Differential role of CYP2E1-mediated metabolism in the lethal and vestibulotoxic effects of cis-crotononitrile in the mouse

Several alkylnitriles are toxic to sensory systems, including the vestibular system, through yet undefined mechanisms. This study addressed the hypothesis that the vestibular toxicity of cis-crotononitrile depends on CYP2E1-mediated bioactivation. Wild-type (129S1) and CYP2E1-null female mice were exposed to cis-crotononitrile at 0, 2, 2.25 or 2.5 mmol/kg (p.o.) in either a baseline condition or following exposure to 1% acetone in drinking water to induce CYP2E1 expression. The exposed animals were assessed for vestibular toxicity using a behavioral test battery and through surface observation of the vestibular sensory epithelia by scanning electron microscopy. In parallel groups, concentrations of cis-crotononitrile and cyanide were assessed in whole blood. Contrary to our hypothesis, CYP2E1-null mice were slightly more susceptible to the vestibular toxicity of cis-crotononitrile than were control 129S1 mice. Similarly, rather than enhance vestibular toxicity, acetone pretreatment actually reduced it slightly in 129S1 controls, although not in CYP2E1-null mice. In addition, significant differences in mortality were recorded, with the greatest mortality occurring in 129S1 mice after acetone pretreatment. The highest mortality recorded in the 129S1 + acetone mice was associated with the lowest blood concentrations of cis-crotononitrile and the highest concentrations of cyanide at 6 h after nitrile exposure, the time when deaths were initially recorded. We conclude that cis-crotononitrile is a CYP2E1 substrate as hypothesized, but that CYP2E1-mediated metabolism of this nitrile is not necessary for vestibular toxicity; rather, this metabolism constitutes a major pathway for cyanide release and subsequent lethality.