LIPOPHILIC FLAVONOIDS FROM ORTHOSIPHON SPICATUS PREVENT OXIDATIVE INACTIVATION OF 15-LIPOXYGENASE

15-Lipoxygenase from soybeans is inactivated by bubbling air through e nzyme 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 eff ects, decreasing loss of activity by 50% at concentrations of 2.0 +/- 0.04, 2.4 +/- 0.3 and 4.3 +/- 1.1 mu M, respectively. There is no sign ificant correlation between enzyme-inhibiting and enzyme-stabilizing e ffect. The Orthosiphon flavonoids show radical-scavenging activity tow ards the diphenylpicrylhydrazyl radical. This is correlated to their e nzyme-stabilizing effect, but not to their inhibitory activity towards 15-lipoxygenase. When the enzyme is inactivated by air bubbling, a lo ss 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.