Purification and binding site characterization of the circulating trophoblastic androgen-binding protein

The trophoblastic androgen-binding protein (t-ABP) was purified 150-fold with a recovery of 51% from serum of patients with hydatidiform mole using various chromatographic techniques, successively affinity on concanavalin A, ion exchange on QAE-Sephadex A 50, gel filtration on Sephadex G 200 and chromatofocusing. The chromatofocusing step eliminated any trace of contaminating sex-hormone binding globulin. Competitive binding experiments using the purified material, [3H]dihydrotestosterone and various steroid derivatives allowed an attempt at characterizing the steroid-binding site of the protein. This latter possess respectively hydrophilic domains facing position 2 and 17 of the steroid molecule, a hydrophilic and proton donor sequence facing position 3 of the steroid molecule, hydrophobic regions facing positions 6, 11 and 16 of the steroid molecule and electron donor domains facing positions 1 and 6 of the steroid molecule. These characteristics are compared with those of the sex hormone-binding globulin (SHBG), rat epididymis androgen-binding protein (RABP) and rat prostate cytoplasmic androgen receptor (CAR) binding sites, respectively. The results of this specificity study indicate that the t-ABP behaves very similarly to CAR, although major differences are likely to exist between the binding sites of both proteins, particularly in the protein domains facing C-1 and C-2 of the steroid.