Association of binding sites for guanine nucleotides with adenylate cyclase activation in rat pancreatic plasma membranes. Interaction of gastrointestinal hormones

1. The activation of rat pancreatic adenylate cyclase by guanosine 5'-(beta-gamma-imido)triphosphate (p[NH]ppG) and GTP, and by the two gastrointestinal hormones pancreozymin (as C-terminal octapeptide) and secretin was correlated with the binding of [8-3H]guanosine 5'-(beta-gamma-imido)triphosphate to rat pancreatic plasma membranes. 2. The low basal adenylate cyclase activity was stimulated 17-fold by p[NH]ppG (after a 2 min lag period), 3,5-fold only by GTP, 21-fold by C-terminal octapeptide of pancreozymin, and 8-fold by secretin. GTP inhibited competitively the activation of adenylate cyclase by p[NH]ppG with a Ki,app almost identical with the Ka,app (0.3 micron). p[NH]ppG and GTP enhanced the stimulation by secretin more markedly than that by the C-terminal octapeptide of pancreozymin, leading to the same maximal activity. Both hormones suppressed the lag period of activation by p[NH]ppG. 3. The binding of [8-3H]p[NH]ppG was dependent on time, temperature and Mg2+ and it was also a saturable and reversible process. Scatchard plots with a concavity upward were linearized after co-addition of ATP, Mg2+ and an ATP-regenerating system that abolished low-affinity sites for p[NH]ppG without saturating higher affinity sites, GTP, ITP and UTP inhibited [8-3H]p[NH]ppG binding to the high-affinity sites in concentration ranges identical with those found for adenylate cyclase activation. Considerable binding of [8-3H]p[NH]ppG was still evident at 20 degrees C, but enzyme activation was not observed any more, except in the presence of hormones.     

Activation of adenylate cyclase in bovine corpus-luteum membranes by human choriogonadotropin, guanine nucleotides and NaF

1. Preincubation of luteal membranes with human choriogonadotropin results in the formation of an activated state of adenylate cyclase which is not reversed by washing and which is limited only by the absence of guanine nucleotides, whereas preincubation with GTP yields only a partially activated adenylate cyclase which requires the presence of both GTP and human choriogonadotropin during assay to demonstrate maximal activity. 2. Preincubation of luteal membranes with GTP and human choriogonadotropin does not lead to a synergistic increase in wash-resistant activity. 3. Luteal membranes that had been preincubated with GTP and hormone exhibited a decreasing rate of cyclic AMP synthesis during the adenylate cyclase assay incubation; addition of GTP during the assay incubation reversed the decrease. 4. Membranes that had been preincubated in the absence of guanine nucleotide and hormone showed a `burst' phase of cyclic AMP synthesis when GTP was present in the assay incubation and a `lag' phase with p[NH]ppG (guanosine 5′-[β,γ-imido]triphosphate) present in the assay. The presence of human choriogonadotropin with either nucleotide in the assay incubation eliminated the curvatures in plots observed with guanine nucleotides alone. 5. Luteal adenylate cyclase was persistently activated by preincubation with p[NH]ppG alone or in combination with human choriogonadotropin; the activation caused by p[NH]ppG alone was still increasing after 70min of preincubation, whereas that caused by p[NH]ppG in the presence of hormone was essentially complete within 10min of preincubation. 6. Luteal adenylate cyclase that had been partially preactivated by preincubation with p[NH]ppG was slightly increased in activity by the inclusion of further p[NH]ppG in the adenylate cyclase assay incubation, but more so with p[NH]ppG and hormone. Human choriogonadotropin alone caused no further increase in the activity of the partially stimulated preparation unless p[NH]ppG was also added to the assay incubation. 7. GTP decreased the activity of adenylate cyclase in membranes that had been partially preactivated in the presence of p[NH]ppG; the decrease in activity was greater when GTP and hormone were present simultaneously in the assay. 8. The results indicate that stable activation states of adenylate cyclase can be induced by preincubation of luteal membranes in vitro with human choriogonadotropin or p[NH]ppG, and that in the presence of p[NH]ppG the hormone may accelerate events subsequent to guanine nucleotide binding. Stable activation of luteal adenylate cyclase by prior exposure to GTP is not achieved. The involvement of GTPase activity and of hormone-promoted guanine nucleotide exchange in the modulation of luteal adenylate cyclase activity is discussed.     

Characteristics of the beta-adrenergic adenylate cyclase system of developing rabbit bone-marrow erythroblasts.

After fractionation of rabbit bone marrow into dividing (early) and non-dividing (late) erythroid cells, the adenylate cyclase activity of membrane ghosts was assayed in the presence of guanine nucleotides ((GTP and its analogue p[NH]ppG (guanosine 5'-[beta, gamma-imido]triphosphate))), the beta-adrenergic agonist L-isoprenaline (L-isoproterenol) and the antagonist L-propranolol. Both GTP and p[NH]ppG increased the adenylate cyclase activity of early and late erythroblasts, whereas the stimulating effect of the beta-adrenergic drug L-isoprenaline was limited to the immature dividing bone-marrow cells. The effect of L-isoprenaline was completely inhibited by the antagonist L-propranolol, confirming that the response was due to stimulation of beta-adrenergic receptors on the plasma membrane. The lack of response of non-dividing erythroblasts to beta-adrenergic stimuli is not due to loss of beta-receptors, since both dividing and non-dividing cells bind the selective ligand [125I]iodohydroxybenzylpindolol with almost equal affinities, the apparent dissociation constants, Kd, being 0.91 X 10(-8)M and 1.0 X 10(-8) M respectively. The number of beta-adrenergic receptors per cell was 2-fold higher in the dividing cells. No significant change in binding affinity for GTP and p[NH]ppG during erythroblast development was observed: the dissociation constants of both guanine nucleotides were almost identical with early and late erythroblast membrane preparations [2-3 (X 10(-7) M]. With dividing cells, however, in the presence of L-isoprenaline the dissociation constants of GTP and p[NH]ppG were lower (6 X 10(-8) M). The dose-response curves for isoprenaline competition in binding of [125I]iodohydroxybenzylpindolol by dividing cells showed that the EC50 (effective concentration for half maximum activity) value for isoprenaline was higher in the presence of p[NH]ppG. With non-dividing cells the EC50 value for isoprenaline was equal in the presence and in the absence of p[NH]ppG and similar to that observed with dividing-cell membranes in the presence of the nucleotide. Thus differentiation of rabbit bone-marrow erythroid cells seems to be accompanied by uncoupling of the beta-adrenergic receptors from the adenylate cyclase catalytic protein as well as by a decrease in the number of receptors per cell, but not by changes in the catecholamine and guanine-nucleotide-binding affinities.     

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.     

Activation of rat liver adenylate cyclase by guanosine 5''-[beta,gamma-imido]triphosphate and glucagon. Existence of reversibly and irreversibly activated states of the stimulatory GTP-binding protein.

The effects of guanosine 5'-[beta-thio]diphosphate (GDP[S]) on the kinetics of activation of rat liver membrane adenylate cyclase by guanosine 5'-[beta,gamma-imido]triphosphate (p[NH]ppG) were examined. GDP[S] caused immediate inhibition of the activation by p[NH]ppG at all time points tested. Substantial inhibition by GDP[S] was observed even after the time required for the enzyme to reach its steady-state activity, but the extent of inhibition became progressively smaller as the preincubation time with p[NH]ppG increased. The rate at which adenylate cyclase became quasi-irreversibly activated was a strictly first-order process. In the presence of glucagon, the formation of the irreversibly activated state was much slower. A combination of GDP[S] and glucagon could partially reverse the quasi-irreversible activation by p[NH]ppG. Glucagon decreased the lag time required for p[NH]ppG to activate adenylate cyclase and increased the extent of activation by p[NH]ppG. This stimulatory effect of the hormone on top of guanine nucleotide decreased on preincubation with p[NH]ppG, but not with GTP. Our results suggest that the activation of adenylate cyclase by non-hydrolysable GTP analogues is a two-stage process: the formation of a reversibly activated form (G rev) is a rapid process, followed by a much slower formation of the quasi-irreversibly activated form (G irr). Glucagon can stimulate G rev but not G irr, and can partially facilitate the formation of the G rev from the G irr state.     

Modifications of the adenylate cyclase complex during differentiation of cultured myoblasts

Alterations in receptor-independent activation of adenylate cyclase during proliferation and differentiation of L6E9 myoblasts were studied using Mn2+, forskolin, and Gpp(NH)p. Analyses were performed 3, 6, and 10 days following subculture, corresponding to onset of proliferation, end of proliferation with start of differentiation, and completion of differentiation, respectively. The apparent activation constant for Mn2+ decreases with the age of the culture; the apparent activation constant for Mg2+ does not. Bimodal activation by Mn2+, i.e., at concentrations greater than 10 mM, results in total adenylate cyclase activity less than the Vmax and occurs exclusively in differentiated cultures. Independent of the presence of Mg2+, forskolin activation occurs with low-and high-affinity constants in differentiated cultures and with a low affinity constant in youngest cultures; intermediate cultures (day 6) demonstrate low- and high-affinity activation only in the presence of high Mg2+. In contrast, the Vmax for forskolin increases with increasing Mg2+ in all culture ages. Although Gpp(NH)p-dependent adenylate cyclase activation occurs with an apparent activation constant independent of culture age and Mg2+, low Mg2+ fosters bimodal activation by Gpp(NH)p, i.e., above 100 microM nucleotide, total adenylate cyclase activity is less than the Vmax. The loss of stimulatory capacity by high Gpp(NH)p is greatest in differentiated cultures. Additional experiments are presented to substantiate that bimodal activation by Gpp(NH)p is specific. Cholera- and pertussis toxin-dependent ADP ribosylation patterns demonstrate a marked decrease in both Ns and Ni in differentiated cultures. The data suggest that alterations in postreceptor activation of adenylate cyclase during the course of differentiation and proliferation are mediated by guanine nucleotide binding proteins as well as by allosteric cation regulatory units.     

Effects of NaCl concentration on adenylate cyclase regulation by prostaglandins and guanine nucleotides

Na+ has been implicated as a requirement for the inhibition of adenylate cyclase by hormones and neurotransmitters. This study examines effects of salt concentration on neuroblastoma plasma membranes that occur in the absence of an inhibitory hormone. The adenylate cyclase response to stimulatory agonists (GTP plus PGE1 (3), PGI2 or PGE2) was influenced by NaCl. As the [NaCl] increased to 150 mM, an increase in maximal activity and a decrease in apparent affinity was observed. At concentrations above 150 mM, NaCl decreased prostaglandin affinity and progressively decreased maximal activation. The GTP requirement was not altered by 30 or 150 mM NaCl in the presence of PGE1 or PGI2. The rate of Gpp(NH)p stimulated activity increased as the [NaCl] was increased in the assay. This increased rate was conserved when membranes activated in the presence of Gpp(NH)p and NaCl were reassayed in the absence of guanine nucleotide or salt. The salt evoked rate increase was proportionally greater at submaximal MgCl2 concentrations. The concentration requirement for Mg2+ was reduced by salt for adenylate cyclase in the presence of GTP or Gpp(NH)p. However, the enzyme stimulated by hormone exhibited a Mg2+ requirement that was low in the absence of salt and could not be further reduced by increased [NaCl]. Alternative monovalent cations (150 mM Li+, K+, Cs+, but not choline or tetramethylammonium) and anions (SO4=) substituted for NaCl. The observed effects were reversible upon washing the membranes and neither ouabain nor tetrodotoxin altered the response. These effects may result from a conformational alteration of a protein particularly sensitive to neutral salts in the assay.     

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.     

Characterization of an ATP-Mg2+-dependent guanine nucleotide-stimulated adenylate cyclase from Neurospora crassa

A novel adenylate cyclase activity was found in crude homogenates of Neurospora crassa. The adenylate cyclase had substantial activity with ATP-Mg2+ as substrate differing significantly from the strictly ATP-Mn2+-dependent enzyme characterized previously. Additionally, the ATP-Mg2+-dependent activity was stimulated two- to fourfold by GTP or guanyl-5'-yl-imido-diphosphate (Gpp(NH)p). We propose that the ATP-Mg2+-dependent, guanine nucleotide-stimulated activity is due to a labile regulatory component (G component) of the adenylate cyclase which was present in carefully prepared extracts. The adenylate cyclase had a pH optimum of 5.8 and both the catalytic and G component were particulate. The Km for ATP-Mg2+ was 2.2 mM in the presence of 4.5 mM excess Mg2+. Low Mn2+ concentrations had no effect on adenylate cyclase activity whereas high concentrations of Mn2+ or Mg2+ stimulated the enzyme. Maximal Gpp(NH)p stimulation required preincubation of the enzyme in the presence of the guanine nucleotide and the K1/2 for Gpp(NH)p stimulation was 110 nM. Neither fluoride nor any of a variety of glycolytic intermediates or hormones, including glucagon, epinephrine, and dopamine, had an effect on ATP-Mg2+-dependent adenylate cyclase activity. However, the enzymatic activity was stimulated not only by GTP but also by 5'-AMP and was inhibited by NADH.     

Effect of Dopamine on Activation of Rat Striatal Adenylate Cyclase by Free Mg2+ and Guanyl Nucleotides1

Abstract: Stimulation of rat striatal adenylate cyclase by guanyl nucleotides was examined utilizing either MgATP or magnesium 5′-adenylylimidodiphos-phate (MgApp(NH) p) as substrate. GTP and 5′- guanylylimidodiphosphate (Gpp(NH) p) stimulate adenylate cyclase under conditions where the guanyl nucleotide is not degraded. The apparent stimulation of adenylate cyclase by GDP is due to an ATP-dependent transphosphorylase present in the tissue which converts GDP to GTP. We conclude that GTP is the physiological guanyl nucleotide responsible for stimulation of striatal adenylate cyclase. Dopamine lowers the K_a for Gpp(NH) p stimulation twofold, from 2.4 μM to 1.2 μM and increases maximal velocity 60%. The kinetics of Gpp(NH) p stimulation indicate no homotropic interactions between Gpp(NH) p sites and are consistent with one nonessential Gpp(NH) p activator site per catalytic site. Double reciprocal plots of the activation by free Mg²⁺ were concave downward, indicating either two sets of sites with different affinities or negative cooperativity (Hill coefficient = 0.3, K_0.5= 23 mM). The data conform well to a model for two sets of independent sites and dopamine lowers the K_a for free Mg²⁺ at the high-affinity site threefold, from 0.21 mM to 0.07 mM. The antipsy-chotic drug fluphenazine blocks this shift in K_a due to dopamine. Dopamine does not appreciably affect the affinity of adenylate cyclase for the substrate, MgApp(NH) p. Therefore, dopamine stimulates striatal adenylate cyclase by increasing the affinity for free Mg²⁺ and guanyl nucleotide and by increasing maximal velocity.     

Forskolin refractoriness. Exposure to the diterpene alters guanine nucleotide-dependent adenylate cyclase and calcium-uptake activity of cells cultured from the rat aorta.

Cells with the morphological properties of endothelial cells were cultured from the rat aorta. The cultured cells accumulated 45Ca2+ from the medium in a manner which was stimulated by forskolin and by 8-bromo-cyclic AMP. Pretreating the cultures for 20 h with forskolin diminished forskolin-dependent Ca2+-uptake activity. Adenylate cyclase activity of cultured cell homogenates was stimulated by guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG) and forskolin, and by isoprenaline in the presence, but not in the absence, of guanine nucleotide. p[NH]ppG increased forskolin sensitivity and caused a leftward shift in the forskolin dose-response curve. Pretreating the cultured cells with forskolin for 20 h, conditions that decreased forskolin-dependent Ca2+ uptake, increased basal and guanine nucleotide-dependent adenylate cyclase activity, but not forskolin-dependent activity determined in the absence of p[NH]ppG. Forskolin pretreatment diminished p[NH]ppG's capacity to increase forskolin sensitivity, but did not have a significant effect on either the sensitivity of adenylate cyclase to p[NH]ppG or its responsiveness to isoprenaline. These results suggest that the Ca2+-uptake mechanism is cyclic AMP-dependent and that guanine nucleotides mediated forskolin-dependent cyclic AMP production by the intact cells. In addition, there may be different guanine nucleotide requirements for hormone-receptor coupling and forskolin activation.     

Catecholamine-stimulated guanosine 5'-O-(3-thiotriphosphate) binding to the stimulatory GTP-binding protein of adenylate cyclase: kinetic analysis in reconstituted phospholipid vesicles

The stimulatory GTP-binding protein of adenylate cyclase, Gs, and beta-adrenergic receptors were reconstituted into unilamellar phospholipid vesicles. The kinetics of the quasiirreversible binding of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) to Gs, equivalent to Gs activation by nucleotide, was studied with respect to the stimulation of this process by beta-adrenergic agonists and Mg2+. The rate of GTP gamma S binding displayed apparent first-order kinetics over a wide range of nucleotide, agonist, and Mg2+ concentrations. In the absence of agonist, the apparent first-order rate constant, kapp, was 0.17-0.34 min-1 and did not vary significantly with the concentration of nucleotide. At 50 mM MgCl2, kapp increased somewhat, to 0.26-0.41 min-1, and remained invariant with the nucleotide concentration. In the presence of agonist, kapp was dependent on nucleotide concentration. At 10(-9) M GTP gamma S, the addition of (-)-isoproterenol caused at most a 2-fold stimulation of kapp. However, kapp measured in the presence of isoproterenol increased as an apparently saturable function of the GTP gamma S concentration, such that isoproterenol caused a 17-fold increase in kapp at 1 microM GTP gamma S. The effect of isoproterenol on kapp also appeared to saturate at high isoproterenol concentration, yielding a kapp approximately 6 min-1 at high concentrations of both nucleotide and agonist. These data suggest that the receptor-agonist complex acts by increasing the rate of conversion of a lower affinity Gs-GTP gamma S complex to the stable activated state.     

Calmodulin regulation of adenylate cyclase activity in human platelet membranes

The mechanism of calmodulin dependent regulation of adenylate cyclase has been studied in human platelet membranes. Calmodulin activated adenylate cyclase exhibited a biphasic response to both Mg2+ and Ca2+. A stimulatory effect of Mg2 on adenylate cyclase was observed at all Mg2+ concentrations employed, although the degree of activation by calmodulin was progressively decreased with increasing concentrations of Mg2+. These results demonstrate that the Vmax of calmodulin dependent platelet adenylate cyclase can be manipulated by varying the relative concentrations of Mg2+ and Ca2+. The activity of calmodulin stimulated adenylate cyclase was always increased 2-fold above respective levels of activity induced by GTP, Gpp(NH)p and/or PGE. The stimulatory influence of calmodulin was not additive but synergistic to the effects of PGE1, GTP and Gpp(NH)p. GDP beta S inhibited GTP-and Gpp(NH)p stimulation of adenylate cyclase but was without effect on calmodulin stimulation. Since the inhibitory effects of GDP beta S have been ascribed to apparent reduction of active N-protein-catalytic unit (C) complex formation, these results suggest that the magnitude of calmodulin dependent adenylate cyclase activity is proportional to the number of N-protein-C complexes, and that calmodulin interacts with preformed N-protein-C complex to increase its catalytic turnover. Our data do not support existence of two isoenzymes of adenylate cyclase (calmodulin sensitive and calmodulin insensitive) in human platelets.     

Attenuation of chick heart adenylate cyclase by muscarinic receptors after pertussis toxin treatment

Pertussis toxin selectively modifies the function of Ni, the inhibitory guanine nucleotide binding protein of the adenylate cyclase complex. In chick heart membranes, guanine nucleotide activation of Ni resulted in a decrease in the apparent affinity of the muscarinic receptor for the agonist oxotremorine, inhibition of basal adenylate cyclase activity, and the attenuation of adenylate cyclase by oxotremorine. Treatment of chicks with pertussis toxin caused the covalent modification of 80-85% of cardiac Ni. After this treatment Gpp(NH)p had no effect on muscarinic receptor affinity and GTP stimulated basal adenylate cyclase activity. In contrast, the GTP-dependent attenuation of adenylate cyclase caused by muscarinic receptors was unaffected.     

Distinct effects of the C-terminal octapeptide of cholecystokinin and of a cholera toxin pretreatment of the kinetics of rat pancreatic adenylate cyclase activity

(1) The kinetic parameters of rat pancreatic adenylate cyclase were evaluated, using GTP, p[NH]ppG or GTP gamma S as nucleotide activator, cholecystokinin as peptide hormone, and GDP beta S and dibutyryl cyclic GMP as inhibitors of guanosine triphosphate and CCK-8, respectively. The time courses of activation and the degree of activation at steady state (EA/ETOT) were compatible with a simple two-state model of activation-deactivation based on a pseudo-monomolecular activation process (rate constant kappa+1), and a deactivation process (rate constant kappa off) that included, depending on the activating nucleotide, the hydrolysis of GTP (rate constant kappa 2) and/or the dissociation of the intact nucleotide (rate constant kappa-1), so that EA/ETOT = kappa+1/(kappa+1 + kappa 2 + kappa-1). (2) The hormone CCK-8 increased the value of kappa+1 with GTP dose-dependently, from 0.2 to 10.9 min-1. The value of kappa-1 increased 0.01 to 0.3 min-1 but the value of kappa 2 was unaltered at 7 min-1, so that EA/ETOT increased 15-fold, from 4% to 61%. (3) A cholera toxin pretreatment at 30 micrograms/ml allowed also a large increase in EA/ETOT with GTP (up to 51%) but the underlying mechanism was different. It consisted of a 14-fold decrease in the kappa off value of the GTP-activated enzyme (from 7 min-1 to 0.5 min-1) that corresponded to a reduction in GTPase activity. When testing the system with p[NH]ppG, two added effects of the cholera toxin pretreatment were observed: a 4-fold increase in the value of kappa+1 (from 0.2 to 0.8 min-1) and the occurrence of a significant 0.3 min-1 value for kappa-1.     

Regulation of human platelet adenylate cyclase by epinephrine, prostaglandin E1, and guanine nucleotides. Evidence for separate guanine nucleotide sites mediating stimulation and inhibition.

A method for preparing human platelet membranes with high adenylate cyclase activity is described. Using these membranes, epinephrine and GTP individually are noted to inhibit adenylate cyclase slightly. When present together, epinephrine and GTP act synergistically to cause a 50% inhibition of basal activity. The epinephrine effect is an alpha-adrenergic process as it is reversed by phentolamine but not propranolol. The quasi-irreversible activation of adenylate cyclase by Gpp(NH)p is time, concentration, and Mg2+-dependent but is not altered by the presence of epinephrine. Adenylate cyclase activated by Gpp(NH)p, and extensively washed to remove unbound Gpp(NH)p, is inhibited by the subsequent addition of Gpp(NH)p, GTP, and epinephrine. This effect of epinephrine is also an alpha-adrenergic phenomenon. In contrast to epinephrine which inhibits the cyclase, PGE1 addition results in enzyme stimulation. PGE1 stimulation does not require GTP addition. PGE1 accelerates the rate of Gpp(NH)p-induced activation. Low GTP concentrations (less than 1 x 10(-6) M) enhance PGE1 stimulation while higher GTP concentrations cause inhibition. These observations suggest that human platelet adenylate cyclase possesses at least two guanine nucleotide sites, one which interacts with the alpha-receptor to result in enzyme inhibition and a second guanine nucleotide site which interacts with the PGE1 receptor and causes enzyme stimulation.     

Purification and partial characterization of two forms of urinary trypsin inhibitor

The activation of bovine thyroid adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) by Gpp(NH)p has been studied using steady-state kinetic methods. This activation is complex and may be characterized by two Gpp(NH)p binding sites of different affinities with measured constants: Ka1 = 0.1 micro M and Ka2 = 2.9 micro M. GDP beta S does not completely inhibit the Gpp(NH)p activation: analysis of the data is consistent with a single GDP beta S inhibitory site which is competitive with the weaker Gpp(NH)p site. Guanine nucleotide effects upon F- activation of adenylate cyclase have been studied. When App(NH)p is the substrate, 10 micro M GTP along with 10 mM NaF gives higher activity than NaF alone, while GDP together with NaF inhibits the activity by 50% relative to NaF. These features are not observed when the complex is assayed with ATP in the presence of a nucleotide regenerating system or when analogs Gpp)NH)p or GDP beta S are used along with NaF. These effects were studied in three other membrane systems using App(NH)p as substrate: rat liver, rat ovary and turkey erythrocyte. No consistent pattern of guanine nucleotide effects upon fluoride activation could be observed in the different membrane preparations. Previous experiments showed that the size of soluble thyroid adenylate cyclase changed whether membranes were preincubated with Gpp(NH)p or NaF. This size change roughly corresponded to the molecular weight of the nucleotide regulatory protein. This finding, coupled with the present data, suggests that two guanine nucleotide binding sites may be involved in regulating thyroid cyclase and that these sites may be on different protein chains.     

Modulation of the receptor-coupled adenylate cyclase system in HeLa cells by sodium butyrate

Exposure of HeLa cells to 5 mM sodium butyrate, but not 0.6 mM, resulted in a more efficient coupling between their beta-adrenergic receptors and the guanine nucleotide binding stimulatory (Ns) component of adenylate cyclase. Both concentrations of the fatty acid, however, caused an increase in receptor number. beta receptors from control and butyrate-treated cells had the same affinity for isoproterenol. Modulation of this affinity by GTP was greatly enhanced, however, in cells treated with 5 mM butyrate compared to untreated and 0.6 mM butyrate treated cells. The concentration of isoproterenol required to half-maximally stimulate adenylate cyclase (Kact) was reduced in cells treated with 5 mM butyrate. In addition, the Kact for GTP in the presence, but not the absence, of isoproterenol was reduced. The effect of butyrate on the coupling between beta receptors and Ns was analyzed in detail by monitoring the activation of Ns by guanine 5'-O-(3-thiotriphosphate) (GTP gamma S) in a two-step assay. In the absence of isoproterenol, Ns from control and 5 mM butyrate treated cells was activated to the same extent with the same time course and Kact for GTP gamma S. In the presence of isoproterenol, Ns from 5 mM butyrate treated cells was activated more rapidly and extensively than Ns from control cells. The Kact for both GTP gamma S and isoproterenol also was reduced. The rate of agonist-mediated activation of Ns was strongly dependent on temperature, which accentuated the differences between 5 mM butyrate treated and control cells. At 4 degrees C, the difference in rate was 8.8-fold.(ABSTRACT TRUNCATED AT 250 WORDS)     

Multiple inhibitory and activating effects of nucleotides and magnesium on adrenal adenylate cyclase.

Adenylate cyclase in particulate fractions from rat adrenal glands is subject to regulation by purine nucleotides, particularly guanine nucleotides. While GTP activates the enzyme, this effect is not evident in all particulate fractions. Following dialysis of the refractory fractions activation by GTP is observed, an indication that endogenous nucleotides may obscure the effects of added GTP. The analog, guanyl-5'-yl imidodiphosphate (Gpp(NH)p gives considerable more activity than does GTP. GDP, on the other hand, is inhibitory, an effect revealed only in the absence of a nucleotide-regenerating solution. GDP blocks the action of both GTP and Gpp(NH)p. These results show that the gamma-phosphate of the nucleotide is required for but need not be metabolized in the activation process. At low substrate concentration (0.1 mM ATP or adenyl-5'-yl imidodiphosphate) stimulation of the enzyme by ACTH occurs only in the presence of added guanine nucleotide (GTP or Gpp(NH)p); the hormone and nucleotide act synergistically. While both GTP and Gpp(NH)p inhibit fluoride-stimulated activity, the level of fluoride required to demonstrate such inhibition appears not to be related to the level of fluoride required for activation of the enzyme. In the presence of GTP, or GTP plus ACTH, the enzyme exhibits normal Michaelis-Menten kinetics with respect to substrate utilization (K-m equal to 0.16 mM). In the activated state, produced with ACTH plus GTP, the enzyme is less susceptible to inhibition by a species of ATP uncomplexed with Mg2+, but is more susceptible to inhibition by Mg2+. These results demonstrate that fundamental differences exist between different states of the adenylate cyclase. The difficulties in describing kinetically the regulation of adenylate cyclase systems in view of the multiple actions of nucleotides and magnesium are discussed.     

Challenge of hepatocytes by glucagon triggers a rapid modulation of adenylate cyclase activity in isolated membranes.

Membrane fractions obtained from hepatocytes treated with glucagon exhibited a decreased glucagon (with or without GTP)-stimulated adenylate cyclase activity. A maximum effect was seen in around 5 min. No change in the rate of cyclic AMP production was observed for the basal, NaF-, p[NH]ppG (guanosine 5'-[beta, gamma-imido]-triphosphate)- and GTP-stimulated states of the enzyme. The lag observed in the p[NH]ppG-stimulated adenylate cyclase activity of native membranes was abolished when membranes from glucagon-pretreated cells were used. When Mn2+ replaced Mg2+ in the assays, the magnitude of the apparent desensitization was decreased. Mn2+ abolished the lag of onset of p[NH]ppG-stimulated activity in native membranes. The desensitization process was dose-dependent on glucagon, which exhibited a Ka of 4 X 10(-10) M. Depletion of intracellular ATP did not affect this process. It is suggested that this desensitization occurs at the level of the guanine nucleotide-regulatory protein.