title: Methylcholanthrene: a possible pseudosubstrate for adrenocortical 17 alpha-hydroxylase and aryl hydrocarbon hydroxylase.
abstract: In cultured bovine adrenocortical cells, loss of 17 alpha-hydroxylase activity was observed after incubation with 3-methylcholanthrene (3-MC). The suppression of 17 alpha-hydroxylase by 3-MC was rapid (50% loss of activity in 10 hr at 1 microM 3-MC), did not exhibit a lag period, and was not affected by cycloheximide. Direct effects of 3-MC on 17 alpha-hydroxylase were observed only at high concentrations, but the concentration for 50% loss of activity was 0.3 microM when 3-MC was added for 24 hr prior to assay of 17 alpha-hydroxylase. High concentrations (to 40 microM) of substrate (progesterone), did not affect the loss of activity due to 3-MC. Loss of 17 alpha-hydroxylase activity was specific; 11 beta-hydroxylase was unaffected and cell growth was unaltered. However, 22-amino-23,24-bisnorchol-5-en-3 beta-ol, an inhibitor of 17 alpha-hydroxylase, partially prevented the loss of 17 alpha-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 17 alpha-hydroxylase over the range of 10 nM to 10 microM. Benz[a]anthracene, 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene, chrysene, and methylphenanthrenes suppressed 17 alpha-hydroxylase at high concentrations (10-50 microM for 50% loss of activity). Some steroids that lack a substituent at position 17 also caused loss of 17 alpha-hydroxylase. Like 17 alpha-hydroxylase, bovine adreno-cortical cell AHH was found to be suppressed by exposure to 3-MC. Compounds that caused loss of 17 alpha-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, alpha-naphthoflavone. This implies a degree of similarity of the cytochrome P-450s for 17 alpha-hydroxylase and adrenal AHH, but the activities were shown to be likely due to different enzymes. The suppression of 17 alpha-hydroxylase and AHH by 3-MC appears not to occur by a receptor-mediated mechanism but to be similar to the suppression of 11 beta-hydroxylase and 21-hydroxylase by steroid pseudosubstrates previously observed.
mesh_terms: D000302:Adrenal Cortex / Q000201:enzymology; D000324:Adrenocorticotropic Hormone / Q000494:pharmacology; D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors*; D002478:Cells, Cultured; D003577:Cytochrome P-450 Enzyme System / Q000032:analysis; D008748:Methylcholanthrene / Q000378:metabolism / Q000494:pharmacology*; D008797:Metyrapone / Q000494:pharmacology; D014176:Protein Biosynthesis; D013254:Steroid 17-alpha-Hydroxylase / Q000037:antagonists & inhibitors*; D013250:Steroid Hydroxylases / Q000037:antagonists & inhibitors*; D013256:Steroids / Q000494:pharmacology; D013997:Time Factors
chemical_list: D013256:Steroids; D008748:Methylcholanthrene; D000324:Adrenocorticotropic Hormone; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; D013254:Steroid 17-alpha-Hydroxylase; D008797:Metyrapone