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

---

Query: EC:3.4.21.5 (thrombin)
33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Stimulation of platelets labeled with [14C]-arachidonate by ionophore A23187 or thrombin produces rapid degradation of specific membrane phospholipids. This is also reflected by the release of [14C]archidonate, which is immediately transformed into products of the cycloxygenase and lipoxygenase enzyme systems, and by increased labeling of phosphatidic acid. Arachidonate metabolism can be effectively prevented by preincubation with indomethacin and eicosatetraynoic acid, but platelet aggregation induced by ionophore A23187 or trombin is not blocked under these conditions. Nevertheless, in the virtually total absence of metabolism of arachidonate, platelet aggregation still occurs concomitantly with phospholipid breakdown and with increased labeling of phosphatidic acid. Increased levels of cyclic AMP block both phospholipase activation and aggregation induced by ionophore A23187 and trombin. These data suggest that some early consequence of phospholipase activation, independent of a metabolic product of arachidonate but possibly related to the production of phosphatidic acid, may play a central, causative role in mediating platelet aggregation.
...
PMID:Ionophore A-23187- and thrombin-induced platelet aggregation: independence from cycloxygenase products. 20 35

Trypsin, thrombin, and ionophore A23187 activate phospholipid breakdown of platelets that have been labeled with [(14)C]arachidonate, releasing their cyclooxygenase and lipoxygenase products. Intact platelets can also very effectively directly degrade low concentrations of exogenous, free [(14)C]arachidonate. Pretreatment of platelets with trypsin, thrombin, or ionophore A23187 for a minimum time of 30 sec leads to complete inactivation of cyclooxygenase activity, as demonstrated by subsequent exposure to [(14)C]arachidonate. Lipoxygenase activity is lost after 5 min. The thrombin-induced inactivation of cyclooxygenase and lipoxygenase is prevented by cyclic AMP (which inhibits the stimulated activity of phospholipase A(2)), although cyclic AMP does not affect the degradation of exogenous [(14)C]arachidonate. Exposure of platelets labeled with [(14)C]arachidonate to unlabeled arachidonate under conditions that lead to use of the latter also results in a similarly rapid inhibition of cyclooxygenase activity, as determined by subsequent challenge with thrombin. Under these conditions lipoxygenase activity is much less markedly inactivated. The arachidonate-induced inhibition of cyclooxygenase activity is not prevented by cyclic AMP. Trypsin does not induce platelet aggregation, and platelets whose cyclooxygenase activity has been inactivated are intact insofar as they are still able to undergo aggregation. These studies demonstrate that operation in intact platelets of the cyclooxygenase pathway, through use of endogenous or exogenous substrate, leads to a very rapid, irreversible inactivation of this enzyme. The lipoxygenase pathway is also progressively impaired, but much less rapidly than the cyclooxygenase enzyme and much less markedly on use of exogenous compared to endogenous substrate. The possible consequences of these physiological processes of spontaneous inactivation are considered.
...
PMID:Rapid inactivation of cyclooxygenase activity after stimulation of intact platelets. 21 91

Platelets enzymatically convert prostaglandin H(3) (PGH(3)) into thromboxane A(3). Both PGH(2) and thromboxane A(2) aggregate human platelet-rich plasma. In contrast, PGH(3) and thromboxane A(3) do not. PGH(3) and thromboxane A(3) increase platelet cyclic AMP in platelet-rich plasma and thereby: (i) inhibit aggregation by other agonists, (ii) block the ADP-induced release reaction, and (iii) suppress platelet phospholipase-A(2) activity or events leading to its activation. PGI(3) (Delta(17)-prostacyclin; synthesized from PGH(3) by blood vessel enzyme) and PGI(2) (prostacyclin) exert similar effects. Both compounds are potent coronary relaxants that also inhibit aggregation in human platelet-rich plasma and increase platelet adenylate cyclase activity. Radioactive eicosapentaenoate and arachidonate are readily and comparably acylated into platelet phospholipids. In addition, stimulation of prelabeled platelets with thrombin releases comparable amounts of eicosapentaenoate and arachidonate, respectively. Although eicosapentaenoic acid is a relatively poor substrate for platelet cyclooxygenase, it appears to have a high binding affinity and thereby inhibits arachidonic acid conversion by platelet cyclooxygenase and lipoxygenase. It is therefore possible that the triene prostaglandins are potential antithrombotic agents because their precursor fatty acids, as well as their transformation products, PGH(3), thromboxane A(3), and PGI(3), are capable of interfering with aggregation of platelets in platelet-rich plasma.
...
PMID:Triene prostaglandins: prostacyclin and thromboxane biosynthesis and unique biological properties. 21 23

Thrombin rapidly induces the formation of labeled phosphatidic acid from platelets prelabeled with [17C]arachidonate or 32PO34- and specifically decreases by 50--75% the content of phosphatidylinositol. Ionophore A23187 also stimulates phosphatidate labeling, but less effectively than thrombin. This effect on phosphatidic acid is blocked by increasing the levels of cyclic AMP by preincubation with dibutyryl cyclic AMP, cyclic AMP-phosphodiesterase inhibitors or prostacyclin. Indomethacin and eicosatetraynoic acid do not alter the production of phosphatidate, indicating independence from cyclooxygenase or lipoxygenase products. Increased turnover of [14C]- or [32P]phosphatidate occurs within 2--5 s after platelet activation by thrombin and is observed before endogenous, 14C-labeled arachidonate can be detected. The rate of phosphatidate formation parallels the induced rate of serotonin release. Release of [3H]serotonin is not affected by eicosatetraynoic acid. Phosphatidate production reflects the generation of diacylglycerol by C-type phospholipase degradation of phosphatidylinositol. Diacylglycerol and phosphatidic acid may participate in the membrane modification related to the early changes in platelet shape, release reactions or aggregation which occur on stimulation.
...
PMID:Stimulation of phosphatidic acid production in platelets precedes the formation of arachidonate and parallels the release of serotonin. 37 88

Various vasoactive agents (e.g., thrombin and bradykinin) and serum stimulate arachidonate production and thus prostaglandin biosynthesis in cultured fibroblasts. Treatment of 3T3 cells with the anti-inflammatory steroid, dexamethasone, inhibits this stimulation but has no inhibitory effect on the basal activity of phospholipase (or on prostaglandin content) in resting, confluent fibroblasts. In intact cells, the proportion of released arachidonic acid converted into prostaglandins is increased by steroid treatment, in quiescent, dense cells and in serum-treated cells, the total incorporation into prostaglandins is increased. Furthermore, the cyclo-oxygenase activity of homogenates from steroid-treated cells is increased very substantially. Thus, although steroids may affect phospholipase (EC 3.1.1.1.4) activities it is possible that these effects may be secondary to a more important stimulatory effect on cyclo-oxygenase activity which leads to selective alterations in prostaglandin biosynthesis. The steroid-induced increase in cyclo-oxygenase activity is not observed in a transformed variant of the same cell line. Fatty acid lipoxygenase (EC 1.13.11.12) activity exists in the particulate rather than the cytosolic fraction of 3T3 cells.
...
PMID:Action of corticosteroids in regulation of prostaglandin biosynthesis in cultured fibroblasts. 41 6

All agents capable of triggering the platelet release reaction also stimulate prostaglandin biosynthesis in these cells. Information concerning the endoperoxides, thromboxanes, and more stable metabolites generated by the action of cyclooxygenase and lipoxygenase on arachidonic acid has accumulated rapidly, but little is known about the preliminary steps in the cleavage and preparation of arachidonic acid for insertion into the enzymatic pathways of prostaglandin synthesis. Studies in this laboratory have shown that the combination of nitroblue tetrazolium (NBT) and vitamin E which prevents oxygenation of arachidonic acid to a free radical also blocks platelet prostaglandin biosynthesis. The present study has evaluated the influence of NBT, vitamin E, and the combination of NBT and vitamin E on the fine structure and biochemistry of platelets during incubation, and the effects of these compounds on the aggregation and secretion of platelets stimulated by collagen, thrombin, epinephrine, and ADP. Results of the study demonstrate that NBT and vitamin E, rather than injuring platelets, appear to protect them during incubation. Together NBT and vitamin E blocked aggregation by epinephrine, collagen, and thrombin, but permitted a small first wave stimulated by ADP. Both ADP and thrombin induced shape change, pseudopod formation, and limited degrees of internal contraction in vitamin E-NBT-treated platelets, whereas epinephrine and collagen failed to significantly alter discoid form. This pattern of response to aggregating agents was identical to reactions observed in platelets pretreated with aspirin and indomethacin, both potent inhibitors of platelet prostaglandin synthesis. In addition, NBT-vitamin E virtually blocked the first wave of aggregation which is not affected by aspirin and indomethacin. The findings support the concept that conversion of arachidonic acid to an activated state is an important step in prostaglandin synthesis and that electron transfer or oxidation-reduction reactions are intimately involved in the development of platelet stickiness.
...
PMID:Effects of nitroblue tetrazolium and vitamin E on platelet ultrastructure, aggregation, and secretion. 87 76

Washed human platelets take up arachidonic acid from plasma and incorporate the fatty acid into the major classes of complex lipids. Thrombin impairs net incorporation. It activates endogenous phospholipases which liberate arachidonic acid from phospholipids. As a consequence of thrombin induced aggregation platelets release arachidonic acid intermediates formed by the action of platelet fatty acid cyclooxygenase and by platelet fatty acid lipoxygenase. Cyclooxygenase, but not lipoxygenase, is inhibited by aspirin and indomethicin. Analysis of the pathways of arachidonic acid metabolism may furnish new insight into platelet function and into disorders of primary hemostasis.
...
PMID:The effect of thrombin on the uptake and transformation of arachidonic acid by human platelets. 98 37

The endoperoxide prostaglandin G2 (PGG2) induced platelet aggregation as well as the platelet release reaction (release of ADP and serotonin) when added to human platelet-rich plasma. Formation of a metabolite of PGG2 [8-(l-hydroxy-3-oxopropyl)-9,12L-dihydroxy-5,10-heptadecadienoic acid] and a lipoxygenase product [12L-hydroxy-5,8,10,14-eicosatetraenoic acid] accompanied the release reaction caused by aggregating agents such as collagen, ADP, epinephrine, and thrombin. Indomethacin inhibited the release reaction and PGG2 formation induced by these agents but had no effect on PGG2-induced release reaction. The aggregating effect of PGG2 was abolished by furosemide, which is a competitive inhibitor of ADP-induced primary aggregation. These data indicate that the aggregating effect of PGG2 is due to release of ADP and that PGG2 synthesis is required for induction of the release reaction by various aggregating agents. A subject with a hemostatic defect due to abnormal release mechanism [decreased aggregation with epinephrine (second wave) and collagen and normal platelet ADP] had a deficiency of the cyclo-oxygenase that catalyzes formation of PGG2. Normal aggregation and release reaction were obtained with added PGG2. Ii is concluded that the endoperoxide (PGG2) is essential in normal hemostasis because of its role in initiating the release reaction required for aggregation by collagen and the second wave of aggregation caused by, e.g., ADP.
...
PMID:Physiological role of an endoperoxide in human platelets: hemostatic defect due to platelet cyclo-oxygenase deficiency. 105 17

We wished to determine whether the metabolism of arachidonic acid, through lipoxygenase and cytochrome P-450 pathways, is involved in production of endothelium-derived relaxing factor(s) (EDRFs) in canine femoral veins. Veins were removed from anesthetized dogs and cut into rings. Endothelium was deliberately removed from some rings. In separate sets of experiments, rings were incubated with either AA861 (10(-5) M) or TMK777 (10(-6) M), inhibitors of 5-lipoxygenase, nordihydroguaiaretic acid (NDGA 3 x 10(-6) M), an inhibitor of lipoxygenase or proadifen (SKF 525A, 10(-6) M), an inhibitor of cytochrome P-450. In addition, some rings were incubated with a combination of indomethacin (10(-5) M) and NG-monomethyl-L-arginine (L-NMMA 10(-4) M) or, where appropriate, a solvent control. Concentration-response curves were obtained for acetylcholine, adenosine diphosphate, thrombin, A23187, and nitric oxide in rings contracted with a submaximal concentration of prostaglandin F2 alpha. AA861 and TMK777 did not alter endothelium-dependent relaxations to the agonists, whether with or without indomethacin and L-NMMA. However, indomethacin plus L-NMMA reduced endothelium-dependent relaxations to thrombin. These results suggest that metabolism of arachidonic acid, through lipoxygenase and cytochrome P-450 pathways, does not produce an EDRF in veins. However, thrombin receptor-activated relaxations are mediated in part by products of the cyclooxygenase pathway and nitric oxide.
...
PMID:Role of lipoxygenase and cytochrome P-450 in production of endothelium-derived relaxing factors in canine femoral veins. 127 84

Thrombin, a potent platelet activating agent, has previously been found to increase intracellular calcium levels and/or thromboxane A2 synthesis in leukemic cell lines exhibiting specific markers of the megakaryocyte/platelet lineage. However, its functional role on these cells has not been defined. As thrombin is implicated in the regulation of cellular proliferation or differentiation in various other cell types, we investigated the functional effects of thrombin on the megakaryoblastic MEG-01 cell line, and further explored its receptor coupling mechanisms on these cells. We observed that thrombin caused in 1% serum containing culture medium, a reduction in the proliferation of MEG-01 cells, without affecting their differentiation stage as determined by the expression of platelet glycoproteins GPIIb/IIIa and GPIb, FVIII-related-antigen and cell-size measurement, which are specific markers for megakaryocyte maturation. In addition, incubation of MEG-01 cells with thrombin resulted in dose-dependent increases in cAMP levels, and in inositol-trisphosphate formation and intracellular Ca2+ levels. All these responses required thrombin proteolytic activity. The lipoxygenase inhibitor, nordihydroguaiaretic acid, blunted thrombin-induced calcium increase without affecting thrombin-induced increase in cAMP levels, suggesting different thrombin coupling mechanisms with these two second messenger pathways. In addition, the inhibitory effect of thrombin on MEG-01 cell growth was mimicked by cAMP level enhancing agents such as forskolin, prostaglandin E1 and Bt2cAMP. These results suggest the involvement of a cAMP-dependent mechanism in the thrombin-induced reduction in MEG-01 cell growth.
...
PMID:Thrombin inhibits proliferation of the human megakaryoblastic MEG-01 cell line: a possible involvement of a cyclic-AMP dependent mechanism. 130 28


1 2 3 4 5 6 7 8 9 10 Next >>

---