Naphthylisothiocyanate disposition in bile and its relationship to liver glutathione and toxicity

1-Naphthylisothiocyanate (ANIT), but not 2-naphthylisothiocyanate (BNIT), produces cholangiolitic hepatitis in rats after a single, oral administration. The mechanisms responsible for the disparate toxic outcomes for these closely related structural isomers are not fully understood. Recent reports suggest that ANIT-induced hepatotoxicity is dependent upon the formation and biliary excretion of a reversible glutathione-ANIT conjugate. To understand better the relationship between hepatic glutathione, secretion into bile and hepatotoxicity, the bile concentrations and hepatotoxicities of ANIT and BNIT were examined in rats with and without pretreatment with buthionine sulfoximine (BSO). ANIT (100 mg/kg, p.o.) caused a 3-fold elevation of plasma alanine aminotransferase activity (ALT), a 6-fold elevation of total plasma bilirubin, and a > 90% reduction in bile flow 24 hr after administration. BNIT, at this same dose and route of administration, did not alter significantly these markers of liver injury. Accumulation of ANIT and BNIT in bile occurred with the same temporal characteristics; however, BNIT accumulated to markedly larger concentrations (292 +/- 83 and 235 +/- 100 microM BNIT and 78 +/- 19 and 29 +/- 13 microM ANIT at 1 and 4 hr, respectively). The accumulation of ANIT and BNIT in bile was coincident with a > 2-fold elevation of reduced glutathione in bile. Pretreatment of rats with BSO decreased hepatic glutathione concentration and reduced the concentration of naphthylisothiocyanates in bile by 85%. Associated with this reduction was an attenuation of ANIT hepatotoxicity. Altogether, these findings indicate that both ANIT and BNIT accumulate in bile in a glutathione-dependent manner, yet they yield different hepatotoxic outcomes. Therefore, the disparity in hepatotoxicities observed with these isomers is not related to a difference in ability to enter bile. Other differences, such as in metabolism, chemical reactivity, conjugate stability and/or cytotoxic potential to bile duct epithelial cells may be more important determinants of hepatotoxicity.