Functional characterization of GPIIb- and GPIIIa-specific monoclonal antibodies. Further evidence for the existence of agonist-specific activated states of the platelet fibrinogen receptor.

In this work human platelet aggregation induced in vitro by ADP, collagen, arachidonic acid and U-46619 (a thromboxane A(2) analogue) was used as a functional test to characterize 19 anti-GPIIb (M series) and anti2 GPIIIa (P series) monoclonal antibodies whose epitope location is known for most of them. Additionally, flow cytofluorimetry was applied to study the epitope expression of these antibodies in resting, EDTA-treated and SFLLRN peptide (thrombin receptor agonist)-activated platelets. Antibodies M6 (epitope located at GPIIbH 657-665), P23-7 (GPIIIa 114-122) and P40 (GPIIIa 262-303) bind weakly to only 43%, 70% and 66%, respectively, of the resting platelet population.

Platelet integrin alpha IIb beta 3 (GPIIb-IIIa) is not implicated in the binding of LDL to intact resting platelets.

It has been suggested that the fibrinogen receptor (glycoprotein [GP] IIb-IIIa or platelet integrin alpha IIb beta 3) could be the binding site for low-density lipoprotein (LDL); however, recent data do not support this. Furthermore, GPIIb and not the GPIIb-IIIa complex is the main binding protein for lipoprotein(a) [Lp(a)]. In the present study, we have investigated the interaction between Lp(a) particles and platelet LDL binding sites and whether platelet integrin alpha IIb beta 3 is implicated.

Platelet LDL receptor recognizes with the same apparent affinity both oxidized and native LDL. Evidence that the receptor-ligand complexes are not internalized.

We have recently demonstrated that the platelet low-density lipoprotein (LDL) receptor is immunologically different from the "classic" receptor of nucleated cells. We undertook the current studies to investigate the interaction of this receptor with oxidized LDL and to determine whether an endocytosis-mediated response is involved in the binding of LDL to platelets. The platelet LDL receptor recognized with the same affinity both native and oxidized LDL particles (IC50, 0.

LDL binding sites on platelets differ from the "classical" receptor of nucleated cells.

Washed human platelets bound radioiodinated low density lipoprotein (125I-LDL) to a class of saturable binding sites; they numbered 1,348 +/- 126 per platelet, and the dissociation constant (KD) was 50.7 +/- 9 nM. 125I-LDL binding to platelets was reversible, and apparent equilibrium was attained within 25 minutes at 22 degrees C and was characterized by forward and reverse rate constants of 1.

Cyclooxygenase activity is increased in platelets from psoriatic patients.

The aim of the present study was to ascertain the relationship between in vitro hyper-aggregability and alterations in arachidonic acid metabolism in platelets from psoriatic patients. We have studied the response to several concentrations of ADP, collagen, and arachidonic acid of intact platelets from psoriatic patients and normal subjects, with and without irreversible inhibition of platelet cyclooxygenase by aspirin. Apparent kinetic constants (apparent Michaelis constant [Km] and apparent maximum velocity [Vmax]) of cyclooxygenase in platelets from both controls and psoriatic patients were also studied.