EARLY PLATELET-THROMBIN RECEPTORS AND THEIR FUNCTIONS
Authors: Simone E R , Davies T A , Zabe N A , Greenberg-seperaky S M , Larsen N E 
Published: August 2018

Human platelets possess less than 1000 high affinity [Kd=10-9]and 50-100,000 receptors of lower [Kd=10-7] affinity for o(α-thrombin. The selective derivatization of thrombin with the bifunctional crosslinking agent, DNCO, has enabled us to identify these receptorsvia covalent binding of either active siteinhibited tosyllyslmethylketothrombin (TLCK-T) or active Ctf-thrombin (T).Kinetic studies of the inhibition of the platelet-thrombin response by covalently and noncovalently bound TLCK-T have helped to elucidate the roles of the high and low affinity thrombin receptors. The activation parameters examined were initial membrane depolarization, cytoplasmic alkalinization,dense granule secretion of serotonin and lysosomal secretion of β-glucuronidase.

Isolation and characterization of the thrombin receptors after covalent photocoupling of the TLCK-T or active T- were performed after solubilization by gel filtration. The intact, high affinity receptor moiety, a glycoprotein, has an approximate molecular weight of∽lSO.OOO daltons; occasionally this protein is found as a dimer of ∽360,000 daltons. When exposed to o(α-T the receptor undergoes proteolysis, leaving a protein of∽80,000 daltons and releasing the remaining glycoprotein into the medium.Higher doses of active T have been shown to bind with lower affinity to a larger protein of approximate molecular weight 600,000 daltons anda smaller protein of 46,000 daltons. Both proteins are nonsusceptible to thrombin proteolysis. Reduction and alkylation of the600,000 dalton complex yielded two and possibly three high molecular weight components (200,000, 160,000, and possibly 145,000daltons) which may correspond to previously suggested GP-Ia and GP-Ib of the GP-I complex. Under different solubilization conditions, two other membrane proteins have been found to be part of the GP-I complex; one which is not a glycoprotein, GP-Ic, while the other is associated with the glycocalyx and is called glycocalicin. Glycocalicin and GP-Icdo inhibit thrombin binding,implying that the low affinity receptor is indeed the previously suggested GP-I complex and does not appear to be directly involved withplatelet activation.

Examination of the effect of dose and duration of incubation with non-covalently binding TLCK-T on subsequent α-thrombin response suggests the existence of positive cooperativity among thrombin receptors.Although TLCK-T has the same affinity for platelets (Kd) as T , the rateof binding and therefore that of dissociation are lower. Thus for incubation times of 1 minute or less with up to a 2x saturating TLCK-T dose, the subsequent depolarization response to a saturating T dose was enhanced. Exposure to higher TLCK-T (5x saturating)doses led to significant inhibition.

Verification of the potentiation observed in noncovalent TLCK-T studies was performed using TLCK-T covalently bound to the platelet receptor with DNCO. Several hundred thrombin molecules were bound to the platelet when a subsaturating dose of TLCK-T(0.0025 U/ml) was used to crosslink, whileseveral thousand resulted with a saturating (0.05 U/ml) TLCK-T dose. Positive cooperativity was observed with low αT doses (0.005 U/ml) when several hundred high affinity receptors are blocked. The parameters studied which exhibited this positive cooperativity were depolarization, pH change and serotonin secretion, α-Glucuronidase secretion was normal. The presence and degreeof enhancement were donor-variableand suggest different threshhold thrombin dose requirements. The enhancement observed can be attributed to either an increased rate of binding (increased affinity) or to an increased number of exposed binding sites. Since little difference was found between the number of TLCK-T molecules bound after30 versus 60 seconds, we conclude that thepotentiation is more likely due to an increased number of exposed binding sites. Results from covalent crosslinks using a fluorescein and rhodamine labeled-TLCK-T and the fluorescence activated cellsorter support this hypothesis. The sensitization of the high affinity binding sitesby partial occupancy implies these bindingsites are responsible for depolarization, pH change and dense granule secretion (the rapid initial activation response), while βglucuronidase secretion, a secondary response, is otherwise controlled.