A pituitary N-acetylgalactosamine transferase that specifically recognizes glycoprotein hormones

The glycoprotein hormones lutropin (LH) and follitropin (FSH), which have common α-subunits but hormone-specific β-subunits, are both synthesized in the gonadotroph. However, they bear Asn-linked oligosaccharides that differ in structure. Those on LH terminate with the sequence SO ₄-4GalNAcβ1→4GlcNAcβ1→2Manα, whereas those on FSH terminate with the sequence sialic acidα- Galβ1→4GlcNAcβ1→2Manα. A GalNAc-transferase was identified in bovine pituitary membranes that recognizes features of the α-subunit peptide and adds GalNAc to its oligosaccharides with an apparent Michaelis constant of 25 micromolar. The different patterns of glycosylation for LH and FSH indicate that access to the protein recognition marker on the α-subunit is modulated by die associated β-subunit. The tightly regulated synthesis of sulfated and sialylated oligosaccharides on the pituitary glycoprotein hormones suggests these oligosaccharides have an important biological role.