Competitive inhibition of phospholipase A2 activity by the platelet-activating factor antagonist Ro 19-3704 and evidence for a novel suppressive effect on platelet activation.
The compound Ro 19-3704 [3-4(R)-2-(methoxycarbonyl) oxy-3-(octadecylcarbamoyl)oxy-propoxy butylthiazolium iodide], initially described as an antagonist of platelet-activating factor, is reported here to directly inhibit rabbit platelet phospholipase (PL) A2 activity, with an IC50 value of 4 to 7 microM. Classical Michaelis-Menten analysis showed that inhibition was reversible and competitive, inasmuch as apparent Km values increased in the presence of Ro 19-3704 (from 0.2-0.4 to 2 microM), whereas Vmax values remained constant (200 +/- 20 nmol/min/10(9) cells). Ro 19-3704 inhibited platelet aggregation, PLA2 release and thromboxane B2 formation induced by thrombin (0.25 U/ml), with IC50 values of 8, 15 and below 5 microM, respectively. Aggregation and PLA2 release by arachidonic acid (100 microM) were also inhibited, but thromboxane B2 formation was unaffected, indicating that Ro 19-3704 does not inhibit cyclooxygenase. Platelet activation by collagen (5 micrograms/ml), the thromboxane mimetic U46619 ([15(S)-hydroxy-11,9(epoxymethano)-prosta-5Z,13E-dienoic acid] 1 microM) and low concentrations of thrombin (0.05-0.1 U/ml) was also inhibited by Ro 19-3704. Inhibition of platelet activation was reversible, suggesting that its suppressive effect was not due to cytotoxicity. Finally, Ro 19-3704 did not stimulate cyclic AMP formation or inhibit phosphodiesterase activity. Ro 19-3704 is a competitive inhibitor of PLA2 activity, and is also endowed with a potent suppressive effect on platelet activation induced by different agonists.
Mounier
C
C
Unités des Venins, Institut Pasteur, Paris, France.
Hatmi
M
M
Faili
A
A
Bon
C
C
Vargaftig
B B
BB
D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism; D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology; D005995:Glyceryl Ethers / Q000494:pharmacology*; D008297:Male; D010741:Phospholipases A / Q000037:antagonists & inhibitors*; D054467:Phospholipases A2; D010972:Platelet Activating Factor / Q000037:antagonists & inhibitors*; D015539:Platelet Activation / Q000187:drug effects*; D011817:Rabbits; D012710:Serum Albumin, Bovine / Q000494:pharmacology; D013844:Thiazoles / Q000494:pharmacology*
D005995:Glyceryl Ethers; D010972:Platelet Activating Factor; D013844:Thiazoles; C053235:Ro 19-3704; D012710:Serum Albumin, Bovine; D016718:Arachidonic Acid; D010741:Phospholipases A; D054467:Phospholipases A2