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DEMONSTRATION OF AN ACTIVE C17-C20 LYASE IN THE SHEEP PLACENTA

Tenovus Institute for Cancer Research, Welsh National School of Medicine, The Heath, Cardiff CF4 4XX

(Received 27th November 1974)

Studies on the sheep have provided more information concerning the hormonal changes that precede parturition than those on any other mammal. The implications of some of the findings are uncertain but a composite picture is gradually emerging.

The ability of sheep placenta to convert C₁₉-steroids to oestrogens (Ainsworth & Ryan, 1966; Pierrepoint, Anderson, Griffiths & Turnbull, 1970a) and their sulphates (Pierrepoint, Anderson, Harvey, Turnbull & Griffiths, 1971) has been well demonstrated although neither a source nor the species of such potential precursors has been found.

Two of the most active C₂₁-steroid-metabolizing enzymes in the sheep placenta are the 20-hydroxysteroid dehydrogenase and the 3β-hydroxysteroid dehydrogenase-⁴-⁵-isomerase with the consequent formation of large amounts of progesterone and 20-dihydroprogesterone (Ainsworth & Ryan, 1967; Pierrepoint et al., 1970b;