Presystemic Effect of ASA

The inhibition of platelet aggregation by ASA results from acetylation and blockade of platelet COX-1 (B). The specificity of this reaction is achieved in the following manner irreversible acetylation of the enzyme already occurs in the blood of the splanchnic region, that is, before the liver is reached. Since ASA is subject to extensive presystemic deacetylation, cyclooxygenases located post-hepatically (e.g., in endothelial cells) are hardly affected. Confinement of COX-1 inhibition to platelets is further accentuated because enzyme can be re-synthesized in normal cells having a nucleus but not in the anuclear platelets. 

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