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We examined the effect of a carbon tetrachloride (CCl4)-induced hepatic injury on the stereoselective N-demethylation of RS-(+/-)-chlorpheniramine (Chp) by cytochrome P450 (CYP) 2C11 isozyme. In the non-treated rat liver microsomes, the stereoselective N-demethylation of racemic Chp was observed. However, in the CCl4-treated (0.5 ml/kg, i.p.) rat liver microsomes, the N-demethylation activities of S-(+)- and R-(-)-Chp decreased continuously up to the third day after the treatment with CCl4, and reached about 9 and 13% of control values, respectively, and the stereoselective N-demethylation of Chp was not observed. Moreover, in the liver microsomes at the 7th day after the treatment with CCl4, the N-demethylation activities of both enantiomers recovered to an original level, and the stereoselective N-demethylation of Chp was again observed. The addition of 30 microliters of the anti-rat CYP2C11 serum to the reaction mixture containing 1 mg of microsomal protein inhibited the formation of monodesmethylchlorpheniramine (DMChp) from both enantiomers to 74 and 57% of the control values for S-(+)- and R-(-)-Chp, respectively. In the liver microsomes of a male rat at the 1st day after the treatment of CCl4, the addition of the anti-rat CYP 2C11 serum (30 microliters) also caused 25% inhibition of the formation of DMChp from S-(+)-Chp, but anti-rat CYP2C11 had no inhibitory effect on the rates of microsomal N-demethylation of R-(-)-enantiomer. On the other hand, in the liver microsomes of a male rat at the 7th day after the treatment with CCl4, the anti-rat CYP2C11 serum had an inhibitory effect on the rates of microsomal N-demethylation of either S-(+)- or R-(-)-enantiomers again. Moreover, it was confirmed by Western blotting analysis that the density of the stained bands of CYP2C11 in the liver microsomes from male rats at the 1st, 2nd and 3rd days after the treatment with CCl4, was thinner than that from non-treatment male rats. These results indicated that the changes of N-demethylation activities of Chp in the CCl4-induced hepatic injury were due to the variation of microsomal CYP2C11.
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