Demonstration of an active C17-C20 lyase in the sheep placenta.

(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\g=a\-hydroxysteroiddehydrogenase and the 3\g=b\-hydroxysteroid dehydrogenase-\g=D\4-\g=D\5-isomerase with the consequent formation of large amounts of progesterone and 20\g=a\-dihydroprogesterone(Ainsworth & Ryan, 1967; Pierrepoint et al., 1970b; Pierrepoint, Anderson, Turnbull & Griffiths, 1973). Steroid 17\g=a\-hydroxylaseand C21-steroid C17-C20 lyase activity have been shown, although the products of their combined actions amounted to less than 1% of the precursors (Pierrepoint et al., 1970b, 1973). Because of these findings and the current acceptance that the 'desmolase' enzyme cannot act without the prior 17a-hydroxylation of the C21-steroid (Slaunwhite & Samuels, 1956; Lynn & Brown, 1958; Ellis & Berliner, 1965), the ability of the sheep placenta to metabolize 17a-hydroxylated C21 -steroid substrates was investigated, with particular regard to the formation of C19and C18-products. Placental tissue was removed at hysterotomy from a ewe at 119 days of gestation and transported to the laboratory on ice. A minced preparation (4 g) was added to 24 ml Krebs-Ringer bicarbonate glucose solution containing 10 μ [7a-3H]17a-hydroxypregnenolone (179-5 Ci/mol) and 2 pCi [4-14C]17ahydroxyprogesterone (35-9 Ci/mol), both supplied by Amersham Radiochemical Centre, Bucks. The mixture was shaken in an atmosphere of 95 % 02:5% C02 for 2 hr in a water bath at 39-5°C. After the incubation period, the reaction was stopped by the addition of 30 ml acetone and refrigeration. Metabolism was assessed by the reverse-isotope dilution method. Steroids to be investigated ( 17a-hydroxypregnenolone 17a-hydroxyprogesterone, dehydroepiandrosterone (DHA), androstenedione, testosterone, epitestosterone, oestradiol-17/?, oestradiol-17a, oestrone, DHA sulphate, oestrone sulphate and