Human Epoxide Hydrolase is the Target of Germander Autoantibodies on the Surface of Human Hepatocytes: Enzymatic Implications

Abstract

Western countries observed an increased interest for the use of herbal medicines because of their supposed safety in contrast to chemical drugs. Wild germander (Teuchrium chamaedrys L) was traditionally used as a folk medicine for its for and antiseptic properties. In 1991, germander consumed to treat obesity, caused an epidemic of cytolytic hepatitis. Thirty cases of hepatotoxicity were first reported including cases with positive rechallengge. For these patients, an early recurrence was observed despite lack of other features of hypersensitivity (Castot and Larrey, 1992). In mice, germander toxicity required CYP3A-dependent metabolism (Loeper et al 1994). More specifically, the metabolic activation of the furan ring of the diterpenoid teucrin A (TA) (Kouzi et al 1994). TA- toxicity was via CYP3A-generated electrophilic metabolites that were detoxified by glutathione conjugation, depleted cellular thiols and caused apoptosis in isolated rat hepatocytes atocytes (Lekehal et al 1996; Fau et al 1997). However, these death processes did not explain the immunologial cal features observed in several cases of germander-induced hepatitis. To explain this immune process, patient’s sera consuming germander tea in great quantity were Tested by Western blot analysis. They contained autoantibodies directed against human microsomal epoxide hydrolase (hmEH), that was located both, in the endoplasmic reticulum and the plasma membrane (PM) of human hepatocyte and hmEH- expressing yeast. Germander-induced autoantibodies (GIAA) were shown to recognize hmEH on the cell surface. To implicate hmEH in the metabolic activation of TA we used a “humanized” yeast strain expressing human P450-reductase and cytochrome b5 transformed with CYP3A4 and/or hmEH cDNAs. TA was metabolized by CYP3A4 into a and hmEH with TA inactivated hmEH in a time-dependent-manner, in agreement with formation of reactive teuchrin A-metabolite that could covalently alter and inhibit hmEH. This modified enzyme may bypass the immunologic tolerance that normally exists for hmEH.