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:3.4.21.1 (chymotrypsin)
10,938 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Considerable evidence indicates that the glycoprotein (GP) IIb/IIIa complex on human platelets functions as a receptor for fibrinogen, but little is known about the mechanism of receptor "exposure." To investigate this mechanism, our previously described murine monoclonal antibody (10E5) and a new monoclonal antibody (7E3), both of which block the binding of fibrinogen to platelets and bind to GPIIb and/or GPIIIa, were radiolabeled and their rates of binding to native and ADP-activated platelets were studied. At low concentrations, 125I-10E5 bound nearly equally rapidly to both native and activated platelets, whereas 125I-7E3 bound slowly to native platelets and much more rapidly to activated platelets. This increased rate of 7E3 binding is unlikely to be due to an increase in the number of GPIIb/IIIa sites on the surface of activated platelets because: (a) the rate of 10E5 binding was unchanged; (b) the total number of surface GPIIb/IIIa sites increased by only 2-10% with activation as judged by equilibrium binding of near-saturating concentrations of 10E5 and 7E3, and (c) there was less than 1% release of platelet factor 4 with activation, indicating minimal fusion of alpha-granule membranes (a potential source of GPIIb/IIIa) with the plasma membrane. Other activators (epinephrine, thrombin, and ionophore A 23187) also increased the rate of 7E3 binding, as did digestion of platelets with chymotrypsin. Aspirin did not affect the rate of binding of 7E3, whereas apyrase, prostaglandin E1, and dibucaine all inhibited the enhancement of the 7E3-binding rate produced by ADP. These data provide evidence for an activation-dependent change in the conformation and/or microenvironment of the GPIIb/IIIa complex, and offer a method of studying the receptor exposure mechanism that does not rely on the binding of fibrinogen itself.
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PMID:A new murine monoclonal antibody reports an activation-dependent change in the conformation and/or microenvironment of the platelet glycoprotein IIb/IIIa complex. 299 35

Incubation of platelets with chymotryptin leads to the exposure of fibrinogen receptors and to the appearance of a 66 kDa membrane component on the surface of platelets. Both glycoprotein IIIa (GP IIIa) and a 66 kDa component were precipitated from detergent extracts of solubilized, surface radiolabeled chymotrypsin-treated platelets by human anti-PlAl antisera. Moreover, the presence of the P1A1 antigen was identified on GP IIIa (but not on GP IIb) and on a 66 kDa protein by means of immunoblot procedures using platelet Triton X-114 extracts and these purified proteins. Anti-PlAl antiserum did not recognize GP IIIa on the surface of intact (untreated) platelets nor the 66 kDa protein on the surface of chymotrypsin-treated platelets of PlAl-negative individuals. The present data demonstrate directly that the 66 kDa protein is derived from GP IIIa and contains the PlAl alloantigen.
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PMID:Identification of PlAl alloantigen domain on a 66 kDa protein derived from glycoprotein IIIa of human platelets. 299 25

Monobromobimane (mBBr, bimane), a compound that penetrates cells and forms a fluorescent adduct with thiol groups, was used to asses the significance of thiols in platelet function. Exposure of washed platelets for 1 min to 100 microM mBBr abolished ADP-induced aggregation; shape change was not inhibited by 500 microM mBBr. The nonpenetrating compound monobromotrimethylammoniobimane was ineffective. Established ADP-induced aggregation was reversed by bimane, and fibrinogen binding to ADP-stimulated platelets was inhibited, an effect mainly due to decreased number of binding sites. Aggregation stimulated by A23187 and arachidonate was less effectively inhibited whereas epinephrine- and collagen-induced aggregation were abolished by 50 microM mBBr. Similar effects on aggregation and secretion were observed in platelet-rich plasma except that higher mBBr concentrations were usually necessary. Aggregation and 14C-serotonin secretion stimulated by 0.1 U/ml thrombin were partially inhibited by pretreatment with bimane. With lower thrombin concentrations, they were often enhanced, as was 3H-arachidonate release. Bimane inhibited epinephrine-induced arachidonate release in gel-filtered platelets, possibly because it abolished the primary aggregation necessary for this release. mBBr did not elevate cyclic AMP but enhanced the increase induced by PGE1 and prevented the subsequent decrease typically caused by ADP. Examination of SDS polyacrylamide gels with ultraviolet light showed that mBBr reacted with many platelet proteins but not with GP IIb or IIIa. This observation, and the fact that bimane did not inhibit the fibrinogen-induced aggregation of DTT- or chymotrypsin-treated platelets suggest that it reacts with thiol group(s) that are involved in "exposing" the fibrinogen receptor.
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PMID:Modification of platelet functions by monobromobimane, a fluorescent thiol group label. 301 18

Tertiary amine local anesthetics modify a variety of platelet membrane-related functions. The present study explored dibucaine (DB)-induced inhibition of platelet cohesion by examining structural and functional alterations of the human platelet membrane glycoprotein IIb-IIIa complex (GPIIb-IIIa) and platelet Ca2+ homeostasis. Complete inhibition of ADP-induced aggregation was achieved five minutes after platelet exposure to 0.10 to 0.25 mmol/L of DB when fibrinogen binding was reduced by 50%. At higher concentrations of DB (approximately 1 mmol/L), ADP-induced fibrinogen binding was completely blocked. Scatchard analysis revealed loss of high-affinity binding sites in addition to reduction in Bmax. In contrast, chymotrypsin-treated platelets sustained 50% inhibition of fibrinogen binding when incubated with 0.4 to 0.5 mmol/L DB, and kinetic analysis showed that the high-affinity platelet-fibrinogen interactions were reduced but not absent. Fibrinogen binding to chymotrypsin-treated platelets could not be completely inhibited even at high DB concentrations (1 mmol/L). The inhibition of fibrinogen binding to chymotrypsin-treated platelets correlated with changes in binding of a monoclonal antibody (10E5) specific for an epitope on the GPIIb-IIIa complex. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and radioelectroimmunoassay of DB-treated platelets, however, showed no evidence of a reduction or degradation of GP IIb or IIIa. Platelet incubation with DB (five minutes, 0.1 to 1.0 mmol/L) was also accompanied by: increased platelet membrane-associated Ca2+ involving low-affinity binding sites [Kd = 5 X 10(-5) mol/L-]; increased 45Ca2+ uptake which correlated with degradation of actin-binding protein (ABP) and digestion of GPIb as visualized on periodic-acid Schiff (PAS)-stained SDS gels and as inferred from decreased binding of a monoclonal antibody (6D1) directed against this glycoprotein; and enhanced Ca2+ exchange. Thus, exposure of platelets to DB results in membrane-related alterations that may contribute to inhibition of platelet cohesion: Decreased fibrinogen receptor exposure by traditional agonists and diminished accessibility of the GPIIb-IIIa complex to extracellular ligands correlate with DB-induced inhibition of platelet aggregation; and increased calcium uptake and exchange across the platelet membrane likely leads to activation of the calcium-dependent protease(s) which was previously shown to correlate with DB-induced inhibition of ristocetin-induced platelet agglutination.
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PMID:Platelet membrane alterations induced by the local anesthetic dibucaine. 301 84

Thrombin is a physiological agonist that promotes platelet aggregation and secretion. In this study we observed that thrombin can also inhibit a function of platelets related to primary hemostasis. Platelet stimulation by thrombin decreased the binding of von Willebrand factor (vWF) to glycoprotein (GP) Ib and decreased ristocetin-induced agglutination, in vitro reactions that correlate with initial platelet adhesion to the vessel wall. Binding of the monoclonal antibody API to GP Ib was also decreased. Cytoskeletal participation in the change of GP Ib was suggested because pretreatment of platelets with cytochalasin to prevent actin filament formation prevented the thrombin-induced decreases in vWF binding. API binding, and ristocetin-induced agglutination. Measurement of GP Ib in detergent extracts by electroimmunoassay demonstrated no loss after thrombin stimulation. Electroimmunoassay also demonstrated that the API epitope of GP Ib on intact thrombin-treated platelets was accessible for complete digestion by chymotrypsin. Therefore GP Ib was neither released from the platelet surface nor internalized by thrombin treatment. A previously recognized effect of thrombin is its induction of receptor sites on platelet surface GP IIb-IIIa for contact-promoting proteins, including vWF that are involved in the platelet spreading and aggregation that follow adhesion. Therefore the action on GP Ib may combine with the effect on GP IIb-IIIa to shift platelet reactivity from GP Ib-vWF-mediated initial contact with the vessel wall to GP IIb-IIIa-mediated spreading and aggregation.
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PMID:Thrombin decreases von Willebrand factor binding to platelet glycoprotein Ib. 325 66

The following lines of evidence suggest that MP100, a putative ADP receptor, and GPIIIa are distinct proteins. [3H]FSBA incorporated equally into normal and thrombasthenic platelets (less than 5% GPIIIa), quantitatively as well as qualitatively. The dose-dependent inhibition of ADP-induced platelet shape change by FSBA is identical for normal and thrombasthenic platelets. Polyclonal rabbit antibodies precipitate MP100 and GPIIIa, but monoclonal antibodies directed against GPIIb/GPIIIa complex GPIIIa and P1A1 fail to precipitate the ADP receptor protein. A monoclonal antibody which inhibits ADP-induced platelet aggregation and fibrinogen binding fails to inhibit ADP-induced shape change. Thus, both functional and immunochemical evidence clearly indicates the distinct character of the ADP and fibrinogen receptors. We hypothesize that conformational changes in an ADP receptor on binding ADP or proteolytic changes as with chymotrypsin digestion may be responsible for exposure of the normally latent fibrinogen receptor (GPIIb/GPIIIa complex).
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PMID:Separation of the 100-kDa membrane protein mediating ADP-induced platelet shape change and activation from glycoprotein IIIa. 360 28

The glycoprotein (GP) localization of a new platelet-specific allo-antigen Yukb is described. The antibody was isolated from serum of a patient with neonatal allo-immune thrombocytopenia. In immunoblot procedure, it bound exclusively to platelet GP IIIa, like anti-PlA1, while the known anti-Baka and anti-Leka reacted with GP IIb. Analysis of GP from chymotrypsin-treated platelets with anti-Yukb revealed no binding in the 68-kilodalton position while anti-PlA1 did. Thus, unlike the PlA1 antigen, the Yukb determinant either resides on the 30-kilodalton fragment of GP IIIa or it is destroyed by chymotrypsin treatment.
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PMID:Identification of the Yukb allo-antigen on platelet glycoprotein IIIa. 366 Jul 69

Trigramin, a highly specific inhibitor of fibrinogen binding to platelet receptors, was purified to homogeneity from Trimeresurus gramineus snake venom. Trigramin is a single chain (approximately 9 kDa) cysteine-rich peptide with the Glu-Ala-Gly-Glu-Asp-Cys-Asp-Cys-Gly-Ser-Pro-Ala NH2-terminal sequence. Chymotryptic fragmentation showed the Arg-Gly-Asp sequence in trigramin. Trigramin inhibited fibrinogen-induced aggregation of platelets stimulated by ADP (IC50 = 1.3 X 10(-7)M) and aggregation of chymotrypsin-treated platelets. It did not affect the platelet secretion. Trigramin was a competitive inhibitor of the 125I-fibrinogen binding to ADP-stimulated platelets (Ki = 2 X 10(-8) M). 125I-Trigramin bound to resting platelets (Kd = 1.7 X 10(-7) M; n = 16,500), to ADP-stimulated platelets (Kd = 2.1 X 10(-8) M; n = 17,600), and to chymotrypsin-treated platelets (Kd = 8.8 X 10(-8) M; n = 13,800) in a saturable manner. The number of 125I-trigramin binding sites on thrombasthenic platelets amounted to 2.7-5.4% of control values obtained for normal platelets and correlated with the reduced number of GPIIb-GPIIIa molecules on the platelet surface. EDTA, monoclonal antibodies directed against the GPIIb-GPIIIa complex, and synthetic peptides (Arg-Gly-Asp-Ser and Tyr-Gly-Gln-Gln-His-His-Leu-Gly-Gly-Ala-Lys-Gln-Ala-Gly-Asp-Val) blocked both 125I-fibrinogen binding and 125I-trigramin binding to platelets. Fibrinogen binding was more readily inhibited by these compounds than was trigramin binding. Monoclonal antibodies directed either against GPIIb or GPIIIa molecules did not block the interaction of either ligand with platelets. Reduced, S-pyridylethyl, trigramin did not inhibit platelet aggregation and fibrinogen binding to platelets and it did not bind to platelets, suggesting that the secondary structure of this molecule is critical for expression of its biological activity.
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PMID:Trigramin. A low molecular weight peptide inhibiting fibrinogen interaction with platelet receptors expressed on glycoprotein IIb-IIIa complex. 368 Feb 47

Human platelets were surface-labeled by the periodate/NaB3H4 method or by lactoperoxidase-catalysed iodination with 125I. The labeled platelets were treated with chymotrypsin under conditions known to give platelets which aggregate with fibrinogen without stimulation with ADP. Platelets and supernatant were then analysed by various gel electrophoretic techniques including isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing or non-reducing conditions and two-dimensional non-reduced/reduced sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by fluorography or indirect autoradiography. Chymotrypsin-treatment of surface-labeled platelets degraded the major glycoproteins Ib, IIb and IIIa but also GP120(4.9-5.4), GPIc and GPV. The membrane-bound fragments of GPIb, IIb and IIIa could be identified and also the supernatant fragments of GPIb and GPV. GPIIIa was also cleaved within a loop structure formed by disulfide bond(s). The fact that remnants of both GPIIb and IIIa are left on chymotrypsin-treated platelets which aggregate spontaneously with fibrinogen may indicate that a complex formed by these remnants constitutes the fibrinogen-binding site on platelets.
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PMID:Identification and characterization of fragments of major glycoproteins from platelet membrane after chymotrypsin treatment. 397 99

Calcium is a cofactor of human platelet aggregation. Moreover a direct correlation between the ability of platelets to bind this divalent cation and to aggregate has been demonstrated. Since magnesium can substitute for calcium in supporting aggregation, especially in the presence of low calcium concentrations, and platelet aggregation is inhibited at low pH, the present study was designed to examine the effects of magnesium and low pH on 45calcium binding to human platelets, and to determine whether such effects might be associated with calcium binding to glycoproteins I (GPI) or IIb/IIIa (GPIIb/IIIa), the putative fibrinogen receptor. 45Calcium binding to aspirin-treated platelets that had been depleted of surface-associated calcium by brief exposure to EDTA was evaluated. Magnesium (5-10 mM) or a change in hydrogen ion concentration to decrease the pH from 7.5 to 6.0 was found to inhibit the binding of 45calcium to platelets from healthy donors by 34 +/- 6 and 32 +/- 8% (mean +/- SD, n = 13), respectively. Similar results were obtained with platelets incubated with chymotrypsin to selectively remove GPI or platelets from a patient with the Bernard Soulier Syndrome, congenitally deficient in GPI. In contrast, calcium binding to platelets from two patients with thrombasthenia, lacking GPIIb/IIIa, was reduced 49 +/- 6% and 42 +/- 8% (n = 4) by magnesium and hydrogen ions, respectively. This apparently increased inhibition was attributed to the combined effects of an overall decrease (approximately 50%) in calcium binding to thrombasthenic platelets compared with that in control platelets, and a similar absolute reduction in calcium binding in the presence of magnesium and/or hydrogen ions. No additional inhibition of 45calcium binding was noted in the presence of magnesium and at low pH, indicating that magnesium and hydrogen ions may affect the same platelet membrane binding sites. The data suggest that although modulation of platelet aggregation by magnesium and pH is accompanied by changes in platelet-associated calcium, calcium binding to the three major platelet membrane glycoproteins, GPI, IIb, and IIIa is unaffected.
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PMID:pH and magnesium alter 45calcium binding to platelets at sites other than glycoproteins I or IIb/IIIa. 399 8


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