Metabolism of C-16-oxygenated steroids by human placenta: the formation of estriol.

Two distinct pathways for the formation of estriol in vitro have recently been described: C-16 hydroxylation of estradiol-17-P by human fetal liver and rat liver (1, 2), and aromatization of A5-androstene-3/?, 16a, 17&triol by human placenta (3). Although the formation in viva of estriol from estradioll and estrone is well known (4), the studies mentioned above present the first convincing data for the direct production of estriol in vitro. The preliminary observation that estriol could be produced by aromatization of a C-16-hydroxylated androgen without involving the classic conversion from estradiol (3), prompted this more extensive study of the metabolism of C-16oxygenated neutral and phenolic steroids by human placenta. The formation of estriol from C-16-hydroxylated androgens has been confirmed and a tentative intermediary role for 16ol-hydroxyestrone can now be made.