ACTIVITY OF STEROID C-17,20 LYASE IN THE OVINE PLACENTA: EFFECT OF EXPOSURE TO FOETAL GLUCOCORTICOID

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Abstract
SUMMARY: Microsomal fractions isolated from post-partum ovine placentae catalysed the synthesis of [3H]oestrone and [3H]oestradiol from [3H]17α-hydroxyprogesterone and NADPH; oestrone and oestradiol were formed in a ratio of approximately 50:1. The expected intermediate, [3H]androstenedione, did not accumulate during these incubations but was shown by trapping experiments to be the intermediate involved. Mean ( ± s.d.) yields of [3H]oestrone (% conversion of substrate) during incubation for 1 h of placentae from five animals in late pregnancy before the onset of labour, from five animals which delivered spontaneously at term and from four animals in which labour was induced by administration of dexamethasone to the foetus were: in tissue obtained before labour, 3·2 ± 0·44; in tissue obtained after the spontaneous onset of labour, 20·6 ± 10·2 (P < 0·01) and in tissue obtained after dexamethasone-induced labour, 24·4 ± 2·13 (P < 0·001). This increase in oestrone synthesis suggests activation of steroid C-17,20 lyase, since this is the step limiting the rate of synthesis of oestrone in vitro. The enzyme is probably activated by foetal glucocorticoid. The findings are discussed in relation to the site of synthesis of oestrogens which in the sheep increase in concentration in the peripheral circulation at term, and with reference to a possible mechanism by which foetal glucocorticoid may control the onset of labour in this species.