Carboxypeptidase digestion of human chorionic gonadotropin

Native human chorionic gonadotropin (hCG) was resistant to carboxypeptidase digestion even in the presence of urea. Isolated alpha subunit of the hormone (hCG-alpha), though unreactive to enzyme treatment in the absence of denaturant, released up to four amino acid residues from the C-terminus on incubation with a mixture of carboxypeptidases A and B in urea. While an hCG-alpha product which lacked Ser-92 recombined completely with intact hCG-beta, hCG-alpha from which Ser-92 recombined completely with intact hCG-beta, hCG-alpha from which Ser-92 and Lys-91 were removed showed only partial recombination. The two recombinants were devoid of any in vivo biologic activity, but retained some of the immunologic activity of the native recombinant. These findings indicate that the integrity of the C-terminal residue of serine in hCG-alpha is essential for the expression of in vivo biologic activity of the native hormone.