Unique oligosaccharides tell glycoprotein hormones where to go and how fast

Levels of circulating lutropin (LtI) rise and fall episodically during the preowdatory surge in hormone levels. This reflects release from storage gramdes and rapid clearance from the circulation. The resulting episodic stimulation of the LIt receptor is essential to obtain its maximal activation in the ovary. The rapid clearance of LH from the blood is mediated by a diffrent receptor in hepatic endothelial cells which recognizes oligosaccharides terminating with S04-4-GalNAcβ,14GlcNAcβ. All vertebrate species express the GaINAc- and sulfo-transferases responsible for synthesis of these unique sulfated structures. The GalNAc-transferase recognizes a peptide recognition determinant which is present within the a subunit of glycoprotein hormones from all vertebrate species. GalNAc- and sulfo-transfrase activities rise and fall in gonadotrophs in response to estrogen during the ovulatory cycle in the same manner as LH. This assures that the hormone is always fully modified by GalNAc-4-SO4. Thus. the structural features of both the oligosaccharide and the protein components of the glycoprotein hormones are essential for their in vivo activity and have been conserved during vertebrate evolution.