A 63-kDa protein with androgen-binding activity is not from the androgen receptor

We have found a new protein in the heart of rat and mice that can be selectively and covalently labelled with the synthetic androgen analog mibolerone. Binding is specific as it can be displaced by excess radioinert ligand. The protein is prominently expressed in liver, kidney, and heart, but not in skeletal muscle. It is water soluble and found in the cytosol. Under denaturing conditions it has a molecular weight of 63,000 and appears on two-dimensional gels with an isoelectric point of 6.3. The protein's affinity for androgen is lower than that of the androgen receptor and it is about 100-fold more abundant than the receptor in the heart. Expression of the protein is not induced by androgen. The presence of this protein in testicular feminization (tfm) mice with a genetical defect of the androgen receptor rules out that it is the androgen receptor or a portion thereof. The biological role of this protein is not yet known.