title: Structural requirements for preventing the aspirin- and the arachidonate-induced inactivation of platelet cyclo-oxygenase: additional evidence for distinct enzymatic sites.
abstract: 2-Hydroxybenzoic acid (salicylic acid) prevents the inhibition by aspirin (ASA) of platelet aggregation and of the generation of thromboxane A2 from arachidonic acid (AA). We studied the ability of 2-hydroxybenzoic acid analogues to block ASA and to prevent the platelet desensitization due to a first exposure to AA. Inactivation was prevented when exposure to AA was done in the presence of reversible inhibitors of cyclo-oxygenase. Phenol, methyl salicylate and L8027 were thus strong inhibitors of AA-induced platelet activation and desensitization. The minimal structural requirement for inhibition of thromboxane A2 generation from AA was a phenol group as benzoic acid was fully inactive. 2-Hydroxybenzoic acid, and to some extent 2,6-dihydroxybenzoic acid were effective against ASA, the most active substances being methyl salicylate and L8027. The minimal structural requirement for blocking ASA was that 2-hydroxybenzoic acid, 2-methoxybenzoic acid should be devoid of activity, which highlights the fact that the hydroxyl group must be available. Our work favours the hypothesis that non-steroidal anti-inflammatory drugs react with two sites of cyclo-oxygenase, which were named the supplementary and the catalytic sites. The interaction of 2-hydroxybenzoic acid and of its analogues with the supplementary site is necessary but not sufficient for the efficacy of these compounds as cyclo-oxygenase inhibitors. The intensity of interaction with the supplementary site and the modifications of the catalytic site determine the potency of these compounds as cyclo-oxygenase inhibitors. For preventing ASA inactivation, an interaction with the supplementary sites is always necessary, but furthermore an appropriate group, preferentially in the position ortho to the hydroxyl, is needed.
mesh_terms: D000818:Animals; D001095:Arachidonic Acids / Q000494:pharmacology*; D001241:Aspirin / Q000494:pharmacology*; D001792:Blood Platelets / Q000201:enzymology / Q000502:physiology*; D016861:Cyclooxygenase Inhibitors; D010974:Platelet Aggregation / Q000187:drug effects; D011451:Prostaglandin-Endoperoxide Synthases / Q000097:blood*; D011817:Rabbits; D012459:Salicylates / Q000494:pharmacology*; D013329:Structure-Activity Relationship; D013928:Thromboxane A2 / Q000096:biosynthesis
chemical_list: D001095:Arachidonic Acids; D016861:Cyclooxygenase Inhibitors; D012459:Salicylates; D013928:Thromboxane A2; D011451:Prostaglandin-Endoperoxide Synthases; D001241:Aspirin