ortho-Phenylphenol (
OPP) is a widely used fungicide and
antibacterial agent that is also known to be highly effective in inducing
bladder tumors in male F344 rats. At present, neither the role of the urinary bladder in the bioactivation of
OPP metabolites nor the nature of the molecular target is understood. To address these issues, we investigated the relationship between
OPP dosage and macromolecular adduct formation in the urinary bladder of male F344 rats. Male F344 rats were treated with 0, 15, 50, 125, 250, 500, 1000 mg/kg of
OPP and its radiocarbon analogue via oral gavage. The dosed rats were euthanized after 24 h, and the
proteins were extracted from the liver, kidney, and bladder. The amount of radioactivity associated with the extracted
protein was quantified using highly sensitive accelerator mass spectrometry. Protein binding in liver and kidney exhibited a linear or modest curvilinear relationship over the dose range studied. In the urinary bladder, however, a pronounced nonlinear relationship between
protein adduct levels and administered dose was observed. The measured
protein adduct levels were in agreement with the predicted concentrations of
phenylbenzoquinone based on a proposed mechanism involving free
phenylhydroquinone autoxidation in the urine. Unlike protein binding,
DNA adducts measured from the same bladder samples did not show a significant difference from the control group. These data are consistent with the hypothesis that
OPP is an indirect acting
carcinogen, and that regenerative
hyperplasia due to
OPP-metabolite cytotoxicity and/or binding of
OPP metabolites to
protein targets may play an important role in
OPP-induced bladder
carcinogenesis.