamma subunits (Gi alpha, G beta gamma). Pertussis toxin blocks hormonal inhibition by catalyzing the ADP-ribosylation of Gi alpha. With purified Gi subunits, but without nucleotides, it was observed that toxin-catalyzed ADP-ribosylation of Gi alpha was negligible in the absence of G beta gamma; ATP, previously shown to increase ADP-ribosylation in membranes, enhanced the ADP-ribosylation of Gi alpha in the absence, more than in the presence, of G beta gamma. Prior studies (Kanaho, Y., Tsai, S.-C., Adamik, R., Hewlett, E.L., Moss, J., and Vaughan, M. (1984) J. Biol. Chem. 259, 7378-7381) had demonstrated that rhodopsin, the retinal photon receptor protein, can replace inhibitory hormone receptors, and stimulate the hydrolysis of GTP by Gi alpha in the presence of G beta gamma. Photolyzed rhodopsin, but not the inactive, dark protein, inhibited ADP-ribosylation of Gi alpha in the presence of G beta gamma. ADP-ribosylation of Gi alpha, in the presence of G beta gamma and photolyzed (but not dark) rhodopsin was increased by guanosine 5'-O-(2-thiodiphosphate) or GDP, but not by (beta, gamma-methylene)guanosine triphosphate or guanosine 5'-O-(3-thiotriphosphate). Presumably, photolyzed rhodopsin and nucleoside triphosphate analogues activate Gi, whereas with dark rhodopsin and nucleoside diphosphates Gi is in the inactive state. The latter appears to be the preferred substrate for pertussis toxin. These observations are consistent with other evidence that rhodopsin and inhibitory hormone receptors are functionally similar.     

Fluoroaluminates mimic guanosine 5''-[gamma-thio]triphosphate in activating the polyphosphoinositide phosphodiesterase of hepatocyte membranes. Role for the guanine nucleotide regulatory protein Gp in signal transduction.

Fluoride and guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) both activate the hepatocyte membrane polyphosphoinositide phosphodiesterase (PPI-pde) in a concentration-dependent manner. AlCl3 enhances the fluoride effect, supporting the concept that [A1F4]- is the active species. Analysis of the products of inositol lipid hydrolysis demonstrate that phosphatidylinositol bisphosphate is the major lipid to be hydrolysed. Guanosine 5'-[beta-thio]diphosphate (GDP beta S) is an inhibitor of activation of PPI-pde by both fluoride and GTP gamma S. These observations suggest that the guanine nucleotide regulatory protein (termed Gp) bears a structural resemblance to the well-characterized G-proteins of the adenylate cyclase system and the cyclic GMP phosphodiesterase system in phototransduction.     

Involvement of Ni protein in the functional coupling of the atrial natriuretic factor (ANF) receptor to adenylate cyclase in rat lung plasma membranes

In the presence of 1 microM atrial natriuretic factor (ANF) and low (0.1 mM) Mg2+ concentrations, the initial rate of binding of [3H]guanosine 5'-[beta, gamma-imido)triphosphate [( 3H]p[NH]ppG) to rat lung plasma membranes was increased twofold to threefold. ANF-dependent stimulation of the initial rate of [3H]p[NH]ppG binding was reduced at high (5 mM) Mg2+ concentrations. Preincubation of membranes with p[NH]ppG (5 min at 37 degrees C) eliminated the ANF-dependent effect on [3H]p[NH]ppG binding whereas ANF-dependent [3H]p[NH]ppG binding was unaffected by similar pretreatment with guanosine 5'-[beta-thio]diphosphate (GDP[beta S]). An increase in ANF concentration from 10 pM to 1 microM caused a 40% decrease in forskolin-stimulated or isoproterenol-stimulated adenylate cyclase activities (IC50 5 nM) in rat lung plasma membranes. GTP (100 microM) was obligatory for the ANF-dependent inhibition of adenylate cyclase, which could be completely overcome by the presence of 100 microM GDP[beta S] or the addition of 10 mM Mn2+. Reduction of Na2+ concentration from 120 mM to 20 mM had the same effect. Pertussis toxin eliminated ANF-dependent inhibition of adenylate cyclase by catalyzing ADP-ribosylation of membrane-bound Ni protein (41-kDa alpha subunit of the inhibitory guanyl-nucleotide-binding protein of adenylate cyclase). The data support the notion that one of the ANF receptors in rat lung plasma membranes is negatively coupled to a hormone-sensitive adenylate cyclase complex via the GTP-binding Ni protein.     

Muscarinic Cholinergic Stimulation of Exogenous Phosphatidylinositol Hydrolysis Is Regulated by Guanine Nucleotides in Rabbit Brain Cortical Membranes

Rabbit brain cortical membranes, which have been extracted with 2 M KCl, hydrolyze exogenously added [3H]phosphatidylinositol [( 3H]PI) in a guanine nucleotide- and carbachol-dependent manner. Both oxotremorine-M and carbachol are full agonists with EC50 values of 8 and 73 microM, respectively. Pirenzepine and atropine inhibit carbachol-stimulated [3H]PI hydrolysis. The hydrolysis-resistant guanine nucleotide analog guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) is the most potent in supporting carbachol-stimulated hydrolysis of PI. There is no effect of carbachol in the absence of guanine nucleotides or in the presence of 100 microM adenosine 5'-O-(3-thiotriphosphate), adenosine-5'-(beta, gamma-imido)triphosphate, or sodium pyrophosphate. Guanylyl-5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] in the presence of carbachol also stimulates PI hydrolysis although much less than that seen with GTP gamma S. GDP and Gpp(NH)p are potent antagonists of the GTP gamma S-dependent carbachol response. Optimal stimulation by carbachol and GTP gamma S was observed at 0.3-1 microM free Ca2+ and 6 mM MgCl2. Limited trypsinization resulted in loss of receptor-regulated PI breakdown and a slight decrease in basal activity. These results demonstrate that phospholipase C hydrolysis of exogenous PI by rabbit cortical membranes may be stimulated by carbachol in a guanine nucleotide-dependent manner.     

Determination of the turn-off reaction for the epinephrine-inhibited human platelet adenylate cyclase

Epinephrine inhibits human platelet adenylate cyclase by an alpha 2-adrenoceptor-mediated and GTP-dependent process. The turn-off reaction for this epinephrine-inhibited enzyme was studied by measuring the rate of cyclic AMP formation upon addition of the alpha2-adrenoceptor antagonist, yohimbine, or upon addition of an excess of the stable GDP analog, guanosine 5'-O-(2-thiodiphosphate) (GDP beta S), which competitively inhibited the action of GTP in the epinephrine-induced inhibition. The decay of the inhibited state of the adenylate cyclase was used to calculate the rate constant of the turn-off reaction. With both methods, almost identical koff values of 0.6-0.7 min-1 at 25 degrees C were obtained for the epinephrine-inhibited platelet enzyme. Cholera toxin, which does not inhibit the epinephrine-induced GTPase stimulation in platelet membranes, did not affect this turn-off reaction. In contrast, the turn-off rate of the prostaglandin-E1-stimulated human platelet adenylate cyclase, measured with GDP beta S, was reduced from about 9 min-1 to 2 min-1 at 25 degrees C by pretreatment of the membranes with cholera toxin, which inhibits the prostaglandin-E1-stimulated GTPase activity. The data strongly suggest that the guanine nucleotide regulatory site, mediating epinephrine-induced adenylate cyclase inhibition, is activated and inactivated by similar mechanisms as is the site mediating adenylate cyclase stimulation, and that cholera toxin affects only the stimulatory site. The findings furthermore suggest that the activity states of these two regulatory sites control the activity of the adenylate cyclase.