Binding of sex steroid binding protein to plasma membranes of human testis

The existence of a specific binding site for sex steroid binding protein (SBP or SHBG) was detected on plasma membranes prepared from the testis of a patient affected by a variant form of testicular feminization. A binding technique using [¹²⁵I]SBP as a tracer allowed us to identify a single set of binding sites, characterized by a K_d of 1.917 × 10⁻¹¹ M. The maximum number of binding sites was 5.2 fmol/mg membrane protein. Membranes were also prepared from a sample of genital skin from the same patient, but no binding for [¹²⁵I]SBP was detectable. The evidence of the SBP membrane receptor in the testis of a patient affected by Morris syndrome extends our knowledge about the tissue distribution of the SBP receptor and suggests the possible implication of SBP and its recognition system in a disorder related to peripheral androgen insensitivity.