Abstract
hCG, a glycoprotein hormone produced by the trophoblast in pregnancy, is composed of two dissimilar subunits, alpha and beta, joined non-covalently. hCG has four O-linked sugar units, all attached to the beta-subunit. The trophoblast also produces a free form of the alpha-subunit, which unlike the alpha-component of hCG, can contain an O-linked sugar unit. The structures of the O-linked sugar units were examined. Four structures were identified on urinary hCG. A hexasaccharide, NeuAc alpha 2-3Gal beta 1-3(NeuAc alpha 2-3Gal beta 1-4GlcNAc beta 1-6)GalNAc- accounting for 13%, a tetrasaccharide, NeuAc alpha 2-3Gal beta 1-3(NeuAc alpha 2-6)GalNAc-, for 34%, a trisaccharide, NeuAc alpha 2-3Gal beta 1-3GalNAc-, for 43% and a disaccharide, NeuAc alpha 2-6GalNAc- for 10% of the total O-linked sugar structures. Similar mixtures were found on peptides containing one, three or four sugar units suggesting a random distribution among attachment sites. The distribution of O-linked sugar structures on hCG and free alpha-subunit from trophoblast explant cultures was compared. The mixture of structures attached at the single site on the free alpha-subunit paralleled that at the four sites on the hCG.
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