Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P01275 (glucagon)
 26,492 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have previously shown that liver plasma membrane (Ca2+-Mg2+)-ATPase activity is inhibited by glucagon. To investigate the possible involvement of a GTP-binding (G) protein in this regulation, we have examined the effects of pertussis toxin and cholera toxin on inhibition of (Ca2+-Mg2+)-ATPase by glucagon. Treatment of liver plasma membranes with pertussis toxin did not affect the sensitivity of (Ca2+-Mg2+)-ATPase to the hormone. In contrast, treatment of plasma membranes or prior injection of animals with cholera toxin prevented inhibition of the (Ca2+-Mg2+)-ATPase by glucagon. Even though adenylate cyclase activity was increased by cholera toxin treatment, addition of cyclic AMP did not mimic the effect of cholera toxin in blocking glucagon-mediated inhibition of (Ca2+-Mg2+)-ATPase activity. These data suggest that a cholera toxin-sensitive protein, perhaps Gs or a Gs-like protein, is involved in the regulation of liver (Ca2+-Mg2+)-ATPase activity. The results emphasize the possible role of Gs-like proteins in regulation of enzymes other than adenylate cyclase and suggest that the study of (Ca2+-Mg2+)-ATPase may provide a useful enzymatic system to examine such regulation.
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PMID:Cholera toxin blocks glucagon-mediated inhibition of the liver plasma membrane (Ca2+-Mg2+)-ATPase. 295 93

Sucrose feeding has been shown previously to alter the plasma concentration of several factors which may regulate beta-adrenergic receptors, including corticosteroids and insulin as well as altered sympathetic nervous system (SNS) tone. For this reason we initiated a study of the effects of sucrose feeding on the beta-adrenergic receptor-adenylate cyclase system in rat liver plasma membranes. Beta-Adrenergic responsiveness was monitored by measuring isoproterenol stimulation of adenylate cyclase activity, while beta-adrenergic receptor characteristics were evaluated by analyzing [125I]iodocyanopindolol [( 125I]CYP) binding. Rats fed rat chow ad lib. supplemented by drinking water containing 10% sucrose solution exhibited a 50-75% reduction in hepatic isoproterenol-sensitive adenylate cyclase activity. This effect of sucrose was also observed in adrenalectomized (ADX) and 6-hydroxydopamine-pretreated animals, ruling out a causal role for corticosteroids or the sympathetic nervous system respectively. No effect was observed on basal, glucagon-, fluoride- or GTP-stimulated adenylate cyclase. A small but significant decrease in [125I]CYP specific binding capacity was observed in liver membranes prepared from sucrose-fed ADX rats, whereas no change in [125I]CYP binding capacity was observed in in sucrose-fed normal rats. These observations suggest that beta-receptor to adenylate cyclase coupling efficiency is decreased by the sucrose diet. The activities of two membrane-associated phospholipid methyltransferases and the content of endogenous S-adenosylmethionine in liver were reduced by sucrose feeding, implying a defect in the methylation pathway for phosphatidylcholine synthesis. The possible relationship between this latter finding and the observed decrease in beta-adrenergic receptor to adenylate cyclase coupling efficiency is discussed.
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PMID:Evidence for a decrease in the efficiency of beta-receptor coupling to adenylate cyclase in liver membranes from sucrose-fed rats. 298 31

To characterize the molecular components of the vasoactive intestinal peptide (VIP) receptor in human intestine, [125I]VIP was covalently bound to human colonic epithelial membranes using dithio-bis(succinimidyl propionate). Sodium dodecyl sulfate-polyacrylamide gel autoradiographic studies of affinity labeled membranes revealed three major bands corresponding to [125I]VIP-protein complexes of 66,000, 33,000, and 16,000 mol wt. Labeling of the 66,000 and 33,000 mol wt complexes was specific, since it was abolished by VIP, while labeling of the 16,000 mol wt complex was not. Densitometric scanning of autoradiographs indicated that labeling of the 66,000 mol wt complex was inhibited by low VIP concentrations in the 10(-10)-10(-8) M range, but was unaffected by glucagon or octa-cholecystokinin. It was also reduced by VIP-(2-28), with a potency 1/100th that of VIP, and by GTP in the concentration range of 10(-7)-10(-3) M. The 33,000 mol wt complex behaved similarly to the 66,000 mol wt complex with respect to specificity and GTP sensitivity, but differed in one major feature, its affinity for VIP. Its labeling was inhibited by native VIP concentrations in the 10(-9)-10(-7) M range. Assuming one molecule of [125I]VIP bound per molecule of protein, two proteins of 63,000 and 30,000 mol wt were identified as VIP-binding sites. The 63,000 mol wt protein had the properties expected for the VIP receptor coupled to adenylate cyclase in human colon, while the 30,000 mol wt protein was a low affinity binding site. Treatment of human colonic membranes with the sulfhydryl reducing agent dithiothreitol before [125I]VIP binding strongly reduced the labeling of the two proteins. This finding does not support the hypothesis that the low affinity 30,000 mol wt binding site may be a monomer of the high affinity binding site.
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PMID:The human vasoactive intestinal peptide receptor: molecular identification by covalent cross-linking in colonic epithelium. 298 95

The effects of Mg2+ and guanine nucleotides on glucagon binding to its receptor were studied using [125I-Tyr10]monoiodoglucagon. Contrary to findings with beta-adrenergic receptors, high affinity binding of the stimulatory hormone was not dependent on Mg2+ and low affinity binding could be obtained on nucleotide addition regardless of presence of Mg2+. GDP, guanyl-5'-yl thiophosphate (GDP beta S), GTP, and guanyl-5'-yl imidodiphosphate (GMP-P(NH)P) were all able to induce low affinity hormone binding. Since the Ns component of adenylyl cyclase, with which the receptor interacts, is inactive in stimulating the catalytic component C of adenylyl cyclase in the absence of Mg2+, both before and after GDP addition, it is suggested that Ns has at least two domains that change conformation independently of each other: a r domain, that interacts with the receptor and confers to it high affinity binding, and a c domain, that interacts with the catalyst C and stimulates it. It is suggested further that Ns is r+c- when stabilizing the receptor in its conformation with high affinity for hormone, and r-c- when under the influence of GDP which results in the receptor adopting the conformation that exhibits low affinity for the hormone. Comparison of potencies of the four nucleotides to induce low affinity binding showed that GDP and GDP beta S were equipotent and 10 times more potent than GTP and 100 times more potent than GMP-P(NH)P. Under the conditions used it was impossible to substantiate that the effects of GTP or GMP-P(NH)P were not due to formation of GDP from GTP or presence of GDP-like material in GMP-P(NH)P. It is suggested that, contrary to widely held opinions, GDP and GDP-like compounds, and not GTP or its analogs, are responsible for the lowering of the affinity of adenylyl cyclase stimulating receptors for their hormones or agonists. Furthermore, the experiments suggest that the c+ conformation of the c domain of Ns co-exists with the r+ and not the r- conformation of its r domain.
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PMID:Regulation of glucagon receptor binding. Lack of effect of Mg and preferential role for GDP. 298 62

GTP-sensitive adenylate cyclases in liver membranes achieved by glucagon and by cholera toxin pretreatment displayed similar responses to added GTP in assay with respect to magnitude and sensitivity. However, their susceptibility to GTP formed during incubation from added GDP catalyzed by membrane-associated nucleoside diphosphate kinase(mNDPK) was different. Adenylate cyclase pretreated with cholera toxin was essentially unaffected by added GDP, while further addition of glucagon produced activation. GTP-stimulated adenylate cyclase activity in toxin-treated membranes was inhibited by added GDP, whereas glucagon addition reduced the inhibitory action of GDP by two orders of magnitude. Since neither pretreatment with toxin nor glucagon addition altered GTP formation by mNDPK, these observations suggest a possible presence of a mechanism by which hormone makes adenylate cyclase susceptible to the GTP formed via mNDPK for activation.
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PMID:Differential susceptibility to GTP formed from added GDP via membrane-associated nucleoside diphosphate kinase of GTP-sensitive adenylate cyclases achieved by hormone and cholera toxin. 299 47

We have analyzed ligand-receptor complexes resulting from (i) the incubation of canine hepatic plasma membranes with [125I]iodoglucagon and (ii) subsequent gentle solubilization of receptor-bound ligand with digitonin. The complexes (molecular weight approximately equal to 500,000) retain the radiolabeled ligand during gel filtration and subsequent manipulation at 4 degrees C in the absence of covalent crosslinking. Affinity chromatography of the glucagon-receptor complexes on columns of wheat germ lectin linked to agarose resulted in two fractions, one of which was not bound by the column and the other of which was specifically eluted by N-acetylglucosamine. The presence of GTP during the incubation of plasma membranes with [125I]iodoglucagon caused about a 50% decrease in total ligand binding but affected only the ligand-receptor complexes that bound to wheat germ lectin. Moreover, it was found that the proportion of the two forms of ligand-receptor complexes identified by chromatography on wheat germ lectin depended on the degree of saturation of the membrane receptor. Thus, both the inhibition by glucagon of radiolabeled glucagon binding to membranes and the concomitantly decreased extent of association of the radiolabeled ligand with solubilized receptor complexes could be modeled in terms of two noninteracting receptor populations (having dissociation constants of about 0.35 and 4.94 X 10(-9) M). We conclude that (i) glucagon-receptor complexes formed on canine hepatic plasma membranes exist in two forms that differ after solubilization by digitonin in their avidities for wheat germ lectin, (ii) the high-and low-affinity binding of glucagon characteristic of hepatic plasma membranes arises from distinct receptor populations that probably differ in glycosylation, and (iii) the effect of GTP to decrease binding of glucagon to membranes arises from interactions of the nucleotide with the receptor complex that binds to wheat germ lectin.
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PMID:Identification of distinct receptor complexes that account for high-and low-affinity glucagon binding to hepatic plasma membranes. 299 90

Specific binding of vasoactive intestinal peptide (VIP) to epithelial cell membranes of rat ventral prostate was reversible, saturable and dependent on time and temperature. The data suggested the presence of two classes of VIP receptors: a class with high affinity (Kd = 1.7 nM) and low binding capacity (0.5 pmol VIP/mg protein), and another class with low affinity (Kd = 36.2 nM) and high binding capacity (7.5 pmol VIP/mg protein). Chicken VIP and porcine secretin recognized VIP receptors but exhibited a 10-fold higher and a 40-fold lower affinity than porcine VIP, respectively. However, glucagon, somatostatin, Met-enkephalin and cholecystokinin were ineffective. GTP inhibited markedly the interaction of VIP with membranes by increasing the rate of dissociation of peptide bound to its receptors. GDP and Gpp(NH)p behaved as GTP but other purine nucleotides and nucleosides did not show any effect.
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PMID:VIP binding to epithelial cell membranes of rat ventral prostate: effect of guanine nucleotides. 299 70

Angiotensin II can inhibit glucagon-stimulated cyclic AMP production in hepatocytes and adenylate cyclase activity in hepatic membranes. Pertussis toxin, an exotoxin produced by Bordetella pertussis, was used to investigate the role of the inhibitory guanine nucleotide-binding regulatory protein of adenylate cyclase (Ni) in coupling angiotensin receptors to the adenylate cyclase system. An assay was developed using [32P] NAD+ to quantitate the amount of Ni protein in the membrane and the extent of its ADP-ribosylation catalyzed by toxin. The ability of angiotensin to inhibit adenylate cyclase and interact with its receptor was compared with the degree of modification of Ni in membranes prepared from isolated hepatocytes. In control membranes angiotensin II inhibited basal adenylate cyclase by 35%. When all of the Ni molecules in the membrane were ADP-ribosylated, angiotensin did not inhibit adenylate cyclase. However, the attenuation of angiotensin's effect on cyclase was not linearly correlated with the degree of modification of Ni; ADP-ribosylation of greater than 80% of the Ni was required before a reduction of the angiotensin effect was observed. A possible explanation for this finding is an excess of Ni molecules in the membrane (approximately 3.4 pmol/mg of membrane protein) over angiotensin II receptors (approximately 1.2 pmol/mg of membrane protein). 125I-angiotensin bound to sites in the membrane with two affinities. Computer fitting of the binding isotherms yielded parameters of N1 = 279 fmol/mg protein, Kd1 = 0.2 nM; N2 = 904 fmol/mg protein, Kd2 = 1.4 nM. When all of the Ni molecules in the membrane were ADP-ribosylated, angiotensin bound to only one site with binding parameters of N = 349 fmol/mg protein, Kd = 0.4 nM. GTP-gamma-S caused a 7-fold increase in the Kd of this site to 2.7 nM. Overall, the data indicate that the Ni protein mediates the effect of angiotensin on adenylate cyclase. The observation that GTP-gamma-S can markedly decrease the affinity of angiotensin receptors when all Ni molecules are ADP-ribosylated suggests that angiotensin receptors may couple to other GTP-binding proteins which may mediate the effects of angiotensin in other signal transduction systems.
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PMID:Role of Ni in coupling angiotensin receptors to inhibition of adenylate cyclase in hepatocytes. 299 49

Adenylate cyclase in the presence of GTP became active by the addition of cholera toxin irrespective of the presence of glucagon, and under the same condition the Gs of these activated enzymes were good acceptor of an ADP-ribose moiety. On the other hand, the cyclase in the presence of GDP remained inactive with cholera toxin but became active by the further addition of glucagon. However, neither of these Gs served as a cholera toxin substrate. Glucagon reduced an inhibitory action of added GDP for cholera toxin plus GTP-stimulated adenylate cyclase activity but did not for toxin plus GTP-enhanced ADP-ribosylation of Gs. These results demonstrate that Gs-GTP complex formation alone is not sufficient for Gs to serve as a cholera toxin substrate, and suggest an additional GTP binding site responsible for ADP-ribosylation by the toxin. Hormone dependent preferential interaction between the GTP binding site on Gs coupled with adenylate cyclase regulation and membrane-associated nucleoside diphosphate kinase is discussed.
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PMID:GTP-activated GTP binding protein(Gs) in membranes achieved by hormone plus GDP does not serve as a substrate for ADP-ribosylation by cholera toxin. 300 74

Islet-activating protein (IAP, a Bordetella pertussis toxin) was employed to test the hypothesis that the inhibitory GTP-binding regulatory protein of adenylate cyclase (Ni) mediates GTP effects on the binding of Ca2+-mobilizing hormones to liver plasma membranes and is involved in calcium mobilization stimulated by these agonists. IAP added to normal liver plasma membranes catalyzed the incorporation of radioactivity from [32P]NAD into a 41,000-Da peptide (presumably the alpha-subunit of Ni). However, no such incorporation was observed in liver membranes prepared from rats 24 hr after intraperitoneal injection of IAP. Angiotensin II attenuated glucagon-stimulated increases in cAMP in hepatocytes prepared from control but not IAP-treated rats. In contrast, following IAP treatment, no changes were observed in the ability of glucagon, vasopressin, angiotensin II, or epinephrine to activate phosphorylase; nor did this treatment alter [3H]vasopressin binding or epinephrine displacement of [3H]prazosin binding. However, IAP treatment decreased [3H]angiotensin II binding affinity when studies were performed in the absence but not the presence of 5'-guanylylimidodiphosphate (GppNHp). This shift was small and represented only 5-8% of the shift in apparent Kd elicited by GppNHp in untreated membranes. In vitro studies with IAP confirmed the results of the radioligand binding studies using in vivo IAP treatment. The effects of NaCl on [3H]angiotensin II binding were also tested but were not typical of other receptors which couple to Ni. The data suggest that, although a small population of hepatic angiotensin II receptors couple to Ni and attenuate glucagon-stimulated increases in cAMP, vasopressin, alpha 1-adrenergic, and the majority of angiotensin II receptors do not interact significantly with Ni. Thus, although there is evidence that agonist-induced Ca2+ mobilization requires a GTP-binding regulatory protein, this protein does not appear to be Ni in rat liver.
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PMID:Effect of islet-activating pertussis toxin on the binding characteristics of Ca2+-mobilizing hormones and on agonist activation of phosphorylase in hepatocytes. 300 28


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