Androgen-binding protein
Androgen-binding protein (ABP) is a glycoprotein (beta-globulin) produced by the Sertoli cells in the seminiferous tubules of the testis that binds specifically to testosterone (T), dihydrotestosterone (DHT), and 17-beta-estradiol. Because ABP binds to T and DHT, these hormones are made less lipophilic and become concentrated within the luminal fluid of the seminiferous tubules. The higher levels of these hormones enable spermatogenesis in the seminiferous tubules and sperm maturation in the epididymis. ABP has the same amino acid sequence as sex hormone-binding globulin (SHBG); the difference is the site of production and the  addition of different sugar moieties. ABP contains 403 amino acids, resulting in a molecular weight of 44,533. Its gene is located on chromosome 17. ABP’s production is regulated under the influence of FSH on Sertoli cells, enhanced by insulin, retinol, and testosterone. The human ABP is not to be confused with a mouce secretoglobin of the same name, which is produced in the saliva.