Antioxidant properties of isotorachrysone isolated from Rhamnus nakaharai

Isotorachrysone inhibited iron-induced lipid peroxidation with an IC₅₀ value of 1.64 ± 0.08 μM in rat brain homogenates, and was comparable in potency to butylated hydroxytoluene and was more potent than α-tocopherol or desferrioxamine. The mechanism of antioxidant properties were then examined. Isotorachrysone could scavenge the stable free radical diphenyl-p-picrylhydrazyl. And it was an efficient direct scavenger of water-soluble peroxyl radicals with stoichiometry factor of 0.53 ± 0.05 in the aqueous phase and also toward lipid-soluble peroxyl radicals in tissue homogenates. The oxygen consumption during peroxidation induced by radicals on human erythrocyte ghosts was suppressed by isotorachrysone. Furthermore, it was reactive towards superoxide anion with a second-order rate constant of 5.06 ± 0.65 x 10⁵ M^-1 S^-1. But it did not react with hydrogen peroxide detected within the sensitivity limit of our assay. Using ascorbate/iron ion/H₂O₂ as a hydroxyl radical generating system and deoxyribose as a probe, isotorachrysone was effective with hydroxyl radicals with a second-order rate constant of 3.88 ± 0.54 x 10¹¹ M^-1 S^-1 under stimulation by iron-EDTA. On the other hand, isotorachrysone retarded the peroxidation of human low density lipoprotein (LDL) initiated by both aqueous and lipophilic peroxyl radicals. And it also suppressed copper-catalyzed human LDL oxidation, as measured by fluorescence intensity, electrophoretic mobility, and thiobarbituric acid-reactive substances formation in a concentration-dependent manner. Our results show that isotorachrysone is potentially an effective and versatile antioxidant, and can help protecting LDL against oxidation.