glucocorticoid receptor in human embryo fibroblasts i kinetic and physicochemical properties the kinetics of dexamethasone binding to l 809 e cell line cytosol have been investigated by means of the protamine sulfate precipitation assay the kdeq for dexamethasone was 1133 nm binding was specific for glucocorticoids the mean association rate constant k1 was 85 x 105 m1 x min1 and the dissociation rate constant was 46 x 105 min1 at 0 degrees c the concentration of binding sites was 03 pmolmg of cytosol protein binding kinetics were compatible with a model of positive cooperativity the receptor sedimented at 759 s in glycerol gradients by a combination of calibrated ultracentrifugation and polyacrylamide gel electrophoresis a stokes radius of 85 nm a molecular weight of 268 000 daltons and a frictional ratio of 18 were determined in low ionic strength conditions when the cells were incubated with 10 nm 3hdexamethasone for 1 h a more than 90 depletion of cytosol receptor and an equivalent accumulation of nuclear dexamethasonereceptor complexes was observed