Schisandrin B elicits the Keap 1-Nrf2 defense system via carbene reactive metabolite which is less harmful to mice liver

Background: Schisandrin B (Sch B) a main active component of Schisandra chinensis, has been shown to act as a liver protectant via activation of the Nrt2 pathway. Nevertheless, it remains unclear whether its reactive metabolite is responsible for Nrf2 activation; also, the effects of its reactive metabolite on liver function are still unknown. Methods: The present study determined and identifed the carbene reactive metabolite of Sch B in human and mice liver microsomes. Its roles in activating Nrf2 pathway and modifying macromolecules were further explored in human liver microsomes. Moreover the potential cytotoxicity and hepatoxicity of carbene on HepG-2 and mice were also investigated. Results: In the present study, cytochromes P450 (CYP450s) metabolized Sch B to carbene reactive metabolite, which, with the potential to modify peptides, were identifed and observed in human and mice liver microsomes. Moreover, the relevance of carbene in Nrf2 activation was verifed by co-incubation in the presence of CYP450 inhibitors in HepG-2 cells, as well as by molecular docking study of carbene and Keap 1. Additionally, the cytotoxicity of Sch Ron HepG-2 cells was significantly aggravated by CYP450 inducer (with LD50 decreasing from 63 to 21 mu M) and signifcantly alleviated by CYP450 inhibitor and glutathione (with LD50 increasing from 63 mu M to 200 mu M). Besides, after oral administration of mice with Sch B (25-100 mg/kg) for 21 days, only the highest dose induced mild hepatotoxicity, which was accompanied by increasing the aminotransferase activity and centrilobular hepatocellular itatration of lymphocytes. In addition, upregulation of CY P450 activity; Nrf2, NQO-1, and GST expression; and glutathione level was observed in Sch B treatment groups. Conclusion: The present study revealed that CYP450s mediate the conversion of Sch B to carbene, which subsequently binds to Keap 1 and elicits Nrt2 pathway, which could further increase the elimination of carbene and thus exhibit a less harmful effect on mice liver.