Calmodulin stimulation of adenylate cyclase of intestinal epithelium

The effect of dicyclohexylcarbodiimide (DCCD) on the proton pumping two-subunit cytochrome c oxidase from Paracoccus denitrificans was investigated. Purified Paracoccus oxidase was reconstituted into phospholipid vesicles by cholate dialysis. Following incubation with increasing amounts of DCCD, proton ejection was recorded in response to reductant pulses with reduced cytochrome c. Concentrations of DCCD which greatly reduced proton pumping by bovine cytochrome c oxidase used as a control were found to exert only a minor effect on proton translocation by Paracoccus oxidase. Similarly, incubation of the bacterial enzyme with [14C]DCCD failed to reveal the specific covalent interaction previously demonstrated to occur with bovine cytochrome c oxidase, and here also shown for the oxidase of yeast. Thus, Paracoccus oxidase differs in its interaction with DCCD from the functionally analogous eukaryotic enzymes.     

Galanin inhibits adenylate cyclase of rat brain membranes.

The ubiquitous neuropeptide, galanin, strongly inhibits adenylate cyclase in rat brain membranes. While basal enzyme activity was not altered, galanin from 10(-11) M to 5 x 10(-7) M decreased forskolin- and VIP-stimulated adenylate cyclase with a half-maximal effect being elicited by 0.7 nM neuropeptide and a maximal 80% inhibition of the enzyme activity. The galanin fragments (2-29) and (1-15) dose-dependently inhibited the forskolin-stimulated adenylate cyclase, while the fragments (3-29) and (10-29) were found inactive. These results indicate that the regulatory action of galanin in the central nervous system involves the coupling of galanin receptors to the inhibition of the adenylate cyclase system.     

The antilipolytic effect of insulin does not require adenylate cyclase or phosphodiesterase action

Insulin antagonized the lipolytic actions of epinephrine in rat epididymal adipocytes when the phosphodiesterase inhibitor, Ro 20-1724, was present. Adipocytes were depleted of functional cAMP by inhibiting adenylate cyclase with N6-phenylisopropyladenosine in the presence of adenosine deaminase such that Ro 20-1724 no longer stimulated lipolysis. The cAMP analogs 8-thioisopropyl-cAMP or 8-thiomethyl-cAMP, which are resistant to phosphodiesterase hydrolysis, were subsequently added to bypass adenylate cyclase and phosphodiesterase action. Under these conditions, insulin antagonized the lipolytic effects of these analogs, even in the presence of Ro 20-1724.     

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.     

Domoic acid inhibits adenylate cyclase activity in rat brain membranes

Adenylate cyclase activity measured by the formation of cyclic AMP in rat brain membranes was inhibited by a shellfish toxin, domoic acid (DOM). The inhibition of enzyme was dependent on DOM concentration, but about 50% of enzyme activity was resistant to DOM-induced inhibition. Rat brain supernatant resulting from 105,000×g centrifugation for 60 min, stimulated adenylate cyclase activity in membranes. Domoic acid abolished the supernatant-stimulated adenylate cyclase activity. The brain supernatant contains factors which modulate adenylate cyclase activity in membranes. The stimulatory factors include calcium, calmodulin, and GTP. In view of these findings, we examined the role of calcium and calmodulin in DOM-induced inhibition of adenylate cyclase in brain membranes. Calcium stimulated adenylate cyclase activity in membranes, and further addition of calmodulin potentiated calcium-stimulated enzyme activity in a concentration dependent manner. Calmodulin also stimulated adenylate cyclase activity, but further addition of calcium did not potentiate calmodulin-stimulated enzyme activity. These results show that the rat brain membranes contain endogenous calcium and calmodulin which stimulate adenylate cyclase activity. However, calmodulin appears to be present in membranes in sub-optimal concentration for adenylate cyclase activation, whereas calcium is present at saturating concentration. Adenylate cyclase activity diminished as DOM concentration was increased, reaching a nadir at about 1 mM. Addition of calcium restored DOM-inhibited adenylate cyclase activity to the control level. Similarly, EGTA also inhibited adenylate cyclase activity in brain membranes in a concentration dependent manner, and addition of calcium restored EGTA-inhibited enzyme activity to above control level. The fact that EGTA is a specific chelator of calcium, and that DOM mimicked adenylate cyclase inhibition by EGTA, indicate that calcium mediates DOM-induced inhibition of adenylate cyclase activity in brain membranes. While DOM completely abolished the supernatant-, and Gpp (NH)p-stimulated adenylate cyclase activity, it partly blocked calmodulin-, and forskolin-stimulated adenylate cyclase activity in brain membranes. These results indicate that DOM may interact with guanine nucleotide-binding (G) protein and/or the catalytic subunit of adenylate cyclase to produce inhibition of enzyme in rat brain membranes.     

Calmodulin activates adenylate cyclase from rabbit heart plasma membranes

It was shown that calmodulin (CM) activates the adenylate cyclase (AC) of rabbit heart light sarcolemma in the presence of micromolar free Ca2+ concentrations and this effect is blocked by trifluoroperazine and troponin I. GTP (in the presence of isoproterenol) and Gpp(NH)p are able to increase the CM-dependent activity of enzyme. It was concluded that there is no special CM-dependent "form' of AC in the heart and the common catalytic component of AC can be regulated both by CM and guanine nucleotide-binding regulatory component (N-protein). In the presence of Ca2+ and guanine nucleotide heart AC exists as a complex: CM-catalytic component-N-protein.     

The role of GTP in prostaglandin E1 stimulation of adenylate cyclase in platelet membranes.

Adenylate cyclase activity in platelet membrane preparations was measured in the presence of prostaglandin E1 (PGE1), GTP and a non-hydrolysable analogue of GDP, guanosine 5'-[beta-thio]diphosphate (GDP[beta S]). A dose-dependent inhibition of adenylate cyclase by GDP[beta S] was observed that could be reversed either by adding increased amounts of GTP or of PGE1.     

A serotonin sensitive adenylate cyclase in mature rat brain synaptic membranes

Results from this study have indicated serotonin-sensitive adenylate cyclase activity in adult rat brain. The enzyme is localized in the synaptosomal plasma membrane and apparently has multiple activation sites for serotonin with specific activity maxima occuring at serotonin concentrations of 5 × 10^?10, 5 × 10^?9, 1 × 10^?8, and 5 × 10^?8 moles/liter. The production of cyclic AMP at these sites appears to be unaffected by 1 × 10^?5M fluphenazine, while 1 × 10^?5M tryptamine, methysergide, and ergonovine decreased the stimulatory effect of 1 × 10^?8M 5-HT by 30 percent, 80 percent, and 57 percent respectively.     

Cholera toxin does not impair hormonal inhibition of adenylate cyclase and concomitant stimulation of a GTPase in adipocyte membranes

Factor H of the human complement system exhibits an unusual circular dichroism spectrum. The CD spectrum of Factor H exhibits a positive extreme at 230 nm and a negative extreme at 190 nm. No apparent alpha-helical or beta-sheet conformations were present in the native protein structure. However, when the disulfide bridges are reduced, followed either by reoxidation or alkylation, the structure of Factor H is modified so that it now exhibits conventional protein secondary structure as determined from its CD spectra in the far ultraviolet region. Factor H also fails to mediate its regulatory function of inhibiting the alternative pathway convertase once the disulfides have been ruptured and conformational rearrangement has occurred. CD studies indicate that minor conformational changes take place when Factor H and C3b associate in free solution.     

Activation of adenylate cyclase by the diterpene forskolin does not require the guanine nucleotide regulatory protein

Forskolin, a novel diterpene activator of adenylate cyclase in membranes and intact cells, activates the enzyme in membranes from mutant cyc-S49 murine lymphoma cells and the soluble enzyme from rat testes. Each of these enzymes consists only of the catalytic subunit and does not have a functional guanine nucleotide-binding protein. In both cases forskolin converts the manganese-dependent enzymes to a form which does not require manganese for activity. Forskolin can also stimulate a detergent-solubilized preparation of adenylate cyclase from rat cerebral cortex. Activation of adenylate cyclase by forskolin is therefore not dependent on a perturbation of membrane structure nor does it require a functional guanine nucleotide-binding subunit.     

Calmodulin regulation of adenylate cyclase activity

Calmodulin-dependent stimulation of adenylate cyclase was initially thought to be a unique feature of neural tissues. In recent years evidence to the contrary has accumulated, calmodulin-dependent stimulation of adenylate cyclase now being demonstrated in a wide range of structurally unrelated tissues and species. Demonstration of the existence of calmodulin-dependent adenylate cyclase has in nearly all instances required the removal of endogenous calmodulin. It is not yet clear whether calmodulin-dependent and calmodulin-independent forms of the enzyme exist and whether some tissues (such as heart) lack a calmodulin-dependent adenylate cyclase. The presence of calmodulin appears largely responsible for the ability of the adenylate cyclase enzyme to be stimulated by submicromolar concentrations of calcium; it may not be relevant to the inhibition of the enzyme which occurs at higher concentrations of calcium. The physical relationship of calmodulin to the plasma membrane bound enzyme (or to the soluble forms of the enzyme) is not known nor is the mechanism of adenylate cyclase activation by calmodulin clear; current data suggest some involvement with both the N and C units of the enzyme. Finally, it is possible that in vivo calcium contributes to the duration of the hormone stimulated cyclic AMP signal. Thus current in vitro data suggest that optimal hormonal activation of calmodulin-dependent adenylate cyclase occurs at very low intracellular calcium concentrations, comparable to those found in the resting cell; conversely the enzyme is inhibited as intracellular calcium increases, following for example agonist stimulation of the cell. These higher calcium concentrations would then activate calmodulin-dependent phosphodiesterase. Such differential effects of calcium on adenylate cyclase and phosphodiesterase would ultimately restrict the duration of the hormone-induced cyclic AMP signal.     

Endogenous GTP and the regulation of epinephrine stimulation of adenylate cyclase.

Epinephrine increased adenylate cyclase activity 10 to 15 fold in lysates of the cultured human astrocytoma cell line 132-1N1. GTP had little effect on adenylate cyclase activity of lysed cell preparations either with or without added epinephrine. However, the epinephrine stimulation of adenylate cyclase was essentially lost (less than 90%) when a washed nuclei-free membrane preparation of the cyclase was assayed. A 10 to 15 fold epinephrine stimulation of the membrane adenylate cyclase could be demonstrated if cytosol of GTP were added to the assay with the hormone. The criteria of anion exchange, cation exchange, gel exclusion and paper chromatography indicated that the cytosolic agents which acted synergistically with hormones were GTP and GDP. The apparent Kact's for the synergistic action of GDP and GTP were essentially identical (1.0 muM) and of all the other nucleotides examined only GDP had a potency similar to GTP. However, the effect of GDP was apparently due to its rapid conversion to GTP even in the absence of a regenerating system. With epinephrine pretreatment of the intact 132-1N1 cells there was a specific loss of epinephrine stimulation of adenylate cyclase activity. The hormone pretreatment did not alter the capacity of the cytosol from these desensitized cells to potentiate epinephrine stimulation of the cyclase. Rather, the alteration was in the particulate fraction of the lysate. The desensitization of the membranous cyclase was stable and not reversed by GTP.     

Prostacyclin stimulation of adenylate cyclase activity in human thyroid membranes

Effect of prostacyclin (PGI2) on adenylate cyclase activity in human thyroid membranes was examined. PGI2 caused a dose- and time-dependent production of cyclic AMP (cAMP) with high potency. When GTP was added in concentrations up to 100 uM, the activation of adenylate cyclase by PGI2 was increased. In the assay medium containing 3 mM ATP, 10 uM GTP and nucleotide regenerating system, the replacement of Mg2+ by increasing concentrations of Mn2+ caused a progressive loss of PGI2 as well as TSH-stimulated adenylate cyclase activities, while high concentrations of Mg2+ (12 or 18 mM) slightly suppressed the activity stimulated by either PGI2 or TSH. Both agents had an additive effect on the stimulation of adenylate cyclase activity in the presence of either 6 mM Mg2+ or 6 mM Mn2+. Gamma-globulin fraction containing non-stimulatory TSH receptor antibody which was prepared from a patient with chronic thyroiditis, suppressed only TSH- but not PGI2-stimulation of the adenylate cyclase activity. These results suggest that PGI2 can stimulate the adenylate cyclase activity in human thyroid tissue, and that PGI2-stimulation may be mediated by the different system from TSH-dependent one.     

Irreversible stimulation of adenylate cyclase activity of fat cell membranes of phosphoramidate and phosphonate analogs of GTP.

The ability of 5'-guanylylimidodiphosphate (Gpp(NH)p) to stimulate irreversibly the adenylate cyclease activity of fat cell membranes has been studied by preincubating the membranes with this or related analogs followed by assaying after thoroughly washing the membranes. Activation can occur in a simple Tris-HCl buffer, in the absence of added divalent cations and in the presence of EDTA. Dithiothreitol enhances the apparent degree of activation, perhaps by stabilization. The importance of utilizing optimal conditions for stabilizing enzyme activity, and of measuring the simultaneous changes in the control enzyme, is illustrated. The organomercurial, p-aminophenylmercuric acetate, inhibits profoundly the activity of the native as well as the Gpp(NH)p-stimulated adenylate cyclase, but in both cases subsequent exposure to dithiothreitol restores fully the original enzyme activity. However, the mercurial-inactivated enzyme does not react with Gpp(NP)p, as evidenced by the subsequent restoration of only the control enzyme activity upon exposure to dithiothreitol. Thus, reaction with Gpp(NH)p requires intact sulfhydryl groups, but the activated state is not irreversibly destroyed by the inactivation caused by sulfhydryl blockade. GTP and, less effectively, GDP and ATP inhibit activation by Gpp(NH)p, but interpretations are complicated by the facts that this inhibition is overcome with time and that GTP and ATP can protect potently from spontaneous inactivation. These two nucleotides can be used in the Gpp(NH)p preincubation to stabilize the enzyme. The Gpp(NH)p-activated enzyme cannot be reversed spontaneously during prolonged incubation at 30 degrees C in the absence or presence of GTP, ATP, MgCl2, glycine, dithiothreitol, NaF or EDTA. The strong nucleophile, neutral hydroxylamine, decreases the Gpp(NH)p-activated enzyme activity and no subsequent activation is detected upon re-exposure to the nucleotide.     

NAD+-mediated stimulation of adenylate cyclase in cardiac membranes

NAD+ significantly enhances adenylate cyclase activity in crude cardiac membrane preparations. This increase is dose-dependent, does not occur in the presence of nicotinamide, ADP-ribose or NADP+, and can be effected by a 30 min pre-incubation period with NAD+. Time course studies are consistent with an enzymatically mediated modification that used NAD+ as substrate. Furthermore, inhibition of NAD+-mediated activation by arginine suggests that this modulation of cardiac adenylate cyclase is analogous to that catalyzed by endogenous ADP-ribosyl transferases.     

Calcitonin-sensitive adenylate cyclase in rat renal tubular membranes.

1. Renal tubular membranes from rat kidneys were prepared, and adenylate cyclase activity was measured under basal conditions, after stimulation by NaF or salmon calcitonin. Apparent Km value of the enzyme for hormone-linked receptor was close to 1 x 10(-8) M. 2. The system was sensitive to temperature and pH. pH was found to act both on affinity for salmon calcitonin-linked receptor and maximum stimulation, suggesting an effect of pH on hormone-receptor binding and on a subsequent step. 3. KCl was without effect areas whereas CoCl and CaCl2 above 100 muM and MnCl2 above 1 muM inhibited F- -and salmon calcitonin-sensitive adenylate cyclase activities. The Ca2+ inhibition of the response reflected a fall in maximum stimulation and not a loss of affinity of salmon calcitonin-linked receptor for the enzyme. 4. The measurement of salmon calcitonin-sensitive adenylate cyclase activity as a function of ATP concentration showed that the hormone increases the maximum velocity of the adenylate cyclase. GTP, ITP and XTP at 200 muM did not modify basal, salmon calcitonin- and parathyroid hormone-sensitive adenylate cyclase activities. 5. Basal, salmon calcitonin- and F- -sensitive adenylate cyclase activities decreased at Mg2+ concentrations below 10 mM. High concentrations of Mg2+ (100 mM) led to an inhibition of the F- -stimulated enzyme. 6. Salmon calcitonin-linked receptor had a greater affinity for adenylate cyclase than human or porcine calcitonin-linked receptors. There was no additive effect of these three calcitonin peptides whereas parathyroid hormone added to salmon calcitonin increased adenylate cyclase activity, thus showing that both hormones bound to different membrane receptors. Human calcitonin fragments had no effect on adenylate cyclase activity. 7. Salmon calcitonin-stimulated adenylate cyclase activity decreased with the preincubation time. This was due to progressive degradation of the hormone and not to the rate of binding to membrane receptors.     

Osteosarcoma cytosol factor promotes parathyroid hormone stimulation of adenylate cyclase independent of GTP

The adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1)-stimulating factor from rat osteosarcoma cytosol was purified 600-fold by ion-exchange chromatography. The factor has an apparent M_r of 20 000, is cold-labile, but retains activity at ?20°C in 10% glycerol.The factor enhanced parathyroid hormone stimulation of adenylate cyclase and restored hormone responsiveness to membranes washed with 0.5 M NaCl. These ‘GTP-like’ effects were not inhibited by 100 μM GDP-β-S, which completely abolished the GTP enhancement of both basal and hormone-stimulated adenylate cyclase.Adenylate cyclase activity in the presence of the stimulating factor was linear with time, and showed hyperbolic dependence on factor concentration. The factor also linearized (in double reciprocal plots) the downward-concave Mg²⁺-dependence of adenylate cyclase, increasing the apparent affinity of the enzyme for Mg²⁺.The presence of the factor in two clonal osteosarcoma cell lines correlated with parathyroid hormone-stimulatable adenylate cyclase. Factor stimulation was absent while GTP stimulation was retained in the hormone-nonresponsive clone. Factor and hormone sensitivity were restored by in vivo passage. This factor thus may represent a guanyl nucleotide-independent path for cellular regulation of hormone response.     

Serotonin sensitive adenylate cyclase in horse brain synaptosomal membranes.

In different membranal preparations isolated from horse brain stritum we have shown the existence of an adenylate cyclase system sensitive to serotonin (5-HT). Activation of the adenylate cyclase was determined by measuring cAMP using a radioimmunoassay. This serotoninergic sensitive enzyme is characterized by a high apparent affinity constant (in the nanomolar range), located on synaptosomal membranes. It is inhibited by antiserotoninergic drugs (cyproheptadine, cinanserin, methysergide, LSD), and synergistically activated by GTP. This serotoninergic activation is clearly additive to the activation induced by dopamine. It appears different from the adenylate cyclase system previously described in the literature which is also activated by 5-HT, but which has a low apparent affinity constant (in the micromolar range); the latter is apparently located in non-synaptosomal membranes, and its activation by 5-HT is non-additive to the activation induced by dopamine.The serotoninergic sensitive adenylate cyclase reported in this study, might be related to the serotoninergic binding system which we have previously described which has similar affinity constant, a similar subcellular distribution and which is inhibited in the same concentration ranges by antiserotoninergic drugs. These two systems might represent a synaptosomal serotoninergic receptor complex.