Dehydroepiandrosterone synthesis by the fetal foal and its importance as an oestrogen precursor

The gonads of the fetal horse were found to be relatively devoid of 3 beta-hydroxysteroid dehydrogenase and other enzymes which metabolize dehydroepiandrosterone (DHA). In short-term in-vitro incubation experiments fetal liver converted DHA to the potential equilin precursor, 7 alpha-hydroxy DHA. DHA was converted to oestrone when incubated with extracts of horse placenta but 7 alpha-hydroxy DHA was not converted to equilin. Levels of DHA measured in peripheral blood of mares throughout pregnancy paralleled those of equilin and oestrone, and DHA concentrations fell rapidly after fetal gonadectomy, as did those of equilin and oestrone. Administration of [4-(14)C]-DHA and [7-3H]DHA to the fetus resulted in the incorporation of both 14C and 3H into maternal urinary oestrone but neither isotope was present in urinary equilin. These findings confirm the role of fetal DHA as a precursor of oestrone, but do not support the previously suggested role of the fetal liver in the synthesis of equilin. They are, however, compatible with the hypothesis that equilin is formed via a pathway which diverges from the terpenoid steroid synthetic route before DHA.