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