Equilin and equilenin biosynthesis. Stereochemistry of aromatization of 3-hydroxy-3,5,7-androstatrien-17-one by horse placenta.

The metabolic pathway leading to equilin and equilenin biosynthesis in the pregnant mare is different from that of estrone and estradiol and it is apparently cholesterol-independent. The precise precursors and intermediates and the stereomechanism of equine placental aromatization have not been established. [1,2-3H, 4-14C]3-Hydroxy-3,5,7-androstatrien-17-one was synthesized as a potential substrate and the 3H-distribution was analyzed by biochemical and chemical derivatization methods. The substrate was converted to equilin, equilenin and Heard's ketone by horse placental microsomes with a sp. act. of 74, 18 and 2.8 pmol/h/mg, respectively, and only to equilin by human placental microsomes with a rate of 26 pmol/h/mg. Analysis of the loss of 3H-labeling during aromatization showed the stereospecific 17 beta,2 beta-cis hydrogen elimination for equine estrogen biosynthesis both by horse and human placental microsomes. This is the same as for estrone and estradiol biosynthesis by both placentas. The biosynthesis of Heard's ketone, a non-phenolic ring-B aromatic C18 steroid, by horse placental microsomes was found to involve none of the four hydrogens at C-1 and C-2. This refutes the previous postulate that Heard's ketone arises from equilenin by reduction of the ring-A.