Investigations on the use of C-21-steroids as precursors for placental oestrogen synthesis in the cow

Summary

Cotyledonary homogenates from 220 and 270 day pregnant and term cows were incubated (NADPH-and NAD⁺-regenerating System) with ³H-pregnenolone and ³H-progesterone, respectively. Identification of metabolites was based on Separation on HPLC and the respective retention times. On days 220/270 the major metabolite formed after incubation with ³H-pregnenolone was progesterone, followed by 17a-hydroxyprogesterone and dehydroepiandrosterone/17a-hydroxypregnenolone; the formation of estrone was low (up to 6%), while it was the major metabolite after incubation of pregnenolone with term placenta. At all stages of pregnancy investigated, the only metabolite found after incubation with ³H-progesterone was 17a-hydroxyprogesterone. These data provide evidence that 17a-hydroxyprogesterone is the endpoint of Steroid biosynthesis in the bovine placenta along the ?4-route and that oestrogen synthesis follows the ?5-pathway. Based on the high avctivity of 3ß-hydroxysteroiddehydrogenase/?_5/4-iso-merase also found on days 220/270, the key enzyme al-lowing for placental oestrogen synthesis in the cow seems to be cytochrome P450cl7a. Thus the Situation in the cow is similar to that in the sheep and the increased turnover of pregnenolone into estrone may explain the decrease of placental progesterone production in the cow towards the end of gestation.

Key words

placenta - steroidbiosynthesis - cow