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

The research article focuses on the biosynthesis process of equilin and equilenin in pregnant mares, which is demonstrated to be independent of cholesterol and follows a different path from estrone and estradiol. The scientists synthesized a potential substrate, 3-Hydroxy-3,5,7-androstatrien-17-one, and evaluated its conversions and distribution. They identified the biochemical pathways leading to equilin, equilenin, and Heard’s ketone, as well as the stereospecific hydrogen elimination involved, refuting a previous notion about Heard’s ketone’s origins.

Research Objectives

• The study aims to understand the biosynthesis process of equilin and equilenin in the pregnant mare. This process is distinct from the biosynthesis of estrone and estradiol and does not rely on cholesterol.
• The researchers also aimed to identify the exact precursors, intermediates, and the stereomechanism of equine placental aromatization.

Experimental Approach

• The team synthesized [1,2-3H, 4-14C]3-Hydroxy-3,5,7-androstatrien-17-one, which they believed to be the potential substrate.
• They treated this substrate with horse and human placental microsomes and studied the distribution of 3H.
• The researchers used biochemical and chemical derivatization methods to analyze the transformations and distribution.

Key Findings

• The substrate, after exposure to horse placental microsomes, was converted to equilin, equilenin, and Heard’s ketone. However, when treated with human placental microsomes, the substrate was only converted into equilin.
• They also found a stereospecific hydrogen elimination (17 beta,2 beta-cis) during aromatization, identical to the elimination seen in estrone and estradiol biosynthesis.
• The production of Heard’s ketone involved none of the four hydrogens at C-1 and C-2, contradicting the existing theory that Heard’s ketone arises from equilenin by the reduction of the ring-A.