Oestrogen receptor of calf mammary gland. Purification by use of sodium bromide and heparin-sepharose.

1. Calf mammary-gland cytosol apparently has a single oestrogen receptor capable of auto- and/or hetero-association of varying complexity. Computation of the dissociation constant for oestradiol-17beta gives Kd = 0.5 nM. The number of binding sites is 40 fmol/mg of cytosol protein. The oestrogen receptor in the presence of NaBr, a chaotropic salt that inhibits the interaction of receptor with other cytosol components, sediments through sucrose density gradients as a single sharp peak at 4S, and it has a Stokes radius of 3.4 nm measured by gel filtration. 2. A large-scale purification procedure of the calf mammary-gland oestrogen receptor based on the inhibition of receptor aggregation by NaBr and interaction with heparin-Sepharose is reported. The receptor is purified more than 1500-fold over that in the 27,000g supernatant of the homogenate, with a 30% yield. In 'low-salt' buffer the purified receptor sediments through sucrose gradients at 4S and the Stokes radius, measured by gel filtration in the presence of heparin, is 3.4 nm. The mol.wt computed from these values is about 60,000, and the frictional ratio is 1.3.