LOSS OF BIOLOGICAL-ACTIVITY OF HUMAN CHORIONIC-GONADOTROPIN (HCG) BY THE AMINO-ACID SUBSTITUTION ON THE CMGCC REGION OF THE ALPHA-SUBUNIT

In order to study the bioactive sites of the glycoprotein hormones, we have prepared five point mutants on the CMGCC (Cys(28)-Met(29)-Gly(30 )-Cys(31)-Cys(32)) legion of the human alpha-subunit by using site-dir ected mutagenesis. Each mutant human chorionic gonadotropin (hCG) agr; cDNA and a wild-type hCG beta cDNA were transcribed by T3 RNA polymer ase, and the mixture of the hCG alpha mRNA and hCG beta mRNA was micro injected into Xenopus laevis oocytes. All five mutant hCGs produced in oocyte culture supernatants were detected as immunoreactive forms by enzyme immunoassay. In contrast, four mutants (Cys(28) --> Tyr(28), Gl y(30) --> Arg(30), Ala(30), Asp(30)) were devoid of biological activit y in vitro bioassay using the production of testosterone with mouse Le ydig cells. These results indicate that the CMGCC region in the alpha- subunit, particularly the cysteine residue at position 28 and the glyc ine residue at position 30, plays an important role in the biosynthesi s of glycoprotein hormones.