Lipophilic flavonoids from Orthosiphon spicatus prevent oxidative inactivation of 15-lipoxygenase

15-Lipoxygenase from soybeans is inactivated by bubbling air through enzyme solutions. This inactivation is prevented by flavonoids from the East Asian medicinal plant Orthosiphon spicatus, previously found to be inhibitors of the enzyme. 5,7,4'-Trimethylapigenin, eupatorin and 5,7,3',4'-tetramethylluteolin show the strongest enzyme-stabilizing effects, decreasing loss of activity by 50% at concentrations of 2.0 +/- 0.04, 2.4 +/- 0.3 and 4.3 +/- 1.1 microM, respectively. There is no significant correlation between enzyme-inhibiting and enzyme-stabilizing effect. The Orthosiphon flavonoids show radical-scavenging activity towards the diphenylpicrylhydrazyl radical. This is correlated to their enzyme-stabilizing effect, but not to their inhibitory activity towards 15-lipoxygenase. When the enzyme is inactivated by air bubbling, a loss of sulfhydryl groups is observed. Sinensetin, a poor stabilizer of the enzyme, shows less efficiency in protecting sulfhydryl groups than tetramethylscutellarein, which stabilizes the enzyme more efficiently. Thus, oxidation of sulfhydryl groups may contribute to the observed air-induced inactivation of 15-lipoxygenase.