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Biomedical subjects

G Schultz

Publications and source records attributed to G Schultz.

At least 289 records · Page 16Linked to original sources
PubMed1983

Hydrogenation of geranylgeraniol : two pathways exist in spinach chloroplasts.

The reduction of geranylgeranylpyrophosphate to phytylpyrophosphate in spinach chloroplasts is described for the first time. The reductase is localized in the chloroplast envelope. By contrast, the reduction of the geranylgeranyl moiety in Chl synthesis is catalyzed in the thylakoids (via Chl synthetase). NADPH functions as electron donor in both reactions. Chl synthetase is firmly bound to the thylakoid membranes, and very little activity is found in the stroma fraction. Chl synthetase in chloroplasts can use the pyrophosphate ester of either phytol, geranylgeraniol, or farnesol, phytylpyrophosphate being the preferred substrate. Exogenous Chlide exhibits no influence on Chl synthesis by chloroplast subfractions.

Journal Article↗
PubMed1983

Inhibitory coupling of hormone and neurotransmitter receptors to adenylate cyclase.

The last few years have provided evidence that beside the familiar and partially well characterized stimulation of the adenylate cyclase the enzyme is also subject to a hormone and neurotransmitter-induced inhibition serving as a general information-transfer system. Similar to hormonal stimulation, the coupling of the inhibitory hormone receptors to adenylate cyclase is mediated by a GTP-dependent process. In some membrane systems, the inhibitory coupling is additionally amplified by sodium ions. Inhibitory hormones stimulate a high affinity GTPase in plasma membranes. The observed correlation suggests that there is a close connection between the hormone-induced adenylate cyclase inhibition and GTPase stimulation. Several lines of evidence are presented suggesting that the guanine nucleotide-dependent component apparently involved in adenylate cyclase inhibition and apparently exhibiting GTPase activity is at least partially different from that involved in adenylate cyclase stimulation by hormones.

Adenylyl Cyclases↗
PubMed1983

Somatostatin-induced stimulation of a high-affinity GTPase in membranes of S49 lymphoma cyc- and H21a variants.

The influence of somatostatin was studied on GTPase activity in membranes of cyc- and H21a variants of S49 lymphoma cells, which are functionally defective in the guanine nucleotide site (Ns) mediating hormonal stimulation of the adenylate cyclase. Somatostatin, which inhibits adenylate cyclase in these membranes by a GTP-dependent process, caused a concomitant activation of a high-affinity GTPase (apparent Km approximately equal to 0.2 microM) by 40-50%. The hormone-stimulated GTPase also exhibited an apparent Km value of about 0.2 microM. GTPase stimulation by somatostatin occurred without an apparent lag phase. There was a close correlation between adenylate cyclase inhibition and high-affinity GTPase stimulation induced by somatostatin. Various other peptide hormones studied and isoproterenol had no effect on GTP hydrolysis. Activation of the enzyme by somatostatin was reduced or abolished by pretreatment of the membranes with the SH reagent, N-ethylmaleimide. In membranes of wild-type S49 lymphoma cells, somatostatin caused an increase in GTPase activity similar to that in cyc- and H21a membranes. The data show that cyc- and H21a membranes, which are more or less defective in Ns, contain a hormone-sensitive, high-affinity GTPase and that the activation of this enzyme is closely related to adenylate cyclase inhibition by somatostatin. The data suggest that, similar to Ns, the activity state of the guanine nucleotide site (Ni), which apparently mediates somatostatin-induced inhibition of the adenylate cyclase, is controlled by a high-affinity GTPase.

Adenylyl Cyclase Inhibitors↗
PubMed1983

Inhibition of adenylate cyclase and stimulation of a high affinity GTPase by the antilipolytic agents, nicotinic acid, acipimox and various related compounds.

In hamster adipocyte ghosts, the influence of the antilipolytic agents, nicotinic acid, 5-methyl-pyrazine-2-carboxylic acid 4-oxide (acipimox) and various related compounds, was studied on adenylate cyclase and low Km GTPase activities. As shown before for hormonal factors and nicotinic acid, the new drug, acipimox, inhibited adenylate cyclase by a GTP-dependent process, which was amplified by sodium ions; half-maximal inhibition occurred at about 10 mumol/l acipimox. For the various compounds studied, the following rank order of potency in inhibition of adenylate cyclase was obtained, nicotinic acid greater than 3-carboxy-5-methylpyrazole greater than acipimox greater than 3-carboxy-5-methylisoxazole; 6-hydroxynicotinic acid and beta-pyridylcarbinol had no effect up to 300 mumol/l. In the same membrane system the antilipolytic drugs increased GTP hydrolysis by stimulation of a low Km GTPase as shown before for antilipolytic hormones. The potency order of the antilipolytic agents studied was identical for GTPase stimulation and adenylate cyclase inhibition. The data suggest that the antilipolytic drugs studied act on adipocyte adenylate cyclase via membrane-bound receptors in a hormone-like manner and that stimulation of a high affinity GTPase is involved in the mechanism of adenylate cyclase inhibition by these agents.

Adenylyl Cyclase Inhibitors↗
PubMed1983

The alpha adrenergic response of Down's syndrome platelets.

The alpha adrenergic responsiveness and pharmacological characteristics of alpha adrenergic receptors were examined using platelets from normal diploid and Down's syndrome (trisomy 21) individuals. Lower than normal doses of epinephrine were required to aggregate Down's syndrome platelets and promote ATP secretion. The concentrations of other platelet aggregating agents (ADP or thrombin) did not distinguish normal from Down's syndrome. Analysis of the alpha adrenergic receptor density and ligand affinity using the radioligand dihydroergocryptine indicated that the Down's syndrome platelets were not significantly different from normal. Likewise, the biochemical characteristics of the adenylate cyclase enzyme, e.g., its sensitivity to stimulation by prostaglandin E1 or inhibition by epinephrine, were not different when comparing Down's syndrome with normal platelet membranes. These data suggest that the enhanced epinephrine sensitivity of Down's syndrome platelets is independent of any change in the alpha adrenergic receptor or the adenylate cyclase enzyme complex.

Adenosine Triphosphate↗
PubMed1982

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

The influence of cholera toxin on hormonal inhibition of adenylate cyclase and concomitant stimulation of low Km GTPase was studied in adipocyte membrane preparations. In hamster adipocyte ghosts, cholera toxin caused an about 8-fold activation of the adenylate cyclase. The antilipolytic hormonal factors, prostaglandin E1 (1 micro M), N6-phenylisopropyladenosine (1 micro M) and nicotinic acid (30 micro M) reduced both basal and cholera toxin-stimulated enzyme activities. Similar data with regard to inhibition of cholera toxin-stimulated adenylate cyclase were obtained in mouse and rat adipocyte ghosts. As studied in hamster adipocyte ghosts, prostaglandin E1 (1 micro M) increased GTP hydrolysis by a low Km GTPase by about 3-4 fold. Pretreatment of the membrane preparation with cholera toxin did not impair prostaglandin E1-induced GTPase stimulation. The data suggest that cholera toxin does not directly affect the GTPase enzyme stimulated by adenylate cyclase inhibitory hormones.

Adenylyl Cyclases↗
PubMed1982

Uncoupling of alpha-adrenoceptor-mediated inhibition of human platelet adenylate cyclase by N-ethylmaleimide.

Human platelet adenylate cyclase is stimulated by prostaglandin E1 (PGE1) and is inhibited by epinephrine via alpha-adrenoceptors. Both agonists, epinephrine more than PGE1, increase the activity of a low Km GTPase in platelet membranes. Pretreatment of intact platelets or platelet membranes with the sulfhydryl reagent, N-ethylmaleimide (NEM), abolished the inhibition of the adenylate cyclase and the concomitant stimulation of the GTPase by epinephrine. In contrast, stimulation of the adenylate cyclase by PGE1 was not affected or even increased by NEM pretreatment; only at high NEM concentrations were both basal and PGE1-stimulated activities decreased. Similarly, the PGE1-induced activation of the low Km GTPase was not or was only partially reduced by NEM. Adenylate cyclase activation by stable GTP analogs, NaF, and cholera toxin was also not decreased by NEM pretreatment. Exposure of intact platelets to NEM did not reduce alpha-adrenoceptor number and affinities for agonists and antagonists, as determined by [3H]yohimbine binding in platelet particles. The data indicate that NEM uncouples alpha-adrenoceptor-mediated inhibition of platelet adenylate cyclase, leaving the receptor recognition site and the adenylate cyclase itself relatively intact. Although the effect of NEM may be based on a reaction with the alpha-adrenoceptor site interacting with a coupling component, the selective loss of the adenylate cyclase inhibition together with an even increased stimulation of the enzyme by PGE1 suggests that there are two at least partially distinct regulatory sites involved in opposing hormonal regulations of adenylate cyclase activity, with that involved in hormonal inhibition being highly susceptible to inactivation by NEM.

Adenylyl Cyclases↗
PubMed1982

Adenosine receptor-mediated stimulation of GTP hydrolysis in adipocyte membranes.

The adenosine derivative, N6-phenylisopropyladenosine (PIA), which inhibits adenylate cyclase in adipocyte membranes by a GTP-dependent and sodium-amplified process, was studied on GTPase activity in hamster adipocyte ghosts. PIA stimulated a high affinity GTPase without apparent lag phase. Both unstimulated and PIA-stimulated GTPases exhibited very similar Km values of about 0.2 microM GTP. PIA-induced low Km GTPase stimulation was amplified by sodium ions and was half-maximal and maximal at about 0.02 and 0.1 microM PIA, respectively. Stimulations of the low Km GTPase by PIA and PGE1, both inhibiting adipocyte adenylate cyclase, were not additive. Similar to PIA-induced adenylate cyclase inhibition, stimulation of the GTPase by PIA but not by PGE1 was prevented by the adenosine receptor antagonist, 3-isobutyl-1-methylxanthine. The data suggest that PIA-induced stimulation of a high affinity GTPase is an essential mechanism of adenosine receptor-mediated adipocyte adenylate cyclase inhibition.

1-Methyl-3-isobutylxanthine↗
PubMed1982

Inactivation of the guanine nucleotide regulatory site mediating inhibition of the adenylate cyclase in hamster adipocytes.

The influence of N-ethylmaleimide and trypsin was studied on stimulatory and inhibitory regulations of the hamster adipocyte adenylate cyclase. Treatment of intact adipocytes or adipocyte ghosts with N-ethylmaleimide decreased basal and forskolin-stimulated adenylate cyclase activities. In the pretreated membrane preparations, inhibition of the enzyme by GTP and by stable GTP analogues was abolished. Concomitantly, activation of the adenylate cyclase by NaCl and its inhibition by the antilipolytic agents, prostaglandin E1 and nicotinic acid, were obliterated. In contrast, adenylate cyclase stimulation by ACTH and stable GTP analogues was not impaired but rather increased. Similarly, the NaCl-induced attenuation of the ACTH-stimulated enzyme activity was increased by the N-ethylmaleimide treatment. Limited proteolysis of hamster adipocyte ghosts with trypsin also obliterated GTP and prostaglandin E1-induced inhibitions and NaCl-induced activation of the adenylate cyclase. In contrast, adenylate cyclase activity stimulated by isoproterenol was increased after trypsin treatment. The data suggest that the activity of the adenylate cyclase is regulated via two distinct guanine nucleotide sites and that treatment with N-ethylmaleimide and limited proteolysis with trypsin functionally eliminates the regulatory site mediating adenylate cyclase inhibition, leading to a state where the enzyme activity is regulated only via the stimulatory site. The differential effects of these treatments on NaCl-induced activation and attenuation of the adenylate cyclase suggest that sodium acts on both regulatory sites in an inhibitory manner, and that by the functional elimination of the inhibitory site, only the sodium-induced attenuation of the adenylate cyclase via the stimulatory site is observed.

Adenylyl Cyclase Inhibitors↗
PubMed1982

Signal transformation involving alpha-adrenoceptors.

Based on different affinities of various agonists and antagonists, alpha-adrenoceptors are subdivided into two subtypes, alpha 1 and alpha 2. Stimulation of alpha 1-adrenoceptors induces several changes, possibly acting as intracellular signals triggering the final cellular response. Among these changes, the increase in the cytosolic calcium ion concentration appears to be the most important signal. Increased membrane phosphatidylinositol turnover and increased cyclic guanosine 3', 5'-monophosphate (cGMP) levels under alpha 1-adrenoceptor stimulation may play a role as signals primary and secondary to the increased calcium influx, respectively. The primary intracellular signal induced by stimulation of alpha 2-adrenoceptors appears to be a fall in cellular cyclic AMP levels as a consequence of inhibition of membrane-bound adenylate cyclase. The molecular mechanisms underlying the alpha 2-adrenoceptor-mediated inhibition of this enzyme are only poorly understood, and apparently involve a guanine nucleotide-binding coupling component, which may be in part different from that involved in beta-adrenoceptor-mediated stimulation of the adenylate cyclase.

Adenylyl Cyclases↗
PubMed1982

Stimulation of a low Km GTPase by inhibitors of adipocyte adenylate cyclase.

The activity of enzymes exhibiting GTPase activity was determined in membrane preparations of hamster adipocytes. Two GTPases with apparent Km values of about 0.2 microM GTP (low Km GTPase) and 180 microM GTP (high Km GTPase) were found. The effects of various agents that stimulate or inhibit adipocyte adenylate cyclase were investigated with these two forms of GTPase. None of the stimulatory or inhibitory agonists studied affected the activity of the high Km GTPase(s). However, factors inhibiting adenylate cyclase, such as prostaglandin E1, nicotinic acid, 3-carboxy-5-methylpyrazole, and N6-phenylisopropyladenosine, stimulated the low Km GTPase by 50--100% without an apparent lag phase. The activity of the stimulated GTPase was half-maximal at about 0.2 microM GTP. NaCl (up to 100 mM) had no effect on the basal activity of this enzyme but amplified the stimulation induced by adenylate cyclase inhibitory agents. There was a good correlation between inhibition of adenylate cyclase and stimulation of the low Km GTPase, both with regard to the concentration required for half-maximal effects on these two enzymes and with regard to the potency order of various prostaglandins studied. In contrast to factors inhibiting adenylate cyclase, the stimulatory hormones, isoproterenol and ACTH, had only small effects on the low Km GTPase activity; potassium fluoride was completely ineffective. The data suggest that an increased GTP hydrolysis by an activated GTPase is an essential mechanism of hormone-induced adenylate cyclase inhibition.

Adenylyl Cyclase Inhibitors↗
PubMed1982

The adrenergic responsiveness of Down syndrome cells.

Platelets and cultured fibroblasts were used to examine the hormonal response of normal diploid and Down syndrome (trisomy 21) cells. An enhanced physiological response to catecholamine hormones was found in the trisomy 21 cells. This effect was specific for the adrenergic agonists since other hormones showed equivalent responses in the normal diploid and trisomy 21 cell types. Radioligand analysis of the alpha-adrenergic receptors in trisomy 21 platelets indicated no significant difference in receptor density or ligand affinity. Enzymatic analysis of the platelet adenylate cyclase activity in vitro showed no major changes in catecholamine sensitivity between the normal and Down syndrome preparations. Somatic cell genetic studies using human-hamster hybrids indicated that a gene coding for the beta-adrenergic receptor was not located on chromosome 21 but on human chromosome 5. These receptor, enzymic, and somatic cell genetic studies argue against a gene-dosage effect involving either the adrenergic hormone receptors or a component of the adenylate cyclase enzyme in the trisomy 21 cells. Identifying the gene(s) on chromosome 21 and the molecular mechanism(s) responsible for the exaggerated response of Down syndrome cells to catecholamines are the current objectives of this research program.

Cells, Cultured↗
PubMed1981

Purification of a soluble, sodium-nitroprusside-stimulated guanylate cyclase from bovine lung.

A soluble, sodium-nitroprusside-stimulated guanylate cyclase as been purified from bovine lung by DEAE-cellulose chromatography, ammonium sulfate precipitation, chromatography on Blue Sepharose CL-6B and preparative gel electrophoresis. Apparent homogeneity was obtained after at least 7000-fold purification with a yield of 3%. A single stained band (Mr 72000) was observed after gel electrophoresis in the presence of sodium dodecyl sulfate. The purified enzyme migrated as one band also under non-denaturing conditions in acrylamide gels (5-12%). The mobility of this band corresponded to an Mr of 145000. The enzyme sedimented on sucrose gradients with an S20, w of 7.0 S. Gel filtration yielded a Stokes' radius of 4.6 nm. These data suggest that the enzyme has an Mr of approximately 150000 and consists of two, presumably identical, subunits of Mr 72000. Sodium nitroprusside stimulated the purified enzyme 15-fold and 140-fold to specific activities of 8.5 and 15.7 mumol of cGMP formed min-1 mg-1 in the presence of Mn2+ and Mg2+, respectively. Formation of cGMP was proportional to the incubation time and to the amount of enzyme added. The stimulatory effect of sodium nitroprusside was half-maximal at about 2 microM, was observed immediately after addition and could be reversed either by dilution or by removal of sodium nitroprusside on a Sephadex G-25 column. The purified enzyme in the absence of catalase was stimulated by sodium nitroprusside, N-methyl-N'-nitro-N-nitrosoguanidine and 3-morpholino-sydnonimine and in the presence of catalase by sodium nitrite and sodium azide. In the presence of Mn2+ and sodium nitroprusside, the purified enzyme catalyzed the formation of cAMP from ATP at a rate of 0.6 mumol min-1 mg-1.

Animals↗