Steroid 16 alpha-hydroxylase from human fetal liver; inhibition by steroids

The 16 alpha-hydroxylase system in fetal liver which used dehydroepiandrosterone (DHA) or pregnenolone as substrate, was apparently inhibited by various endogenous and synthetic steriods: DHA, pregnenolone, their sulfates, androstenediol, androstenetriol, estrone, estradiol-17 beta, ethynylestradiol and chlormadinone-acetate. The inhibition constants (Ki) towards DHA were as follows; pregnenolone 22 muM, DHA-sulfate 13 muM, pregnenolone-sulfate 21 muM, androstenediol 16 muM, androstenetriol 53 muM estrone 32 muM, estradiol-17 beta 74 muM, ethynylestradiol 22 muM and chlormadinone-acetate 27 muM. The Ki values towards pregnenolone were DHA 6.3 muM, DHA-sulfate 8.3 muM, pregnenolone-sulfate 3.9 muM, androstenediol 8.7 muM, androstenetriol 14.7 muM, estradiol-17 beta 15.4 muM and ethynylestradiol 16.0 muM, respectively. The reaction products, 16 alpha OH-DHA and 16 alpha OH-pregnenolone, showed little inhibitory effect upon the 16 alpha-hydroxylase.