Inhibition mode of soybean lipoxygenase-1 by quercetin.

Quercetin noncompetitively inhibited the peroxidation of linoleic acid catalyzed by soybean lipoxygenase-1 (EC 1.13.11.12, Type 1) with an IC50 value of 4.8 μM (1.45 μg/ml). This inhibition is considered to proceed in sequential order, by initially reducing the ferric form of the enzyme to an inactive ferrous form and then, by chelating the iron of the active site of the enzyme. In the pseudoperoxidase assay, quercetin was slowly oxidized by hydroperoxides to a rather stable intermediate, 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxybenzofuran-3(2H)-one, and this oxidized intermediate still inhibited the enzymatic oxidation, probably as a chelator. Rutin and kaempferol also exhibited lipoxygenase-1 inhibitory activity, but to a much lesser extent than quercetin.