Guaiacol is a phenolic natural product first isolated from Guaiac resin, as well as the oxidation of lignin. Guaiacol is readily oxidized by the heme iron of peroxidases including the peroxidase of cyclooxygenase (COX) enzymes, hence it serves a reducing co-substrate for COX reactions.1 The color yield per peroxide at 470 nm is 6,650 (M peroxide reduced)-1 cm-1.2,3,4 Two moles of zguaiacol are oxidized for each mole of hydroperoxide reduced by the peroxidase. The resulting guaiacol chromophore can be used for the colorimetric determination of hydroperoxidase activity.A phenolic natural product and reducing co-substrate for COX reactions. Guaiacol is a phenolic natural product first isolated from Guaiac resin, as well as the oxidation of lignin. Guaiacol is readily oxidized by the heme iron of peroxidases including the peroxidase of cyclooxygenase (COX) enzymes, hence it serves a reducing co-substrate for COX reactions.1 The color yield per peroxide at 470 nm is 6,650 (M peroxide reduced)-1 cm-1.2,3,4 Two moles of zguaiacol are oxidized for each mole of hydroperoxide reduced by the peroxidase. The resulting guaiacol chromophore can be used for the colorimetric determination of hydroperoxidase activity. A phenolic natural product and reducing co-substrate for COX reactions.