A SYNTHETIC PEPTIDE CORRESPONDING TO HUMAN-FSH-BETA-SUBUNIT 33-53 BINDS TO FSH RECEPTOR, STIMULATES BASAL ESTRADIOL BIOSYNTHESIS, AND IS A PARTIAL ANTAGONIST OF FSH

We have previously shown that hFSH-β 34-37 (KTCT) and 49-52 (TRDL) inhibit binding of 125I-hFSH to FSH receptor in calf testis membranes and that hFSH-β 33-53, which encompasses these tetrapeptides, inhibits binding with increased potency. hFSH-β 33-53 rapidly dimerizes under conditions utilized in the receptor binding assay (pH 7.5) so that the binding inhibition reported earlier was due to the hFSH-β 33-53 dimer rather than the monomer. At pH 6.5, conversion to dimer does not occur, and binding inhibition could be unequivocally attributed to the monomer. Radioiodinated and alkylated hFSH-β 33-53 binds to the FSH receptor with a Kd = (5.5 ± 1.4) × 10−5 M. The biological activity of hFSH-β 33-53 was assessed by its ability to affect the conversion of androstenedione to estradiol in rat Sertoli cells cultures. FSH-β 33-53 behaved as a partial antagonist of the FSH-induced estradiol synthesis. The required incubation medium, however, contains cystine as well as cysteine, which rapidly forms a hFSH-β Cys-(51)-S-S-Cys derivative at the pH of the incubation, 7.4. When hFSH-β 33-53 was converted either to the hFSH-β Cys(51)-S-S-Cys or to a carboxymethylated derivative, inhibition of FSH-induced estradiol synthesis still was observed. This result demonstrates that the free R-SH group at Cys51 is not responsible for the inhibition. FSH-β 33-53 also significantly stimulated basal levels of estradiol synthesis, but not to maximal levels observed with FSH (partial agonist). Neither the carbohydrate content of hFSH-β nor the α subunit of FSH appears to be essential for signal transduction and expression of the hormone effect of FSH-β 33-53. © 1990, American Chemical Society. All rights reserved.