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Participation of the nonreducing terminal β‐galactosyl residues of the neutral N‐linked carbohydrate chains of porcine zona pellucida glycoproteins in sperm–egg binding

The zona pellucida (ZP) surrounding the mammalian oocyte is composed of three glycoprotein components (ZPA, ZPB, and ZPC). Mammalian sperm bind to carbohydrate chains of a ZP glycoprotein in the initial phase of fertilization. Sperm‐ligand carbohydrate chains have been characterized in mouse, cow, and pig. In pigs, triantennary/tetraantennary neutral complex‐type chains from ZPB/ZPC mixture possess stronger sperm‐binding activity than those of biantennary chains (Kudo et al., 1998: Eur J Biochem 252:492–499). Most of these oligosaccharides have β‐galactosyl residues at the nonreducing ends. This study used two in vitro competition assays to investigate the participation of the nonreducing terminal β‐galactosyl residues of the ligand active chains in porcine sperm binding. The removal of the nonreducing terminal β‐galactosyl residues from either the ligand active carbohydrate chains or endo‐β‐galactosidase‐digested glycoproteins significantly reduced their inhibition of sperm–egg binding, indicating that the β‐galactosyl residues at the nonreducing ends are involved in porcine sperm–egg binding. A correlation between the sperm‐binding activity and in vitro fertilization rate is also presented. Mol. Reprod. Dev. 70: 222–227, 2005. © 2005 Wiley‐Liss, Inc.