Sulfation of the choriogonadotropin alpha subunit in human placental explants.

Incubation of first trimester placental explants with [35S]O4 resulted in incorporation of radioactive sulfate into free and dimer forms of alpha subunit of human chorionic gonadotropin. Sulfate was not attached to N-linked oligosaccharides since it was not released by endoglycosidase F. Analysis of pronase digest revealed the presence of tyrosine-O-[35S]O4. Comparison of tryptic peptides of alpha subunit labeled with several amino acids identified the penultimate carboxyterminal peptide as the sulfation site. Since the C-terminal region of the hCG alpha plays a critical role in receptor binding of the hormone, modification in this region may regulate hormonal activity.