An oligosaccharide of the O-linked type distinguishes the free from the combined form of hCG [alpha] subunit

Abstract

JAR malignant trophoblast cells produce a free [alpha] subunit in addition to an [alpha] combined with [beta] subunit as hCG. The free [alpha] is larger by gel chromatography and SDS-PAGE than combined [alpha] and is unable to associate with [beta] subunit to form hCG. A tryptic fragment, representing amino acid residues 36-42, derived from free [alpha] was larger than the corresponding fragment from combined [alpha]. After neuraminidase treatment, the fragment from free [alpha] bound peanut lectin agarose, which is specific for Gal[beta]1-3GalNAc as found in O-linked oligosaccharides. The fragment also contained Gal and GalNAc (and a lesser amount of GlcNAc) as determined by glycosidase sensitivity and amino sugar analyses. Removal of this tryptic fragment ablated the size difference between free and combined [alpha] subunits.