Background: Koelreuteria henryi Dummer is an indigenous plant in Taiwan. The species has been used in traditional folk medicine for the promotion of liver functions and for treating malaria and urethritis. The present study investigated the antioxidant activity of flower extract of Koelreuteria henryi Dummer. The extraction conditions were optimized by the contents of total phenolic acids and total flavonoids, and antioxidant activity assays. Moreover, an in vitro study for investigating antioxidant activity of K. henryi flower extract was demonstrated by hydrogen peroxide (H₂O₂)-induced apoptosis.

Results: K. henryi flower extracted for 150 min showed high contents of total phenolic acids and total flavonoids. In an in vitro model, L929 cells were pretreated with K. henryi flower extract, and then treated with H₂O₂ to induce oxidative damage. Results demonstrated that H₂O₂-induced apoptosis was inhibited by the treatment of 200 µg/ml K. henryi flower extract through the mitochondria-mediated pathway and mitogen-activated protein kinase (MAPK) pathway. The caspase 8/9 activity and expression of p-p38 and pERK were repressed by K. henryi flower extract. In addition, the prevention of H₂O₂-induced apoptosis by K. henryi flower extract activated the nuclear factor-erythroid 2-related factor (Nrf2) stress response pathway to transcript heme oxygenase 1 (HO-1). Also, K. henryi flower extract prevented H₂O₂-induced apoptosis through HO-1 production, as evident by the use of HO-1 inhibitor.

Conclusions: The present study demonstrated that K. henryi flower extract could inhibit the H₂O₂-induced apoptosis in L929 cells through the activation of the Nrf2/HO-1 pathway.