Role of isozymes of cytochrome P-450 in the metabolism of N,N-dimethyl-4-aminoazobenzene in the rat.

The metabolism of N,N-dimethyl-4-aminoazobenzene (DAB) was investigated in vitro by use of hepatic 10,000g supernatant fraction, microsomes, and purified cytochromes P-450 prepared from rats. Position-selective metabolism was studied in response to induction by 3-methylcholanthrene (MC), phenobarbital (PB), beta-naphthoflavone (BNF), and pregnenolone-16 alpha-carbonitrile (PCN) as well as inhibition by SKF 525-A, metyrapone, alpha-naphthoflavone, and piperonyl butoxide. The principal phase I pathways are demethylation of the tertiary (DAB) and secondary (MAB) amines and ring hydroxylation. When metabolism was measured with 10,000g supernatant fractions, each pathway responded differently and often independently to the inducers and inhibitors, suggesting that they are catalyzed preferentially by different isozymes of cytochrome P-450. Microsomes from PB-treated animals demethylated and hydroxylated DAB at the same rate as did control microsomes, based on cytochrome P-450 content, whereas microsomes from BNF- or MC-treated animals demethylated more rapidly and hydroxylated more slowly. Microsomes from PB-treated animals demethylated the secondary amine, MAB, more rapidly than the tertiary amine, DAB. Purified cytochrome P-448 from MC-treated animals catalyzed DAB demethylation very readily but hydroxylation very poorly. The turnover number was 10 times that seen in microsomes from MC-treated animals. Only one of the four cytochrome P-450 fractions isolated from PB-treated animals had significant activity with DAB and the turnover number of one of these (fraction B) was approximately that seen in microsomes. This study supports the concept of selectivity of various isozymes of cytochrome P-450 for the different steps in phase I metabolism of DAB. Furthermore, it is apparent that the association of certain inhibitors with specific isozymes of cytochrome P-450 is a generalization that requires qualification in terms of the substrates(s) involved.