Protamine-precipitated estrogen receptor: a solid-phase ligand exchange assay.

Cytoplasmic estrogen receptor can exist either free (R) or bound to estradiol-17beta (RE). Both forms can be precipitated from cytosols by protamine sulfate. After protamine precipitation R binds 3-H-estradiol-17beta quantitatively at either 0 degrees or 30 degrees, while precipitated RE binds 3-H-estradiol-17beta only at 30 degrees by exchanging with previously bound hormone. Using these observations, we have developed a method for separate determination of both cytoplasmic R and RE. This method should also be applicable for assay of other steroid receptors, especially in cases where interfering components are present in the whole cytosol.