Human Chorionic Gonadotropin: Acetylation of Tyrosyls with N-Acetylimidazole

Abstract

When human chorionic gonadotropin (hCG) was treated with a 20-fold.molar excess of N-acetylimidazole in aqueous solution, two tyros, yls were acetylated, resulting in a 50% reduction in in vivo biological activity. With increased quantities of reagent, the number of tyrosyls acety lated increased but with no further decrease in the biological activity. In vitro biological activity (binding to hCG receptors) of the hormone was not affected at all. Acetylation in urea increased tyrosyl modification, accompanied by acetylation of some lysyls, yielding a product in which all seven tyrosyls were acetylated and both in vivo and in in vitro biological activities were completely abolished. Deacylatlon with hydroxylamine partially restored biological activity of some but not all of the modified products.

When individual hCG subunits were treated with the same reagent, the number of tyrosyls acetylated in each subunit again increased with increasing amounts of reagent, up to three in the asubunit and two in the gsubunit in the absence of urea. The tyrosyls in the β subunit appeared less reactive to the reagent than those in the α subunit. Subunits modified to these extents retained ability to recombine as examined by gel electrophoresis, but the recombined products varied considerably in both in vivo and in vitro biological activities. A completely tyrosyl-acetylated product of hCG-α did not combine with intact hCG-β, while fully modified hCG-β did with intact hCG-α.