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1.
Summary We investigated the influence of Mg2+ and Mn2+ on the effects of adenosine and some derivatives on basal adenylate cyclase activity in rat fat cell membranes as well as on enzyme activity stimulated by isoprenaline or sodium fluoride. Adenosine and derivatives modified in the ribose function were inhibitory, irrespective of the stimulant used, both in the presence of MgCl2 or MnCl2. Inhibition of basal and sodium fluoride stimulated adenylate cyclase activity was more pronounced in the presence of MnCl2 than in the presence of MgCl2. N6-substituted adenosine analogs proved to be inhibitory in the presence of 5 MM MgCl2, but in the presence of 1 mM MnCl2 the fluoride stimulated adenylate cyclase activity was potentiated, while basal and isoprenaline stimulated activity were not significantly inhibited. These effects of adenosine and derivatives could not be blocked by theophylline with or without guanyl nucleotides.The potentiating effect of N6-substituted adenosine derivatives on sodium fluoride activated adenylate cyclase is dependent on the structure of the N6-substitutent and consists of an enhancement of Vrnax in combination with a small decrease of the Km for MnATP2–, indicative of an allosteric effect on adenylate cyclase. No potentiation by N6-phenylisopropyladeno sine of sodium fluoride stimulated cyclase was found on digitonin solubilized cyclase, while the inhibitory effect of adenosine was retained. The relevance of these findings is discussed in connection with the current hypothesis concerning the presence of two adenosinesensitive sites on rat fat cell membranes.  相似文献   

2.
—Some basic kinetic properties of adenylate cyclase in cell free preparations of mouse neuroblastoma were investigated. Production of cAMP from ATP by the enzyme requires the presence of either Mg2+ or Mn2+ in addition to ATP. In the presence of Mg2+, the Km for ATP is 120 ± 15 μM and the interaction of ATP and adenylate cyclase appears to be non-cooperative (Hill coefficient of 1). Magnesium ion concentrations in excess of the ATP concentration cause stimulation although similar excess concentrations of Mn2+ cause inhibition. Prostaglandin E1 and 2-chloroadenosine activate the enzyme. The Km of the cyclase for 2-chloroadenosine is 6 μm . Activation by 2-chloroadenosine leads to an increase in Vmax but does not effect the Km for ATP. At a fixed ATP concentration, the extent of activation caused by prostaglandin E1 and 2-chloroadenosine is inversely related to the Mg2+ concentration. Calcium ion causes inhibition of adenylate cyclase from 0.1 to 4mM with a Ki of 5 ± 10?4m . Ca2+ interaction with the enzyme in the absence or presence of either 2-chloroadenosine or prostaglandin E1 appears cooperative (i.e. Hill coefficients of ?2). Ca2+ inhibition is non-competitive with respect to either ATP or 2-chloroadenosine but is progressively diminished by increasing Mn2+ concentrations. Divalent cation effects and activation by 2-chloroadenosine and prostaglandin E1 of the neuroblastoma adenylate cyclase are compared with ion effects and hormone activation of the enzyme obtained from non-neuronal tissue.  相似文献   

3.
Certain biochemical characteristics of an adenylate cyclase that is activated by low concentrations of histamine (Ka, 8 μm) and that is present in cell-free preparations from the dorsal hippocampus of guinea pig brain have been studied. Histamine increased the maximal reaction velocity of adenylate cyclase without altering the Km (0.18 mm) for its substrate, MgATP. Increasing concentrations of free Mg2+ stimulated enzymatic activity; the kinetic properties of this activation by Mg2+ suggest the existence of a Mg2+ allosteric site on the enzyme. Histamine increased the affinity of this apparent site for free Mg2+. Free ATP was a competitive inhibitor with respect to the MgATP substrate. The apparent potency of free ATP as an inhibitor increased in the presence of histamine. In the presence of Mg2+, low concentrations of Ca2+ markedly inhibited adenylate cyclase activity; half-maximal inhibition of both basal and histamine-stimulated enzyme activity occurred at 40 μm Ca2+. Other divalent cations, including Zn2+, Cu2+, and Cd2+, were also inhibitory. Of the divalent cations tested, only Co2+ and Mn2+ could replace Mg2+ in supporting histamine-stimulated adenylate cyclase activity. The nucleoside triphosphates GTP and ITP increased basal adenylate cyclase activity and markedly potentiated the stimulation by histamine. Preincubation of adenylate cyclase with 5′-guanylylimidodiphosphate dramatically increased enzyme activity; in this activated state, the adenylate cyclase was relatively refractory to further stimulation by histamine or F?. The subcellular distribution of histamine-sensitive adenylate cyclase activity was studied in subfractions from guinea pig cerebral cortex. The highest total and specific activities were observed in those fractions enriched in nerve endings, while adenylate cyclase activity was not detectable in the brain cytosol fraction. A possible physiological role for this histamine-sensitive adenylate cyclase in neuronal function is discussed.  相似文献   

4.
Low concentrations of Mn2+ supported the basal adenylate cyclase activity in crude and purified sarcolemmal membranes from cardiac muscle more effectively than did relatively high concentrations of Mg2+; at saturating concentrations the cyclase activities obtained with Mg2+ or Mn2+ were similar. In contrast, Mg2+ supported the basal cyclase activities of crude membrane fractions and purified sarcolemmal membranes from skeletal muscle far more effectively than did Mn2+; at saturating concentrations of either metal ion the Mg2+-supported cyclase activities were 5- to 10-fold greater than Mn2+-supported activities. Further, compared to Mg2+, Mn2+ supported the cyclase activities very poorly in all the primary subcellular fractions of skeletal muscle, whereas this cation was at least as effective as Mg2+ in all fractions of cardiac muscle. The apparent affinities of the cyclase for Mn2+ in heart as well as skeletal muscle appeared to be greater compared to those for Mg2+. The skeletal muscle cyclase displayed greater apparent affinity for MnATP2? (app. Km 0.10 mm) compared to MgATP2? (app. Km 0.32 mm) whereas the heart enzyme displayed greater apparent affinity for MgATP2? (app. Km 0.07 mm) compared to MnATP2? (app. Km 0.19 mm). Following preactivation with guanyl-5′-yl imidodiphosphate and isoproterenol, Mn2+ (0.15 to 2 mm) supported the cyclase activity of skeletal muscle even more effectively than did optimally effective concentrations of Mg2+. With the heart enzyme the relatively greater potency of Mn2+ persisted following preactivation. Significant enhancement in the Mn2+-sensitivity of skeletal muscle cyclase was also observed when assayed in the presence of GTP and isoproterenol or in the presence of NaF. Preactivation of both heart and skeletal muscle cyclases caused selective enhancement in the enzyme's apparent affinity for free Me2+ (Mg2+ or Mn2+) without influencing the apparent Km for MeATP2? (MgATP2? or MnATP2?). Evidences were obtained to show that the poor effectiveness of Mn2+ in supporting the basal activity of skeletal muscle cyclase is not related to (a) potentiation by Mn2+ of adenosine-mediated inhibition of the cyclase, (b) Mn2+-induced lability of the cyclase, (c) indirect effects of Mn2+ on ATP-regenerating system, or (d) the presence of different cation-specific molecular forms of the cyclase. It is also shown that the onset of enhanced Mn2+ sensitivity of the skeletal muscle enzyme following preactivation is not accompanied by a general loss of cation specificity of the cyclase. These results suggest that cations support the catalytic activity of adenylate cyclase by interacting with an enzymeregulatory free metal binding site and that the differential cation sensitivity of nonactivated (basal) cyclases from heart and skeletal muscle is likely due to differences in the properties of such an allosteric metal site. Furthermore, the metal site appears to undergo a conformational change following interaction of the cyclase system with the guanyl nucleotide and isoproterenol since the cation sensitivity of the cyclase and the relative potency of cations depend on the conformational status of the enzyme.  相似文献   

5.
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 Ka 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, K0.5= 23 mM). The data conform well to a model for two sets of independent sites and dopamine lowers the Ka 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 Ka 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.  相似文献   

6.
We have studied β-adrenergic stimulation of cyclic AMP formation in fragmented membranes and in unsealed or resealed ghosts prepared from rat reticulocytes. The maximal rate of isoprenaline-stimulated cyclic AMP formation with saturating MgATP concentrations and in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine was 5–8 nmol/min per ml ghosts are remained constant for at least 15 min. Transition from resealed ghosts to fragmented membranes was associated with a shift of the activation constant (Ka) for (±)-isoprenaline from 0.1 to 0.6 μM. The apparent dissociation constant for propranolol (0.01 μM) remained unchanged. The Ka values for isoprenaline in native reticulocytes and in resealed ghosts were identi The stimulating effect of NaF on cyclic AMP formation in resealed ghosts reached 15% of maximal β-adrenergic stimulation. Cyclic AMP formation, both in fragmented membranes and in ghosts, was half-maximally inhibited with Ca2+ concentrations ranging between 0.1 and 1 μM. GTP stimulated iosprenaline-dependent cyclic AMP formation in unsealed ghosts and in fragmented reticulocyte membranes by a factor of 3–5 but did not change the Ka value for isoprenaline. Ka values for the guanylnucleotides in different experiments varied between 0.3 and 2 μM. Ca2+ concentrations up to 4.6 μM reduced the maximal activation by GTP and Gpp(NH)p but did not affect their Ka values. Compared to GTP, maximal activation by Gpp(NH)p was higher in fragmented membranes, but much lower in ghosts. Our results suggest that the native β-receptor adenylate cyclase system of reticulocytes is more closely approximated in the ghost model than in fragmented membrane preparations. Membrane properties seem to modulate the actions of guanylnucleotides on isoprenaline-dependent cyclic AMP formation in ghosts. Some of these effects are not observed in isolated membranes.  相似文献   

7.
The inhibition of rat liver adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) by Pb2+ could be separated into an irreversible and a reversible component.Evidence was obtained that both types of inhibition were due to free Pb2+, rather than Pb/ATP, and that Pb2+ did not act via the site wherein Mg2+ and Mn2+ activate the cyclase.Guanine nucleotides strongly counteracted the reversible inhibition of cyclase by Pb2+, providing onother example of guanine nucleotide effects on adenylate cyclase function.It is suggested that the Pb2+-inhibited cyclase may be of value in the study of guanine nucleotide-cyclase interactions.  相似文献   

8.
Magnesium-dependent adenosine triphosphatase, purified from sheep kidney medulla using digitonin, has been characterized in a series of kinetic and magnetic resonance studies. Kinetic studies of divalent metal activation using either Mg2+ or Mn2+ indicate a biphasic response to divalent cations. Apparent Km values of 23 μm for free Mg2+ and 3.3 μm for free Mn2+ are obtained at low levels of added metal, while Km values of 0.50 mm for free Mg2+ and 0.43 mm for free Mn2+ are obtained at much higher levels of divalent cations. In all cases the kinetic data indicate that the binding of divalent metals is independent of the substrate, ATP. Kinetic studies of the substrate requirements of the Mg2+-ATPase also yield biphasic Lineweaver-Burk plots. At low ATP concentrations, kinetic studies yield apparent Km values for free ATP of 6.0 and 1.4 μm with Mg2+ and Mn2+, respectively, as the activating divalent metals. At much higher levels of ATP the response of the enzyme to ATP changes so that Km values for free ATP of 8.0 and 2.0 mm are obtained for Mg2+ and Mn2+, respectively. In both cases, however, the binding of ATP is independent of added metal. ADP inhibits the Mg2+-ATPase and the kinetic data indicate that ADP competes with ATP at both the high and low affinity sites. Dixon plots of the data are consistent with competitive inhibition at both ATP sites, with Ki values of 10.5 μm and 4.5 mm. Electron paramagnetic resonance and water proton relaxation rate studies show that the enzyme binds 1 g ion of Mn2+ per 469,000 g of protein. The Mn2+ binding studies yield a KD for Mn2+ at the single high affinity site of 2 μm, in good agreement with the kinetically determined activator constant for Mn2+ at low Mn2+ levels. Moreover, the EPR binding studies also indicate the existence of 34 weak sites for Mn2+ per single high affinity Mn2+ site. The KD for Mn2+ at these sites is 0.55 mm, in good agreement with the kinetic activator constant for Mn2+ of 0.43 mm, consistent with additional activation of the enzyme by the large number of weaker metal binding sites. The enhancement of water proton relaxation by Mn2+ in the presence of the enzyme is also consistent with the tight binding of a single Mn2+ ion per 469,000 Mr protein and the weaker binding of a large number of divalent metal ions. Analysis of the data yields a value for the enhancement for bound Mn2+ at the single tight site, ?b, of 5 and an enhancement at the 34 weak sites of 11. The frequency dependence of water proton relaxation by Mn2+ at the single tight site yields a dipolar correlation time (constant from 8–60 MHz) of 3.18 × 10?9 s. The kinetics and metal binding studies, together with the effect of temperature on ATPase activity at high and low levels of ATP, are consistent with the existence in this preparation of a single Mg2+-ATPase, with high and low affinity sites for divalent metals and for ATP. Observations of both high and low affinities for ATP have been made with two other purified ATPases. The similarities of these systems to the Mg2+-ATPase described here are discussed.  相似文献   

9.
Ca2+ inhibited the Mg2+-dependent and K+-stimulated p-nitrophenylphosphatase activity of a highly purified preparation of dog kidney (Na+ + K+)-ATPase. In the absence of K+, however, a Mg2+-dependent and Ca2+-stimulated phosphatase was observed, the maximal velocity of which, at pH 7.2, was about 20% of that of the K+-stimulated phosphatase. The Ca2+-stimulated phosphatase, like the K+-stimulated activity, was inhibited by either ouabain or Na+ or ATP. Ouabain sensitivity was decreased with increase in Ca2+, but the K0.5 values of the inhibitory effects of Na+ and ATP were independent of Ca2+ concentration. Optimal pH was 7.0 for Ca2+-stimulated activity, and 7.8–8.2 for the K+-stimulated activity. The ratio of the two activities was the same in several enzyme preparations in different states of purity. The data indicate that (a) Ca2+-stimulated phosphatase is catalyzed by (Na+ + K+)-ATPase; (b) there is a site of Ca2+ action different from the site at which Ca2+ inhibits in competition with Mg2+; and (c) Ca2+ stimulation can not be explained easily by the action of Ca2+ at either the Na+ site or the K+ site.  相似文献   

10.
The K+-dependent p-nitrophenylphosphatase activity catalyzed by purified (Na+ + K+)-ATPase from pig kidney shows substrate inhibition (Ki about 9.5 mM at 2.1 mM Mg2+). Potassium antagonizes and sodium favours this inhibition. In addition, K+ reduces the apparent affinity for substrate activation, whereas p-nitrophenyl phosphate reduces the apparent affinity for K+ activation. In the absence of Mg2+, p-nitrophenyl phosphate, as well as ATP, accelerates the release of Rb+ from the Rb+ occluded unphosphorylated enzyme. With no Mg2+ and with 0.5 mM KCl, trypsin inactivation of (Na+ + K+)-ATPase as a function of time follows a single exponential but is transformed into a double exponential when 1 mM ATP or 5 mM p-nitrophenyl phosphate are also present. In the presence of 3 mM MgCl2, 5 mM p-nitrophenyl phosphate and without KCl the trypsin inactivation pattern is that described for the E1 enzyme form; the addition of 10 mM KCl changes the pattern which, after about 6 min delay, follows a single exponential. These results suggest that (i) the shifting of the enzyme toward the E1 state is the basis for substrate inhibition of the p-nitrophenulphosphatase acitivy of (Na+ + K+)-ATPase, and (ii) the substrate site during phosphatase activity is distinct from the low-affinity ATP site.  相似文献   

11.
In hamster adipocyte ghosts, ACTH stimulates adenylate cyclase by a GTP-dependent process, whereas prostaglandin E E1, α-adrenergic agonists and nicotinic acid inhibit the enzyme by a mechanism which is both GTP- and sodium-dependent. The influence of the divalent cations Mn2+ and Mg2+, was studied on these two different, apparently receptor-mediated effects on the adipocyte adenylate cyclase. At low Mn2+ concentrations, GTP (1 μM) decreased enzyme activity by about 80%. Under this condition, ACTH (0.1 μM) stimulated the cyclase by 6- to 8-fold, and NaCl (100 mM) caused a similar activation. In the presence of both GTP and NaCl, prostaglandin E1 (1 or 10 μM) and nicotinic acid (30 μM) inhibited the enzyme by about 70–80% and epinephrine (300 μM, added in combination with a β-adrenergic blocking agent) by 40–50%. With increasing concentrations of Mn2+, the GTP-induced decrease and the NaCl-induced increase in activity diminished, with a concomitant decrease in prostaglandin E1?, nicotinic acid- and epinephrine-induced inhibitions as well as in ACTH-induced stimulation. At 1 mM Mn2+, inhibition of the enzyme was almost abolished and stimulation by ACTH was largely reduced, whereas activation of the enzyme by KF (10 mM) was only partially impaired. The uncoupling action of Mn2+ on hormone-induced inhibition was half-maximal at 100–200 μM and appeared not to be due to increased formation of the enzyme substrate, Mn · ATP. It occurred without apparent lag phase and could not be overcome by increasing the concentration of GTP. Similar but not identical findings with regard to adenylate cyclase stimulation and inhibition by hormonal factors were obtained with Mg2+, although about 100-fold higher concentrations of Mg2+ than of Mn2+ were required. The data indicate that Mn2+at low concentrations functionally uncouples inhibitory and stimulatory hormone receptors from adenylate adenylate cyclase in membrane preparations of hamster adipocytes, and they suggest that the mechanism leading to uncoupling involves an action of Mn2+ on the functions of the guanine nucleotide site(s) in the system.  相似文献   

12.
Magnesium-dependent adenosine triphosphatase has been purified from sheep kidney medulla plasma membranes. The purification, which is based on treatment of a kidney plasma membrane fraction with 0.5% digitonin in 3 mm MgCl2, effectively separates the Mg2+-ATPase from (Na+ + K+)-ATPase present in the same tissue and yields the Mg2+-ATPase in soluble form. The purified enzyme is activated by a variety of divalent cations and trivalent cations, including Mg2+, Mn2+, Ca2+, Co2+, Fe2+, Zn2+, Eu3+, Gd3+, and VO2+. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme shows two bands with Rf values corresponding to molecular weights of 150,000 and 77,000. The larger peptide is phosphorylated by [γ-32P]ATP, suggesting that this peptide may contain the active site of the Mg2+-ATPase. The Mg2+-ATPase activity is unaffected by the specific (Na+ + K+)-ATPase inhibitor ouabain.  相似文献   

13.
Summary The characteristics of the cholera toxin-stimulated adenylate cyclase of toad (Bufus marinus) and rat erythrocyte plasma membranes have been examined, with special emphasis on the response to purine nucleotides, fluoride, magnesium and catecholamine hormones. Toad erythrocytes briefly exposed to low concentrations of cholera toxin (40,000 to 60,000 molecules per cell) and incubated 2 to 4 hr at 30°C exhibit dramatic alterations in the kinetic and regulatory properties of adenylate cyclase. The approximateK m for ATP, Mg++ increases from about 1.8 to 3.4mm in the toxinstimulated enzyme. The stimulation by cholera toxin increases with increasing ATP, Mg++ concentrations, from 20% at low levels (0.2mm) to 500% at high concentrations (greater than 3mm). Addition of GTP, Mg++ (0.2mm) restores normal kinetic properties to the toxin-modified enzyme, such that stimulation is most simply explained by an elevation ofV max. GTP enhances the toxin-treated enzyme activity two-to fourfold at low ATP concentrations, but this effect disappears at high levels of the substrate. At 0.6mm ATP and 5mm MgCl2 the apparentK a for GTP, Mg++ is 5 to 10m. The control (unstimulated) enzyme demonstrates a very small response to the guanyl nucleotide. 5-ITP also stimulates the toxin-treated enzyme but cGMP, guanine, and the pyrimidine nucleotides have no effect. Cholera toxin also alters the activation of adenylate cyclase by free Mg++, decreasing the apparentK a from about 25 to 5mm. (–)-Epinephrine sensitizes the toad erythrocyte adenylate cyclase to GTP and also decreases the apparentK a for free metal. Sodium fluoride, which cause a 70- to 100-fold activation of enzyme activity, has little effect on sensitivity to GTP, and does not change the apparentK a for Mg++; moreover, it prevents modulation of these parameters by cholera toxin. Conversely, cholera toxin severely inhibits NaF activation, and in the presence of fluoride ion the usual three- to fivefold stimulation by toxin becomes a 30 to 60% inhibition of activity. The toxin-stimulated enzyme can be further activated by catecholamines; in the presence of GTP the (–)-epinephrine stimulation is enhanced by two- to threefold. The increased catecholamine stimulation of toad erythrocyte adenylate cyclase induced by cholera toxin is explained primarily by an increase in the maximal extent of activation by the hormones. Rat erythrocyte adenylate cyclase is also modified by cholera toxin. In the mammalian system the apparent affinity for the hormone appears to be increased. Cholera toxin thus induces profound and nearly permanent changes in adenylate cyclase by a unique process which mimics the stimulation by hormones in important ways, and which also accentuates the normal hormonal response. The relevance of these findings to the mechanism of action of cholera toxin is considered.Part of this work was reported at the 1974 meeting of the Federation of American Societies for Experimental Biology (Bennett & Cuatrecasas, 1974).  相似文献   

14.
Isolated hepatocytes converted exogenous [α-32P]ATP to cyclic [32P]AMP at high rates. This system was used for kinetic studies of the effects of glucagon, fluoride, free magnesium and free ATP4? on adenylate cyclase. In the absence or presence of glucagon, free Mg2+ activated adenylate cyclase by decreasing the Km for MgATP2? without changing V. Free ATP4? was not a potent inhibitor of adenylate cyclase and the only effect of glucagon was to increase V.Fluoride also increased the V of adenylate cyclase, but, in contrast to the results obtained with glucagon, the effect increased as the concentration of free Mg2+ increased. One explanation of the effect of fluoride, consistent with the idea that free Mg2+ activates adenylate cyclase and free ATP is not an inhibitor, is that fluoride increases the affinity of the enzyme for Mg2+. Weak inhibition of adenylate cyclase by ATP4? in the presence of fluoride cannot be excluded.  相似文献   

15.
We report that the adenylate cyclase system in human platelets is subject to multiple regulation by guanine nucleotides. Previously it has been reported that GTP is either required for or has little effect on the response of the enzyme to prostaglandin E1. We have found that when platelet lysates were prepared in the presence of 5 mM EDTA, GTP lowered the basal and prostaglandin E1-stimulated adenylate cyclase activity when the enzyme was assayed in the presence of Mg2+. The basal and prostaglandin E1-stimulated adenylate cyclase activities were also increased by washing, which presumably removes endogenous GTP. The analog, guanyl-5′-yl-imidodiphosphate mimics the inhibitory effect of GTP on prostaglandin E1-stimulated adenylate cyclase activity but it stimulates basal enzyme activity. The onset of the inhibitory effect of GTP on the adenylate cyclase system is rapid (1 min) and is maintained at a constant rate during incubation for 10 min. GTP and guanyl-5′-yl-imidodiphosphate were noncompetitive inhibitors of prostaglandin E1. An increase in the concentration of Mg2+ gradually reduces the effect of GTP while having little influence on the effect of guanyl-5′-yl-imidodiphosphate. Neither the substrate concentration nor the pH (7.2–8.5) is related to the inhibitory effect of guanine nucleotides. The inhibition by nucleotides was found to show a specificity for purine nucleotides with the order of potency being guanyl-5′-yl-imidodiphosphate > dGTP > GTP > ITP > XTP > CTP > TTP. The inhibitory effect of GTP is reversible while the effect of guanyl-5′-yl-imidodiphosphate is irreversible. The GTP inhibitory effect was abolished by preparing the lysates in the presence of Ca2+. However, the inhibitory effect of guanyl-5′-yl-imidodiphosphate persisted. Substitution of Mn2+ for Mg2+ in the assay medium resulted in a diminution of the inhibitory effect of GTP on basal activity and converted the inhibitory effect of GTP on prostaglandin E1-stimulated activity to a stimulatory effect. At a lower concentration of Mn2+ (less than 2 mM) guanyl-5′-yl-imidodiphosphate inhibited prostaglandin E1-stimulated adenylate cyclase activity, but at a higher concentration of Mn2+, it caused an increase in enzyme activity exceeding that occuring in the presence of prostaglandin E1. In the presence of Mn2+, dGTP mimics the effect of GTP and is 50% as effective as GTP. Our data suggest that the inhibitory effect of GTP on prostaglandin E1-stimulated adenylate cyclase is mainly due to its direct effect on the enzyme itself, whereas the stimulatory effect of GTP on prostaglandin E1-stimulated adenylate cyclase is due to enhancement of the coupling between the prostaglandin E1 receptor and adenylate cyclase. These studies also indicate that the method of preparation of platelet lysates can profoundly alter the nature of guanine nucleotide regulation of adenylate cyclase.  相似文献   

16.
Bovine lung soluble guanylate cyclase was purified to apparent homogeneity in a form that was deficient in heme. Heme-deficient guanylate cyclase was rapidly and easily reconstituted with heme by reacting enzyme with hematin in the presence of excess dithiothreitol, followed by removal of unbound heme by gel filtration. Bound heme was verified spectrally and NO shifted the absorbance maximum in a manner characteristic of other hemoproteins. Heme-deficient and heme-reconstituted guanylate cyclase were compared with enzyme that had completely retained heme during purification. NO and S-nitroso-N-acetylpenicillamine only marginally activated heme-deficient guanylate cyclase but markedly activated both heme-reconstituted and heme-containg forms of the enzyme. Restoration of marked activation of heme-deficient guanylate cyclase was accomplished by including 1 μM hematin in enzyme reaction mixtures containing dithiothreitol. Preformed NO-heme activated all forms of guanylate cyclase in the absence of additional heme. Guanylate cyclase activation was observed in the presence of either MgGTP or MnGTP, although the magnitude of enzyme activation was consistently greater with MgGTP. The apparent Km for GTP in the presence of excess Mn2+ or Mg2+ was 10 μM and 85–120 μM, respectively, for unactivated guanylate cyclase. The apparent Km for GTP in the presence of Mn2+ was not altered but the Km in the presence of Mg2+ was lowered to 58 μM with activated enzyme. Maximal velocities were increased by enzyme activators in the presence of either Mg2+ or Mn2+. The data reported in this study indicate that purified guanylate cyclase binds heme and the latter is required for enzyme activation by NO nitroso compounds.  相似文献   

17.
The interdependent effects of divalent cations, pH, and various activators of adenylate cyclase were examined in partially purified plasma membranes from rat liver. This adenylate cyclase was found to exhibit largely alkaline pH optima, in the range of 8.3 to 9.3, for the expression of basal activity, and activities with GTP, GPP(NH)P, prostaglandin E1 and GTP, and N6-(phenylisopropyl)adenosine and GTP. Glucagon and GTP, while increasing activity 8- to 10-fold, shifted the optimum activity to about pH 7.5. However, stimulation of the enzyme by 10 mm NaF or 3 mm Na3VO4 was strikingly dependent on pH. In both cases activation was optimal at pH values between 6.3 and 7.3, though above about pH 8.5 fluoride was barely stimulatory and vanadate was slightly inhibitory. This effect of elevated pH to reduce fluoride- or vanadate-stimulated activity could be prevented by glucagon plus guanine nucleotide, but could not be reversed once activity was lowered during preincubation. The data suggest that this effect was not due to the formation of an inhibitor of adenylate cyclase per se, nor to an artifact of assay methods. The effect of elevated pH was more pronounced with Mn2+ as activating cation than with Mg2+. With fluoride and lower pH adenylate cyclase was essentially Mn2+ requiring, whereas with fluoride and higher pH activity was comparable with either cation. The data suggested that combinations of pH, divalent cation, and activating ligand dictate the interactions of the constitutive subunits of the adenylate cyclase and provide additional criteria with which current models for the regulation of adenylate cyclase may be tested.  相似文献   

18.
ATP and the divalent cations Mg2+ and Ca2+ regulated K+ stimulation of the Ca2+-transport ATPase of cardiac sarcoplasmic reticulum vesicles. Millimolar concentrations of total ATP increased the K+-stimulated ATPase activity of the Ca2+ pump by two mechanisms. First, ATP chelated free Mg2+ and, at low ionized Mg2+ concentrations, K+ was shown to be a potent activator of ATP hydrolysis. In the absence of K+ ionized Mg2+ activated the enzyme half-maximally at approximately 1 mM, whereas in the presence of K+ the concentration of ionized Mg2+ required for half-maximal activation was reduced at least 20-fold. Second MgATP apparently interacted directly with the enzyme at a low affinity nucleotide site to facilitate K+-stimulation. With a saturating concentration of ionized Mg2+, stimulation by K+ was 2-fold, but only when the MgATP concentration was greater than 2 mM. Hill plots showed that K+ increased the concentration of MgATP required for half-maximal enzymic activation approx. 3-fold.Activation of K+-stimulated ATPase activity by Ca2+ was maximal at anionized Ca2+ concentration of approx. 1 μM. At very high concentrations of either Ca2+ or Mg2+, basal Ca2+-dependent ATPase activity persisted, but the enzymic response to K+ was completely inhibited. The results provide further evidence that the Ca2+-transport ATPase of cardiac sarcoplasmic reticulum has distinct sites for monovalent cations, which in turn interact allosterically with other regulatory sites on the enzyme.  相似文献   

19.
Smooth muscle adenylate cyclase of a membrane preparation of canine gastric antrum has been characterized, and the effect of hormonal and neuronal agents examined. The enzyme is active in the presence of Mg2+ or Mn2+, but is inhibited by Ca2+. The Km is 0.5 mM ATP, similar to the Km of skeletal muscle adenylate cyclase. The enzyme is activated by isoproterenol but not norepinephrine, consistent with a β2-catecholamine receptor-adenylate cyclase interaction. Secretin activates the enzyme in concentrations as low as 1 · 10?11 M, while glucagon was effective only at 1 · 10?6 M. Prostaglandin E1 and E2 have a biphasic effect with activation of adenylate cyclase at 1 · 10?5 M and a small but significant inhibition of enzyme activity at 1 · 10?11 M.  相似文献   

20.
Calmodulin-depleted isotonic erythrocyte ghosts contain 200 ng residual calmodulin/mg protein which is not removed by extensive washings at pCa2+ > 7. Specific activity and Ca2+-affinity of the (Ca2+ + Mg2+)ATPase increase at increasing calmodulin, with K0.5 Ca of 0.38 μM at calmodulin concentrations corresponding to that in erythrocytes. High Ca2+ concentrations inhibit the enzyme. Specific activity and Ca2+-affinity of the enzyme decrease at increasing Mg2+ concentrations. The Ca2+ ? Mg2+ antagonism is likewise observed at inhibitory Ca2+ concentrations.  相似文献   

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