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S M Yeung

Publications and source records attributed to S M Yeung.

7 recordsLinked to original sources
PubMed1990

Irreversible inactivation of adenylyl cyclase by the "P"-site agonist 2',5'-dideoxy-,3'-p-fluorosulfonylbenzoyl adenosine.

2',5'-Dideoxy,3'-p-fluorosulfonylbenzoyl Adenosine (2',5'-dd3'-FSBA) was synthesized and found to be an agonist and affinity label for the "P"-site of adenylyl cyclase. This compound irreversibly inactivated both a crude detergent-dispersed adenylyl cyclase from rat brain and the partially purified enzyme from bovine brain. The irreversible inactivation by 100 to 200 microM 2',5'-dd3'-FSBA was blocked in a concentration-dependent manner by several established P-site inhibitors of adenylyl cyclase, 2',5'-dideoxyadenosine, 2'-d3'-AMP, adenosine, and 2'-deoxyadenosine, but not by inosine, N6-(phenylisopropyl)adenosine, adenine, 2'-d3':5'-cAMP, or 5'-AMP, agents known not to act at the P-site. Moreover, irreversible inactivation by 2',5'-dd3'-FSBA occurred in the presence of ATP at concentrations up to 3 mM, making it unlikely that inactivation was due to an effect on the enzyme's catalytic site. Adenylyl cyclase was also irreversibly inactivated by 5'-FSBA, although modestly (less than 20%) and apparently nonspecifically. Dithiothreitol protected the enzyme from irreversible inactivation by 2',5'-dd3'-FSBA, but reversible inhibition of the enzyme was still observed, although with reduced potency. When 2 mM dithiothreitol was added after a 30-min preincubation with 2',5'-dd3'-FSBA, the rat brain enzyme was partially (approximately 80%) reactivated. The data suggest that 2',5'-dd3'-FSBA may irreversibly inactivate adenylyl cyclase by reacting with a cysteinyl moiety in proximity to the P-site domain of the enzyme. These data together with results of studies of P-site inhibition kinetics published elsewhere (Johnson, R. A., and Shoshani, I. (1990) J. Biol. Chem. 265, 11595-11600) strongly suggest that the P-site and catalytic site are distinct domains on the enzyme. 2',5'-dd3'-FSBA, and especially its radiolabeled analog, should prove to be a useful probe for structural studies of adenylyl cyclase, particularly with regard to the P-site.

Adenylyl Cyclase Inhibitors↗
PubMed1989

Ammonium ions enhance proteolytic activation of adenylate cyclase and decrease its sensitivity to inhibition by "P"-site agonists.

A detergent-dispersed adenylate cyclase from rat brain was used to study the effects of ammonium salts and polyamines on the proteolytic activation of the enzyme by a sperm protease and on the sensitivity of adenylate cyclase to inhibition via its "P"-site. A purified preparation of a trypsin-like, serine protease from bovine sperm was used to activate solubilized adenylate cyclase in the presence of guanosine 5'-O-(3-thiotriphosphate (GTP gamma S). The proteolytically activated form of adenylate cyclase was found to be particularly sensitive to further activation by ammonium bicarbonate. The activation by NH4HCO3 was found to be due to the NH+4 cation and was characterized by an increased Vmax and by a decreased sensitivity of adenylate cyclase to inactivation by elevated concentrations of the sperm protease or by trypsin. NH4Cl and (NH4)2SO4 also caused biphasic effects on adenylate cyclase, which mimicked but were less effective than those caused by NH4HCO3. Consistent with observations of others, adenylate cyclase activity was enhanced by ammonium ions whether in the presence of reversible (Mn2+) or irreversible (GTP gamma S) activators. Mn2+- and GTP gamma S-stimulated activities were similarly optimally enhanced by 30 mM (NH4)2SO4 and by 30 to 150 mM NH4Cl or NH4HCO3. Ammonium ions did not increase the activity of the purified catalytic unit. Moreover, the effect of ammonium ions was not accompanied by an increased rate of activation by GTP gamma S, suggesting that the activation of Gs (guanine nucleotide-dependent stimulatory component) may not be the primary cause of stimulation by ammonium salts. Several polyamines at millimolar concentrations blocked the stimulatory effect of NH+4. This was observed when adenylate cyclase was activated by Mn2+, but not when it was activated by GTP gamma S or by the sperm protease + GTP gamma S. The inhibitory effect of polyamines was not due to the formation of a complex with ATP. Both the increase in Vmax of the Mn2+-stimulated enzyme by NH+4 and the decrease in Vmax caused by spermine were accompanied by an increase in the enzyme's Km MnATP app. Spermine increased the IC50 for inhibition of Mn2+-activated adenylate cyclase by 2',5'-dideoxyadenosine (2',5'-ddAdo) from 0.75 to 4.6 microM, consistent with the idea that increased sensitivity of P-site-mediated inhibition is associated with increased enzyme activity. In contrast, activation of Mn2+-stimulated adenylate cyclase by 30 mM (NH4)2SO4 also reduced sensitivity to inhibition by 2',5'-ddAdo(IC50 1.1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Acrosin↗
PubMed1989

Cation and structural requirements for P site-mediated inhibition of adenylate cyclase.

The cation and structural requirements of the intracellular inhibitory "P" site of adenylate cyclase were investigated in human platelet membranes, bovine sperm particles, and detergent-solubilized and purified preparations from rat and bovine brain. Sensitivity of adenylate cyclase to P site-mediated inhibition was enhanced by reversible and irreversible activators of the enzyme. The most effective sensitization of the platelet and brain adenylate cyclases was observed with Mn2+ and upon proteolysis with inhibin in the presence of guanosine 5'-O-(3-thiotriphosphate). These resulted in IC50 values for (2',5'dideoxy-adenosine (2',5'-dd-Ado) and 2'-deoxy-3'-AMP of approximately 1-2 microM. The data were consistent with the ideas that P site-mediated inhibition of adenylate cyclase is dependent on divalent cation and is a function of enzyme activity. A number of nucleosides and nucleotides were synthesized and used to define structural requirements for P site-mediated inhibition of a detergent-solubilized adenylate cyclase from rat brain. The data suggest a strict requirement for an intact adenine moiety and a beta-glycosidic linkage for the ribosyl moiety. 2'-Deoxy-and especially 2',5'-dideoxy-ribosyl moieties enhanced sensitivity and a strong preference for phosphate at the 3'-position was exhibited. Substitutions at the 5'-ribose position impaired sensitivity. The order of potency and IC50 values of the more potent adenosine analogs were 2',5'-dideoxy-3'-AMP (congruent to 0.1 microM) greater than 2'-deoxy-3'-AMP (congruent to 1 microM) greater than 2',5'-dd-Ado (congruent to 3 microM) greater than 3'-AMP (congruent to 9 microM) greater than 2'-deoxy-adenosine (congruent to 15 microM) greater than adenosine (congruent to 80 microM). Large substitutions at the 3'-ribose position were tolerated, e.g., dApdN di- and dAp(dN)4 penta-nucleotides and succinyl- and p-fluoro-sulfonyl-benzoyl- moieties. The purified adenylate cyclase from bovine brain was inhibited by P site agonists with IC50 values of 34 and 45 microM for 2'-deoxy-3'-AMP and 2',5'-dd-Ado, respectively. The data imply, first, that the locus of the P site is the catalytic subunit of adenylate cyclase and, second, that the increased sensitivity observed with Mn2+ is due to an effect of the cation on the catalytic subunit. In contrast with adenylate cyclases from other mammalian tissues, the enzyme from bovine sperm exhibited only weak sensitivity to P site agonists; 2'-deoxy-3'-AMP congruent to 2',5'-dd-Ado greater than adenosine, each with IC50 values greater than 1000 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine↗
PubMed1987

Hydrodynamic properties of adenosine Ri receptors solubilized from rat cerebral-cortical membranes.

Adenosine Ri receptors and inhibitory guanine-nucleotide-regulatory components were solubilized from rat cerebral-cortical membranes with sodium cholate. (-)-N6-Phenylisopropyl[2,8-3H]adenosine [( 3H]PIA) binds with high affinity to the soluble receptors, which retain the pharmacological specificity of adenosine Ri receptors observed in membranes. The binding is regulated by bivalent cations and guanine nucleotides. Bivalent cations increase [3H]PIA binding by increasing both the affinity and the apparent number of receptors. Guanine nucleotides decrease agonist binding by increasing the dissociation of the ligand-receptor complex. Adenosine agonists stabilize the high-affinity form of the soluble receptor. The hydrodynamic properties of the adenosine receptor were determined with cholate extracts of membranes that were treated with [3H]PIA. Sucrose-gradient-centrifugation analysis indicates that the receptor has a sedimentation coefficient of 7.7 S. The receptor is eluted from Sepharose 6B columns with an apparent Stokes radius of 7.2 nm. Labelling of either sucrose-gradient or gel-filtration-column fractions with pertussis toxin and [32P]-NAD+ reveals that both the 41,000- and 39,000-Mr substrates overlap with the receptor activity. These studies suggest that the high-affinity adenosine-receptor-binding activity in the cholate extract represents a stable R1-N complex.

Adenosine↗
PubMed1986

Target size of the adenosine Ri receptor.

The adenosine receptor of rat cerebral-cortical membranes was examined by radiation inactivation. In control membranes the receptor is distributed between high- and low-affinity states, that can be preferentially expressed by Mg2+ ions and guanine nucleotides respectively. Upon exposure to increasing doses of radiation, the high-affinity receptor decayed linearly as a function of radiation dose. This decay rate corresponded to a target size of 63,000 Da, when compared with the decay of the muscarinic cholinergic receptor that was also measured in these membranes.

Animals↗
PubMed1985

Magnesium ion exerts a central role in the regulation of inhibitory adenosine receptors.

Guanine nucleotides and Mg2+ differentially regulate agonist binding to adenosine (Ri) receptors in fat-cell plasma membranes. GTP alone decreases binding of the agonist ligand [3H]N6-cyclohexyladenosine (CHA) by increasing the dissociation constant (Kd). Mg2+ alone also decreases [3H]CHA binding, which is associated with a decrease in the number of receptors and in the dissociation constant. In the presence of Mg2+, the effect of GTP is to increase [3H]CHA binding by increasing the total number of receptors. It thus appears that Mg2+ acts specifically at a bivalent-cation site which, with GTP, regulates agonist binding. This putative Mg site is highly sensitive to alkylating agents. Mild treatment with N-ethylmaleimide (NEM) abolishes the characteristic GTP effect on agonist binding in the presence of Mg2+. In addition, the effect of Mg2+ alone is also eliminated. The effect of GTP alone is largely unaltered. Studies of the adenylate cyclase activity indicate that this NEM treatment also abolishes the inhibition of basal activity by adenosine analogues, whereas guanylyl imidodiphosphate inhibition of forskolin-stimulated activity is only slightly impaired at this NEM concentration. These observations indicate that a Mg2+ 'site' or 'component' is required for the integration of receptor (Ri) occupancy with regulation of catalytic activity (C). The regulatory role of Mg2+ is more demonstrable in receptor-GTP-regulatory-protein (Ri-Ni) interactions than in GTP-regulatory-protein-catalytic-unit (Ni-C) interactions.

Adenosine↗