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Inhibitory effect of carbon monoxide on the N- and ring-hydroxylation of 2-acetamidofluorene by hamster hepatic microsomal preparations.

ABSTRACT: The effect of CO on N-, 3-, 5- and 7-hydroxylation of 2-acetamidofluorene by liver microsomal fractions from control and 3-methylcholanthrene-pretreated hamsters was studied. All hydroxylations were inhibited by CO, but the degree of inhibition was different for each hydroxylation. The ratios of CO to O(2) needed for 50% inhibition of the N-, 3-, 5- and 7-hydroxylations by control preparations were 8.0:1, 8.2:1, 4.2:1 and 7.1:1 respectively and by preparations from treated animals were 4.2:1, 8.9:1, 2.3:1 and 3.2:1 respectively. These results are discussed in terms of the possible presence of more than one type of cytochrome P-450 involved in hydroxylations of 2-acetamidofluorene by liver microsomal fractions from both control and pretreated hamsters.

SUBMITTER: Lotlikar PD

PROVIDER: S-EPMC1168511 | biostudies-other | 1974 Nov

REPOSITORIES: biostudies-other

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Project description:Rat hepatic cytochrome P-450 form 3 (testosterone 7 alpha-hydroxylase; P-450 gene IIA1) and P-450 form RLM2 (testosterone 15 alpha-hydroxylase; P-450 gene IIA2) are 88% identical in primary structure, yet they hydroxylate testosterone with distinct and apparently unrelated regioselectivities. In this study, androstenedione and progesterone were used to assess the regioselectivity and stereospecificity of these two P-450 enzymes towards other steroid substrates. Although P-450 RLM2 exhibited low 7 alpha-hydroxylase activity with testosterone or progesterone as substrate (turnover number less than or equal to 1-2 nmol of metabolite/min per nmol of P-450), it did catalyse androstenedione 7 alpha-hydroxylation at a high rate (21 min-1) which exceeded that of P-450 3 (7 min-1). However, whereas P-450 3 exhibited a high specificity for hydroxylation of these steroids at the 7 alpha position (95-97% of total activity), P-450 RLM2 actively metabolized these compounds at four or more major sites including the nearby C-15 position, which dominated in the case of testosterone and progesterone. The observation that androstenedione is actively 7 alpha-hydroxylated by purified P-450 RLM2 suggested that this P-450 enzyme might make significant contributions to microsomal androstenedione 7 alpha-hydroxylation, an activity that was previously reported to be associated with immunoreactive P-450 3. Antibody inhibition experiments were therefore carried out in liver microsomes using polyclonal anti-(P-450 3) antibodies which cross-react with P-450 RLM2, and using a monoclonal antibody that is reactive with and inhibitory towards P-450 3 but not P-450 RLM2. P-450 3 was thus shown to catalyse only around 35% of the total androstenedione 7 alpha-hydroxylase activity in uninduced adult male rat liver microsomes, with the balance attributed to P-450 RLM2. The P-450-3-dependent 7 alpha-hydroxylase activity was increased to approximately 65% of the total in phenobarbital-induced adult male microsomes, and to greater than 90% of the total in untreated adult female rat liver microsomes. These observations are consistent with the inducibility of P-450 3 by phenobarbital and with the absence of P-450 RLM2 from adult female rat liver respectively. These findings establish that P-450 RLM2 and P-450 3 can both contribute significantly to microsomal androstenedione 7 alpha-hydroxylation, thus demonstrating that the 7 alpha-hydroxylation of this androgen does not serve as a specific catalytic monitor for microsomal P-450 3.

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Inhibitory effect of carbon monoxide on the N- and ring-hydroxylation of 2-acetamidofluorene by hamster hepatic microsomal preparations.

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