Relative inhibitory potencies of flavonoids on 12-lipoxygenase of fish gill

The relative efficacy of 10 flavonoid compounds and some common antioxidants in inhibiting 12-lipoxygenase of fish gill was determined. Lipoxygenase activity was measured by oxygen consumption with polarography and formation of hydroxy fatty acid using thin layer chromatography with autoradiography. Generally, the inhibition of 12-lipoxygenase by flavonoids was noncompetitive. The lipoxygenase product pattern did not change in the presence of flavonoids. Fisetin (3,3′,4′,7-tetrahydroxyflavone) and quercetin (3,3′,4′,5,7-pentahydroxyflavone) were the most potent inhibitors of 12-lipoxygenase, with IC₅₀'s of 0.25 and 0.4 μM respectively. These were compared with IC₅₀'s values of 5, 1000 μM for BHA (t-butylhydroxyanisole) and BHT (t-butylhydroxytoluene) respectively. Possible inhibitory mechanisms and relationships between flavonoid structure and inhibitory potencies are discussed.