In low doses, aspirin inhibits which of the following mediators?

Arachdonic acid metabolites are formed by two classes of enzymes such as Cycloxygenases and Lipoxygenases. Cycloxygenase pathway is mediated by COX1 and COX2 enzymes leading to the formation of Prostaglandins and Thromboxanes. In low doses, aspirin inhibit cycloxygenase irreversibly by acetylating one of its serine residues. Return of cycloxygenase activity depends upon synthesis of fresh enzyme. Ref: Essentials of Medical Pharmacology, 5th Edition, Pages 168, 173-4.