Attempt to map the receptor binding sites of human follicle‐stimulating hormone using disulfide peptides of its β‐subunit indicates major involvement of the regions around disulfide bonds Cys28–Cys82 and Cys32–Cys84 in receptor binding of the hormone

Abstract: Follicle‐stimulating hormone (FSH) is a heterodimeric glycoprotein hormone secreted by the anterior pituitary. It plays a very important role in folliculogenesis in females and is responsible for spermatogenesis in males. The α‐subunit which is common within a species and the β‐subunit which is hormone‐specific are held together by noncovalent association. This association is very essential for the biological activity of the hormone. Each of these subunits are highly cross‐linked by disulfide bonds which appear to stabilize the tertiary structures required for the noncovalent association of the subunits to generate hormonal activity. This study was initiated to delineate the role of the disulfide bonds of hFSHβ in receptor binding of the hormone. Five intermolecular and one intramolecular disulfide peptides corresponding to the disulfide bonds found in hFSHβ were synthesized and screened along with their linear counterparts, for their ability to competitively inhibit the radiolabelled [¹²⁵I]hFSH from binding to the FSH receptor containing membranes from the testis of immature rats. The disulfide peptides Cys²⁸–Cys⁸² and Cys³²–Cys⁸⁴ were found to be the most potent in inhibiting radiolabelled hFSH from binding to its receptor. The results suggest the involvement of the regions around disulfide bonds Cys²⁸–Cys⁸² and Cys³²–Cys⁸⁴ in receptor binding of the hormone. The studies also suggest the involvement of βL2 and βL3 loop regions in receptor binding of the hormone. This study is the first of its kind to use disulfide peptides rather than linear peptides to map the receptor binding regions of hFSH.