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

Diosbulbin B: An important component responsible for hepatotoxicity and protein covalent binding induced by Dioscorea bulbifera L

BACKGROUND

Dioscorea bulbifera L. (DBL) is an herbal medicine used for the treatment of thyroid diseases and tumors in China. However, the hepatotoxicity of DBL limits its wide safe use. Diosbulbin B (DSB) is the most abundant diterpene lactone occurring in DBL. Numbers of studies showed that this furanoterpenoid plays an important role in DBL-induced liver injury and that DSB is metabolized to a cis-enedial intermediate which reacts with protein to form protein covalent binding and induces hepatotoxicity.

PURPOSE

The present study aimed to define the association of DSB content in DBL with the severity of DBL hepatotoxicity to ensure the safe use of the herbal medicine in clinical practice and to determine the role of DSB in DBL-induced liver injury.

METHODS

Chemical chromatographic fingerprints of DBL were established by UPLC-MS/MS. Their hepatotoxicity potencies were evaluated in vitro and in vivo. Metabolic activation of DSB was evaluated by liver microsomal incubation. Protein modification was assessed by mass spectrometry and immunostaining.

RESULTS

The contents of DSB in DBL herbs collected from 11 locations in China varied dramatically with as much as 47-fold difference. The hepatotoxicity potencies of DBL herbs were found to be proportional to the contents of DSB. Intensified protein adduction derived from the reactive metabolite of DSB was observed in mice administered DBL with high contents of DSB.

CONCLUSION

The findings not only demonstrated that contents of DSB can be quite different depending on harvest location and special attention needs to pay for quality control of DBL but also suggest DSB is a key contributor for DBL-induced hepatotoxicity.

Asparagine and Glutamine Residues Participate in Protein Covalent Binding by Epoxide Metabolite of 8-Epidiosbulbin E Acetate In Vitro and In Vivo

Dioscorea Bulbifera L. (DBL), an effective traditional Chinese medicine, has been restricted because of multiple reports that it can cause severe hepatotoxicity. 8-Epidiosbulbin E acetate (EEA), one of the main components of DBL, can induce severe liver injury. It has been reported that EEA can be metabolized by CYP3A to the corresponding cis-enedial intermediate which alkylates the lysine residues of proteins to form pyrroline derivatives. The present study unexpectedly found that the reactive intermediate reacted with the amide groups of asparagine (Asn) and glutamine (Gln) residues of hepatic proteins of mice treated with EEA. The amide-derived protein modification increased with the increase in the dose administered. Like the adduction of the primary amine of lysine residues, the electrophilic metabolite reacted with the amide groups of Asn and Gln residues to offer the corresponding pyrrolines. The structures of the pyrrolines were confirmed by mass spectrometry and nuclear magnetic resonance spectroscopy.

Evidence for adduction of biologic amines with reactive metabolite of 8-epidiosbulbin E acetate in vitro and in vivo

Dioscorea bulbifera L. (DBL) is one of traditional Chinese medicines and has been used for the treatment of goiter, tumor and carbuncles. However, clinic application of the herbal medicine has been limited, due to reported severe hepatotoxicity. 8-Epidiosbulbin E acetate (EEA), one of the major components of DBL, can cause severe liver damage. The furan ring of EEA is metabolized by CYP3A4 to a cis-enedial reactive intermediate prone to react amino and/or thiol groups of amino acid residues. In this study, we investigated the interaction of the reactive intermediate with biologic amines. EEA-derived biologic amine adducts, including spermidine, spermine, putrescine, ornithine, lysine and glutamine were detected in cultured mouse primary hepatocytes treated with EEA. Only spermidine adduct was observed in bile of mice given EEA. The detection of the adducts was established by labeling with bromobenzyl mercaptan and LC-MS/MS analysis. Exposure of EEA resulted in concentration dependent cytotoxicity in hepatocytes. Pretreatment with spermidine attenuated the susceptibility of cells to the cytotoxicity of EEA, because of the compensation of the depleted spermidine.