Methylcholanthrene: A possible pseudosubstrate for adrenocortical 17α-hydroxylase and aryl hydrocarbon hydroxylase

In cultured bovine adrenocortical cells, loss of 17α-hydroxylase activity was observed after incubation with 3-methylcholanthrene (3-MC). The suppression of 17α-hydroxylase by 3-MC was rapid (50% loss of activity in 10 hr at 1 μM 3-MC), did not exhibit a lag period, and was not affected by cycloheximide. Direct effects of 3-MC on 17α-hydroxylase were observed only at high concentrations, but the concentration for 50% loss of activity was 0.3 μM when 3-MC was added for 24 hr prior to assay of 17α-hydroxylase. High concentrations (to 40 μM) of substrate (progesterone), did not affect the loss of activity due to 3-MC. Loss of 17α-hydroxylase activity was specific; 11β-hydroxylase was unaffected and cell growth was unaltered. However, 22-amino-23,24-bisnorchol-5-en-3β-ol, an inhibitor of 17α-hydroxylase, partially prevented the loss of 17α-hydroxylase at 1-30 nM. 3-MC is thought to induce cytochrome P-450s via a receptor with high affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). TCDD was without effect on 17α-hydroxylase over the range of 10 nM to 10 μM. Benz[a]anthracene, 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene, chrysene, and methylphenanthrenes suppressed 17α-hydroxylase at high concentrations (10-50 μm for 50% loss of activity). Some steroids that lack a substituent at position 17 also caused loss of 17α-hydroxylase. Like 17α-hydroxylase, bovine adrenocortical cell AHH was found to be suppressed by exposure to 3-MC. Compounds that caused loss of 17α-hydroxylase caused loss of AHH, with a similar order of potency and at similar concentrations. Suppression of AHH by 3-MC did not require protein synthesis and was prevented by an inhibitor of enzymatic activity, α-naphthoflavone. This implies a degree of similarity of the cytochrome P-450s for 17α-hydroxylase and adrenal AHH, but the activities were shown to be likely due to different enzymes. The suppression of 17α-hydroxylase and AHH by 3-MC appears not to occur by a receptor-mediated mechanism but to be similar to the suppression of 11β-hydroxylase and 21-hydroxylase by steroid pseudosubstrates previously observed.