Steroid metabolism in the yolk sac placenta and endometrium of the tammar wallaby, Macropus eugenii.

Yolk sac and endometrial tissue were obtained from tammar wallabies between 11 and 25 days after the removal of pouch young. Tissues were examined histologically and steroid-metabolizing enzymes were identified by incubation for 3 h at 37 degrees C in Medium 199 containing labelled steroid precursors. Yolk sac membrane (YSM) incubated with labelled pregnenolone produced a small amount of progesterone and pregnanediols; 80.5 +/- 8.4 (S.E.M.) % of the original substrate remained unmetabolized. Labelled androstenedione was metabolized to 5 alpha-androstane-3,17-dione and androsterone, and only 5.8 +/- 3.8% of the original substrate remained at the end of incubation. Incorporation of androstenedione or dehydroepiandrosterone (DHA) into phenolic compounds was low (0.5 +/- 0.1%). There was no evidence for the enzymes, arylsulphatase or sulphotransferase, in YSM. Endometrial tissue from the same animals metabolized pregnenolone, DHA and androstenedione, converted progesterone to androstenedione, and produced aqueous-soluble steroid conjugates. The results demonstrated that YSM contains enzymes associated predominantly with steroid catabolism and with incipient progesterone synthesis. The findings are discussed in relation to the histological appearance of the tissues and compared with placental steroid synthesis in eutherian mammals.