Received for publication November 29, 2006.
Revised January 10, 2007.
Accepted for publication January 17, 2007.

DEACETYLCLIVORINE: A GENDER-SELECTIVE METABOLITE OF CLIVORINE FORMED IN FEMALE SD RAT LIVER MICROSOMES

Abstract

Clivorine, a naturally occurring pyrrolizidine alkaloid, causes liver toxicity via its metabolic activation to generate toxic metabolite (pyrrolic ester). Female SD rats are reported to be less susceptible to clivorine intoxication than male SD rats. However, the biochemical mechanism causing such gender difference is largely unknown. The present study investigated hepatic microsomal metabolism of clivorine in female rats to delineate the mechanism of the gender difference. Two pathways, which directly metabolize clivorine, were observed. Firstly the metabolic activation to produce the toxic pyrrolic ester followed by formations of bound-pyrroles, DHR, 7-GSH-DHR and clivoric acid were found in female rats, and CYP3A1/2 isozymes were identified to catalyze the metabolic activation. Compared with male rats (~21%), the metabolic activation in female rats was significantly lower (~4%) possibly due to significantly lower CYP3A1/2 levels expressed in female rats. Secondly a direct hydrolysis to generate a novel female rat-specific metabolite deacetylclivorine was demonstrated as the predominant pathway (~16% clivorine metabolism) in female rat liver microsomes and was determined to be mediated by microsomal hydrolase A. Furthermore, when the metabolic activation was completely inhibited by ketoconazole, the amount of deacetylclivorine formed in 1-hour incubation significantly increased from 19.44±3.00 to 54.87±9.30 nmol/mg protein, suggesting that the two pathways compete with each other. Therefore, the lower susceptibility of female SD rats to clivorine intoxication is suggested to be due to significantly higher extent of the direct hydrolysis as well as a lower degree of the metabolic activation.

Key words: bioactivation, gender differences, hepatotoxicity, idiosyncratic drug reactions, liver microsomes, metabolite identification

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