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.