Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.1.4.1 (phosphodiesterase)
18,767 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Cyclic GMP is central to visual excitation in vertebrate retinal rod cells. Sodium channels in the plasma membrane of the outer segment are kept open in the dark by a high level of cGMP. Light closes these channels by activating an enzymatic cascade that leads to the rapid hydrolysis of cGMP. Photoexcited rhodopsin triggers transducin by catalyzing the exchange of GTP for bound GDP. The activated GTP-form of transducin then switches on the phosphodiesterase by overcoming an inhibitory constraint. The overall gain of this cascade is about 10(5). The cascade is turned off by the GTPase activity of transducin and by the action of rhodopsin kinase and arrestin. One of the challenges now is to delineate the interplay of cGMP, calcium ion, and phosphoinositides in excitation and adaptation. Transducin belongs to a family of signal-coupling proteins that includes the G proteins of the hormone-regulated adenylate cyclase cascade. The initial events in visual excitation in molluscs and arthropods are probably similar to those of vertebrates. The triggering of transducin by photoexcited rhodopsin is a recurring motif in visual transduction. The coming together of electrophysiology, biochemistry, and molecular genetics affords new opportunities in unraveling the molecular mechanism of visual transduction.
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PMID:Cyclic GMP cascade of vision. 242 11

Seven monoclonal antibodies to the alpha subunit (G alpha) of the frog photoreceptor guanyl nucleotide-binding protein (transducin or G-protein) have been characterized as to their effect on G-protein function, and this has been correlated in the accompanying paper (Deretic, D., and Hamm, H. E. (1987) J. Biol. Chem. 262, 10839-10847) with the antibody-binding sites on G alpha tryptic fragments. Antibodies 4A, 7A, 7B, 7C, and 7D are members of a class of antibodies that block G-protein activation by light and therefore also block activation of the cGMP phosphodiesterase. All these blocking antibodies also block the interaction of G-protein with rhodopsin as measured by the light-scattering "binding signal," and as measured by the stabilization of meta-rhodopsin II by bound G-protein (extra-meta-rhodopsin II). The antibodies (or Fab fragments) also solubilize G alpha beta gamma from the membrane in the dark under isosmotic conditions and thus interfere with G alpha interaction with the membrane. Antibody 4A also blocks the extra-meta-rhodopsin II generated by G-protein-rhodopsin interaction in detergent solubilized membranes. Thus, even in the absence of phospholipids, antibody 4A blocks G-protein-rhodopsin interaction. Therefore, we suggest that the antibodies recognize a region of G alpha involved with binding to rhodopsin. An alternative hypothesis is that this antigenic site is a region of interaction between the alpha and beta gamma subunits, disruption of this interaction leading to removal of both the alpha and beta gamma subunits from the membrane and blocking interaction with rhodopsin. This does not seem to be the case because the antibodies immunoprecipitate the alpha beta gamma complex, and not just the alpha subunit. Other antibodies, 4C and 4H, do not block phosphodiesterase activation, the light-scattering signal, extra-meta-rhodopsin II formation, or interaction with the membrane in the dark and therefore recognize other sites on G alpha.
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PMID:Mechanism of action of monoclonal antibodies that block the light activation of the guanyl nucleotide-binding protein, transducin. 244 Aug 75

The rod photoreceptors of vertebrate retinas contain a cGMP phosphodiesterase (PDE) that is activated by light. The light is absorbed by rhodopsin that activates an intermediate GTP-binding protein; this species then activates the PDE. Photo-excited rhodopsin passes through a series of transient states, and the purpose of this study is to identify the earliest state that interacts with the GTP-binding protein and thus activate the PDE. The majority of evidence points to this state being metarhodopsin II (MII), but PDE activation is seen at low temperatures where the rhodopsin reaction sequence is not expected to pass beyond the metarhodopsin I (MI) stage. Light thresholds for PDE activation have been determined under conditions where little MII is generated, and these are compared with the concentration of MII. The conclusion is that for a criterion threshold of PDE activity, the MII concentration is constant, irrespective of the amount of MI present, which suggests that MI cannot activate the PDE system.
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PMID:Can metarhodopsin I activate rod outer segment phosphodiesterase? 245 51

Aluminum ion perturbs the activity of a number of physiologically important enzymes, including members of a family of guanine nucleotide-binding proteins (G-proteins). G-proteins couple cellular receptor proteins to a variety of effector enzymes (including adenylate cyclase, phospholipase C, and the rod photoreceptor phosphodiesterase). We show herein that subnanomolar concentrations of free aluminum ion, produced in a carefully defined and kinetically stable manner through the buffering of total aluminum at 0.1-1.0 mM with calculated ratios of chelating agents, inhibit both the receptor-mediated activation and the self-inactivating GTPase activity of the rod photoreceptor G-protein, Gv. In the presence of 4 X 10(-10) M free aluminum ion, GTPase activity is inhibited from about 25-60% as the magnesium ion concentration is reduced from 10(-3) to about 5 X 10(-5) M. The principal effect of aluminum ion upon Gv is to inhibit receptor catalyzed nucleotide exchange. Binding of the GTP analog 5'-guanylyl imidodiphosphate can be reduced by as much as 90% by aluminum ion following subsaturating rhodopsin stimulation. Aluminum ion can produce either competitive or mixed noncompetitive inhibition of rhodopsin-catalyzed Gv activation and GTPase activity, as a function of whether Gv undergoes single (competitive), or multiple (mixed noncompetitive) nucleotide exchanges. The rod photoreceptor phosphodiesterase is only slightly inhibited by similar aluminum ion activities. Light- and Gv-coupled phosphodiesterase activation exhibits both a lower maximum rate of cyclic guanosine monophosphate hydrolysis and a slower inactivation in the presence of aluminum ion activities from about 10(-12) - 10(-10) M. These data suggest that intracellular free aluminum ion concentrations in the subnanomolar range could markedly affect the ability of cells to transduce extracellular signals. Interestingly, the combination of Al3+ and F- to produce the fluoro-aluminate species (AlFx) also inhibits the GTPase of G-proteins, although the mechanism of inhibition (e.g. binding to the G-protein.Mg2+.GDP complex) is totally distinct from that observed for free Al3+ and the overall effect on signal transduction (e.g. enhanced signal amplification) is in complete opposition to that observed for free Al3+.
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PMID:Inhibition of transducin activation and guanosine triphosphatase activity by aluminum ion. 253 40

According to present-day concepts an important and, presumably, a key role in signal transmission in photoreceptor cells is ascribed to a system containing the photosensitive protein rhodopsin, GTP-binding protein transducin and cyclic GMP phosphodiesterase which in many features is similar to the adenylate cyclase system from other eukaryotic cells. The experimental and literary data concerning the already established and hypothetical mechanisms of transmission, enhancement and switch-off of the signal in the rhodopsin----transducin----phosphodiesterase chain are reviewed.
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PMID:[The enzymology of visual reception: phosphodiesterase cascade of signal amplification]. 254 57

In this work we have characterized the ability of a carboxyl peptide-specific antibody (AS/7), raised against the alpha subunit of transducin (alpha T), to potentiate the stimulation of the cyclic GMP phosphodiesterase (PDE) by transducin. The complexation of the purified guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)-bound form of alpha T (alpha T.GTP gamma S) with AS/7 results in a 2-5-fold enhancement in the total levels of cyclic GMP hydrolysis measured after 1 min. This potentiation by AS/7 cannot be attributed simply to an increase in the apparent affinity of alpha T.GTP gamma S for the effector enzyme, nor to an increased affinity of the enzyme for the substrate cyclic GMP. The AS/7-induced potentiation is specific for alpha T.GTP gamma S-PDE interactions; this antibody has no effect on the activity of the trypsin-activated PDE nor on the ability of the GDP-bound form of alpha T to inhibit the trypsin-activated enzyme (Kroll, S., Phillips, W. J., and Cerione, R. A. (1989) J. Biol. Chem. 264, 4490-4497). Phosphatidylcholine vesicles also will enhance the alpha T.GTP gamma S-stimulated PDE activity (1.5-2-fold) relative to that measured in the absence of a lipid milieu. However, the potentiations of alpha T-stimulated cyclic GMP hydrolysis elicited by AS/7 and lipids represent separate events. Titration profiles describing the AS/7-induced potentiation, as a function of the amount of antibody added to the assay mixtures, indicate that maximal activity occurs when there is one molecule of AS/7 per two molecules of alpha T.GTP gamma S; the AS/7-induced potentiation is lost when AS/7 much greater than alpha T. GTP gamma S, i.e. conditions which favor the formation of monovalent AS/7-alpha T.GTP gamma S complexes. When the AS/7 is papain-treated to yield monovalent antibody molecules, complexation between these monovalent antibodies and alpha T still occurs (as reflected by the ability of these antibodies to block rhodopsin-alpha T coupling); however, the potentiation of the alpha T.GTP gamma S-stimulated PDE activity is lost. Taken together, these results suggest that the AS/7-induced potentiation of alpha T-stimulated activity is dependent on the bivalent nature of the antibody, and maximal stimulation of PDE activity is achieved by the interactions of two activated-alpha T molecules with a single molecule of PDE.
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PMID:An antibody-induced enhancement of the transducin-stimulated cyclic GMP phosphodiesterase activity. 255 Apr 53

We present a quantitative kinetic model for the transient velocity (microM of cGMP hydrolyzed/s) response of retinal rod outer segment (ROS) cGMP phosphodiesterase (v(t) versus t) to a stimulating light pulse in the linear response range. The model gives an excellent fit to experimental v(t) versus t data for ROS suspensions at different concentrations of GTP and GDP and clarifies experimental results which are difficult to understand in the absence of such a model. It contains the minimum number of steps required to fit our experimental data and consists of one rate-limiting step with specific rate kL for the production of active phosphodiesterase (PDE), PDE*, by photoactivated rhodopsin, R*, and deactivation processes for R* and PDE* with lifetimes tau R and tau P, respectively. The experimental graphs of v(t) versus t at each concentration of GTP and GDP are characterized by a fast rise to a peak value, vpeak, followed by a slow decay to zero level. The minimal kinetic model allows us to characterized completely the effects of GTP and GDP, and any other pertinent species, in terms of their effects on the parameters kL, tau R, and tau P. Our kinetic model indicates that for "washed" ROS preparations (a) the risetime of v(t) is determined by tau P which has a value of about 2 s and is insensitive to [GTP]. (b) The decay of v(t) is determined by tau R which decreases with [GTP] and has a value greater than 300 s at low [GTP] and a limiting value of 50 s at high [GTP]. We attribute the greater than 300 s lifetime to the complex R*G (where G is ROS G protein) and the 50-s lifetime to free R*. (c) The rate kL increases hyperbolically with [GTP] with a half-maximal value of 56 microM and kL.max = 22-45 s-1. (d) Peak velocity is given by the expression vpeak alpha kL tau P which is consistent with the dependence of kL on [GTP] and the experimental finding that vpeak varies hyperbolically with [GTP]. The minimal model has also allowed us to (a) develop clear definitions of amplification for the light-triggered enzymatic cascade and (b) clarify experimental methods for measuring gain.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Transient response of retinal rod outer segment phosphodiesterase to actinic light pulses. I. Simple quantitative kinetic model. 255 31

In the accompanying article (Schmidt, J.A., and Yguerabide, J. (1989) J. Biol. Chem. 264, 19790-19803), we presented a minimal quantitative kinetic model with one rate-limiting step for the transient response of rod outer segment (ROS) phosphodiesterase (PDE) to stimulating light pulses of low fractional bleach (linear response range) and showed that the model was in excellent quantitative agreement with experimental results. The model characterizes the PDE response in terms of the specific rate constant of the rate-limiting step, kL, the lifetime of photoactivated rhodopsin, tau R, and the lifetime of activated PDE, tau P, but makes no predictions on how these kinetic parameters should depend on the concentrations of the various reactive species involved in the PDE response to light and does not reveal the nature of the rate-limiting step. However, we established by curve fitting experimental data to theoretical expressions from the model that kL increases hyperbolically with [GTP], tau R decreases with [GTP], and tau P is independent of GTP. In this report we present three detailed kinetic models which make specific quantitative predictions on how the kinetic parameters of the minimal model should depend on nucleotide and G protein concentrations and test the models against experimental data. Each model consists of one rate-limiting step. The first detailed model postulates that the rate-limiting step is the dissociation of R*GT into R* and GT (T stands for GTP). The second model postulates that the rate-limiting step is the binding of GTP to R*G, and the third model postulates that the rate-limiting step is the encounter rate of R* and G on the ROS disc membrane. We find that only the first detailed model is consistent with the experimental results as characterized by the minimal model. Using this detailed model we (a) define kL and tau R in terms of more fundamental equilibrium and rate parameters, (b) develop a theory for the systematic evaluation of amplification or gain of the PDE light response from light-stimulated GTP-binding data as well as v(t) versus t graphs, and (c) clarify methods which have been used in the past to evaluate gain experimentally.
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PMID:Transient response of rod outer segment cGMP phosphodiesterase to actinic light pulses. II. Detailed quantitative kinetic model. 255 32

In both of the early onset systems, the rd mouse and the rcdl Irish Setter, early elevation of cyclic GMP may be the ultimate cause of accelerated photoreceptor degeneration. This would be consistent with the data utilizing in vitro systems in which retinal samples, in culture, undergo degeneration in response to constant exposure to high levels of this nucleotide. However, the ultimate cause of the elevated cyclic GMP in the rd mouse or in the rcdl Irish Setter still remains a mystery. It appears that all of the necessary proteins of the visual cascade are produced, although they are lost at different rates. The phosphodiesterase appears to be reduced faster than other proteins. This may, in turn, account for the elevation in cyclic GMP levels. The cause of this enhanced disappearance could reside in the phosphodiesterase protein itself, or in other more distal components. The alteration in rhodopsin reaction to the specific rhod-4 antisera suggests that this protein is not properly oriented in the disc membrane. Although this may or may not alter the visual cascade, it does suggest that these membranes are not identical to those of the normal dog retina. Future studies should focus on the individual functional activities of each component, on their structures, and on their proper assembly within the disc.
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PMID:Analysis of normal and rcdl Irish setter retinal proteins. 255 84

Photoreceptor membranes in the bovine retina may be substituted in reconstructed systems by pigment fractions from Chlamydomonas reinhardtii which account for GTP-dependent activation of cGMP-phosphodiesterase. Fractions from carotinoid-less mutant did not exhibit this capacity. The results obtained reveal significant structural and functional similarity between rhodopsin-like pigment of Ch. reinhardtii and rhodopsin from vertebrates.
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PMID:[The functional similarity of vertebrate rhodopsin and of a photosensitive pigment from the unicellular flagellate alga Chlamydomonas reinhardtii]. 256 Mar 8


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