Interaction between steroids and membranes, uptake of steroids and steroid sulphates by resealed erythrocyte ghosts

Reconstituted cells were formed by haemolysis of human erythrocytes in hypotonic media followed by resealing of the membranes in isotonic solution. These so-called “resealed erythrocyte ghosts” could be filled with different protein solutions present in the haemolysing medium. Reconstituted cells containing bovine serum albumin or diluted plasma with a large amount of corticosteroid binding globulin (CBG) were prepared by this method.Uptake of steroids by reconstituted cells was measured using equilibrium dialysis and incubation studies and was compared with the uptake by cells not containing proteins. After washing of the cells containing CBG, the steroid specifically bound by this protein inside the cell could be determined.Free steroids (testosterone, dihydrotestosterone, androstenedione, dehydroepiandrosterone, oestradiol, progesterone, cortisol) penetrated quickly into the reconstituted cells, compared with the rather slow penetration of conjugated steroids (dehydroepiandrosterone sulphate and oestrone 3-sulphate).A difference between the amount of cortisol and progesterone bound to CBG in resealed ghosts was only observed if plasma proteins were also present outside the reconstituted cells. Under these conditions the rate of uptake by binding sites inside the ghost was much slower for cortisol than for progesterone. This was not due to differences in membrane permeability but mainly to different distribution coefficients of the steroids between medium and ghosts.