Aromatization of Norethindrone to Ethinyl Estradiol by Human Placental Microsomes*

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Abstract
The interaction of 19-norethindrone [4-estren-17α-ethinyl-17β-ol, 3-one (NET)] with human placental micro-somes was investigated using enzymatic and spectral techniques. The incubation of [6, 7-3H]norethindrone with human placental microsomes, NADPH, and molecular oxygen resulted in the production of ethinyl estradiol [l,3,5-(10)estratrien-17α-ethinyl-3,17β-diol (EE)]. The reaction was linear with respect to time and protein concentration. Androstenedione inhibited the enzymatic aromatization of NET to EE. The product was identified by thin layer chromatography, recrystallization to constant specific activity, and derivative formation. No acid or base was used in any step of product identification. To ensure that spontaneous aromatization of metabolites of NET did not contribute to our results, representative samples were treated with sodium boro-hydride before processing. Sodium borohydride reduces the 4-en-3-one grouping of the A-ring, thereby preventing chemical aromatization. Sodium borohydride treatment did not reduce our observed yields of EE from NET. The addition of NET to a preparation of solubilized, partially purified placental microso-mal cytochrome P-450 yielded a type I cytochrome P-450 binding spectrum. The apparent spectral dissociation constant for NET binding to cytochrome P-450 was 28 μM. These results suggest that NET is enzymatically aromatized to EE by human placental microsomes.