title: Cytochrome P-450 and the aromatization of 16alpha-hydroxytestosterone and androstenedione by human placental microsomes.
abstract: When measured in vitro using human placental microsomal preparations, the aromatization of 16alpha-hydroxytestosterone to estriol and androstenedione to estrone and estradiol proceeds at almost identical initial rates. Important differences between 16alpha-hydroxytestosterone and adrostenedione aromatization are evident, however. While substantial findings have implicated cytochrome P-450 in placental aromatization, the aromatizaiton of androstenedione is insensitive to CO although it is competitively inhibited by metyrapone. 16alpha-Hydroxytestosterone aromatization, in contrast, is inhibited 50-60% by CO and is strongly inhibited by metyrapone. 16alpha-hydroxytestosterone aromatization is strongly inhibited in a competitive manner by androstenedione, while 16alpha-hydroxytestosterone has essentially no effect on androstenedione aromatization, althogh at very high 16alpha-hydroxytestosterone concentrations (65 muM) and subsaturating androstenedione concentrations, 16alpha-hydroxytestosterone appears to noncomptitively inhibit androstenedione aromatization. The apparent Km for the aromatization of androstenedione is 95 nM and for 16alpha-hydroxytestosterone, 7 muM. Both androstenedione and 16alpha-hydroxytestosterone cause type I spectral perturbations associated with binding to cytochrome P-450 when added to placental microsomes; however, the deltaA390-420 is twice as great in response to saturating amounts of androstenedione than in response to 16alpha-hydroxytestosterone. If androstenedione is added to 16alpha-hydroxytestosterone, the same spectral change as that caused by androstenedione alone is expressed. The apparent spectral dissociation constant for androstenedione is 93 nM while for 16alpha-hydroxytestosterone it is 11muM; both essentially the same as the comparable apparent Kms for aromatization. The evidence suggests the presence of two aromatase P-450's in human placenta.
mesh_terms: D000735:Androstenedione / Q000378:metabolism*; D001141:Aromatase / Q000378:metabolism; D002248:Carbon Monoxide / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005260:Female; D006801:Humans; D006915:Hydroxytestosterones / Q000378:metabolism*; D007700:Kinetics; D008797:Metyrapone / Q000494:pharmacology; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism*; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D013053:Spectrophotometry; D013739:Testosterone / Q000031:analogs & derivatives*
chemical_list: D006915:Hydroxytestosterones; D013739:Testosterone; D000735:Androstenedione; D002248:Carbon Monoxide; D003577:Cytochrome P-450 Enzyme System; D001141:Aromatase; D008797:Metyrapone