The glycoprotein hormones (LH, hCG, FSH, and TSH) have a common 92-amino acid ɑ-subunit whiph is noncovalently linked to a hormone-specific β-subunit. Synthetic peptides of the ɑ-subunit have been shown to inhibit binding of [12 5I]iodo-hCG to rat ovarian membrane and [12 5I]iodo-TSH to human thyroid membrane preparations. Synthetic overlapping peptides of the ɑ-subunit of hCG were prepared by solid phase techniques and tested in a standard in vitro rat Leydig cell bioassay. Three regions in the a-subunit (ɑl-15, ɑ30-45, and ɑ71-85) were found to stimulate testosterone production. All three regions correlate with inhibition of hCG binding to ovarian receptors, but subtle differences exist between the binding sites and effector sites. These data indicate that the glycoprotein a-subunit has intrinsic bioactivity.
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