title: Kinetics of inhibition of estrogen 2-hydroxylase by various haloestrogens.
abstract: Inhibitors of estrogen 2-hydroxylase can be utilized in studying the kinetics of this cytochrome P450 enzyme complex and in elucidating the structural requirements of the active site. The conversion of estrogens to 2-hydroxyestrogens in rat liver microsomal preparations was examined using two radiotracer assays, the conversion of [4-14C]-estradiol to [4-14C]-2-hydroxyestradiol and the release of 3H2O from [2-3H]-estradiol. Using the microsomal fraction from male rat liver, the apparent Km for the substrate estradiol was 2.2 microM. Competitive inhibition was observed for 2-halo- and 2,4-haloestrogens (apparent Ki's of 1.6 to 3.7 microM), while 4-haloestrogens did not produce normal inhibition patterns. Employing female rat liver microsomes in which nonclassical enzyme kinetics was observed, the synthetic steroids increased the sigmoidal character of the velocity curve. Multiple inhibition studies with 2-haloestrogens and 4-haloestrogens with the male rat liver microsomal fraction indicated that these compounds are mutually exclusive inhibitors of the 2-hydroxylase activity.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002250:Carbon Radioisotopes; D019363:Cytochrome P-450 CYP1A1; D004958:Estradiol / Q000031:analogs & derivatives*; D005260:Female; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D051381:Rats; D011919:Rats, Inbred Strains; D013250:Steroid Hydroxylases / Q000037:antagonists & inhibitors*; D013329:Structure-Activity Relationship
chemical_list: D002250:Carbon Radioisotopes; D004958:Estradiol; D013250:Steroid Hydroxylases; D019363:Cytochrome P-450 CYP1A1; C020120:estrogen 2-hydroxylase