Relationship between ligand structure and affinity for androgen binding protein

Androgen binding protein (ABP) is an FSH stimulated secretory product of the Sertoli cell of the testis1. Under the dual control of FSH and androgen, ABP is secreted into the blood and seminiferous tubules. Once in the seminiferous tubule, ABP travels through the rete testis to the caput epididymis. It is in the initial segment and the caput epididymis that ABP is largely reabsorbed out of the luminal fluid. In the rat ABP binds testosterone and dihydrotestosterone (DHT), and is presumed to help maintain the high intraluminal concentration of these androgens in the seminiferous tubules and to transport them to the initial segment of the epididymis. This sequelae of events suggested that ABP may play a role in the androgen-dependent maturation of epididymal spermatozoa and male fertility in general. ABP is also found in much lower concentration in the serum of the rat, rabbit and other species. The study of serum ABP is confounded by a similar but not identical liver protein, testosterone-oestrogen binding globulin (TeBG), in many species but not the rat2. Serum hABP can be distinguished from serum hTeBG by concanavalin A chromatography3. Rat ABP has a molecular weight of about 90000, an isoelectric point of 4.6-4.7 pH and an R f of approximately 0.54 by polyacrylamide gel electrophoresis (PAGE)2. It is composed of protomers of 47000 and 41000 daltons. Although the physical-chemical properties of ABP have been well defined, the biological function remains to be elucidated.