A SYNTHETIC PEPTIDE CORRESPONDING TO GLYCOPROTEIN HORMONE ALPHA-SUBUNIT RESIDUES 32-46 INHIBITS GONADOTROPIN BINDING TO RECEPTOR

A synthetic peptide strategy was used to study structure-function rela tionships between residues 32 to 46 of the glycoprotein hormone alpha subunit (GPH alpha) and the testicular follicle-stimulating hormone (F SH) and luteinizing hormone (LH/hCG) receptors. A peptide amide corres ponding to this region [GPH-alpha-(32-46)] inhibited boil? I-125-hFSH and I-125-hCG binding to their respective calf testis membrane recepto rs. The concentration at which GPH-alpha-(32-46) peptide amide inhibit ed FSH binding by 50% (IC50) was 36 mu M, and for hCG it was 54 mu M. GPH-alpha-(32-46) peptide amide also inhibited FSH-stimulated estradio l biosynthesis in cultured I nr Sertoli cells. in order to determine t he involvement of individual residues within this region of the glycop rotein hormone alpha subunit iii receptor binning inhibitory activity, truncated and alanine-substituted peptide analogs were synthesized an d rested in both FSH and hCG radioligand receptor competition assays. Based on the relative potency of each peptide, we conclude that Phe-33 , Arg-35, Arg-42, Ser-43 and Lys-44 may be important, and Cys-32 is re quired, for inhibition of FSH and hCG binding to their respective rece ptor. Our results demonstrate involvement of the glycoprotein hormone alpha-subunit in receptor binding, identify residues 32 to 46 as a rec eptor binding domain, and define the relative importance of specific r esidues within this region of the alpha subunit for hormone-receptor i nteraction.