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Figure 1 Oxidative metabolism of salicylic acid: acetylsalicylic acid (ASA) is rapidly hydrolyzed to SA by esterases and for 60% remains unmodified and can undergo hydroxyl radical (^·OH) attack to produce two derivates—namely, 2,3-dihydroxybenzoic acid (DHBA) and 2,5-DHBA. Enzymatic pathways through the cytochrome P-450 system can also produce the latter acid, whereas the former acid is solely formed by direct ^·OH attack. Therefore, measurement of 2,3-DHBA or the 2,3-DHBA/SA ratio, after administration of ASA, is recognized as a sensitive and specific method to determine ^·OH production in vivo.