Immunochemical Detection of Protein Modification Derived from Metabolic Activation of 8-Epidiosbulbin E Acetate

Furanoid 8-epidiosbulbin E acetate (EEA) is one of the most abundant diterpenoid lactones in herbal medicine Dioscorea bulbifera L. (DB). Our early work proved that EEA could be metabolized to EEA-derived cis-enedial (EDE), a reactive intermediate, which is required for the hepatotoxicity observed in experimental animals exposed to EEA. Also, we found that EDE could modify hepatic protein by reaction with thiol groups and/or primary amines of protein. The present study was inclined to develop polyclonal antibodies to detect protein modified by EDE. An immunogen was prepared by reaction of EDE with keyhole limpet hemocyanin (KLH), and polyclonal antibodies were raised in rabbits immunized with the immunogen. Antisera collected from the immunized rabbits demonstrated high titers evaluated by enzyme-linked immunosorbent assays (ELISAs). Immunoblot analysis showed that the polyclonal antibodies recognized EDE-modified bovine serum albumin (BSA) in a hapten load-dependent manner but did not cross-react with native BSA. Competitive inhibition experiments elicited high selectivity of the antibodies toward EDE-modified BSA. The antibodies allowed us to detect and enrich EDE-modified protein in liver homogenates obtained from EEA-treated mice. The developed immunoprecipitation technique, along with mass spectrometry, enabled us to succeed in identifying multiple hepatic proteins of animals given EEA. We have successfully developed polyclonal antibodies with the ability to recognize EDE-derived protein adducts, which is a unique tool for us to define the mechanisms of toxic action of EEA.