Occurence of a sialylglycopeptide and free sialylglycans in hen's egg yolk.

Free sialylglycans (FSGs) and a sialylglycopeptide (SGP) as components of hen's egg yolk were found and their chemical structures were determined. SGP and FSGs were isolated from fresh egg yolk by treatment with phenol, gel filtration and successive chromatographies on columns of anion- and cation-exchangers. They were localized in the yolk plasma. The glycan moiety of SGP, which was liberated by PNGase digestion, was studied for the chemical structure by HPLC mapping with p-aminobenzoic ethylester-derivatization, sugar composition analysis, 1H nuclear magnetic resonance and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry and the glycomoiety was found to be an N-linked disialyl-biantennary glycan. The amino acid sequence of the peptide moiety of SGP was determined to consist of Lys-Val-Ala-Asn-Lys-Thr, the Asn of which is modified with the disialylglycan moiety. FSGs were determined to be two free disialyl-biantennary glycans whose reducing end was either Man beta1-4GlcNAc (FSG-I) or Man beta1-4GlcNAc beta1-4GlcNAc (FSG-II). Since the molar value of SGP present in one egg yolk (2.8 micromol) is comparable to those of well-known major yolk proteins, low density lipoprotein, lipovitellins and phosvitin, it can be considered that SGP is one of the major components in hen's egg yolk.