ROLE OF ISOZYMES OF CYTOCHROME-P-450 IN THE METABOLISM OF N,N-DIMETHYL-4-AMINOAZOBENZENE IN THE RAT

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  • 1 January 1982
    • journal article
    • research article
    • Vol. 10  (2) , 102-109
Abstract
The metabolism of N,N-dimethyl-4-aminoazobenzene (DAB) was investigated in vitro by use of hepatic 10,000 g 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 10,000 g 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 1 of the 4 cytochrome P-450 fractions isolated from PB-treated animals had significant activity with DAB, and the turnover number of 1 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. 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 substrate(s) involved. [DAB is a carcinogen.].