cGMP-stimulated Cyclic Nucleotide Phosphodiesterase Research Articles

CGMP-stimulated cyclic nucleotide phosphodiesterase regulates the basal calcium current in human atrial myocytes.

EHNA (Erythro-9-[2-hydroxy-3-nonyl]adenine) is a wellknown inhibitor of adenosine deaminase. Recently, EHNA was shown to block the activity of purified soluble cGMPstimulated phosphodiesterase (PDE2) from frog, human, and porcine heart with an apparent Ki value of approximately 1 microM and with negligible effects on Ca2+/calmodulin PDE (PDE1), cGMP-inhibited PDE (PDE3), and low Km cAMP-specific PDE (PDE4) (Méry, P.F., C. Pavoine, F. Pecker, and R. Fischmeister. 1995. Mol. Pharmacol. 48:121-130; Podzuweit, T., P. Nennstiel, and A. Muller. 1995. Cell. Signalling. 7:733- 738). To investigate the role of PDE2 in the regulation of cardiac L-type Ca2+ current (ICa), we have examined the effect of EHNA on ICa in freshly isolated human atrial myocytes. Extracellular application of 0.1-10 microM EHNA induced an increase in the amplitude of basal ICa ( approximately 80% at 1 microM) without modification of the current-voltage or inactivation curves. The maximal stimulatory effect of EHNA on ICa was comparable in amplitude with the maximal effect of isoprenaline (1 microM), and the two effects were not additive. The effect of EHNA was not a result of adenosine deaminase inhibition, since 2'-deoxycoformycin (1-30 microM), another adenosine deaminase inhibitor with no effect on PDE2, or adenosine (1-10 microM) did not increase ICa. In the absence of intracellular GTP, the substrate of guanylyl cyclase, EHNA did not increase ICa. However, under similar conditions, intracellular perfusion with 0.5 microM cGMP produced an 80% increase in ICa. As opposed to human cardiomyocytes, EHNA (1-10 microM) did not modify ICa in isolated rat ventricular and atrial myocytes. We conclude that basal ICa is controlled by PDE2 activity in human atrial myocytes. Both PDE2 and PDE3 may contribute to keep the cyclic nucleotides concentrations at minimum in the absence of adenylyl and/or guanylyl cyclase stimulation.

Partial purification of cyclic guanosine monophosphate phosphodiesterase from sea urchin ( Strongylocentrotus purpuratus) sperm

1. 1. A cGMP-stimulated cyclic nucleotide phosphodiesterase has been extracted from intact sea urchin sperm by means of detergents. 2. 2. CHAPS and octyl glucoside inhibited enzyme activity less than did Triton X-100, Lubrol PX, Nonidet P-40 and sodium deoxycholate. 3. 3. Homogenization in 1% CHAPS with a sperm concentration of 10 mg/ml resulted in maximal solubilization (55%) of the cGMP phosphodiesterase from whole sperm. 4. 4. Partial purification of cGMP phosphodiesterase from intact sperm was achieved by 1% CHAPS extraction and chromatography on DEAE-Sephacel. 5. 5. Chromatography separated cAMP and cGMP phosphodiesterases from each other, with an increase in specific activity of the cGMP enzyme up to 9-fold. 6. 6. The partially purified cGMP phosphodiesterase activity was markedly unstable. 7. 7. It displayed Michaelis-Menten kinetic for cGMP dependence with a K m of 66 μM. With cGMP as substrate, both Mg 2+ and Mn 2+ but not Ca 2+ were effective activators of the partially purified cGMP phosphodiesterase. 8. 8. The K 0.5 of Mg 2+ and of Mn 2+ were 0.50 and 0.32 mM, respectively.

Selective inhibition of the cGMP-stimulated cyclic nucleotide phosphodiesterase from pig and human myocardium

Selective inhibition of the cGMP-stimulated cyclic nucleotide phosphodiesterase from pig and human myocardium

Direct Photolabeling of the cGMP-stimulated Cyclic Nucleotide Phosphodiesterase

cGMP-stimulated phosphodiesterase (PDE) has been directly photolabeled with [32P]cGMP using UV light. Sequence analysis of peptide fragments obtained from partial proteolysis or cyanogen bromide cleavage indicate that two different domains are labeled. One site, on a Mr = 36,000 chymotryptic fragment located near the COOH terminus, has characteristics consistent with it being close to or part of the catalytic site of the enzyme. This peptide contains a region of sequence that is highly conserved in all mammalian cyclic nucleotide PDEs and has been postulated to contain the catalytic domain of the enzyme. The other site, on a Mr = 28,000 cyanogen bromide cleavage fragment located near the middle of the molecule, probably makes up part of the allosteric site of the molecule. Labeling of the enzyme is concentration dependent and Scatchard analysis of labeling yields a biphasic plot with apparent half labeling concentrations of about 1 and 30 microM consistent with two types of sites being labeled. Limited proteolysis of the PDE by chymotrypsin yields five prominent fragments that separate by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) at Mr = 60,000, 57,000, 36,000, 21,000, and 17,000. Both the Mr = 60,000 and 57,000 apparently have blocked NH2 termini suggesting that the Mr = 57,000 fragment is a subfragment of the Mr = 60,000 fragment. Primary sequence analysis indicates that both the Mr = 21,000 and 17,000 fragments are subfragments of the Mr = 36,000 fragment. Autoradiographs of photolabeled then partially proteolyzed enzyme show labeled bands at Mr = 60,000, 57,000, and 36,000. Addition of 5 microM cAMP prior to photolabeling eliminates photolabeling of the Mr = 36,000 fragment but not the Mr = 60,000 or 57,000 fragments. The labeled site not blocked by cAMP is also contained in a Mr = 28,000 cyanogen bromide fragment of the enzyme that does not overlap with the Mr = 36,000 proteolytic fragment. Limited chymotryptic proteolysis also increases basal activity and eliminates cGMP stimulation of cAMP hydrolysis. The chymotryptic fragments can be separated by either ion exchange high performance liquid chromatography (HPLC) or solid-phase monoclonal antibody treatment. A solid-phase monoclonal antibody against the cGMP-stimulated PDE removes the Mr = 60,000 and 57,000 labeled fragments and any intact, unproteolyzed protein but does not remove the Mr = 36,000 fragment or the majority of activity. Ion exchange HPLC separates the fragments into three peaks (I, II, and III). Peaks I and II contain activity of approximately 40 and 100 units/mg, respectively. Peak II is the undigested or slightly nicked native enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of pH on allosteric and catalytic properties of the Guanosine Cyclic 3',5'-phosphate cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver

We have investigated effects of pH on the catalytic and allosteric properties of the cGMP-stimulated cyclic nucleotide phosphodiesterase purified from calf liver. In the "activated" state, i.e., with 0.5 microM [3H]cAMP plus 1 microM cGMP or at saturating substrate concentrations (250 microM [3H]cAMP or [3H]cGMP), hydrolysis was maximal at pH 7.5-8.0 in assays of different pH. Hydrolysis of concentrations of substrate not sufficient to saturate regulatory sites and below the apparent Michaelis constant (Kmapp), i.e., 0.5 microM [3H]cAMP or 0.01 microM [3H]cGMP, was maximal at pH 9.5. Although hydrolysis of 0.5 microM [3H]cAMP increased with pH from 7.5 to 9.5, cGMP stimulation of cAMP hydrolysis decreased. As pH increased or decreased from 7.5, Hill coefficients (napp) and Vmax for cAMP decreased. Thus, assay pH affects both catalytic (Vmax) and allosteric (napp) properties. Enzyme was therefore incubated for 5 min at 30 degrees C in the presence of MgCl2 at various pHs before assay at pH 7.5. Prior exposure to different pHs from pH 6.5 to 10.0 did not alter the Vmax or cGMP-stimulated activity (assayed at pH 7.5). Incubation at high (9.0-10.0) pH did, in assays at pH 7.5, markedly increase hydrolysis of 0.5 microM [3H]cAMP and reduce Kmapp and napp. After incubation at pH 10, hydrolysis of 0.5 microM [3H]cAMP was maximally increased and was similar in the presence or absence of cGMP. Thus, after incubation at high pH, the phosphodiesterase acquires characteristics of the cGMP-stimulated form. Activation at high pH occurs at 30 degrees C but not 5 degrees C, requires MgCl2, and is prevented but not reversed by ethylenediaminetetraacetic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the kinetics of cyclic AMP hydrolysis by the cyclic GMP-stimulated cyclic nucleotide phosphodiesterase.

The kinetics of cAMP hydrolysis by the purified calf liver cGMP-stimulated cyclic nucleotide phosphodiesterase were analyzed in the absence or presence of a number of competitive inhibitors of the methylxanthine type according to a two-site competitive model for allosteric enzymes. Methylxanthines were also classified by graphical analysis of classical competition kinetics at saturating cAMP. This treatment yielded Km/KI ratios which estimated the relative effectiveness of the binding of substrate and inhibitors to the "high affinity" (ES complex) state without establishing individual equilibrium-binding constants of cAMP and inhibitors for specific enzyme states. Individual binding constants for substrate and inhibitors were estimated directly by fitting primary data to the rate equation for the two-site competitive model. The equilibrium dissociation constants for cAMP to the "high" (KS) and "low affinity" (AKS) states were 2.4 +/- 0.8 and 410 +/- 140 microM, respectively. Dissociation constants for various inhibitors to the high (BKI) and low affinity (KI) states were also estimated. The ratio KS/BKI, which directly compared the equilibrium-binding constants of substrate and inhibitors to the high affinity state (ES complex), was in excellent agreement with Km/KI ratios derived from graphical analysis. Whereas a number of the methylxanthine analogues were more effective or as effective as cAMP in binding to the low affinity or "ligand-free" state, only isobutylmethylxanthine was effective as cAMP in binding to the high affinity state (1-methyl-3-isopropylxanthine, and 1,3-dipropylxanthine were somewhat less effective). These findings suggested that allosteric transitions might alter the topography of specific hydrophobic domains at cyclic nucleotide-binding sites and that structural determinants were more stringent for binding to the high affinity state than to the low affinity state.

Effects of temperature on allosteric and catalytic properties of the cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver.

We have investigated effects of temperature on the catalytic and allosteric properties of the cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver. Vmax for cAMP and cGMP increased as assay temperature increased from 5 to 45 degrees C. At substrate concentrations below Kmapp, however, hydrolysis increased as temperature decreased from 45 to 5 degrees C and was much greater at 5 degrees C than at 45 degrees C. As assay temperature decreased, Kmapp for cAMP and cGMP decreased. Hill coefficients for cAMP and cGMP were approximately 1.9 at 45 degrees C and 1.2-1.0 at 5 degrees C. cGMP stimulated hydrolysis of 0.5 microM [3H]cAMP at all assay temperatures. Although maximal activity stimulated by cGMP, like Vmax, was lowest at 5 degrees C, presumably because of the effect of temperature on catalytic activity, the apparent activation constant (K alpha app) for cGMP stimulation was lower at 5 degrees C than at 45 degrees C. Thus, affinity for both substrate and effector was increased at 5 degrees C, suggesting that low temperature promotes transitions of the cGMP-stimulated phosphodiesterase to a "high affinity" state. That cGMP stimulated cAMP hydrolysis at 5 degrees C suggests that temperature-induced transitions are incomplete and/or readily reversible. In assays at 30 degrees C competitive inhibitors, like substrates, induce allosteric transitions which result in enhanced hydrolysis of low substrate (1.0 microM [3H] cAMP) concentrations. At higher substrate concentrations (50 microM [3H]cAMP), with the enzyme in the "activated" state, inhibitors compete with substrate at catalytic sites and reduce hydrolysis. At 45 degrees C, as at 30 degrees C, 1-methyl-3-isobutylxanthine (IBMX) and papaverine increased hydrolysis of 1.0 microM [3H]cAMP and reduced hydrolysis of 50 microM [3H]cAMP. At 5 degrees C, however, IBMX and papaverine inhibited hydrolysis of both 1.0 and 50 microM [3H]cAMP. Enzyme activity was relatively more sensitive to inhibition by IBMX at 5 degrees C than at 45 degrees C. Taken together, these observations support the notion that low temperature induces incomplete or readily reversible transitions to the high affinity state for substrates, effectors, and inhibitors. These observed effects of temperature also point out that enzyme determinants and topographical features responsible for transitions to the high affinity state and expression of catalytic activity can be regulated independently.

Characterization of cyclic nucleotide phosphodiesterases in Xenopus laevis ovary

1. 1. A cGMP-stimulated cyclic nucleotide phosphodiesterase present in cytosol of Xenopus laevis ovary has been purified and characterized. 2. 2. A cAMP-specific phosphodiesterase which is not activated by either cGMP or calmodulin, has also been characterized. 3. 3. Brief exposure of intact oocytes to 10 μM progesterone results in an increase in activity of the cAMP-specific enzyme. The cGMP-stimulated and the calmodulin-activated phosphodiesterases are not altered. 4. 4. Changes in cyclic nucleotide levels during progesterone-induced maturation of oocytes may be modulated by these isoenzymes.

Opposite effects of cyclic GMP and cyclic AMP on Ca2+ current in single heart cells

The slow inward Ca2+ current, ICa, is fundamental in the initiation of cardiac contraction and neurohormonal regulation of cardiac function. It is increased by beta-adrenergic agonists, which stimulate synthesis of cyclic AMP (cAMP) and cAMP-dependent phosphorylation. The neurotransmitter acetylcholine reduces ICa by an unknown mechanism. There is strong evidence that acetylcholine reduces ICa by decreasing adenylate cyclase activity, but cGMP has also been implicated as ACh stimulates cGMP accumulation and activates cGMP-dependent protein kinase. Application of cGMP decreases contractile force, decreases Ca flux, shortens the duration of action potentials and inhibits Ca-dependent action potentials. Other studies, however, have concluded that cGMP levels do not correlate with contractile force and that cGMP has no effect on ICa. We have therefore examined the effects of intracellular perfusion of cGMP on ICa using isolated, voltage-clamped cells from frog ventricle. We find that cGMP has negligible effects on basal ICa, but greatly decreases the ICa that had been elevated by beta-adrenergic agonists or by intracellular perfusion with cAMP. The decrease of ICa is mediated by cAMP hydrolysis via a cGMP-stimulated cyclic nucleotide phosphodiesterase.

The identification and characterization of two cyclic nucleotide phosphodiesterases from bovine adrenal medulla

Two soluble cyclic nucleotide phosphodiesterase activities, designated Peak I ( M r = 216,000) and Peak II ( M r = 230,000), have been isolated from bovine adrenal medulla by DEAE-cellulose chromatography. Peak I has Ca 2+-independent, cGMP-specific phosphodiesterase activity and Peak II has cGMP-stimulated cyclic nucleotide phosphodiesterase activity. Peak I hydrolyzes cGMP with hyperbolic kinetics and demonstrates a K m of 23 μ m. Peak II hydrolyzes cGMP with hyperbolic kinetics but hydrolyzes cAMP with slightly sigmoidal kinetics and demonstrates K m values of 54 ± 0.7 μ m cGMP and 38 ± 6 μ m cAMP. Cyclic AMP and cGMP are competitive inhibitors of each other's hydrolysis, suggesting that these nucleotides may be hydrolyzed at the same catalytic site. Micromolar concentrations of cGMP cause a 5-fold stimulation of the hydrolysis of subsaturating concentrations of cAMP by the Peak II phosphodiesterase. Half-maximal activation occurs at 0.5 μ m cGMP and the result of activation is a decrease in the apparent K m for cAMP. Stimulation of the hydrolysis of subsaturating concentrations of cGMP by cAMP was also detected; however, cAMP is a less potent activator of the enzyme than cGMP. Cyclic AMP causes a 1.5-fold stimulation of cGMP hydrolysis and half-maximal activation occurs at 2.5 μ m cAMP.

Effects of fatty acids on activity of cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver

Effects of fatty acids, prostaglandins, and phospholipids on the activity of purified cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver were investigated. Prostaglandins A 2, E 1, E 2, F 1α, and F 2α, thromboxane B 2, and most phospholipids were without effect; lysophosphatidylcholine was a potent inhibitor. Several saturated fatty acids (carbon chain length 14–24), at concentrations up to 1 m m, had little or no effect on hydrolysis of 0.5 μ m [ 3H]cGMP or 0.5 μ m [ 3H]cAMP with or without 1 μ m cGMP. In general, unsaturated fatty acids were inhibitory, except for myristoleic and palmitoleic acids which increased hydrolysis of 0.5 μ m [ 3H]cAMP. The extent of inhibition by cis-isomers correlated with the number of double bonds. Increasing concentrations of palmitoleic acid from 10 to 100 μ m increased hydrolysis of [ 3H]cAMP with maximal activation (60%) at 100 μ m; higher concentrations were inhibitory. Palmitoleic acid inhibited cGMP hydrolysis and cGMP-stimulated cAMP hydrolysis with IC 50 values of 110 and 75 μ m, respectively. Inhibitory effects of palmitoleic acid were completely or partially prevented by equimolar α-tocopherol. Palmitelaidic acid, the trans isomer, had only slightly inhibitory effects. The effects of palmitoleic acid (100 μ m) were dependent on substrate concentration. Activation was maximal with 1 μ m [ 3H]cAMP and was reduced with increasing substrate; with >10 μ m cAMP, palmitoleic had no effect. Inhibition of cGMP hydrolysis was maximal at 2.5 μ m cGMP and was reduced with increasing cGMP; at >100 μ m cGMP palmitoleic acid increased hydrolysis slightly. Palmitoleic acid did not affect apparent K m or V max for cAMP hydrolysis, but increased the apparent K m (from 17 to 60 μ m) and V max for cGMP hydrolysis with little or no effect on the Hill coefficient for either substrate. These results suggest that certain hydrophobic domains play an important role in modifying the catalytic specificity of the cGMP-stimulated phosphodiesterase for cAMP and cGMP.

Complex effects of inhibitors on cyclic GMP-stimulated cyclic nucleotide phosphodiesterase.

We have investigated the effects of several phosphodiesterase inhibitors on the activity of a cGMP-stimulated cyclic nucleotide phosphodiesterase purified from calf liver supernatant. Theophylline, RO 20-1724, and MY 5445 were not effective inhibitors. With 0.5 microM [3H]cGMP as substrate or with 0.5 microM [3H]cAMP in the presence of 1 microM cGMP, activity was inhibited by papaverine, dipyridamole, isobutylmethylxanthine (IBMX), and cilostamide. With 0.5 microM [3H]cAMP as substrate, however, only cilostamide was inhibitory; papaverine, dipyridamole, and IBMX increased activity. The increase was dependent on both drug and substrate concentration with maximal stimulation (150-180%) at concentrations of cAMP between 0.5 and 2.5 microM. At higher cAMP concentrations, the three drugs were inhibitory; inhibition was maximal at approximately 40 microM and decreased at higher cAMP concentrations. Inhibition of cGMP hydrolysis was maximal at approximately 3 microM and decreased at higher concentrations. Papaverine, IBMX, dipyridamole, and cilostamide inhibited [3H] cGMP hydrolysis competitively with Ki values of 3, 6.5, 7, and 11.5 microM, respectively. Papaverine, IBMX, or dipyridamole reduced the Hill coefficient for cAMP hydrolysis from 1.8 to 1.1-1.2, and Lineweaver-Burk plots were linear or nearly linear. With cilostamide, however, Lineweaver-Burk plots remained curvilinear. Thus, three competitive inhibitors, papaverine, dipyridamole, and IBMX, can mimic substrate and effect allosteric transitions that increase catalytic activity, whereas another, cilostamide, apparently cannot. Differences in the actions of these inhibitors presumably reflect differences in the molecular requirements for effective interaction at catalytic and allosteric sites on phosphodiesterase, i.e. differences in the structure of these sites.

Identification of cGMP-stimulated cyclic nucleotide phosphodiesterase in lung tissue with monoclonal antibodies.

Monoclonal antibodies (CGS-1, 2, 3, and 4) and rabbit antisera have been produced against bovine heart cyclic GMP-stimulated cyclic nucleotide phosphodiesterase. The monoclonal antibodies were all of the IgG1 subclass and had relatively high affinities (kd values, 1-5 X 10(-10) M), making them suitable for many sensitive immunological and analytical procedures. Most of the monoclonal antibodies did not inhibit the enzyme, which allowed a rapid and specific assay for this form of phosphodiesterase to be developed. All of the activity could be adsorbed from purified preparations of cGMP-stimulated phosphodiesterase by each of the antibodies indicating antigenic homogeneity of the cGMP-stimulated phosphodiesterase. Neither the monoclonal antibodies nor the antisera inhibited cGMP-binding to the phosphodiesterase. Substrate specificity, Electrophoresis, and cyclic GMP binding studies carried out on immunoadsorbed material indicated that a major part of the cyclic nucleotide phosphodiesterase activity present in bovine lung was due to the cyclic GMP-stimulated form. The presence of this enzyme was difficult to identify by other more common procedures and has not been previously demonstrated in this tissue. These studies also indicated that the cGMP-stimulated phosphodiesterase was immunologically distinct from the other major cGMP-binding proteins present in lung extract and that it could be separated from these binding proteins by chromatography on DEAE-cellulose. Finally, the data suggest that none of the other peaks of cyclic nucleotide phosphodiesterase activity, seen upon DEAE-cellulose fractionation of lung extracts, contain the immunological determinants present on the cGMP-stimulated form which are recognized by the antibodies.

Purification and characterization of a cyclic GMP-stimulated cyclic nucleotide phosphodiesterase from bovine tissues.

A cGMP-stimulated cyclic nucleotide phosphodiesterase has been purified to near homogeneity from bovine adrenal and heart tissues. The purification procedure utilizes chromatography on DEAE-cellulose and cGMP affinity resin. The procedure can be completed within 2 days and is easily adapted to large scale. To obtain pure enzyme, an 8,000-9,000-fold increase in specific activity was required in adrenal tissues and 15,000-30,000-fold in cardiac muscle. A single band of protein having an apparent Mr = 105,000-107,000 was seen on sodium dodecyl sulfate gel electrophoresis. At equilibrium, native polyacrylamide gradient gel electrophoresis revealed a single major band having an apparent Mr = 240,000. Cyclic GMP binding and phosphodiesterase activity co-migrated with the protein band on native polyacrylamide gradient gels. The enzyme bound cGMP with high affinity reaching a maximum binding of 1.02 mol of cGMP bound/mol of enzyme dimer. Titration curves of the binding data indicated at least two classes of binding sites with 10% maximal binding occurring at 7 nM and 90% maximal binding at 4 microM cGMP. Kinetic analysis indicated the enzyme can hydrolyze both cAMP and cGMP with similar maximal rates. The nucleotide concentration at half-maximal velocity were 30 and 10 microM for cAMP and cGMP, respectively. The hydrolyses of both nucleotides exhibit positive homotropic cooperativity with Hill coefficients of 1.9 for cAMP and 1.3 for cGMP. The rate of cAMP hydrolysis by the purified enzyme when measured at 10 microM cAMP was enhanced 5- to 6-fold by low levels of cGMP.