Binding of 2-azidoadenosine [beta-32P]diphosphate to the receptor on intact human blood platelets which inhibits adenylate cyclase

Importance of measurement of platelet reactivity to ADP in patients with coronary artery disease: an historical account

The pivotal roles of platelets in physiological hemostasis and pathological thrombosis at the site of plaque rupture are well established. The latter roles provide the fundamental basis for the most widely implemented pharmacologic management of coronary artery disease--dual antiplatelet therapy with aspirin to inhibit platelet thromboxane A2 generation, and a P2Y12 receptor inhibitor to prevent adenosine diphosphate (ADP)-induced platelet activation. Although suboptimal pharmacodynamic efficacy, also described as high on-treatment platelet reactivity to ADP, has been associated with greater risk for post-stenting ischemic event occurrence, enhanced responsiveness is associated with higher risk for bleeding in selected patients. In this review article, we aim to provide an historical account of the one and a half century long journey starting with the first description of platelets through the first report of ex vivo measurement of ADP-induced platelet aggregation, the first demonstration of an association between ADP-induced platelet aggregation and post-stenting ischemic event occurrence, and finally to the most recent description of a 'therapeutic window' concept for P2Y12 receptor inhibitor therapy.

Photoaffinity labeling with 2-azidoadenosine diphosphate of a tight nucleotide binding site on chloroplast coupling factor 1

An analog of ADP containing an azido group at the C-2 position of the purine ring has been synthesized and used as an affinity probe of the membrane-bound coupling factor 1 of spinach chloroplast thylakoid membranes. The 2-azido-ADP inhibited light-induced dark binding of ADP at the tight nucleotide binding site on the thylakoid membranes. The 2-azido-ADP itself bound tightly to the thylakoid membranes, with 1 muM as the concentration giving 50% maximum binding. Tight binding of the analog required the thylakoid membranes to be energized, and the nucleotide remained bound after repeated washings of the membranes. The maximum extent of tight binding of the analog (1,2-1.3 nmol/mg of chlorophyll) was stoichiometric with the known coupling factor 1 content of thylakoid membranes but somewhat higher than that observed for ADP (0.5-0.9 nmol per mg of chlorophyll). Tight binding of 2-azido-ADP was decreased by the simultaneous addition of ADP. UV photolysis of washed thylakoid membranes containing tightly-bound 2-azido-[beta-(32)P]ADP resulted in the covalent incorporation of label into the membranes. Isolation of the chloroplast coupling factor 1 from these membranes followed by NaDodSO(4) gel electrophoresis demonstrated that the analog was covalently bound to the beta subunit of the coupling factor complex.

Agonist concentration-dependent differential responsivity of a human platelet purinergic receptor: pharmacological and kinetic studies of aggregation, deaggregation and shape change responses mediated by the purinergic P2Y1 receptor in vitro

Platelet shape change (SC), aggregation and deaggregation responses are integral components of hemostasis that are elicited and modulated in vivo by the simultaneous activation of several membrane receptors. Selective activation of the purinergic P2Y1 receptor in vivo elicits a sustained SC and a small, transient aggregation response that is reversed rapidly by a robust deaggregation response (Platelets 2003; 14: 89). Using a kinetics-based turbidimetric approach to study the modulation of these concurrent components of human platelet responses, we demonstrate that these P2Y1 receptor-related responses and a number of their kinetic and steady-state characteristics are differentially elicited and modulated. P2Y1 receptor agonist concentrations that elicited aggregation (pEC50 for ADP, 2-MeSADP; 5.88, 6.69) were 10-fold greater than those that elicited SC (7.33, 7.67). The magnitude of the aggregation response was agonist concentration-dependent, saturable and was associated with an agonist concentration-dependent deceleration of the deaggregation response. Gi-coupled receptor (alpha 2A-adrenoceptor, EP3 and P2Y12 receptors) agonists also enhanced aggregation through deceleration of the deaggregation response, and an inhibitor of PI3K activity (wortmannin) inhibited aggregation through acceleration of the deaggregation response. Neither treatment affected the extent or the kinetics of the SC response. The aggregation but not the SC response was rapidly desensitized by P2Y1 receptor activation by ADP. The affinity of the presence of a single P2Y1 receptor subtype. The differential characteristics and modulation of the SC and aggregation responses by a single receptor support the idea that different signaling pathways activated at different occupancy states of the same receptor underlie the two responses. P2Y1 receptor-mediated platelet aggregation and SC responses provide a convenient model for studying the phenomenon of agonist-directed signaling by differential occupancy of the same membrane receptor.

ADP-induced platelet aggregation and inhibition of adenylyl cyclase activity stimulated by prostaglandins: signal transduction mechanisms

ADP is the oldest and one of the most important agonists of platelet activation. ADP induces platelet shape change, exposure of fibrinogen binding sites, aggregation, and influx and intracellular mobilization of Ca2+. ADP-induced platelet aggregation is important for maintaining normal hemostasis, but aberrant platelet aggregation manifests itself pathophysiologically in myocardial ischemia, stroke, and atherosclerosis. Another important aspect of ADP-induced platelet activation is the ability of ADP to antagonize adenylyl cyclase activated by prostaglandins. ADP-induced inhibition of the stimulated adenylyl cyclase activity does not appear to play a role in ADP-induced platelet aggregation in vitro or in vivo. It is believed that a single ADP receptor mediates the above two ADP-induced platelet responses in platelets. The ADP receptor mediating ADP-induced platelet aggregation and inhibition of the stimulated adenylyl cyclase activity has not been purified. Therefore, the nature of molecular mechanisms underlying the two seemingly unrelated ADP-induced platelet responses remains either unclear or less well understood. The purpose of this commentary is to examine and make suggestions concerning the role of phospholipases and G-proteins in the molecular mechanisms of signal transduction underlying the two ADP-induced platelet responses. It is hoped that such discussion would stimulate thinking and invite future debates on this subject, and energize investigators in their efforts to advance our knowledge of the details of the molecular mechanisms of ADP-induced platelet activation.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.

Functional expression of a P2T ADP receptor in Xenopus oocytes injected with megakaryocyte (CMK 11-5) RNA

Since the P2T purinergic (ADP) receptor is unique to the megakaryocytic/platelet lineage, cells of this lineage were screened for the relative effects of ADP and ATP in intracellular Ca2+ levels. Like platelets, CMK 11-5 cells responded with an increase in intracellular Ca2+ mobilization in response to ADP but not to ATP or adenosine. In contrast, both nucleotides increased intracellular Ca2+ mobilization in the megakaryoblastic cell lines MO7E and Meg-01, indicating that they contain P2Y receptors or a mixed complement of purinergic receptors. Pharmacological responsiveness of CMK 11-5 cells to nucleotides paralleled those of platelets, in which ADP and ADP-alpha-S are active as agonists and ATP and ATP-alpha-S are inactive as agonists but act as antagonists. [3H]ADP and 35S-ATP-alpha-S bound to CMK 11-5 cells at a high-affinity site (Kd1 and Ki1, 262 and 125 nmol/L, respectively) and a low-affinity site (Kd2 and Ki2, 10,100 and 5400 nmol/L, respectively) with 2 x 10(6) to 6 x 10(6) sites per cell. ADP bound at both sites was competed with ADP, ATP, and ATP-alpha-S with affinities in a rank order similar to that found for platelets (ATP-alpha-S approximately ATP approximately ADP > or = ADP-beta-S approximately adenosine), suggesting the presence of a P2T receptor on CMK 11-5 cells. Photoaffinity labeling of intact CMK 11-5 cells with 35S-ATP-alpha-S resulted in the labeling of the alpha-subunit of GP IIb as found with platelets, although this was confirmed to be independent of ADP receptors. After RNA from CMK 11-5 cells was microinjected into Xenopus oocytes, only ADP and ADP-alpha-S stimulated 45Ca2+ efflux, which was not observed with ATP, 2-methylthio-ATP, alpha, beta-methylene-ATP, ATP-gamma-S, ATP-alpha-S, or adenosine. In addition, incubation of RNA-injected oocytes with ATP or ATP-alpha-S but not adenosine blocked the 45Ca2+ response to ADP. These experiments demonstrate that a nascent receptor that responded specifically to ADP but not to other P1, P2Y, P2X, and P2U agonists was expressed in functional form on Xenopus oocytes.

Photocross-linking of nucleic acids to associated proteins

Photocross-linking is a useful technique for the partial definition of the nucleic acid-protein interface of nucleoprotein complexes. It can be accomplished by one or two photon excitations of wild-type nucleoprotein complexes or by one photon excitation of nucleoprotein complexes bearing one or more substitutions with photoreactive chromophores. Chromophores that have been incorporated into nucleic acids for this purpose include aryl azides, 5-azidouracil, 8-azidoadenine, 8-azidoguanine, 4-thiouracil, 5-bromouracil, 5-iodouracil, and 5-iodocytosine. The various techniques and chromophores are described and compared, with attention to the photochemical mechanism.

Ultrastructural studies of platelet aggregates from human subjects receiving clopidogrel and from a patient with an inherited defect of an ADP-dependent pathway of platelet activation

Our study investigated the effect of the antithrombotic drug clopidogrel (75 mg/d for 7 days) on the ultrastructure of platelet aggregates induced by ADP or 2-methylthio-ADP (2-MeS-ADP) in citrated platelet-rich plasma and examined the activation state of the GP IIb/IIIa complexes. Results were compared with those obtained for patient M.L., who has a congenital disorder characterized by a reduced and reversible platelet response to ADP. When untreated normal platelets were stimulated with high-dose ADP, electron microscopy revealed large and stable aggregates often surrounded by a layer of what appeared to be degranulated platelets. The reversible aggregates of platelets from subjects receiving clopidogrel or from patient M.L. did not show this layer. Electron microscopy showed that in both situations, the aggregates were composed of loosely bound platelets with few contact points. Immunogold labeling of ultrathin sections of Lowicryl-embedded aggregates formed by ADP or 2-MeS-ADP showed a much decreased platelet surface staining by (1) a polyclonal anti-fibrinogen antibody and (2) AP-6, a murine anti-ligand-induced binding site monoclonal antibody specific for GP IIb/IIIa complexes occupied with fibrinogen. Similar findings were seen after disaggregation, when many single platelets were present that showed no signs of secretion. Flow cytometry confirmed that the number of ligand-occupied GP IIb/IIIa complexes was much lower on platelets stimulated with ADP or 2-MeS-ADP after clopidogrel treatment. As expected from previous studies, ADP-induced platelet shape change and Ca2+ influx were unaffected by clopidogrel. These results agree with the hypothesis that platelet activation by ADP is biphasic and highlight a receptor-induced activation pathway affected by clopidogrel (or congenitally impaired in patient M.L.) that is necessary for the full activation of GP IIb/IIIa and the formation of stable macroaggregates.

Platelet activation by 2-(4-bromo-2,3-dioxobutylthio)adenosine 5'-diphosphate is mediated by its binding to a putative ADP receptor, aggregin

Platelet responses induced by ADP are mediated by a unique P21-purinergic receptor. Although a variety of ADP analogs, substituted at C2, have been used to delineate pharmacological properties of the ADP-binding site(s), the identity of the receptor protein has not been firmly established. 2-(4-Bromo-2,3-dioxobutylthio)- ADP [2-BrCH2(CO)2CH2-S-ADP], a well-characterized ADP analog, has been previously used as an affinity label to examine the structure/function relationship of ADP-requiring enzymes [Kapetanovic, E., Bailey, J.B. & Colman, R.F. (1985) Biochemistry 24, 7586-7593]. We found that it induced platelet shape change, aggregation, exposure of fibrinogen binding sites, secretion and mobilization of intracellular calcium, but was less potent than ADP. Under non-stirring conditions, incubation of platelets with this analog for longer time periods blocked ADP-induced shape change, aggregation, and the ability to ADP to antagonize the rise in intracellular levels of cAMP induced by iloprost (a prostaglandin I2 analog). Of a variety of agonists examined, only ADP-induced aggregation was almost completely inhibited in platelets irreversibly modified by the analog. An autoradiogram of the gel obtained by SDS/PAGE of solubilized platelets modified by the ADP analog followed by reduction of the dioxo group by NaB[3H], showed the presence of a single radiolabeled protein band at 100 kDa. Platelets incubated first with either ADP, ATP, or 2-methylthio-ADP were not labeled by 2-BrCH2(CO)2CH2S-ADP and NaB[3H]4-8-BrCH2(CO)2CH2-S-ADP was previously shown by us to irreversibly antagonize ADP-induced platelet responses by selectively modifying aggregin. Incubation of platelets with 2-BrCH2(CO)2CH2S-ADP completely blocked labeling of aggregin in platelets by 8-BrCH2(CO)2CH2S-[32P]ADP. These results show that 2-BrCH2(CO)2CH2S-ADP initially interacts reversibly with aggregin (100kDa), a putative ADP receptor, and induces platelet shape change and aggregation, and at longer periods of incubation reacts irreversibly to block the ability of ADP to antagonize stimulated adenylate cyclase activity. In contrast, 6-BrCH2(CO)2CH2S-ADP was found to be a weak and reversible inhibitor of ADP-induced platelet aggregation. Prior incubation of platelets with the latter analog reduced labeling of aggregin by 8-BrCH2(CO)2CH2S-[32P]ADP. Taken together, the results further show that substitution by the BrCH2(CO)2CH2 group at the C2 and C8 positions is tolerated, while the presence of a free amino function at the C6 position is essential for its interaction with aggregin.

Inhibition of ADP-induced platelet responses by covalent modification of aggregin, a putative ADP receptor, by 8-(4-bromo-2,3-dioxobutylthio)ADP

ADP is an important platelet agonist which initiates platelet shape change, aggregation, exposure of fibrinogen receptors, and calcium mobilization. Because of the limitations of previously used affinity analogs and photo-labeling studies as well as controversies surrounding the identity of an ADP receptor on platelets, we have used an affinity label capable of alkylating a putative exofacial receptor on platelets. We now report that 8-(4-bromo-2,3-dioxobutylthio)adenosine-5'-diphosphate (8-BDB-TADP), which is an analog of the natural ligand ADP, blocked ADP-induced platelet shape change, aggregation, exposure of fibrinogen-binding sites, secretion, and calcium mobilization. Following modification by 8-BDB-TADP, the rates of aggregation of platelets induced by thrombin, a calcium ionophore (A23187) or a stimulator of protein kinase C (phorbol myristate acetate) were minimally affected. However, the 8-BDB-TADP-modified platelets exhibited decreased rates of aggregation in response to ADP, as well as collagen and a thromboxane mimetic (U46619), both of which partially require ADP. Autoradiograms of the gels obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized platelets modified by either [beta-32P]8-BDB-TADP, or 8-BDB-TADP and NaB[3H]4 showed the presence of a single radiolabeled protein band at 100 kDa. The intensity of this band was reduced when platelets were preincubated with ADP, ATP, and 8-bromo-ADP prior to labeling by the radioactive 8-BDB-TADP. The results show that 8-BDB-TADP selectively and covalently labeled aggregin (100 kDa), a putative ADP receptor, resulting in a loss of ADP-induced platelet responses.

Adenine nucleotide binding and photoincorporation in Glanzmann's thrombasthenia platelets

Adenosine 5'-(1-thiotriphosphate) (ATP alpha S) binds to about 25,000 high affinity sites in platelets (Kd approximately 3 nM), competes fully in inhibiting the binding of ADP and, despite the absence of a specific photoactivatable substituent, is directly photoincorporated into a specific 18 kDa domain beginning at Tyr-198 in the alpha chain of glycoprotein IIb (GPIIb alpha) following ultraviolet irradiation of fresh unfixed platelets (Greco et al. (1991) J. Biol. Chem. 266, 13627-13633). 8-azido ATP has now been shown to have similar binding parameters (Kd 8 nM, 20,000 sites/platelet) but, in this case, photoincorporation occurred equally in GPIIb and GPIIIa. To determine the possible function of GPIIb alpha in ADP-induced activation, platelets were isolated from two Glanzmann's thrombasthenia patients whose platelets contain approximately 6% of normal levels of GPIIb. ADP and ATP alpha S bound to intact, formaldehyde-fixed Glanzmann's platelets at high affinity sites with dissociation constants of approximately 30 nM and approximately 2 nM, respectively. Both nucleotides also bound to low affinity sites with dissociation constants of approximately 2 microM: these values are similar to those obtained with control platelets. ATP alpha S antagonized the shape ADP-induced shape change response of Glanzmann's platelets (EC50 5 microM) indicating that it bound to the P2T (ADP) receptor. However, photoincorporation was low (approximately 7% of control) similar to their content of GPIIb alpha. These results show that ADP binding and photoincorporation are occurring at different sites on the platelet surface but suggest that the ADP binding site may be located in proximity to GPIIb alpha.

An inherited bleeding disorder linked to a defective interaction between ADP and its receptor on platelets. Its influence on glycoprotein IIb-IIIa complex function

Much discussion has concerned the central role of ADP in platelet aggregation. We now describe a patient (M.L.) with an inherited bleeding disorder whose specific feature is that ADP induces a limited and rapidly reversible platelet aggregation even at high doses. Platelet shape change and other hemostatic parameters were unmodified. A receptor defect was indicated, for, while epinephrine normally lowered cAMP levels of PGE1-treated (M.L.) platelets, ADP was without effect. The binding of [3H]2-methylthio-ADP decreased from 836 +/- 126 molecules/platelet for normals to 30 +/- 17 molecules/platelet for the patient. Flow cytometry confirmed that ADP induced a much lower fibrinogen binding to (M.L.) platelets. Nonetheless, the binding in whole blood of activation-dependent monoclonal antibodies showed that some activation of GP IIb-IIIa complexes by ADP was occurring. Platelets of a patient with type I Glanzmann's thrombasthenia bound [3H]2-methylthio-ADP and responded normally to ADP in the presence of PGE1. Electron microscopy showed that ADP-induced aggregates of (M. L.) platelets were composed of loosely bound shape-changed platelets with few contact points. Thus this receptor defect has a direct influence on the capacity of platelets to bind to each other in response to ADP.

Evidence for the existence of two different ADP-binding sites on rat platelets

[3H]-2-Methylthio-ADP ([3H]-2-MeS-ADP), a stable analogue of ADP bound to one type of specific binding sites on rat platelets (KD = 0.77 +/- 0.07 nM, Bmax = 160 +/- 11 fmol/10(8) cells). 2-MeS-ADP and ADP antagonized [3H]-2-MeS-ADP binding, showing respective Ki values of 1.4 +/- 0.1 nM and 486 +/- 78 nM. Clopidogrel, a potent and specific inhibitor of ADP-induced platelet aggregation partially inhibited (approximately 70% inhibition) the binding of [3H]-2-MeS-ADP at the same time it abrogated 2-MeS-ADP- and ADP-induced adenylyl cyclase inhibition and aggregation. A population of clopidogrel-resistant [3H]-2-MeS-ADP binding sites was detected on platelets from treated animals. These receptor sites (KD = 0.9 +/- 0.2 nM, Bmax = 47 +/- 5 fmol/10(8) platelets) which showed high affinity for both ADP and 2-MeS-ADP (Ki values in the nanomolar range) might be involved in the ADP-induced shape change, a clopidogrel-resistant ADP-induced event. Using clopidogrel which acts via a direct and irreversible inhibition of ADP binding to its adenylyl cyclase-coupled receptor sites on platelets, we were able to discriminate between two types of ADP receptor sites. The former which was clopidogrel-sensitive represented about 70% of the total [3H]-2-MeS-ADP receptors and was responsible for ADP-induced platelet aggregation and adenylyl cyclase inhibition. The latter which was not affected by clopidogrel might be involved in ADP-induced shape-change.

Effects of extracellular divalent cations on responses of human blood platelets to adenosine 5'-diphosphate

The effects of extracellular divalent cations on the responses of human platelets to adenosine 5'-diphosphate (ADP) and on its inhibition by the competitive antagonist adenosine 5'-triphosphate (ATP) were investigated. Two responses were studied, shape change and the inhibition of prostaglandin E1 (PGE1)-stimulated adenylate cyclase, and experiments were carried out in the presence of divalent cations (Ca2+ and Mg2+, 1 mM) or in their absence. For each response there was a small leftward shift of the concentration-response curve to ADP in the absence of divalent cations compared to that in their presence, and this leftward shift disappeared when the results were plotted in terms of ADP3- rather than total ADP concentration. The shape change results were, however, complicated by a reduction in the maximal response to ADP in the absence of divalent cations. For each response there was also a marked increase in the pA2 value of ATP in the absence of divalent cations compared to that in their presence, and this difference disappeared if the results were calculated in terms of ATP4- instead of total ATP. These results suggest that the human platelet ADP receptor, in common with other receptors for adenine nucleotides, recognises predominantly the uncomplexed forms of ADP and ATP as ligands.

Characterization of specific binding sites for [3H]2-MeS-ADP on megakaryocytoblastic cell lines in culture

Binding of [3H]2-methyl thio-adenosine 5' diphosphate ([3H]2-MeS-ADP), a stable analogue of adenosine 5' diphosphate (ADP) to DAMI and Meg-01, two megakaryocytoblastic cell lines, was time-dependent, reversible and saturable. Scatchard analysis of the saturation binding data indicated that [3H]2-MeS-ADP bound to one class of specific binding sites with high affinity (dissociation constants = 45.3 +/- 13.4 and 48.2 +/- 17.7 nM, and maximum binding capacities = 341.2 +/- 31.1 and 903 +/- 98 fmole/10(6) cells for DAMI and Meg-01, respectively) (N = 3). Unlabelled 2-MeS-ADP competitively and selectively inhibited the specific binding of [3H]2-MeS-ADP on DAMI and Meg-01 with inhibitory constant values of 118 +/- 11 and 38 +/- 11 nM, respectively (N = 3). ADP was 3 to 10 times less potent than 2-Mes-ADP in displacing [3H]2-MeS-ADP from its binding sites on DAMI and Meg-01, whereas other ADP analogues, such as AMP, GDP, UDP, adenosine or FSBA, did not interfere with the binding of [3H]2-MeS-ADP, suggesting that DAMI and Meg-01 contain ADP-specific receptors.

Receptors for ADP on human blood platelets

It is well established that ADP causes aggregation of human blood platelets, and indeed it was the first aggregating agent to be studied, but the ways in which platelets respond to ADP are still relatively obscure. Although it is apparent that increases in intracellular Ca2+ concentrations are of major importance in activating platelets, it is not clearly understood how ADP causes these increases and what other signal transduction mechanisms it uses. It is not even clear whether ADP causes its effects by interacting with only one receptor, or whether multiple receptors for ADP exist on platelets. In this review, Susanna Hourani and David Hall examine some of the conflicting evidence in this field, and draw some tentative conclusions about the number and nature of receptors for ADP on human platelets.

Effects of suramin on increases in cytosolic calcium and on inhibition of adenylate cyclase induced by adenosine 5'-diphosphate in human platelets

The effects of the P2-purinoceptor antagonist, suramin, on ADP-induced increases in human platelet cytosolic calcium concentration ([Ca2+]i) and inhibition of prostaglandin E1 (PGE1)-stimulated adenylate cyclase activity were investigated. Suramin (50-200 microM) acted as an antagonist of ADP-induced increases in [Ca2+]i, causing parallel, rightward shifts of the log concentration-response curve to ADP with no apparent depression of the maximal response. However, the slope of the Schild plot was 2.3 +/- 0.3, similar to that obtained in previous studies on aggregation, indicating that the antagonism was not simply competitive. The apparent pA2 for suramin, taken from the Schild plot, was 4.63, similar to that for suramin's inhibition of aggregation, which suggests that these two effects are closely related. Suramin was not specific for the ADP receptor, however, as it was also able to inhibit, non-competitively, increases in [Ca2+]i induced by 5-hydroxytryptamine. Suramin (50-400 microM) also inhibited the effect of ADP on PGE1-stimulated accumulation of cyclic AMP, causing parallel shifts of the log concentration-response curve to ADP, with a Schild plot slope of 1.00 +/- 0.10, suggesting competitive antagonism, and a pA2 value of 5.09. Suramin (400 microM) did not reduce the inhibition of cyclic AMP accumulation by adrenaline, although it was able to inhibit the accumulation of cyclic AMP caused by PGE1, again showing that suramin has some non-specific effects. These data suggest that suramin is an antagonist at the platelet ADP receptor mediating increases in [Ca2+]i and inhibition of adenylate cyclase, but that it also shows non-specific effects and can depress platelet responses to other agonists. In addition, the similar pA2 value of suramin for the two effects of ADP does not support suggestion that they are mediated by two different receptors on human platelets.

Identification of a receptor for ADP on blood platelets by photoaffinity labelling

The synthesis of a new analogue of ADP, 2-(p-azidophenyl)-ethythioadenosine 5'-diphosphate (AzPET-ADP), is described. This compound contains a photolabile phenylazide group attached to the ADP molecule by a thioether link at the purine 2 position. It has been prepared in radioactive form with 32P in the beta-phosphate at a specific radioactivity of 100 mCi/mumol. The reagent activated platelets, causing shape change and aggregation, with somewhat lower affinity than ADP. On photolysis the affinity was increased. The reagent also inhibited platelet adenylate cyclase stimulation by prostaglandin E1, with considerably higher affinity than ADP. On photolysis the affinity was decreased. AzPET-ADP competitively inhibited the binding of 2-methylthio[beta-32P]ADP, a ligand for the receptor by which ADP causes inhibition of adenylate cyclase. In the dark, AzPET-[beta-32P]ADP bound reversibly and with high affinity to a single population of sites similar in number to the sites that bind 2-methylthio[beta-32P]ADP. Binding was inhibited by ADP and by ATP and by p-chloromercuribenzenesulphonic acid (pCMBS). On exposure to u.v. light in the presence of platelets, AzPET-[beta-32P]ADP was incorporated covalently but non-specifically into several platelet proteins, although prominent intracellular proteins were not labelled. Specific labelling was confined to a single region of SDS/polyacrylamide gels, overlying but not comigrating with actin. Incorporation of radioactivity into this region was inhibited by ADP and by ATP as well as by ADP beta S, ATP alpha S and pCMBS, but not by adenosine, GDP or AMP. Inhibition of AzPET-[beta-32P]ADP incorporation was closely correlated with inhibition of equilibrium binding of 2-methylthio[beta-32P]ADP. These results suggests that the labelled protein, which migrates with an apparent molecular mass of 43 kDa in reduced gels, is the receptor through which ADP inhibits adenylate cyclase.

Effects of analogues of adenine nucleotides on increases in intracellular calcium mediated by P2T-purinoceptors on human blood platelets

1. By use of a number of analogues of adenine nucleotides, the structure-activity relationships of the human platelet receptor for adenosine 5'-diphosphate (ADP) mediating increases in intracellular calcium were investigated, and compared with the known structure-activity relationships for induction by ADP of platelet aggregation. 2. ADP, 2-methylthioadenosine 5'-diphosphate (2-methylthio-ADP), adenosine 5'-O-(1-thiodiphosphate) (ADP-alpha-S) and adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S) each induced increases in intracellular calcium in a manner similar to their reported ability to induce human platelet aggregation. The effects of these agonists were antagonized by ATP, with a pA2 value in each case consistent with the inhibition by ATP of ADP-induced aggregation. In the case of ADP, the inhibition by ATP of increases in intracellular calcium was shown to be competitive by Schild analysis. 3. Of the analogues tested as inhibitors of the effect of ADP on intracellular calcium, 2-chloroadenosine 5'-triphosphate (2-chloro-ATP), adenosine 5'-O-(1-thiotriphosphate) (ATP-alpha-S), P1, P5-diadenosine pentaphosphate (Ap5A) and adenylyl 5'-(beta,gamma-methylene)diphosphonate (AMPPCP) were apparently competitive antagonists, although only one concentration of each antagonist was used. There was a good correlation between the pA2 values found here for these antagonists including ATP, and their pA2 values reported for inhibition of ADP-induced aggregation. Adenosine 5'-(alpha, beta-methylene)triphosphate (AMPCPP) and uridine 5'-triphosphate (UTP) (100 microM) were only very weak inhibitors of the effect of ADP on intracellular calcium, and this is consistent with their weak actions as inhibitors of aggregation. 2-Methylthioadenosine 5'-triphosphate (2-methylthio-ATP) (50 microM) non-competitively inhibited the effect of ADP on intracellular calcium, in a very similar way to its inhibition of ADP-induced aggregation.4. The good correspondence found for these analogues between their effect on intracellular calcium and on aggregation confirms that there is a causal relationship between these actions of ADP, and that they are mediated by the same receptor on platelets. These findings cast further doubt on the use of the affinity reagent 5'-fluorosulphonylbenzoyladenosine (FSBA) as an antagonist and label for the ADP receptor, as this compound has been reported to inhibit aggregation but not ADP-induced increases in intracellular calcium.

Photoaffinity labeling of integrin alpha IIb beta 3 (glycoprotein IIb-IIIa) on intact platelets with 8-azido-[gamma-32P]ATP

The fibrinogen receptor GPIIb-IIIa plays a crucial role in platelet aggregation. Here we show that the adenine nucleotide, 8-azido-ATP, inhibits ADP-induced conformational change of the platelet fibrinogen receptor GPIIb-IIIa (integrin alpha IIb beta 3). Photoaffinity labeling of intact platelets with 8-azido-[gamma-32P]ATP exclusively modifies two plasma-membrane glycoproteins which are identical with both subunits of GPIIb-IIIa. The presence of adenine-nucleotide-binding sites on GPIIb-IIIa implies that the platelet fibrinogen receptor is directly regulated by extracellular adenine nucleotides.

Clopidogrel inhibits the binding of ADP analogues to the receptor mediating inhibition of platelet adenylate cyclase

Clopidogrel, like the homologous thienopyridine derivative ticlopidine, selectively inhibits platelet aggregation induced by ADP. We have previously described two nucleotide-binding sites on platelets related to ADP-mediated platelet responses. The first is a high-affinity binding site for 2-methylthio-ADP (2-MeSADP) that is linked to the inhibition of stimulated adenylate cyclase. The second is the 100-kd exofacial membrane protein aggregin, which is labeled by the reactive ADP analogue 5'-p-fluorosulfonylbenzoyl adenosine (FSBA) that is related to shape change and aggregation. We set out to determine if either of these sites is blocked in vivo by clopidogrel or its active metabolite. Six subjects were given clopidogrel (75 mg/day for 10 days) in a double-blind crossover experiment. All of the subjects developed prolonged bleeding times while taking the drug. The rate of onset of the effect on bleeding time varied among subjects. Platelet aggregation induced by ADP or thrombin was significantly impaired by the drug treatment, but no effect was detected on shape change. The incorporation of [3H]FSBA into aggregin was also unaffected. Inhibition of adenylate cyclase by ADP or by 2-MeSADP was greatly reduced in all subjects, and in the case of 2-MeSADP, there was evidence for a noncompetitive effect. Inhibition of adenylate cyclase by epinephrine was unaffected. In the three subjects for whom binding measurements were made, the number of binding sites for [32P]2-MeSADP was reduced from 534 +/- 44 molecules per platelet during control and placebo periods (11 determinations) to 199 +/- 78 molecules per platelet during drug treatment (three determinations). There was no consistent change in the binding affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the P2-purinoceptor antagonist, suramin, on human platelet aggregation induced by adenosine 5'-diphosphate

1. The effects of suramin, a trypanocidal drug which has been reported to be a P2-purinoceptor antagonist on smooth muscle, were investigated in human platelets, where adenosine 5'-diphosphate (ADP) induces aggregation by acting on a subtype of purinoceptors which has been called P2T. 2. Suramin (100 microM) had no inhibitory effect on ADP-induced platelet aggregation in plasma, even after 40 min incubation in the presence of bacitracin, a peptidase inhibitor, and did not affect the ability of adenosine 5'-triphosphate (ATP) (40 microM) to inhibit competitively ADP-induced aggregation. This lack of effect of suramin on platelets in plasma is probably due to its extensive binding to plasma proteins. 3. In washed platelets, suramin (50-400 microM) acted as an apparently competitive antagonist, causing parallel shifts to the right of the log concentration-response curve to ADP. No depression of the maximal response to ADP was observed at concentrations of suramin (50-150 microM) for which full log concentration-response curves to ADP could be obtained, but the slope of the Schild plot was around 2, indicating that this antagonism was not simply competitive. The apparent pA2 value for suramin, taken from this Schild plot, was 4.6. 4. Suramin (200-400 microM) also noncompetitively inhibited aggregation induced by U46619 (a thromboxane receptor agonist) or by 5-hydroxytryptamine in the presence of adrenaline (100 microM), and caused a depression of the maximal response to these agonists. This nonspecific effect of suramin may explain the high Schild plot slope obtained against ADP.5. These results provide evidence that the ADP receptor on human platelets is indeed similar to the P2-purinoceptors responding to adenine nucleotides on smooth muscle and other tissues, and show that suramin cannot distinguish between the proposed subtypes of the P2-purinoceptors.

A new cGMP phosphodiesterase isolated from bovine platelets is substrate for cAMP- and cGMP-dependent protein kinases: evidence for a key role in the process of platelet activation

The biochemical differences among cGMP phosphodiesterases in platelets have not been thoroughly examined, primarily due to the lack of sufficient purified material. This report describes a simple method developed to isolate a specific bovine platelet cGMP phosphodiesterase. This enzyme is cytosolic in its native form and was purified to an apparent homogeneity by ion-exchange chromatography, affinity chromatography, and density gradient centrifugation. Cyclic GMP binds to a "pseudo-site" when the catalytic site is deprived of Mg++. The affinity for cGMP at alkaline pH in presence of EDTA and IBMX (Kd = 60 nM) suggests that the removal of Mg++ by EDTA converts the catalytic site to a binding site. A ligand affinity chromatography was designed to take advantage of these features. The core enzyme has a molecular weight 190,000 composed of 2 subunits (MW 95,000) and has a specific activity of 2.5 mumol/min/mg. Moreover, this enzyme was phosphorylated by cAMP- and cGMP-dependent protein kinases, suggesting that its activity could be indirectly regulated by cyclic nucleotides. Agents elevating cGMP and cAMP inhibit platelet activation by inhibiting protein kinase C and thrombin induced hydrolysis of phosphatidylinositol 4,5 diphosphate. The antiaggregating properties of some of these agents might therefore be attributed to the fact that they are inhibitors of phosphodiesterases.

Inhibition of ADP-induced platelet shape change and aggregation by o-phthalaldehyde: evidence for covalent modification of cysteine and lysine residues

Platelets play a major role in the hemostatic process following vascular injury. Chemical modification of cysteine and/or lysine residues in platelet proteins has been shown to cause loss of platelet aggregation induced by diverse agonists; however, these investigations have not addressed the identity of the specific proteins affected. o-Phthalaldehyde (OPTH) is a unique chemical modification reagent that forms and permits the identification of fluorescent isoindole derivatives with proteins by covalently and simultaneously modifying closely spaced cysteine and lysine residues. We found that OPTH inhibited platelet aggregation induced by ADP, collagen, and U46619 (an analog of prostaglandin H2), but had minimal effect on platelet aggregation induced by thrombin, plasmin, chymotrypsin, A23187 (a calcium ionophore), PMA (phorbol 12-myristate 13-acetate), and PMA + A23187. Since platelet aggregation induced by ADP, collagen, and U46619 has been shown to involve binding of endogenous or exogenous ADP to the platelet receptor, our further studies focused on platelet aggregation induced by ADP. OPTH inhibited ADP-induced shape change and aggregation in a concentration-dependent manner. The second-order rate constant for the inhibition of ADP-induced platelet shape change (Ksc = 1.0 X 10(3) M-1 s-1) was lower than that for aggregation (Kagg = 5.4 X 10(3) M-1 s-1). Fluorescence excitation and emission spectra of OPTH-platelet adduct exhibited maxima at 346 and 437 nm, respectively, consistent with the formation of an isoindole derivative(s). The nonpenetrating thiol-specific reagent, p-chloromercuribenzenesulfonate (pCMBS) (0.8 mM), is known to block the inhibition of stimulated adenylate cyclase induced by ADP but not the ADP-induced platelet shape change. The inhibition of ADP-induced platelet shape change (Ksc = 1.5 X 10(3) M-1 s-1) by OPTH was not affected by pCMBS. OPTH, at concentrations (15-50 microM) that inhibited ADP-induced platelet aggregation and shape change did not raise the intracellular levels of adenosine cyclic 3',5'-monophosphate (cAMP) in platelets nor did it impair the ability of iloprost (a stable analog of prostaglandin I2) to raise the platelet cAMP level. Thus, OPTH under these conditions did not interact with platelet adenylate cyclase. 5'-p-fluorosulfonylbenzoyladenosine (FSBA) has been previously shown to inhibit ADP-induced platelet shape change and aggregation by covalently modifying aggregin (Mr = 100 kDa), a putative ADP receptor on platelet surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of the ATP-binding site in the 23-kDa and 20-kDa regions of the heavy chain of the skeletal muscle myosin head

Three kinds of ATP analogues were synthesized. These ATP analogues can be classified into two conformations, i.e. syn and anti forms with respect to the N-glycosidic bond between adenine and ribose groups of ATP. 3'-O-(N-Methylanthraniloyl)-2-azidoadenosine 5'-triphosphate (MantN2(3)ATP) is recognized as the anti form, as ATP, and the other two, 3'-O-(N-methylanthraniloyl)-8-azidoadenosine 5'-triphosphate (MantN8(3)ATP) and 1,N6-etheno-8-azidoadenosine 5'-triphosphate (epsilon N8(3)ATP) are both syn forms. Mant and etheno groups are both fluorescent which allows detection of their binding to proteins. The photochemical binding of azido groups in ATP analogues to the myosin active site, examined in the presence and absence of ATP, showed that all the analogues bound to the site of myosin ATPase. These analogues also acted as substrates of the ATPase and were hydrolyzed in the active site, as judged by competitive inhibition of the ATPase and by their ATPase activities. Of these analogues, MantN2(3)ATP is very similar to ATP in divalent-cation dependence of its hydrolysis rate and in its ability to trap ADP in the active site with vanadate, while the other two are different from ATP in these respects. The photochemical binding sites of ATP analogues were localized by gel electrophoresis of trypsinized myosin ATPase with photocross-linked ATP analogues and/or by isolating the modified peptides. MantN2(3)ATP was found in the 23-kDa fragment which has a structure common to ATP-binding proteins, i.e. Gly-Xaa-Xaa-Gly-Xaa-Gly-Lys-Thr. Mant N8(3)ATP was found in a region of the 20-kDa fragment where actin is reported to attach.

Preparation of 2-azidoadenosine 3',5'-[5'-32P]bisphosphate for incorporation into transfer RNA. Photoaffinity labeling of Escherichia coli ribosomes

2-Azidoadenosine was synthesized from 2-chloroadenosine by sequential reaction with hydrazine and nitrous acid and then bisphosphorylated with pyrophosphoryl chloride to form 2-azidoadenosine 3',5'-bisphosphate. The bisphosphate was labeled in the 5'-position using the exchange reaction catalyzed by T4 polynucleotide kinase in the presence of [gamma-32P]ATP. Polynucleotide kinase from a T4 mutant which lacks 3'-phosphatase activity (ATP:5'-dephosphopolynucleotide 5'-phosphotransferase, EC 2.7.1.78) was required to facilitate this reaction. 2-Azidoadenosine 3',5'-[5'-32P]bisphosphate can serve as an efficient donor in the T4 RNA ligase reaction and can replace the 3'-terminal adenosine of yeast tRNAPhe with little effect on the amino acid acceptor activity of the tRNA. In addition, we show that the modified tRNAPhe derivative can be photochemically cross-linked to the Escherichia coli ribosome.

Characterization of the normal bovine platelet aggregation response

1. Bovine platelets are more sensitive to stimulation by platelet activating factor (PAF) than adenosine-di-phosphate (ADP) or thrombin. 2. While epinephrine, arachidonic acid and serotonin are ineffective by themselves as aggregatory stimulants of bovine platelets they enhance the aggregation response of other platelet agonists. 3. There is no correlation between thromboxane A2 production and release and the extent of platelet aggregation in bovine platelets. 4. The dependence of bovine platelet aggregation on a phospholipid pathway and calcium mobilization is indicated.

Nucleotide binding to the rod outer segment rim protein

The rod photoreceptor outer segment maintains a remarkable morphology. Two of the proteins which have been implicated in the maintenance of this structure are the 240 kDa spectrin-like protein, and the 220 kDa glycoprotein often referred to as the rim protein. We have probed rat rod outer segment proteins with light-activated (azido-labeled) radioactive nucleotides and found a nucleotide binding site(s) on the rim protein which has a preference for guanine nucleotides. Binding to this site is stimulated by the divalent cations zinc, manganese and magnesium, but not calcium. This site is under investigation and may play a role in stabilizing protein structure.

8-Azido-ATP (alpha 32P) binding to rod outer segment proteins

ATP has important roles in the vertebrate rod outer segment (ROS) physiological response to light. One of them is the quench of light-activated cGMP-phosphodiesterase activity. How ATP quenches PDE is not established; however, leading hypotheses favor the intervention of a 48-kDa ATP-binding protein and/or an ATP-utilizing rhodopsin kinase in this reaction. We have investigated the binding of [alpha 32P]8-azido-ATP to rat ROS proteins in the presence and absence of various divalent cations and competitive nucleotides. An event we have detected which might further clarify the role of ATP in PDE inactivation is a zinc-induced binding of azido-ATP to rhodopsin. Manganese is also effective in inducing this binding, while magnesium and calcium are not. The azido-ATP binding is eliminated by the addition of ATP, but not GTP, UTP, cGMP, or cAMP. A nucleotide-binding site on the rim protein is also suggested from these studies.

Covalent modification of Escherichia coli ADPglucose synthetase with 8-azido substrate analogs

Two photoaffinity labeling agents, 8-azido-ATP and 8-azido-ADPglucose, are substrate site specific probes of the Escherichia coli ADPglucose synthetase. In the presence of light (254 nm), the analogs specifically and covalently modify the enzyme with concomitant loss of catalytic activity. The substrate ADPglucose completely protects the enzyme from covalent modification by these 8-azido analogs. ATP, another substrate, also provides nearly 100% protection from 8-azido-ATP inactivation but is less efficient in protection of inactivation by 8-azido-ADPglucose. In the absence of light, however, ADPglucose synthetase can utilize either 8-azido-ATP or 8-azido-ADPglucose as substrates.

Exploration of nucleotide binding sites in the mitochondrial membrane by 2-azido-[alpha-32P]ADP

The ADP/ATP carrier of beef heart mitochondria is able to bind 2-azido-[alpha-32P]ADP in the dark with a Kd value of congruent to 8 microM. 2-Azido ADP is not transported and it inhibits ADP transport and ADP binding. Photoirradiation of beef heart mitochondria with 2-azido-[alpha-32P]ADP results mainly in photolabeling of the ADP/ATP carrier protein; photolabeling is prevented by carboxyatractyloside, a specific inhibitor of ADP/ATP transport. Upon photoirradiation of inside-out submitochondrial particles with 2-azido-[alpha-32P]ADP, both the ADP/ATP carrier and the beta subunit of the membrane-bound F1-ATPase are covalently labeled. The binding specificity of 2-azido-[alpha-32P]ADP for the beta subunit of F1-ATPase is ascertained by prevention of photolabeling of isolated F1 by preincubation with an excess of ADP.

Tautomerism of 2-azidoadenine nucleotides. Effects on enzyme kinetics and photoaffinity labeling

The 2-azidoadenine nucleotides show promise as photoaffinity probes. Substitution at the C-2 position should favor an anti conformation and enable binding of the analogue to enzyme sites which exhibit low affinity for the 8-azidoadenine derivatives. The 2-azidoadenine nucleotides were found to be substrates for pyruvate kinase, phosphofructokinase, adenylate kinase, hexokinase and the mitochondrial F1-ATPase. However, tautomerism of 2-azidoadenine nucleotides to two nonphotoreactive tetrazole forms complicates kinetic analyses and their use as photoaffinity probes. An analysis of the ultraviolet spectra of these analogues enables an estimation of the tetrazolo isomer content and the rates of tautomerization. The photoreactive azido isomer was found to represent only 45% of the total analogue population in neutral aqueous solution. The azidoazomethine-tetrazole equilibrium favors the azido isomer in acidic or nonpolar solutions. The first-order rate constants at 25 degrees C were determined to be 0.017 min-1 and 0.021 min-1 for tautomerism to the azido and tetrazolo isomers, respectively. Prior equilibration of the probe in various solvents thus allows investigation of the analogue's behavior with an enzyme system at different, essentially fixed, isomer ratios. The determination of the impact of the tetrazolo tautomers on the system allows optimization of conditions for photoaffinity-labeling experiments.