Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.4.21.5 (thrombin)
 33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

PAF-acether (platelet-activating factor) was hypothesized as the mediator of the ADP and thromboxane-independent activation of platelets induced by thrombin (Thr) and by the snake venom glycoprotein convulxin (Cx). Aspirinized rabbit platelets self-desensitized to PAF-acether were less responsive to low amounts of Thr, as expected if PAF-acether would be formed, but were hyper-reactive to Cx, in contradiction with its hypothesized mediating role. Aggregation by higher concentrations of Thr overcame inhibition. Experiments with ADP-depleted platelets showed that secretion is neither involved with desensitization to PAF-acether nor with hyper-reactivity to Cx. Those effects required the presence of PAF-acether in the platelet suspension and persisted when transformation of PAF-acether into its recognized metabolite alkyl-acyl-glycerophosphorylcholine was inhibited. The ADP and thromboxane-independent activation of rabbit platelets by low and medium concentrations of Thr may be accounted for by platelet formation of PAF-acether, but overall the contrasting effects of platelet desensitization to PAF-acether on responsiveness to Thr and to Cx suggest that the third pathway of aggregation requires other explanations.
 ...
PMID:PAF-acether may not mediate the third pathway of platelet aggregation since self-desensitization reduces the effects of low thrombin but enhances those of convulxin. 399 25

It has been shown [Touqui, Jacquemin & Vargaftig (1983) Thromb. Haemostasis 50, 163; Touqui, Jacquemin & Vargaftig (1983) Biochem. Biophys. Res. Commun. 110, 890-893; Alam, Smith & Melvin (1983) Lipids 18, 534-538; Pieroni & Hanahan (1983) Arch. Biochem. Biophys. 224, 485-493] that rabbit platelets inactivate exogenous PAF (platelet-activating factor, PAF-acether) by a deacetylation-reacylation mechanism. The deacetylation step is catalysed by an acetyl hydrolase sensitive to the serine-hydrolase inhibitor PMSF (phenylmethanesulphonyl fluoride) [Touqui, Jacquemin, Dumarey & Vargaftig (1985) Biochim. Biophys. Acta 833, 111-118]. We report here that human platelets can produce PAF on thrombin stimulation. This production is marginal and transient, reaching a maximum at 10 min and decreasing thereafter. In contrast, 10-12 times more PAF is produced when platelets are treated with PMSF and stimulated with thrombin. Under these conditions, the maximum formation is observed at 30 min and no decline occurs for up to 60 min after stimulation. In addition, these platelets (treated with PMSF and stimulated with thrombin) incorporate exogenous labelled acetate in the 2-position of PAF, probably by an acetyltransferase-dependent mechanism. Production of PAF by human platelets during physiological stimulation can be demonstrated when PAF degradation is suppressed by the acetyl-hydrolase inhibitor PMSF.
 ...
PMID:Human platelets stimulated by thrombin produce platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) when the degrading enzyme acetyl hydrolase is blocked. 405 28

The interaction of beta 2-glycoprotein-I (beta 2-G-I), a plasma constituent of unknown function, with blood platelets was studied. The following results were obtained: 1) beta 2-G-I binds to washed human platelets isolated by centrifugation (WP) at one kind of specific, saturable binding sites. The dissociation constant was found to be approx. 1 X 10(-6) M. In the presence of physiological concentrations of Ca++ (2.5 mM), this specific binding is markedly reduced. Unspecific binding of beta 2-G-I to platelets, however, is not influenced by Ca++. Platelets prepared by gel filtration (GFP), differing in their in vitro aggregability from WP, exhibit no specific binding of beta 2-G-I. Binding to GFP is also not induced by activation with thrombin, collagen or ADP. beta 2-G-I causes significant alteration of the ADP-induced aggregation of GFP. Aggregation induced by thrombin, collagen, arachidonic acid or PAF-acether, however is not altered by beta 2-G-I. It is suggested, that pelleting during centrifugation causes irreversible rearrangements in the membrane of platelets.
 ...
PMID:Interaction of beta 2-glycoprotein-I with human blood platelets: influence upon the ADP-induced aggregation. 408 80

Platelet aggregation can be measured quantitatively by continuous recording of the transmission of a beam of light across a suspension of platelets in constant agitation in an aggregometer. In routine clinical investigation, the study of platelet aggregation is performed on platelet-rich citrated plasma (PRPc). The blood sample has to be excellent to eliminate all traces of thrombin. The blood is collected in 3.8% sodium dihydrate citrate (1 volume for 9 volumes of blood). It is centrifuged at 190 g for 15 minutes at ambiant temperature. The PRPc obtained can be diluted with platelet-poor plasma (PPP) to adjust the concentration of platelets to 3 X 10(5)/microliters. The PRPc is stored at ambiant temperature in stoppered tubes under CO2 to avoid variations in pH. The maximal delay between the collection of the blood and the end of the study of aggregation is 3 hours. In certain cases, in order to define the platelet lesion and to eliminate the influence of plasma proteins, clotting factors and anti-coagulants, a suspension of washed platelets is used. The blood is collected on acid-citrate-dextrose (ACD) and centrifuged at + 37 degrees C to obtain the PRPc. The platelet deposit obtained by centrifugation of the PRPc is washed twice, according to Mustard's method, in Tyrode-albumin buffer at a concentration of 0.35% at + 37 degrees C. It is re-suspended in the same buffer solution at + 37 degrees C in the presence of apyrase and the platelet concentration is adjusted to 3 X 10(5)/microliters. The aggregation or agglutination of the platelets is induced by several agents: ADP, adrenalin, collagen, thrombin, arachidonic acid, ionophor A 23187, PAF-acether and ristocetin. The quantitative study of the aggregation curves of human platelets allows us to study the physiological and biochemical mechanisms control platelet aggregation, to recognize and classify the hereditary or acquired platelet abnormalities which lead to clinical haemorrhagic or thrombotic manifestations and to study the effect of drugs which inhibit platelet aggregation and to understand their mechanism of action.
 ...
PMID:[Platelet aggregation: a tool for clinical investigation and pharmacological study. Methodology]. 635 9

We studied the effect of the methanol extract of garlic bulbs (EOG) and of three pure components isolated from it (F1, F2, F3), on human platelet aggregation induced by ADP, epinephrine, collagen, thrombin, arachidonate, PAF, and the ionophore A-23187. Incubation of PRP with EOG, either in methanol or in homologous PPP, inhibits platelet aggregation induced by all of the above mentioned agonists. F1, F2, and F3 also inhibit platelet aggregation, however, F3 was about four times more potent. Addition of EOG or F3 to platelets that have already been irreversibly aggregated by 10 microM ADP, induces rapid deaggregation. Inhibition of aggregation was still present after three hours. The inhibitory effect persisted even after the treated platelets were Gel-Filtered (GFP) or separated from plasma through a metrizamide gradient and resuspended in new homologous PPP. Thrombin-induced release of ATP from GFP was inhibited by 75-80% after EOG or F3 treatment. Incorporation of [3-H]-arachidonate by intact platelets was decreased by 50-60% in treated platelets. However, platelets incubated with the inhibitors after incorporation of radiolabeled arachidonate, although did not aggregate, produced, after thrombin activation similar amounts of radiolabeled TXB2 and lipoxygenase products as the controls. Electron microscopy of inhibited platelets, in the presence of thrombin, showed no degranulation but an increase of spherical forms. Our results suggest that the effects described might be mediate by a perturbation of the physicochemical properties of the plasma membrane rather than by affecting arachidonate or calcium metabolism in the cells. Chemical structures of F1, F2 and F3 have been provisionally assigned: F1 is diallytrisulfide, F2 is 2-vinyl-1,3-dithiene, and F3 is most probably allyl 1,5-hexadienyltrisulfide.
 ...
PMID:Effects of garlic extract and of three pure components isolated from it on human platelet aggregation, arachidonate metabolism, release reaction and platelet ultrastructure. 641 74

AGEPC (PAF), at 1.9 x 10(-8) M or higher, induced concentration-dependent aggregation and release in human platelet-rich plasma. Comparative studies with arachidonate, collagen, ionophore, and ADP suggested that AGEPC was a strong stimulus for platelet aggregation and probably a moderate agonist for release, as well as a relatively weak inducer of TXA2 production. The initial phase of AGEPC-induced aggregation was independent of ADP release and TXA2 formation, since it was not inhibited by ASA, apyrase, or CP/CPK. Whereas irreversible aggregation always required ADP release, TXA2 formation was not essential in each instance. Thus, in several experiments, full aggregation responses took place in AGEPC-stimulated platelets that had been pretreated with ASA. AGEPC-induced release of 5-HT, beta -thromboglobulin and PF-4 occurred in parallel and were inhibited by both apyrase and ASA. Washed human platelets did not respond to exogenous AGEPC in the absence of ADP and did not appear to generate significant quantities of AGEPC upon stimulation with thrombin or ionophore.
 ...
PMID:Effects of acetyl glyceryl ether phosphorylcholine on human platelet function in vitro. 645 58

In vivo platelet aggregation has been studied using a novel, minimally invasive technique. No aggregatory effects of heparin were observed on normal circulating platelets nor was there enhancement of aggregation of platelets during activation by intravenous injection of ADP, collagen, PAF acether or thrombin. On the contrary, high doses of heparin were found to inhibit platelet accumulation induced by ADP, collagen or PAF-acether. Inhibition of these responses necessitated doses of heparin in excess of those required for anti-coagulant effects. The present experiments do not establish a mechanism for such inhibition. Extension to other species, including man, is needed before attributing clinical relevance to the present observations.
 ...
PMID:Inhibition by heparin of platelet accumulation in vivo. 649 56

The amino sugars glucosamine, galactosamine and mannosamine (30 mM) inhibited aggregation of human or rabbit platelets induced by ADP, collagen, thrombin, PAF or high concentrations of sodium arachidonate. 125I-fibrinogen binding during ADP-induced aggregation, and release of amine storage granule contents were also inhibited. Increasing the calcium concentration of the suspending medium to 5 mM did not overcome the inhibitory effect on the release reaction. The amino sugars deaggregated rabbit platelets that had been aggregated by ADP, collagen or thrombin, but deaggregated human platelets readily only when ADP was used as the aggregating agent. Fibrinogen-induced aggregation of chymotrypsin-treated platelets was blocked by the amino sugars. They did not inhibit platelet adherence to a collagen-coated glass surface, nor affect release of granule contents from the adherent platelets. Aggregation and release induced by low concentrations of sodium arachidonate or the divalent cation ionophore A23187 were potentiated, indicating that the effects of the amino sugars on platelets are more complex than simple inhibition of the lectin-like activity that becomes available on the surface of platelets that have undergone the release reaction. One of the effects of the amino sugars, however, is interference with the binding of fibrinogen to platelets. The effects of the amino sugars are shared by other primary amines.
 ...
PMID:Effect of amino sugars on platelet aggregation and on fibrinogen binding. 649 68

Molecular mechanisms underlying the ability of heparin to enhance the platelet-aggregating effect of various agonists were studied. Heparin potentiates the aggregating effect of adenosine diphosphate (ADP) and epinephrine, but it is uneffective on the aggregation induced by ristocetin and collagen. Heparin inhibits aggregation induced by thrombin in the presence of plasma, but it is uneffective, or sometimes stimulates aggregation, in the absence of plasma. The effects on the platelet-activating factor- (PAF-acether) induced aggregation are very variable. The late phase of the ADP-induced aggregation is sensitive to proteinase inhibitors, but heparin overcomes this inhibitory effect. Drugs which inhibit remodeling of membrane phospholipids abolish the potentiating effect of heparin, while cyclooxygenase inhibitors do not. The proaggregating effect of heparin subfractions correlates with the lipoprotein lipase activity and, slightly, with the molecular weight, but it does not correlate with the anticoagulant activity. Platelets prelabelled with phosphatidyl[U14C]inositol show a very rapid effect of heparin in triggering phosphatidylinositor breakdown and a cooperative effect with ADP, a known agonist of the 'phosphatidylinositol cycle'. Heparin is also effective in stimulating the labelling of polyphosphoinositides in platelets prelabelled with 32Pi. These results, together with the selective sensitivity to drugs, lead to the conclusion that a stimulatory effect on the very early events of remodeling of membrane phospholipids is involved in the platelet proaggregating effect of heparin.
 ...
PMID:Molecular events involved in the proaggregating effect of heparin on human platelets. 649 25

Cultured human endothelial cells produce platelet-activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine; PAF) when stimulated with human thrombin. The response to thrombin is dose dependent, with a half-maximal effect at 0.17 unit/ml. The product is identified as PAF by the incorporation of radiolabeled precursors, its behavior in chromatographic systems, the recovery of biological activity, and the effect of treatment with phospholipase A2. Incorporation of [3H]acetate into PAF is maximal by 5 min and decreases thereafter. Endothelial cells produce 0.10-0.17 nmol of PAF per 10(6) cells in a 5-min exposure to thrombin, as judged by the amount of neutrophil-aggregating activity. The production of this potent agonist for platelet activation and neutrophil chemotaxis by endothelial cells suggests that it may play a role in the maintenance of vascular integrity and perhaps in pathological events such as thrombosis and atherogenesis.
 ...
PMID:Human endothelial cells in culture produce platelet-activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) when stimulated with thrombin. 658 68


<< Previous 1 2 3 4 5 6 7 8 9 10