scholarly journals Biochemical and functional consequences of dissociation of the platelet membrane glycoprotein IIb-IIIa complex

Blood ◽  
1985 ◽  
Vol 66 (1) ◽  
pp. 92-98 ◽  
Author(s):  
SJ Shattil ◽  
LF Brass ◽  
JS Bennett ◽  
P Pandhi
Keyword(s):  
Platelet Aggregation ◽  
Surface Membrane ◽  
Broad Band ◽  
Adenosine Diphosphate ◽  
Platelet Membrane ◽  
Membrane Glycoproteins ◽  
Fibrinogen Receptor ◽  
Activated Platelets ◽  
Discrete Band ◽  
Functional Consequences

Abstract The platelet membrane glycoproteins, IIb and IIIa, form a Ca2+- dependent heterodimer complex that functions as the fibrinogen receptor in activated platelets to mediate platelet aggregation. Little is known about factors that affect the IIb-IIIa complex within the platelet membrane. It has been observed that platelets incubated with ethylene glycol tetra-acetic acid (EGTA) at 37 degrees C are unable to aggregate or to bind monoclonal antibodies specific for the IIb-IIIa complex. To determine whether this is due to a dissociation of IIb from IIIa, we developed a method for quantitating the complex on nondenaturing, polyacrylamide gradient gels. Platelets were surface-labeled with 125I and then solubilized and electrophoresed in 0.2% Triton and 10 mmol/L CHAPS. Under these conditions and in the presence of 1 mmol/L Ca2+, glycoproteins IIb and IIIa migrated on the gels as a discrete band at Rf = 0.33. Protein that was eluted from this band bound to an immunoaffinity column specific for the IIb-IIIa complex. In contrast, when the IIb-IIIa complex was solubilized and then dissociated with EGTA, the discrete band at Rf = 0.33 was no longer present, and IIb and IIIa were now found in a broad band at Rf = 0.45 to 0.50. To study IIb and IIIa within the surface membrane, the 125I-labeled platelets were first incubated with 0.5 mmol/L EGTA (1 nmol/L free Ca2+) at 22 degrees C and then solubilized in the absence of EGTA. The IIb and IIIa from these platelets migrated at Rf = 0.33, indicating the presence of the intact IIb-IIIa complex. In contrast, when the platelets were incubated at 37 degrees C for one hour with the EGTA, the discrete band at Rf = 0.33 representing the IIb-IIIa complex gradually disappeared. This phenomenon could not be reversed by adding Ca2+ back to the platelets before solubilization and electrophoresis. This loss of the IIb-IIIa complex from intact platelets was accompanied by (a) a progressive and irreversible decrease in adenosine diphosphate (ADP)-induced platelet aggregation and (b) decreased binding of a complex-dependent monoclonal antibody to the platelets. These studies demonstrate that when platelets are exposed to low Ca2+ at 37 degrees C, the IIb-IIIa heterodimer complexes in their surface membranes are irreversibly disrupted. Because intact IIb-IIIa complexes are required for platelet aggregation, the loss of these complexes may account for the failure of these platelets to aggregate in response to ADP.

scholarly journals Biochemical and functional consequences of dissociation of the platelet membrane glycoprotein IIb-IIIa complex

Blood ◽  
1985 ◽  
Vol 66 (1) ◽  
pp. 92-98
Author(s):  
SJ Shattil ◽  
LF Brass ◽  
JS Bennett ◽  
P Pandhi
Keyword(s):  
Platelet Aggregation ◽  
Surface Membrane ◽  
Broad Band ◽  
Adenosine Diphosphate ◽  
Platelet Membrane ◽  
Membrane Glycoproteins ◽  
Fibrinogen Receptor ◽  
Activated Platelets ◽  
Discrete Band ◽  
Functional Consequences

The platelet membrane glycoproteins, IIb and IIIa, form a Ca2+- dependent heterodimer complex that functions as the fibrinogen receptor in activated platelets to mediate platelet aggregation. Little is known about factors that affect the IIb-IIIa complex within the platelet membrane. It has been observed that platelets incubated with ethylene glycol tetra-acetic acid (EGTA) at 37 degrees C are unable to aggregate or to bind monoclonal antibodies specific for the IIb-IIIa complex. To determine whether this is due to a dissociation of IIb from IIIa, we developed a method for quantitating the complex on nondenaturing, polyacrylamide gradient gels. Platelets were surface-labeled with 125I and then solubilized and electrophoresed in 0.2% Triton and 10 mmol/L CHAPS. Under these conditions and in the presence of 1 mmol/L Ca2+, glycoproteins IIb and IIIa migrated on the gels as a discrete band at Rf = 0.33. Protein that was eluted from this band bound to an immunoaffinity column specific for the IIb-IIIa complex. In contrast, when the IIb-IIIa complex was solubilized and then dissociated with EGTA, the discrete band at Rf = 0.33 was no longer present, and IIb and IIIa were now found in a broad band at Rf = 0.45 to 0.50. To study IIb and IIIa within the surface membrane, the 125I-labeled platelets were first incubated with 0.5 mmol/L EGTA (1 nmol/L free Ca2+) at 22 degrees C and then solubilized in the absence of EGTA. The IIb and IIIa from these platelets migrated at Rf = 0.33, indicating the presence of the intact IIb-IIIa complex. In contrast, when the platelets were incubated at 37 degrees C for one hour with the EGTA, the discrete band at Rf = 0.33 representing the IIb-IIIa complex gradually disappeared. This phenomenon could not be reversed by adding Ca2+ back to the platelets before solubilization and electrophoresis. This loss of the IIb-IIIa complex from intact platelets was accompanied by (a) a progressive and irreversible decrease in adenosine diphosphate (ADP)-induced platelet aggregation and (b) decreased binding of a complex-dependent monoclonal antibody to the platelets. These studies demonstrate that when platelets are exposed to low Ca2+ at 37 degrees C, the IIb-IIIa heterodimer complexes in their surface membranes are irreversibly disrupted. Because intact IIb-IIIa complexes are required for platelet aggregation, the loss of these complexes may account for the failure of these platelets to aggregate in response to ADP.

scholarly journals A murine monoclonal antibody that blocks fibrinogen binding to normal platelets also inhibits fibrinogen interactions with chymotrypsin- treated platelets

Blood ◽  
1984 ◽  
Vol 64 (1) ◽  
pp. 59-63 ◽  
Author(s):  
EI Peerschke ◽  
BS Coller
Keyword(s):  
Monoclonal Antibody ◽  
Platelet Aggregation ◽  
Adenosine Diphosphate ◽  
Human Platelets ◽  
Membrane Glycoproteins ◽  
Molecular Weights ◽  
Fibrinogen Receptor ◽  
Fibrinogen Binding ◽  
Unstimulated Control ◽  
Lines Of Evidence

We recently described a monoclonal antibody, 10E5 , that completely blocks adenosine diphosphate (ADP) induced fibrinogen binding to platelets and aggregation induced by ADP, epinephrine, and thrombin. Multiple lines of evidence indicate that 10E5 binds to platelet membrane glycoproteins IIb and/or IIIa. Because it has been reported that platelets treated with chymotrypsin aggregate when fibrinogen is added, we tested the effect of 10E5 antibody on chymotrypsin-induced fibrinogen binding and platelet aggregation. Aspirin-treated human platelets were washed in modified Tyrode's buffer (pH 7.5), incubated for 5 minutes at 22 degrees C with 300 micrograms/mL chymotrypsin, and washed again. The amount of 10E5 antibody bound to these platelets (37,232 +/- 2,928 molecules/platelet; mean +/- SEM, N=9) was similar to that bound to unstimulated control platelets (36,910 +/- 2,669) and did not differ significantly from the amount of antibody bound to ADP- treated platelets (P less than .01, N = 5). The amount of 10E5 bound to chymotrypsin-treated platelets correlated directly with the amount of fibrinogen bound to separate aliquots of the same platelet samples (r = .876, P less than .001). The 10E5 antibody caused virtually complete inhibition of both the binding of fibrinogen to chymotrypsin-treated platelets and the aggregation induced by exogenous fibrinogen. Immunoprecipitation studies of 125I-labeled chymotrypsin-treated platelets revealed that the 10E5 antibody bound proteins with molecular weights characteristic of glycoproteins IIb and IIIa. These data suggest that the fibrinogen receptor on chymotrypsin-treated platelets is identical to that on ADP-treated platelets and that this receptor is either near to, or on, the glycoprotein IIb/IIIa complex.

scholarly journals A murine monoclonal antibody that blocks fibrinogen binding to normal platelets also inhibits fibrinogen interactions with chymotrypsin- treated platelets

Blood ◽  
1984 ◽  
Vol 64 (1) ◽  
pp. 59-63 ◽  
Author(s):  
EI Peerschke ◽  
BS Coller
Keyword(s):  
Monoclonal Antibody ◽  
Platelet Aggregation ◽  
Adenosine Diphosphate ◽  
Human Platelets ◽  
Membrane Glycoproteins ◽  
Molecular Weights ◽  
Fibrinogen Receptor ◽  
Fibrinogen Binding ◽  
Unstimulated Control ◽  
Lines Of Evidence

Abstract We recently described a monoclonal antibody, 10E5 , that completely blocks adenosine diphosphate (ADP) induced fibrinogen binding to platelets and aggregation induced by ADP, epinephrine, and thrombin. Multiple lines of evidence indicate that 10E5 binds to platelet membrane glycoproteins IIb and/or IIIa. Because it has been reported that platelets treated with chymotrypsin aggregate when fibrinogen is added, we tested the effect of 10E5 antibody on chymotrypsin-induced fibrinogen binding and platelet aggregation. Aspirin-treated human platelets were washed in modified Tyrode's buffer (pH 7.5), incubated for 5 minutes at 22 degrees C with 300 micrograms/mL chymotrypsin, and washed again. The amount of 10E5 antibody bound to these platelets (37,232 +/- 2,928 molecules/platelet; mean +/- SEM, N=9) was similar to that bound to unstimulated control platelets (36,910 +/- 2,669) and did not differ significantly from the amount of antibody bound to ADP- treated platelets (P less than .01, N = 5). The amount of 10E5 bound to chymotrypsin-treated platelets correlated directly with the amount of fibrinogen bound to separate aliquots of the same platelet samples (r = .876, P less than .001). The 10E5 antibody caused virtually complete inhibition of both the binding of fibrinogen to chymotrypsin-treated platelets and the aggregation induced by exogenous fibrinogen. Immunoprecipitation studies of 125I-labeled chymotrypsin-treated platelets revealed that the 10E5 antibody bound proteins with molecular weights characteristic of glycoproteins IIb and IIIa. These data suggest that the fibrinogen receptor on chymotrypsin-treated platelets is identical to that on ADP-treated platelets and that this receptor is either near to, or on, the glycoprotein IIb/IIIa complex.

A Comparison of the Effect of Decorsin and Two Disintegrins, Albolabrin and Eristostatin, on Platelet Function

1995 ◽  
Vol 74 (05) ◽  
pp. 1316-1322 ◽  
Author(s):  
Mary Ann McLane ◽  
Jagadeesh Gabbeta ◽  
A Koneti Rao ◽  
Lucia Beviglia ◽  
Robert A Lazarus ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Platelet Function ◽  
Sequence Similarity ◽  
Platelet Aggregation Inhibitors ◽  
Antiplatelet Drug ◽  
Rgd Sequence ◽  
Fibrinogen Receptor ◽  
Activated Platelets

SummaryNaturally-occurring fibrinogen receptor antagonists and platelet aggregation inhibitors that are found in snake venom (disintegrins) and leeches share many common features, including an RGD sequence, high cysteine content, and low molecular weight. There are, however, significant selectivity and potency differences. We compared the effect of three proteins on platelet function: albolabrin, a 7.5 kDa disintegrin, eristostatin, a 5.4 kDa disintegrin in which part of the disintegrin domain is deleted, and decorsin, a 4.5 kDa non-disintegrin derived from the leech Macrobdella decora, which has very little sequence similarity with either disintegrin. Decorsin was about two times less potent than albolabrin and six times less potent than eristostatin in inhibiting ADP- induced human platelet aggregation. It had a different pattern of interaction with glycoprotein IIb/IIIa as compared to the two disintegrins. Decorsin bound with a low affinity to resting platelets (409 nM) and to ADP-activated platelets (270 nM), and with high affinity to thrombin- activated platelets (74 nM). At concentrations up to 685 nM, it did not cause expression of a ligand-induced binding site epitope on the (β3 subunit of the GPIIb/IIIa complex. It did not significantly inhibit isolated GPIIb/IIIa binding to immobilized von Willebrand Factor. At low doses (1.5-3.0 μg/mouse), decorsin protected mice against death from pulmonary thromboembolism, showing an effect similar to eristostatin. This suggested that decorsin is a much more potent inhibitor of platelet aggregation in vivo than in vitro, and it may have potential as an antiplatelet drug.

Aprotinin: is it prothrombotic?

Perfusion ◽  
2001 ◽  
Vol 16 (5) ◽  
pp. 401-409 ◽  
Author(s):  
M Poullis ◽  
R C Landis ◽  
K M Taylor
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Mechanism Of Action ◽  
Adenosine Diphosphate ◽  
Platelet Membrane ◽  
Calcium Flux ◽  
Thrombin Receptor ◽  
Proteolytic Activation ◽  
Agonist Peptide ◽  
Receptor Family

Controversy continues as to whether aprotinin (Trasylol) is prothrombotic. The recent discovery of the thrombin receptor family, known as the protease-activated receptor family (PAR) has been essential in aiding our understanding of the mechanism of action of aprotinin. Our results show that aprotinin has no effect on platelet aggregation induced by adrenaline, adenosine diphosphate, phorbol-12-myristate-13-acetate, collagen or PAR 1 agonist peptide. However, aprotinin inhibits thrombin-induced platelet activation as assessed by macroaggregation, microaggregation and platelet membrane calcium flux. Aprotinin inhibits proteolytic activation of platelets, but platelets can still be activated by non-proteolytic mechanisms.

Antibodies Against Platelet Membrane Glycoproteins: Effect On Ristocetin-Induced Platelet Aggregation

1981 ◽  
Author(s):  
E F Ali-Briggs ◽  
C S P Jenkins ◽  
K J Clemetson
Keyword(s):  
Platelet Aggregation ◽  
Affinity Chromatography ◽  
Platelet Membrane ◽  
Receptor Activity ◽  
Free Medium ◽  
Membrane Glycoproteins ◽  
Lectin Affinity Chromatography ◽  
Lectin Affinity ◽  
Fab Fragments ◽  
Induced Aggregation

Some membrane glycoproteins (GPs) have been isolated by lectin-affinity chromatography and antibodies towards them have been raised. Platelets that have lost glycocalicin no longer respond to ristocetin-human VIII:WF, bovine VIIIR:WF, or to anti-glycocalicin or anti-GPs la and lb antibodies but are still agglutinated by anti-GPs lib and Ilia antibodies. Anti-GPs la and lb and anti-glycocalicin antibodies, IgG and Fab' fragments inhibited ristocetin- human VIIIR:WF- and bovine VIIIR:WF-induced aggregation of fixed, washed platelets and of platelets in plasma while anti-GPs Hb and Ilia antibodies were without effect.Crossed immunoelectrophorectic studies showed that glycocalicin was present on whole platelets in only trace amounts; anti-glycocalicin antibodies, however, recognized a slower migrating component. Platelets incubated in an EDTA-free medium no longer respond to ristocetin-human VIIIRrWF. Membranes isolated from such platelets contained glycocalicin which cross-reacted with a remnant of the slower migrating component. Anti-GPs la and lb antibodies gave more complex patterns but it was possible to identify the slower moving component recognized by the anti-glycocalicin antibodies.These results show that glycocalicin is not normally found as such on whole platelets but is present as a precursor which is most likely GP lb. On degradation of this precursor, glycocalicin is released from the membrane and VIIIRrWF-receptor activity is lost.

scholarly journals Analysis of platelet aggregation disorders based on flow cytometric analysis of membrane glycoprotein IIb-IIIa with conformation-specific monoclonal antibodies

Blood ◽  
1990 ◽  
Vol 76 (10) ◽  
pp. 2017-2023 ◽  
Author(s):  
MH Ginsberg ◽  
AL Frelinger ◽  
SC Lam ◽  
J Forsyth ◽  
R McMillan ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Ligand Binding ◽  
Cell Activation ◽  
Flow Cytometric Analysis ◽  
Adenosine Diphosphate ◽  
Receptor Activation ◽  
Affinity Column ◽  
Primary Defect ◽  
Fibrinogen Receptor ◽  
Fibrinogen Binding

Abstract Normal primary platelet aggregation requires agonist-mediated activation of membrane GPIIb-IIIa, binding of fibrinogen to GPIIb-IIIa, and cellular events after ligand binding. PAC1 monoclonal antibody distinguishes between resting and activated states of GPIIb-IIIa, and other antibodies preferentially recognize GPIIb (PMI-1) or IIIa (anti- LIBS1) after the binding of fibrinogen or fibrinogen-mimetic peptides, such as GRGDSP. Using these antibodies and platelet flow cytometry, we studied two distinct persistent platelet aggregation abnormalities. Platelets from a thrombasthenic variant, which contained near-normal amounts of GPIIb-IIIa, failed to aggregate or bind PAC1 in response to agonists. In addition, GRGDSP, which binds to normal GPIIb-IIIa without prior cell activation, failed to increase the binding of PMI-1 or anti- LIBS1 to the thrombasthenic platelets, suggesting a primary defect in ligand binding. Chromatography of detergent-solubilized platelets on a KYGRGDS affinity column confirmed that the patient's GPIIb-IIIa lacked the fibrinogen binding site. In another patient with myelofibrosis and defective aggregation, PAC1 failed to bind to adenosine diphosphate- stimulated platelets, but did bind when protein kinase C was directly activated with phorbol myristate acetate. Furthermore, the binding of PMI-1 and anti-LIBS1 increased in response to GRGDSP, confirming a defect in agonist-mediated fibrinogen receptor activation rather than in fibrinogen binding or events distal to binding. These studies indicate that this immunochemical approach is useful in classification of clinical abnormalities of platelet aggregation as defects in either (a) fibrinogen receptor activation, (b) fibrinogen binding, or (c) postoccupancy events.

Interspecies Comparison of Platelet Aggregation, LIBS Expression and Clot Retraction: Observed Differences in GPIIb-IIIa Functional Activity

1995 ◽  
Vol 74 (06) ◽  
pp. 1551-1556 ◽  
Author(s):  
Lisa K Jennings ◽  
Melanie M White ◽  
Timothy D Mandrell
Keyword(s):  
Platelet Aggregation ◽  
Conformational Changes ◽  
Low Dose ◽  
Species Differences ◽  
Receptor Occupancy ◽  
Adenosine Diphosphate ◽  
Human Platelets ◽  
Clot Retraction ◽  
Fibrinogen Receptor ◽  
Functional Aspects

SummaryWe examined interspecies differences in the function of the platelet fibrinogen receptor, GPIIb-IIIa, by comparing platelet aggregation responses to adenosine diphosphate (ADP) added alone or in combination with a GPIIIa specific monoclonal antibody (mAb), D3. D3 can activate the GPIIb-IIIa receptor in the absence of platelet activation, and it preferentially binds to a region on the GPIIIa subunit after the GPIIb-IIIa complex is occupied by ligand. Using human, monkey, dog, rabbit and pig platelets, we examined whether all species’ platelets bound the D3 mAb similarly, and if the binding of Arg-Gly-Asp-Ser (RGDS) peptides induced the exposure of the anti-LIBS (D3) epitope as previously described for human platelets. We also evaluated how blocking of this neoantigenic region by the D3 mAb affected clot retraction, a process that requires linkage of GPIIb-IIIa with fibrin(ogen) and the platelet cytoskeleton. We found that all species tested bound the D3 mAb. Only in human and monkey platelets did D3 cause aggregation as well as inhibit clot retraction. However, in all species tested, except for pig, D3 prevented disaggregation of platelets typically observed when platelets are treated with low dose ADP. With the exception of pig platelets, there was increased D3 binding to platelets in the presence of RGDS peptides. We propose that this region of GPIIIa is important in the conformational changes that GPIIb-IIIa undergoes during the binding of ligand in most species tested. Our studies suggest 1) there are measurable inter-species differences in GPIIb-IIIa mediated platelet aggregation and clot retraction, 2) LIBS expression due to receptor occupancy is a common but not all-inclusive response and 3) interspecies comparisons may be useful in understanding structural and functional aspects of platelet GPIIb-IIIa.

scholarly journals Zinc-induced platelet aggregation is mediated by the fibrinogen receptor and is not accompanied by release or by thromboxane synthesis

Blood ◽  
1985 ◽  
Vol 66 (1) ◽  
pp. 213-219 ◽  
Author(s):  
P Heyns A du ◽  
A Eldor ◽  
R Yarom ◽  
G Marx
Keyword(s):  
Platelet Aggregation ◽  
Dense Body ◽  
Platelet Rich Plasma ◽  
Adenosine Monophosphate ◽  
Adenosine Diphosphate ◽  
Creatine Phosphate ◽  
Calcium Flux ◽  
Fibrinogen Receptor ◽  
Induced Aggregation

Abstract We demonstrate that zinc (0.1 to 0.3 mmol/L) induces aggregation of washed platelet suspensions. Higher concentrations (1 to 3 mmol/L) of zinc were needed to aggregate platelets in platelet-rich plasma obtained from blood anticoagulated with low-molecular-weight heparin, probably due to the binding of zinc to the plasma proteins. Zinc- induced aggregation of normal washed platelets required added fibrinogen and no aggregation occurred with thrombasthenic platelets or with normal platelets pretreated with a monoclonal antibody (10E5) that blocks the platelet fibrinogen receptor. These data indicate that the platelet membrane fibrinogen receptor-glycoproteins IIb and IIIa mediate the effect of zinc. Zinc-induced aggregation was blocked by the agent TMB-8, which interferes with the internal calcium flux, and by prostacyclin, which elevates platelet cyclic adenosine monophosphate levels. Zinc-induced aggregation was not accompanied by thromboxane synthesis or by the secretion of dense-body serotonin and was not affected by preexposure of platelets to acetylsalicylic acid. Experiments with creatine phosphate/creatine phosphokinase showed that the zinc effect on platelets was independent of extracellular adenosine diphosphate (ADP). Zinc had an additive effect when platelet aggregation was stimulated with subthreshhold concentrations of collagen or ADP. Together with the known effects of nutritional zinc on in vivo bleeding, on platelet aggregation, and on lipid metabolism, the results suggest that zinc may have an important bearing on normal hemostasis, thrombosis, and atherosclerosis.

Recognition of platelet-associated fibrinogen by polyclonal antibodies: correlation with platelet aggregation

Blood ◽  
1992 ◽  
Vol 79 (8) ◽  
pp. 2028-2033
Author(s):  
EI Peerschke
Keyword(s):  
Platelet Aggregation ◽  
Time Course ◽  
Polyclonal Antibodies ◽  
Surface Membrane ◽  
Adenosine Diphosphate ◽  
Dependent Manner ◽  
Membrane Structures ◽  
Platelet Surface ◽  
Fibrinogen Binding ◽  
Qualitative Changes

Progressive decreases in platelet-bound fibrinogen accessibility to antibody and enzymes were recently reported to occur after adenosine diphosphate (ADP)-induced fibrinogen binding. Because previous studies also indicated that platelets that are activated but not aggregated by ADP in the presence of fibrinogen lose their ability to aggregate in a time-dependent manner despite negligible changes in fibrinogen binding, the present study examined the relationship between platelet aggregation and accessibility of platelet-bound fibrinogen to specific polyclonal antibody F(ab')2 fragments over a 60-minute time course. Although 125I-fibrinogen binding remained virtually unchanged, comparison of antifibrinogen antibody F(ab')2 binding and platelet aggregation 5 minutes and 60 minutes after platelet stimulation with ADP or thrombin showed decreases in F(ab')2 binding of 62% +/- 13% and 73% +/- 7% (mean +/- SD, n = 5), respectively, and decreases of 65% +/- 16% and 60% +/- 10% in platelet aggregation. In contrast, platelets stimulated with A23187 or chymotrypsin retained 87% +/- 16% and 76% +/- 12% of their ability to aggregate over the same time course, and lost only 39% +/- 14% and 36% +/- 12% of their ability to bind antifibrinogen antibody F(ab')2 fragments, respectively. Pretreatment of ADP-stimulated platelets with chymotrypsin largely prevented the progressive loss of platelet aggregability and the accompanying decreased recognition of bound fibrinogen by antifibrinogen F(ab')2 fragments. Preincubation of platelets with cytochalasin D (30 micrograms/mL) also inhibited the decrease in platelet aggregation after exposure of ADP-treated platelets to fibrinogen over a 60-minute time course. This was accompanied by only a 25% +/- 18% decrease in antifibrinogen antibody F(ab')2 binding. Present data support the hypothesis that qualitative changes in platelet-bound fibrinogen correlate with loss of the ability of platelets to aggregate, and implicate both the platelet cytoskeleton and chymotrypsin-sensitive surface membrane structures in modulating qualitative changes in bound fibrinogen on the platelet surface.

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