Site-selective oxidation of strychnine by phenobarbital inducible cytochrome P-450.

Abstract

The metabolism of strychnine was studied using liver microsomes of rats treated with phenobarbital or 3-methylcholanthrene (MC). The phenobarbital-treatment resulted in 7.9-fold and 4.8-fold increases in 2-hydroxylation and N-oxidation of strychnine, respectively. The formation of 16-hydroxystrychnine, strychnine 21,22-epoxide and 22-hydroxystrychnine was induced about 2-fold. MC-treatment resulted in only 1.4-fold induction of each oxidation activity. In addition, strychnine 2-hydroxylation activity was markedly induced in liver microsomes of phenobarbital-treated mice, guinea pigs, rabbits and dogs (2.5-10.5-fold). Induction of N-oxidation activity was also higher than that of the three other oxidation activities. A reconstituted system of strychnine metabolism using cytochrome P-450 isozymes, P-450I (P450IIB1) and P-450II (P450IIB2), purified from liver microsomes of phenobarbital-treated rats showed significantly high and selective activities towards 2-hydroxylation and N-oxidation of strychnine. This characteristic metabolism of strychnine appears to occur in common with interspecies P450IIB gene subfamily. The pH Optima of 2-hydroxylation and N-oxidation of strychnine were between 8.4 and 8.6 in the microsomes of phenobarbital-treated rats, while that of N-demethylation of benzphetamine, a typical substrate of phenobarbital-inducible cytochrome P-450, was between 7.4 and 7.6. In a reconstituted system with P-450I, strychnine oxidation was little affected by pH change, while benzphetamine N-demethylation activity was decreased in the alkaline side. Among several oxidations tested, only ethylmorphine N-demethylation underwent the same pH effect as did strychnine oxidation with the microsomes and reconstituted system.(ABSTRACT TRUNCATED AT 250 WORDS)