C19 steroidal precursors of estrogens.

The pathways of biosynthesis of the estrogens from C19 steroids has been examined by in vitro kinetic experiments using human placental microsomes and two precursors, each labeled with a different radioisotope. The results indicate that under the conditions employed, androstenedione is an obligatory intermediate in the conversion of 3 beta-hydroxy-5-androsten-17-one (dehydroisoandrosterone) or its sulfate, dehydroisoandrosterone sulfate, into estrone and estradiol. The same result was obtained when the microsomal fraction was replaced by either a 1000 x g supernatant preparation from human placenta or a homogenate of placental mitochondria. If alternative pathways involving C19-hydroxylated derivatives of dehydroisoandrosterone or its sulfate exist, they appear to be of minor quantitative significance.