Cyclic GMP and photoreceptor function.

A single photon can be detected by a rod photoreceptor cell. The absorption of light by rhodopsin triggers a cascade of reactions that amplifies the photon signal and results in ion channel closure with hyperpolarization of the rod photoreceptor cell. Light-induced conformational changes in rhodopsin facilitate the binding of a guanosine nucleotide-binding protein, transducin, which then undergoes a GTP-GDP exchange reaction and dissociation of the transducin complex. A subunit of transducin then activates a phosphodiesterase complex that hydrolyzes cyclic GMP. In darkness, cyclic GMP binds to cation channels of the photoreceptor plasma membrane, maintaining them in an open configuration. The light-induced reduction in cyclic GMP concentration dissociates the bound cyclic GMP, resulting in channel closure and hyperpolarization. Down-regulation of the cascade involves other proteins that block the interaction of transducin with rhodopsin and another protein that may interfere with transducin recycling. Cone photoreceptors possess a light-activated cascade that follows the rod format, but it is composed of proteins that are homologous to those of rod photoreceptors. Phototransduction in invertebrate photoreceptors uses rhodopsin to activate a cascade that uses phosphoinositides and calcium ion to regulate membrane polarization.     

A light-stimulated increase of cyclic GMP in squid photoreceptors

Photoreceptor outer segments isolated from squid retina are known to contain a light-activated GTP-binding protein. Here it is shown that these photoreceptors contain around 0.01 mol cyclic GMP per mol rhodopsin. Adding GTP in the dark stimulates the production of 0.0003-0.001 mol cyclic GMP/mol rhodopsin per min. GTP and light cause a 2-fold faster increase in cyclic GMP. These results show that either (1) squid rhodopsin activates a guanylate cyclase, or (2) there is a constant guanylate cyclase activity and photoexcited rhodopsin inhibits a cyclic GMP phosphodiesterase.     

Cyclic GMP directly regulates a cation conductance in membranes of bovine rods by a cooperative mechanism

Cyclic GMP causes the release of endogenous Ca2+ from rod outer segments, whose plasma membrane has been made permeable, or from isolated discs. Approximately 11,000 Ca2+ ions are released per disc at saturating concentrations of cyclic GMP. The velocity and the amplitude of the release of Ca2+ are dependent on the concentration of cyclic GMP. The maximal rate of the Ca2+ efflux is approximately 7 X 10(4) Ca2+ ions s-1 rod-1. The Ca2+ release by cyclic GMP is independent of light. The activation of the efflux occurred within a narrow range of the cyclic GMP concentration (30-80 microM) and does not obey a simple Michaelis-Menten scheme. Instead, the kinetic analysis of the Ca2+ efflux suggests that a minimum number of 2 molecules of cyclic GMP activates the ion conductance in a cooperative fashion. The release of Ca2+ by cyclic GMP requires a gradient of Ca2+ ions across the disc membrane. If the endogenous Ca2+ gradient is dissipated by means of the ionophore A23187, the release of Ca2+ by cyclic GMP is abolished. Ca2+ is released by analogues of cyclic GMP which are either modified at the 8-carbon position of the imidazole ring or by the deaza-analogue of cyclic GMP. Congeners of cyclic GMP which are modified at the ribose, phosphodiester, or pyrimidine portion of the molecule are ineffective. The hydrolysis of cyclic GMP by the light-regulated phosphodiesterase of rod outer segments is not a necessary condition for the Ca2+ release because 8-bromo-cyclic GMP, a congener resistant to hydrolysis, is a more powerful activator of the release than cyclic GMP itself. Ca2+ release by cyclic GMP is inhibited by organic and inorganic blockers of Ca2+ channels. The l-stereoisomer of cis-diltiazem blocks the release of Ca2+ at micromolar concentrations, whereas the d-form is much less effective. These results suggest that disc membranes contain a cationic conductance which is permeable to Ca2+ ions and which is regulated through the cooperative binding of at least 2 molecules of cyclic GMP to regulatory sites of the transport protein. By this mechanism, subtle changes in the concentration of cyclic GMP could promote large changes in the flux of Ca2+ ions across the disc membrane.     

Evidence for convertible forms of soluble uterine cyclic nucleotide phosphodiesterase

The cyclic nucleotide phosphodiesterase (3':5'-cyclic nucleotide 5'-nucleotidohydrolase, EC 3.1.4.17) systems of many tissues show multiple physical and kinetic forms. In contrast, the soluble rat uterine phosphodiesterase exists as a single enzyme form with non-linear Lineweaver-Burk kinetics for cyclic AMP (app. Km of approx. 3 and 20 microM) and linear kinetics for cyclic GMP (app. Km of approx. 3 microM) since the two hydrolytic activities are not separated by a variety of techniques. In uterine cytosolic fractions, cyclic AMP is a non-competitive inhibitor of cyclic GMP hydrolysis (Ki approx. 32 microM). Also, cyclic GMP is a non-competitive inhibitor of cyclic AMP hydrolysis (Ki approx 16 microM) at low cyclic GMP/cyclic AMP substrate ratios. However, cyclic GMP acts as a competitive inhibitor of cyclic AMP phosphodiesterase (Ki approx 34 microM) at high cyclic GMP/cyclic AMP substrate ratios. When a single hydrolytic form of uterine phosphodiesterase, separated initially by DEAE anion-exchange chromatography, is treated with trypsin (0.5 microgram/ml for 2 min) and rechromatographed on DEAE-Sephacel, two major forms of phosphodiesterase are revealed. One form elutes at 0.3 M NaOAc- and displays anomalous kinetics for cyclic AMP hydrolysis (app. Km of 2 and 20 microM) and linear kinetics for cyclic GMP (app. Km approx. 5 microM), kinetic profiles which are similar to those of the uterine cytosolic preparations. A second form of phosphodiesterase elutes at 0.6 M NaOAc- and displays a higher apparent affinity for cyclic AMP (app. Km approx. 1.5 mu) without appreciable cyclic GMP hydrolytic activity. These data provide kinetic and structural evidence that uterine phosphodiesterase contains distinct catalytic sites for cyclic AMP and cyclic GMP. Moreover, they provide further documentation that the multiple forms of cyclic nucleotide phosphodiesterase in mammalian tissues may be conversions from a single enzyme species.     

Binding of inositol phosphates to arrestin.

Arrestin binds to phosphorylated rhodopsin in its light-activated form (metarhodopsin II), blocking thereby its interaction with the G-protein, transducin. In this study, we show that highly phosphorylated forms of inositol compete against the arrestin-rhodopsin interaction. Competition curves and direct binding assays with free arrestin consistently yield affinities in the micromolar range; for example, inositol 1,3,4,5-tetrakisphosphate (InP4) and inositol hexakisphosphate (InP6 bind to arrestin with dissociation constants of 12 microM and 5 microM, respectively. Only a small control amount of inositol phosphates is bound, when arrestin interacts with phosphorylated rhodopsin. This argues for a release of bound inositol phosphates by interaction with rhodopsin. Transducin, rhodopsin kinase, or cyclic GMP phosphodiesterase are not affected by inositol phosphates. These observations open a new way to purify arrestin and to inhibit its interaction with rhodopsin. Their physiological significance deserves further investigation.     

Control of the cyclic GMP phosphodiesterase of frog photoreceptor membranes

The light-activated cyclic GMP phosphodiesterase (PDE) of frog photoreceptor membranes has been assayed in isolated outer segments suspended in a low-calcium Ringer's solution. Activation occurs over a range of light intensity that also causes a decrease in the permeability, cyclic GMP levels, and GTP levels of isolated outer segments. At intermediate intensities, PDE activity assumes constant intermediate values determined by the rate of rhodopsin bleaching. Washing causes an increase in maximal enzyme activity. Increasing light intensity from darkness to a level bleaching 5 x 10(3) rhodopsin molecules per outer segment per second shifts the apparent Michaelis constant (Km) from 100 to 900 microM. Maximum enzyme velocity increases at least 10-fold. The component that normally regulates this light- induced increase in the Km of PDE is removed by the customary sucrose flotation procedures. The presence of 10(-3) M Ca++ increases the light sensitivity of PDE, and maximal activation is caused by illumination bleaching only 5 x 10(2) rhodopsin molecules per outer segment per second. Calcium acts by increasing enzyme velocity while having little influence on Km. The effect of calcium appears to require a labile component, sensitive to aging of the outer segment preparation. The decrease in the light sensitivity of PDE that can be observed upon lowering the calcium concentration may be related to the desensitization of the permeability change mechanism that occurs during light adaptation of rod photoreceptors.     

Inhibition of cardiac slow action potentials by 8-bromo-cyclic GMP occurs independent of changes in cyclic AMP levels

Cyclic GMP inhibits the slow inward Ca current of cardiac cells. This effect could be due to a cyclic GMP-mediated phosphorylation of the Ca channel (or some protein modifying Ca channel activity), or alternatively, to enhanced degradation of cyclic AMP owing to stimulation of a phosphodiesterase by cyclic GMP. To test the latter possibility, we examined the effect of extracellular 8-bromo-cyclic GMP on cyclic AMP levels in guinea pig papillary muscles, in parallel with electrophysiological experiments. Isoproterenol (10(-6) M) significantly increased the cyclic AMP levels and induced Ca-dependent slow action potentials. Superfusion with 8-bromo-cyclic GMP (10(-3) M) inhibited the slow action potentials induced by isoproterenol. However, muscles superfused with 8-bromo-cyclic GMP had cyclic AMP levels identical to those of muscles superfused with isoproterenol alone. Similarly, 8-bromo-cyclic GMP had no effect on the increase in cyclic AMP levels of muscles treated with forskolin (10(-6) M) or histamine (10(-6) M). We conclude that the inhibitory effect of cyclic GMP on slow Ca channels in guinea pig ventricular cells is not due to a decrease in the cyclic AMP levels. We hypothesize that a cyclic GMP-mediated phosphorylation is the most likely explanation for the Ca channel inhibition observed in this preparation.     

Physiological evidence that light-mediated decrease in cyclic GMP is an intermediary process in retinal rod transduction

Brief, intracellularly injected pulses of cyclic GMP transiently depolarized toad retinal rod outer segments (ROS). The depolarization is antagonized by light, perhaps by the activation of phosphodiesterase (PDE), as shown in the biochemical studies of others. As measured by the antagonism of cyclic GMP pulses by light, PDE activity peaks after the peak of the receptor potential and has approximately the same recovery time as the membrane voltage after weak illumination, but recovers more slowly than the membrane potential after strong illumination, as sensitivity does in other preparations. A cyclic GMP pulse delivered just after the hyperpolarizing phase of the receptor potential tends to turn off the light response. The kinetics of recovery from this turnoff are similar to those of the initial phase of the receptor potential. This similarity suggests that the initial phase of the receptor potential is controlled by light-activated PDE. Both EGTA and saturating doses of cyclic GMP block the light response, but only cyclic GMP increases response latency, which suggests that if calcium is involved in transduction, it is controlled by the hydrolysis of cyclic GMP. After brief pulses of cyclic AMP, a new steady state of increased depolarization occasionally develops. The effects described above also occur under these conditions. The results are consistent with the hypothesis that light-activated hydrolysis of cGMP is an intermediary process in transduction.     

In vitro studies of the testis: the effect of LH on cyclic nucleotides.

This study evaluated the relationship between LH, cyclic AMP, cyclic GMP, and testosterone using in vitro incubation of decapsulated rat testes and sampling incubation medium. With added LH (1.0, 5.0, 100, and 500 mIU/ml) there were statistically significant increases in cyclic AMP at 5 mIU/ml or more LH, and progressively greater titers of this nucleotide were produced as LH was increased. For cyclic GMP all levels of added LH caused significant increments in titers of nucleotide; however, peak cyclic GMP concentrations occurred with 5 mIU/ml of LH. The addition of 10(-3) and 10-(4)M 8-bromo-cyclic AMP caused significant increases in testosterone production, while no changes in production of this androgen were found with 10(-3), 10(-4), or 10(-5)M 8-bromo-cyclic GMP. Neither cyclic AMP nor cyclic GMP titers were altered by the addition of 1 to 50 micrograms/ml of testosterone to medium bathing the rat testes. The dose response curves of cyclic AMP and cyclic GMP to LH are different. Progressive increments in added LH cause parallel increases of cyclic AMP and a biphasic change of cyclic GMP, 8-bromo-cyclic GMP does not cause testosterone generation, suggesting that cyclic GMP does not result in androgen synthesis. However, cyclic GMP may be involved in other Leydig cell functions.     

Cyclic nucleotide hydrolysis in the thyroid gland. General properties and key role in the interrelations between concentrations of adenosine 3':5'-monophosphate and guanosine 3':5'-monophosphate.

Phosphodiesterase activities of horse (and dog) thyroid soluble fraction were compared with either cyclic AMP (adenosine 3':3'-monophosphate) or cyclic GMP (guanosine 3':5'-monophosphate) as substrate. Optimal activity for cyclic AMP hydrolysis was observed at pH 8, and at pH 7.6 for cyclic GMP. Increasing concentrations of ethyleneglycol bis(2-aminoethyl)-N,N'-tetraacetic acid inhibited both phosphodiesterase activities; in the presence of exogenous Ca2+, this effect was shifted to higher concentrations of the chelator. In a dialysed supernatant preparation, Ca2+ had no significant stimulatory effect, but both Mg2+ and Mn2+ increased cyclic nucleotides breakdown. Mn2+ promoted the hydrolysis of cyclic AMP more effectively than that of cyclic GMP. For both substrates, substrate velocity curves exhibited a two-slope pattern in a Hofstee plot. Cyclic GMP stimulated cyclic AMP hydrolysis, both nucleotides being at micromolar concentrations. Conversely, at no concentration had cyclic AMP any stimulatory effect on cyclic GMP hydrolysis. 1-Methyl-3-isobutylxanthine and theophylline blocked the activation by cyclic GMP of cyclic GMP of cyclic AMP hydrolysis, whereas Ro 20-1724 (4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone), a non-methylxanthine inhibitor of phosphodiesterases, did not alter this effect. In dog thyroid slices, carbamoylcholine, which promotes an accumulation of cyclic GMP, inhibits the thyrotropin-induced increase in cyclic AMP. This inhibitory effect of carbamoylcholine was blocked by theophylline and 1-methyl-3-isobutylxanthine, but not by Ro 20-1724. It is suggested that the cholinergic inhibitory effect on cyclic AMP accumulation is mediated by cyclic GMP, through a direct activation of phosphodiesterase activity.     

Protein activator of cyclic AMP phosphodiesterase and cyclic nucleotide phosphodiesterase in bovine retina and bovine lens. Activity, subcellular distribution and kinetic parameters.

We have examined the activity of cyclic AMP phosphodiesterase, cyclic GMP phosphodiesterase and the protein activator of cyclic AMP phosphodiesterase in various anatomic and subcellular fractions of the bovine eye. Cyclic GMP hydrolysis was 1.6--12 times faster than hydrolysis of cyclic AMP in the subcellular fractions of the retina and in the precipitate of the rod outer segment. An opposite pattern was seen in the bovine lens, where the hyrolysis of cyclic AMP occurred 17 and 169 times faster than that of cyclic GMP in the supernatant and precipitate of lens, respectively. The activity of cyclic AMP phosphodiesterase was not affected by ethylene-glycol bis(beta-aminoethylether)-N,N'-tetraacetic acid in any fractions except in the retinal supernatant, suggesting that the phosphodiesterase exists primarily as a Ca2+-independent, activator-independent form. However, the protein activator of cyclic AMP phosphodiesterase existed in all fractions examine. A complex kinetic patternwas observed for both cyclic AMP and cyllic GMP hydrolysis by the 105000 times g lens supernatant. The Michaelis constants for both cyclic AMP (1.3-10(-6) and 9.I-10(-6) M) and cyclic GMP (1.04-10(6) AND 1.22 10(-5) M) appeared to be similar.     

Properties of the cyclic GMP phosphodiesterase from rat lung. Inhibition by unsaturated fatty acids

Catalytic and regulatory properties of the major form of cyclic GMP phosphodiesterase (3':5'-cyclic-GMP 5'-nucleotidohydrolase, EC 3.1.4.35) from rat lung were studied. The enzyme partially purified by a DEAE-Sepharose chromatography displayed a much higher affinity toward cyclic GMP than toward cyclic AMP, the apparent Km values being 5.7 microM and 482 microM for the guanylic and the adenylic cyclic nucleotide, respectively. In contrast, the V value for cyclic AMP was about 3-times higher than the V value for cyclic GMP. Linear double reciprocal plots of initial velocity were observed with each cyclic nucleotide. From 10(-8) to 3.3 X 10(-6) M, cyclic GMP did not change the hydrolysis of 1 or 10 microM cyclic [3H]AMP, while it became inhibitory at higher concentrations. In contrast with a calmodulin-sensitive phosphodiesterase prepared from rat brain, the lung enzyme was not stimulated by a heat-stable Ca2+-dependent factor from rat lung or by rat brain calmodulin or by lipids including fatty acids and lysophosphatidylcholine. Various unsaturated 18- and 20-carbon fatty acids inhibited at varying degrees the cyclic GMP phosphodiesterase from rat lung. The inhibitory potency increased with the number of double bonds in the hydrocarbon chain. In contrast, the methyl esters of the unsaturated fatty acids and the saturated fatty acids of variable hydrocarbon chain lengths had no appreciable effects. A linear Hill plot of phosphodiesterase inhibition with a slope of unity was obtained with arachidonic acid up to 30 microM, suggesting only one type of inhibitory site. In this range of concentrations the inhibition was entirely reversible. Kinetics analysis demonstrated that up to 30 microM arachidonic acid was a purely competitive inhibitor with an apparent Ki of 20 microM. Over 30 microM, the Hill coefficient increased progressively, indicating the binding to other inhibitory sites, while the reversibility disappeared.     

Association of cyclic nucleotide phosphodiesterase of buffalo spermatozoa with sperm chromatin

Buffalo sperm heads contain more than 50% of the total cyclic AMP-phosphodiesterase activity (EC 3.1.4.17) present in spermatozoa. Its distribution in sperm heads revealed no activity in acrosome and other membrane structures present in the head. All the cyclic AMP-phosphodiesterase activity was found firmly bound to sperm chromatin which could not be solubilized. In addition to cyclic AMP, cyclic GMP was also hydrolysed by chromatin preparation. The rate of hydrolysis was 2.5-times more rapid with cyclic AMP than with cyclic GMP at their optimum pH of 7.5 and 8.0, respectively. The pH and heat stability profiles, inhibition studies and the effect of divalent metal ions indicated that the two activities are not associated with the same protein. Mixed substrate analysis showed two sites at which the hydrolysis of cyclic AMP and cyclic GMP is catalysed. Chromatin cyclic nucleotide phosphodiesterases exhibited kinetics typical of one enzyme species both for cyclic AMP (K m = 100 microM; V = 1.0 nmol/min per mg protein) and cyclic GMP (Km = 23 microM; V = 0.4 nmol/min per mg protein). Each cyclic nucleotide was found to be a competitive inhibitor of the hydrolysis of the other with a Ki value of 30.18 microM for cyclic AMP hydrolysis and 256 microM for cyclic GMP hydrolysis. Hill coefficients of 1.0 obtained in the presence of cyclic AMP for cyclic GMP hydrolysis and vice-versa indicated no allosteric interactions. It is suggested that chromatin cyclic nucleotide phosphodiesterase may have a role post fertilization in cell growth and differentiation with no role in sperm motility which is regulated by similar enzymes present in sperm flagella.     

Stimulation of rhodopsin phosphorylation by guanine nucleotides in rod outer segments

Porcine rod outer segment (ROS) proteins were phosphorylated in the presence of [gamma-32P]ATP and Mg2+, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and detected by autoradiography. The phosphorylation of rhodopsin, the major protein-staining band (Mr approximately 34 000-38 000), was markedly and specifically increased by exposure of rod outer segments to light; various guanine nucleotides (10 microM) including GMP, GDP, and GTP also specifically increased rhodopsin phosphorylation (up to 5-fold). Adenine nucleotides (cyclic AMP, AMP, and ADP at 10 microM) and 8-bromo-GMP (10 microM) or cyclic 8-bromo-GMP (10 microM) had no detectable stimulatory effect on rhodopsin phosphorylation. GTP increased the phosphorylation of rhodopsin at concentrations as low as 100 nM, and guanosine 5'-(beta, gamma-imidotriphosphate), a relatively stable analogue of GTP, was nearly as effective as GTP. Maximal stimulation of rhodopsin phosphorylation by GTP was observed at 2 microM. GMP and GDP were less potent than GTP. Both cyclic GMP and GMP were converted to GTP during the time period of the protein phosphorylation reaction, suggestive of a GTP-specific effect. Transphosphorylation of guanine nucleotides by [32P]ATP and subsequent utilization of [32P]GTP as a more effective substrate were ruled out as an explanation for the guanine nucleotide stimulation. With increasing concentrations of ROS proteins, the phosphorylation of rhodopsin was nonlinear, whereas in the presence of GTP (2 microM) linear increases in rhodopsin phosphorylation as a function of added ROS protein were observed. These results suggest that GTP stimulates the phosphorylation of rhodopsin by ATP and that a GTP-sensitive inhibitor (or regulator) of rhodopsin phosphorylation may be present in ROS.     

Phosphorylation of rhodopsin and quenching of cyclic GMP phosphodiesterase activation by ATP at weak bleaches

Light and GTP-dependent cyclic GMP phosphodiesterase activation of rod disk membranes is rapidly quenched by ATP. Maximum speed of this effect occurs only with the weakest bleaches. Though it has been proposed that ATP mediates its effect through rapid phosphorylation of bleached rhodopsin, previous workers have found phosphorylation kinetics too slow by more than an order of magnitude to be causal in quenching of cyclic GMP phosphodiesterase activation. In this report, we use preparations retaining more endogenous rhodopsin kinase, higher specific activity ATP, and cyclic GMP phosphodiesterase quenching conditions to show that ATP-dependent multiple phosphorylation of rhodopsin at very weak bleaches (10(-5)) is complete in less than 2 s, easily compatible with cyclic GMP phosphodiesterase quench times of 4 s measured under identical conditions. Thus, it seems likely that previous efforts to achieve high 32P counts by using large bleaches have produced conditions of substrate saturation where much longer times to completion are caused by a very large ratio of substrate to enzyme velocity. Such conditions are not appropriately compared to those that support rapid quenching. We conclude that the speed of rhodopsin phosphorylation is, in fact, adequate to explain ATP quenching of cyclic GMP phosphodiesterase activation.     

Light adaption of the cyclic GMP phosphodiesterase of frog photoreceptor membranes mediated by ATP and calcium ions

The light-activated guanosine 3',5'-cyclic monophosphate (cyclic GMP) phosphodiesterase (PDE) of frog photoreceptor membranes has been assayed by measuring the evolution of protons that accompanies cyclic GMP hydrolysis. The validity of this assay has been confirmed by comparison with an isotope assay used in previous studies (Robinson et al. 1980. J. Gen. Physiol. 76: 631-645). The PDE activity elicited by either flash or continuous dim illumination is reduced if ATP is added to outer segment suspensions. This desensitization is most pronounced at low calcium levels. In 10(-9) M Ca++, with 0.5 mM ATP and 0.5 mM GTP present, PDE activity remains almost constant as dim illumination and rhodopsin bleaching continue. At intermediate Ca++ levels (10-7-10-5M) the activity slowly increases during illumination. Finally, in 10(-4) and PDE activity is more a reflection of the total number of rhodopsin molecules bleached than of the rate of the rhodopsin bleaching. At intermediate or low calcium levels a short-lived inhibitory process is revealed by observing a nonlinear summation of responses of the enzyme to closely spaced flashes of light. Each flash makes PDE activity less responsive to successive flashes, and a steady state is obtained in which activation and inactivation are balanced. It is suggested that calcium and ATP regulation of PDE play a role in the normal light adaption processes of frog photoreceptor membranes.     

Cyclic nucleotide phosphodiesterase in rat neostriatum: multiple isoelectric forms with similar kinetic properties

Separation of multiple forms of cyclic nucleotide phosphodiesterase from the soluble supernatant fraction of rat neostriatum by isoelectric focusing yielded five separate peaks of cyclic nucleotide hydrolysing activity. Each separated enzyme form displayed a complex kinetic pattern for the hydrolysis of both cyclic AMP and cyclic GMP, and there were two apparent Km's for each nucleotide. At 1 microM substrate concentration, four enzyme forms exhibited higher activity with cyclic AMP than with cyclic GMP, while one form yielded higher activity with cyclic GMP than with cyclic AMP. Cyclic AMP and cyclic GMP were both capable of almost complete inhibition of the hydrolysis of the other nucleotide in all the peaks separated by isoelectric focusing; the IC50's for this interaction correlated well with the relative rates of hydrolysis of each nucleotide in each peak. The ratio of activity at 1 microM substrate concentration for the five enzyme forms separated by isoelectric focusing was 10:10:5:15:1 for cyclic AMP hydrolysis; and 6:6:4:8:2 for cyclic GMP hydrolysis; and the isoelectric points of the five peaks were 4.3, 4.45, 4.7, 4.85, and 5.5, respectively. Known phosphodiesterase inhibitors did not preferentially inhibit any of the separated forms of activity for either cyclic AMP or cyclic GMP hydrolysis, at either high (100 microM) or low (1 microM) substrate concentrations. Preliminary examination of the subcellular distribution of the different forms of enzyme activity indicated a different degree of attachment of the various forms to particulate tissue components. Isoelectric focusing of the soluble supernatant of rat cerebellum gave rise to a slightly different pattern of isoelectric forms from the neostriatum, indicating a different cellular distribution of the isoelectric forms of PDE in rat brain. Polyacrylamide disc gel electrophoresis of the soluble supernatant of rat neostriatum also generated a characteristic pattern of five separate peaks of cyclic nucleotide phosphodiesterase activity, each of which hydrolysed both cyclic AMP and cyclic GMP. Polyacrylamide gel electrophoresis of single enzyme forms previously separated by isoelectric focusing gave single peaks, with a marked correspondence between the enzyme forms produced by isoelectric focusing and those produced by gel electrophoresis, suggesting that both protein separation procedures were isolating the same enzyme forms. The results indicate the existence of multiple isoelectric forms of cyclic nucleotide phosphodiesterase in the soluble supernatant fraction of rat neostriatum, all of which exhibit similar properties. In this tissue a single kinetic form of this enzyme appears to exist displaying complex kinetic behaviour indicative of negative cooperativity and hydrolysing both cyclic AMP and cyclic GMP, with varying affinities.     

Ca2+-independent cyclic GMP phosphodiesterases from rat liver and HTC hepatoma cells.

We have separated and characterized a Ca2+- and calmodulin-insensitive cyclic nucleotide phosphodiesterase from rat liver supernatant as well as an analogous enzyme from HTC hepatoma cells. Chromatography of rat liver supernatant on DEAE-cellulose in the presence and subsequently in the absence of 0.1 mM-CaCl2 resulted in the separation of two distinct phosphodiesterase activities, both of which preferentially hydrolysed cyclic GMP rather than cyclic AMP. One enzyme, E-Ib, was activated in the presence of Ca2+ and calmodulin, and the other, E-Ia, was not. The E-Ia enzyme, which did not bind to calmodulin-Sepharose, had Mr 325 000 and displayed anomalous kinetic behaviour [Km (cyclic GMP) 1.2 microM; Km (cyclic AMP) 15.4 microM]. The E-Ib enzyme, which bound to calmodulin-Sepharose in the presence of Ca2+, had Mr 150 000 and exhibited Michaelis-Menten kinetics for hydrolysis of cyclic GMP [Km (basal) 6.5 microM; Km (activated) 12.0 microM]. E-Ia activity was diminished by incubation with alpha-chymotrypsin and was unaffected by the action of a rat kidney lysosomal proteinase. Partial hydrolysis of E-Ib enzyme by alpha-chymotrypsin or the kidney proteinase resulted in irreversible activation of the enzyme. The E-I enzyme isolated from HTC hepatoma cells was similar to the rat liver E-Ia enzyme in many respects. Its apparent Mr was 325 000. Its activity was unaffected by calmodulin in the presence of Ca2+ or by incubation with the kidney proteinase, and was decreased by digestion with alpha-chymotrypsin. Unlike the liver E-Ia enzyme, however, the hepatoma enzyme exhibited normal kinetic behaviour, with Km (cyclic GMP) 3.2 microM. Although HTC cells contain two other phosphodiesterases analogous to those in rat liver and a calmodulin-like activator of phosphodiesterase, no calmodulin-sensitive phosphodiesterase was detected.     

Rod outer segment structure influences the apparent kinetic parameters of cyclic GMP phosphodiesterase

Cyclic GMP hydrolysis by the phosphodiesterase (PDE) of retinal rod outer segments (ROS) is a key amplification step in phototransduction. Definitive estimates of the turnover number, kcat, and of the Km are crucial to quantifying the amplification contributed by the PDE. Published estimates for these kinetic parameters vary widely; moreover, light-dependent changes in the Km of PDE have been reported. The experiments and analyses reported here account for most observed variations in apparent Km, and they lead to definitive estimates of the intrinsic kinetic parameters in amphibian rods. We first obtained a new and highly accurate estimate of the ratio of holo-PDE to rhodopsin in the amphibian ROS, 1:270. We then estimated the apparent kinetic parameters of light-activated PDE of suspensions of disrupted frog ROS whose structural integrity was systematically varied. In the most severely disrupted ROS preparation, we found Km = 95 microM and kcat = 4,400 cGMP.s-1. In suspensions of disc-stack fragments of greater integrity, the apparent Km increased to approximately 600 microM, though kcat remained unchanged. In contrast, the Km for cAMP was not shifted in the disc stack preparations. A theoretical analysis shows that the elevated apparent Km of suspensions of disc stacks can be explained as a consequence of diffusion with hydrolysis in the disc stack, which causes active PDEs nearer the center of the stack to be exposed to a lower concentration of cyclic GMP than PDEs at the disc stack rim. The analysis predicts our observation that the apparent Km for cGMP is elevated with no accompanying decrease in kcat. The analysis also predicts the lack of a Km shift for cAMP and the previously reported light dependence of the apparent Km for cGMP. We conclude that the intrinsic kinetic parameters of the PDE do not vary with light or structural integrity, and are those of the most severely disrupted disc stacks.