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

The binding of alpha 2-macroglobulin (alpha 2M) to human peripheral blood monocytes was investigated. Monocytes, the precursors of tissue macrophages, were isolated from fresh blood by centrifugal elutriation or density gradient centrifugation. Binding studies were performed using 125I-labeled alpha 2M. Cells and bound ligand were separated from free ligand by rapid vacuum filtration. Nonlinear least-squares analysis of data obtained in direct binding studies at 0 degrees C showed that monocytes bound the alpha 2M-thrombin complex with a Kd of 3.0 +/- 0.9 nM and the monocyte had 1545 +/- 153 sites/cell. Thrombin alone did not compete for the site. Binding was divalent cation dependent. Direct binding studies also demonstrated that monocytes bound methylamine-treated alpha 2M in a manner similar to alpha 2M-thrombin. Competitive binding studies showed that alpha 2M-thrombin and methylamine-treated alpha 2M bound to the same sites on the monocyte. In contrast, native alpha 2M did not compete with alpha 2M-thrombin for the site. Studies done at 37 degrees C suggested that after binding, the monocyte internalized and degraded alpha 2M-thrombin and excreted the degradation products. Receptor turnover and degradation of alpha 2M-thrombin complexes were blocked in monocytes treated with chloroquine, an inhibitor of lysosomal function. Our results indicate that human monocytes have a divalent cation dependent, high-affinity binding site for alpha 2M-thrombin and methylamine-treated alpha 2M which may function to clear alpha 2M-proteinase complexes from the circulation.
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PMID:Binding of alpha 2-macroglobulin-thrombin complexes and methylamine-treated alpha 2-macroglobulin to human blood monocytes. 245 79

Specific binding of purified histidine rich glycoprotein (HRGP) to human platelets stimulated with either bisdiazoniumbenzidine-crosslinked immunoglobulin G (BDB-IgG), with thrombin or with collagen was dose- and divalent cation dependent. A 5-10-fold increase of platelet bound 125I-HRGP was obtained when 0.5-0.8 x 10(9) platelets/ml were activated with 100 micrograms BDB-IgG/ml, 0.1 U thrombin/ml or 15 micrograms collagen/ml. At maximal binding tested 16,000 molecules of HRGP became bound per platelet, but saturation was not achieved. Such platelet inhibitors as acetylsalicylic acid, prostaglandin E1 and cytochalasin B reduced the capacity of platelets to bind ligand, and by kinetic experiments involving enzymatic digestion of radiolabelled bound HRGP the ligand revealed to remain surface bound rather than being taken up to inner parts of the cell.
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PMID:Histidine-rich glycoprotein binding to activated human platelets. 319 Oct 32

The binding of fibrinogen to its GPIIb-IIIa receptor is divalent-cation dependent. In addition to Ca+2 and Mg+2, Mn+2 has been shown to modulate adhesive protein interactions with integrins. This study examined the effect of Mn+2 on fibrinogen interactions with intact platelets. Compared with that of control platelets in buffer containing 1 mmol/L Mg+2, fibrinogen binding to adenosine diphosphate- or thrombin-stimulated platelets decreased 23% +/- 12% and 15% +/- 9% (mean +/- SD, n = 4), respectively, after addition of 1 mmol/L Mn+2. No change in binding affinity was noted, but the stability of platelet-fibrinogen interactions was diminished markedly. Ethylenediaminetetraacetic acid dissociated 68% +/- 8% of fibrinogen bound to ADP-treated platelets (p < 0.05) during a 60-minute incubation with fibrinogen and 1 mmol/L Mn+2, compared with 40% +/- 13% of fibrinogen bound to control platelets and 29% +/- 8% of fibrinogen bound in the presence of Ca+2 (mean +/- SD, n = 6). Mn+2 also diminished the stabilization of fibrinogen interaction with thrombin-stimulated platelets and inhibited the recovery of bound fibrinogen with the Triton X-100 (Union Carbide Corp., Danbury, Conn.) insoluble cytoskeleton. Only 31% +/- 10% of fibrinogen bound to thrombin-stimulated platelets for 60 minutes in the presence of Mn+2 associated with the cytoskeleton (p < 0.05), compared with 61% +/- 14% and 75% +/- 20% of fibrinogen bound to control platelets incubated with and without Ca+2, respectively. Mn+2 further inhibited large adenosine diphosphate- or thrombin-induced platelet aggregate formation and reduced the ability of platelets to retract fibrin clots. These data suggest that Mn+2 alters GPIIb-IIIa function relative to native fibrinogen and support a role for the stabilization of platelet-fibrinogen interactions in platelet aggregation and clot retraction.
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PMID:Stabilization of platelet-fibrinogen interactions: modulation by divalent cations. 842 76

Evidence is emerging for the regulation of platelet function at sites of vascular injury or thrombosis by multiple platelet recognition sites in fibrinogen. This study examined the interaction of platelets with immobilized fibrinogen degradation products, fragments D and E. A 60 kDa D fragment (D60) and 30 kDa fragment E supported the adhesion of activated platelets in a static system, despite the absence of gamma chain 400-411 dodecapeptide and RGD sequences. Moreover, platelet adhesion to these fragments was incompletely inhibited by EDTA. In the absence of divalent cations, ADP-stimulated platelet adhesion to fragments D60 or E constituted 31 +/- 12% and 33 +/- 10% (mean +/- SD,n = 23) of adhesion to intact fibrinogen in the presence of divalent cations, respectively. This EDTA-resistant adhesion was distinctly modulated by thrombin which preferentially supported platelet adhesion to fragment E, and chymotrypsin which selectively supported platelet adhesion to fragment D60. Furthermore, two potent inhibitors of fibrinogen binding, the 10E5 monoclonal antibody directed against the GPIIb-IIIa complex and the RGDF peptide, inhibited EDTA-resistant platelet adhesion to fragment D60 but not to fragment E. These data suggest the presence of novel, non-RGD, non-dodecapeptide containing platelet recognition sequences in both fibrinogen D and E domains which support divalent cation dependent and independent platelet adhesion via potentially unique binding mechanisms.
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PMID:Platelet adhesion to late fibrinogen degradation products. 873 44

Evidence is increasing that platelets can initiate and propagate inflammatory processes by interacting with leucocytes and the vascular endothelium. Platelets have been shown to bind to neutrophils, existing as platelet/neutrophil complexes (PNC) within the circulation. We describe a simple flow cytometric method for assessing and investigating platelet interactions with neutrophils in small volumes of whole blood. Twenty-five percent (sd 6%) of circulating neutrophils from healthy adults were associated with platelets. Formation of these platelet-neutrophil complexes was CD62P (P-selectin) and divalent cation dependent. Platelet activation (with ADP or thrombin) caused a rapid and sustained rise in %PNC which differed from the pattern of free platelet activation as assessed by CD62P expression. F-met-leu-phe induced neutrophil activation but did not increase the percentage PNC. Platelet activation also caused increased neutrophil CD11b/CD18 expression which was most marked on neutrophils complexed with platelets. This straightforward technique is simple, reproducible, and allows assessment of platelet-neutrophil interactions and activation of neutrophils. It may also provide a method for estimating platelet activation in whole blood.
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PMID:Investigation of platelet-neutrophil interactions in whole blood by flow cytometry. 946 29

P-selectin, an inducible cell adhesion molecule, mediates rolling of neutrophils on activated vascular endothelium. Because rolling is an early event of the inflammatory response, therapeutic applications of selectin antagonists have been of broad interest. There are, however, no truly satisfactory therapeutic candidates among known inhibitors. Consequently, we have used Systematic Evolution of Ligands by Exponential Enrichment (SELEX) technology, a process based on oligonucleotide combinatorial chemistry and in vitro selection, to develop aptamer antagonists of P-selectin. Equilibrium dissociation constants for aptamer/P-selectin binding range from 16 to 710 pM, a 10(5)-10(6)-fold improvement compared with the minimal carbohydrate ligand, sialyl Lewis X (sLeX). Aptamer binding is divalent cation dependent and, unlike sLeX, is specific for P-selectin. The selectivity for human P-selectin relative to human E-selectin or human L-selectin is 10(4)-10(5). In vitro, aptamers bind with subnanomolar affinities to P-selectin expressed on thrombin-activated platelets, inhibit the binding of P-selectin-IgG chimera to sLeX and to neutrophils, and block the binding activated platelets to neutrophils in flow cytometry and in hydrodynamic assays. Extrapolating from their in vitro characteristics, these novel P-selectin-specific antagonists may be suitable candidates for therapeutic development.
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PMID:Oligonucleotide inhibitors of P-selectin-dependent neutrophil-platelet adhesion. 974 65

Proton inventory studies of the thrombin-catalyzed fibrinogen activation to fibrinopeptide A are most consistent with a two-proton bridge forming at the transition state probably between Ser195 OgammaH and His57 Nepsilon2 and His57 Ndelta1 and Asp102 COObeta- at the active site, with fractionation factors 0.66 +/- 0.03 under enzyme saturation with substrate and 0.64 +/- 0.03 at fibrinogen concentration at 0.2 Km, at pH 8.0, pD 8.6, and 25.0 +/- 0.1 degrees C. Strongly inverse solvent isotope effects (SIEs) result from inverse lag times and maximal slopes of blood clotting plots, which are also anion and cation dependent. The blood clot is much coarser in D2O, as indicated in clotting curves with 3-9 times shorter lag time and steeper slopes with respect to H2O. The finer the particles, the weaker the H-bonds interlocking the fibrin mesh and/or in water structure around fibrin. Proton inventories of inverse lag times and maximal slopes of blood clotting curves in buffers containing Na+ and Cl- ions give the best fit to an exponential dependence on deuterium content in the buffer and give fractionation factors 5.6 +/- 0.5 and 7.8 +/- 0.6 at pH 8.0 and 25.0 +/- 0.1 degrees C. The thrombin-catalyzed activation of protein C (PC) to APC is associated with inverse kinetic SIEs (KSIEs) of 0.75 +/- 0.09 and 1.02 +/- 0.06 in 0.3 M NaCl and 0.3 M choline chloride, respectively, at substrate concentrations = 0.2 Km. In comparison, thrombin-catalyzed hydrolysis of chromogenic substrates gives greater KSIEs (Enyedy, E. I.; Kovach. I. M J. Am. Chem. Soc. 2004, 126, 6017-6024) and more complex proton inventories than the ones reported here for the first time for natural substrates. The present study illuminates differences in the character of the rate-determining transition state for the initial phase of the two physiological reactions catalyzed by thrombin.
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PMID:Full and partial deuterium solvent isotope effect studies of alpha-thrombin-catalyzed reactions of natural substrates. 1577 10

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