Abstract
The radiolabeled thromboxane A2/prostaglandin H2 (TXA2/PGH2) agonist 125I-BOP bound to the TXA2/PGH2 receptor on human platelet membranes. Scatchard analysis showed that pretreatment of platelet membranes with the reducing agent dithiothreitol (DTT) (10 mM) for 10 min decreased maximal 125I-BOP binding (Bmax) from 1.51 +/- 0.11 pmol/mg to 0.51 +/- 0.05 pmol/mg (p = 0.001) and increased the affinity of the remaining binding sites (Kd = 647 +/- 64 pM (untreated), 363 +/- 46 pM (treated), p = 0.006). Prolonged incubation of membranes with DTT (10 mM) for 40 min further reduced the Bmax to 0.23 +/- 0.08 pmol/mg (p = 0.001 from untreated), and the binding affinity remained elevated (Kd = 334 +/- 117 pM, p = 0.035 from untreated). Kinetic analysis of 125I-BOP binding indicated that the apparent increase in binding affinity after DTT treatment was due exclusively to an increase in the rate of ligand-receptor association with no change in dissociation rate. The effects of DTT on 125I-BOP binding were dose-dependent with an EC50 of 8.1 +/- 0.2 mM. DTT inactivation of TXA2/PGH2 receptors was time-dependent with a second order rate constant (k2) of 0.123 M-1 s-1 at 20 degrees C. The platelet membrane 125I-BOP binding site was partially protected from DTT inactivation by prior occupation with the ligand. TXA2/PGH2 receptor protection by I-BOP was dose-dependent and linearly related (r = 0.97, p = 0.002) to the proportion of receptors occupied, but was incomplete since agonist occupation of 89% of the total number of receptors resulted in only a 38% protective effect. Inhibition of 125I-BOP binding after reduction with DTT could be made permanent by addition of the sulfhydryl alkylating agent N-ethylmaleimide (25 mM), but was completely reversed by reoxidation with dithionitrobenzoic acid (DTNB) (5 mM). Oxidation of untreated receptors with DTNB resulted in a 64% increase in 125I-BOP binding sites from 1.65 +/- 0.12 pmol/mg to 2.70 +/- 0.08 pmol/mg (p = 0.013) without affecting binding affinity. DTNB-induced increases in 125I-BOP binding were concentration-dependent with an EC50 of 668 +/- 106 microM and occurred in less than 1 min at 37 degrees C. In the absence of DTT, alkylation of free sulfhydryl groups with N-ethylmaleimide reduced 125I-BOP Bmax in platelet membranes to 0.85 +/- 0.08 pmol/mg (p = 0.003), but did not change the affinity of the remaining receptors. The EC50 for N-ethylmaleimide inactivation of TXA2/PGH2 receptors was 139 +/- 8 mM, and the k2 in time course experiments was 0.067 M-1 s-1 at 20 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)
Highlights
The radiolabeled thromboxaneThe EC& for iv-ethylmaleimide inactivation of TXA2/PGHZ receptors was 139 2 8 mM, and the k2 in time course experiments was 0.067 Mm1 s-l at 20 “C
TXA2/PGHZ receptor protection by I-BOP was dose-dependent and linearly related (r = 0.97, p = 0.002) to the proportion of receptors occupied, but was incomplete since agonist occupation of 89% of the total number of receptors resulted in only a 38% protective effect
The possibility that disulfide-sulfhydry1 reactions play a role in the regulation of human platelet TXAZ/PGHZ receptors was tested by examining the effects of the reducing agent DTT, the oxidizing agent 5,5’-dithiobis(nitrobenzoic acid) (DTNB), and the sulfhydryl alkylating agent Ar-ethylmaleimide (NEM) on binding of the radioligand TXAZ/PGHZ agonist 1251-BOP[18] to human platelet membranes
Summary
The EC& for iv-ethylmaleimide inactivation of TXA2/PGHZ receptors was 139 2 8 mM, and the k2 in time course experiments was 0.067 Mm1 s-l at 20 “C These data indicate that the human platelet TXA2/PGH2 receptor possesses disulfide bond(s) and sulfhydryl group(s) located at or near the ligand binding site. The possibility that disulfide-sulfhydry reactions play a role in the regulation of human platelet TXAZ/PGHZ receptors was tested by examining the effects of the reducing agent DTT, the oxidizing agent 5,5’-dithiobis(nitrobenzoic acid) (DTNB), and the sulfhydryl alkylating agent Ar-ethylmaleimide (NEM) on binding of the radioligand TXAZ/PGHZ agonist 1251-BOP[18] to human platelet membranes. Disulfide bond reduction reversibly decreased the total number of membrane receptors and increased the agonist affinity of the remaining receptors
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