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The mouse placenta possesses a soluble oestrogen sulphotransferase activity which increases markedly from at least 12 days of gestation until term. At about 16 days of gestation, a similar activity is found in the uterus. This activity also increases until term and disappears rapidly post partum. The uterine enzyme activity appears to require the presence of the foetal unit for its onset, since unoccupied horns, whether their endometrial stromal cells are differentiated to decidual cells or not, are essentially devoid of it. Uterine cytosols from non-pregnant mice are also inactive in this respect. In late gestation, the uterine sulphotransferase is confined to the decidua basalis, the areas to which the placentas are attached. The sulphotransferase(s) of placenta and uterus has an absolute requirement for 3'-phosphoadenosine 5'-phosphosulphate, and possesses little activity in the absence of exogenous thiol groups. Stimulation is also seen in the presence of Mn2+, Mg2+ or Ca2+. Oestrone and oestradiol, and to a lesser degree oestriol, are substrates for the enzyme(s), whereas testosterone, cortisol and dehydroepiandrosterone are not. Oestrone and oestradiol at higher concentrations (1.0-1.5 microM) completely inhibit the enzyme(s). These enzymes could play a role in altering tissue concentrations of active oestrogens during gestation in the mouse. Oestrogen sulphotransferase activity is low or absent in reproductive tissues of the pregnant rat.

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