Phase III elimination: another two-edge sword.

Excretion into bile is well recognized as a major pathway for the elimination of amphipathic, hydrophobic, high molecular weight xenobiotics and thus complements the renal elimination of hydrophilic compounds of low molecular weight. For many xeno-biotics, excretion in bile, which has been termed a phase III elimination reaction (1), begins with an oxidation reaction (phase I), followed by conjugation (phase II), usually to glucuronide or glutathione. conjugates, which are good substrates for biliary excretory pathways. Until recently, there was little understanding Iu9on ol regarding the mechanisms by which substrates are concentrated in bile or regarding what determines the sub-strate specificity of the transporters. The identification of distinct ATP-dependent and voltage-dependent transporters in the canalicular membrane over the last few years has begun to provide answers to these questions. Important questions that have not been addressed, and which are critical for toxicology, relate to the regulation of Excretion in b the activities of these transporters by endogenous, or more importantly, envi-Phase Ill elimination reactions i ronmental agents. Two examples illus-liver (phase I), followed by conij trate the importance of concentrative transport into bile for toxicity and how regulation of this process and the subsequent fate of these conjugates might influence toxicity. Technical-grade dinitrotoluene (DNT) consists primarily of 2,4-DNT (75.8%) and 2,6-DNT (19.5%) and is hepatocarcinogenic when fed to Fischer-344 rats. After administration of 35 mg DNT/kg/day for 55 weeks, the incidence of hepatocellular carcinomas in female rats was half that in males (2). In an elegant series of studies, Rickert and colleagues suggested a bioactivation pathway involving hepatic metabolism to dinitrobenzyl glucuronide followed by biliary excretion, deconjugation, and nitroreduction by intestinal microflora, and finally reabsorption of the resulting aminonitrobenzyl alcohol, which was then oxidized to reactive metabolites which were genotoxic. A key rate-determining step in the development of hepatocarcinoma was the rate and extent to which the glucuronide conjugate was delivered to the microflora in the gastrointestinal tract, which was several-fold lower in female rats. Why is biliary excretion of this substrate lower in female rats? Based on the decreased biliary excretion of many substrates during pregnancy, and following treatment with estradiol, ethinyl estradi-ol, or diethylstilbestrol, it seems most likely that estrogens are important regulators of phase III elimination. The mechanism by which estrogens inhibit this process is not known, but the selective changes induced by treatment of rats with ethinyl estradiol (i.e., inhibition of some transporters; ATP-dependent transport of bile …