Secretion of oestrone sulphate into milk

A Golgi apparatus-enriched fraction obtained from lactating rat mammary glands has been used to investigate the binding transport of oestrone sulphate using radio-labelled ligand. The fraction showed good galactosyl transferase activity and was substantially intact when assayed for lactose synthase activity. Binding sites for the hormone on the Golgi membrane were demonstrated and at 4 degrees C binding was a linear function of concentration over the range 0.2 mol 1(-1) to 25 mmol 1(-1). The hormone was readily displaced by high concentrations of oestrone sulphate but not by oestrone, 17-beta-oestradiol or progesterone. Lysis of the vesicle prior to binding of oestrone sulphate allowed additional hormone to bind but only when lysis conditions were mild. Nevertheless, the hormone could be readily displaced by simple dilution of the reaction medium suggesting that binding was to the cytosolic surface of the membrane. No evidence of the transport of the hormone into the lumen of the vesicle was obtained.