Multiple non-reducing chain termini isolated from bovine corneal keratan sulfates

Keratan sulfate-containing proteoglycans were isolated from bovine cornea (15-month-old to 3-year-old animals) and digested with the enzyme, keratanase II. The released oligosaccharides, which included non-reducing termini and repeat region oligosaccharides but not linkage regions, were reduced with alkaline borohydride and fractionated on a Spherisorb column. These oligosaccharides were examined by 600-MHz 1H NMR spectroscopy using one- and two-dimensional methods and, in addition to some oligosaccharide alditols previously recovered from skeletal keratan sulfate, the following new capping structures were identified: NeuAcalpha 2-6Galbeta 1-4GlcNAc(S)-ol, NeuAcalpha 2-3Gal(S)beta 1 -4GlcNAc(S)beta 1-3Galbeta 1-4GlcNAc(S)-ol, NeuGcalpha 2-6Galbeta 1- 4GlcNAc(S)beta 1-3Galbeta 1-4GlcNAc(S)-ol, NeuGcalpha 2-3Galbeta 1-4GlcNAc(S)beta 1-3Galbeta 1-4GlcNAc(S)-ol, NeuGcalpha 2-3Gal- (S)beta 1-4GlcNAc(S)beta 1-3Galbeta 1-4GlcNAc(S)-ol, NeuGcalpha 2-3Gal(S)beta 1-4GlcNAc(S)beta 1 -3Gal(S)beta 1-4GlcNAc(S)-ol, Galalpha 1-3 Galbeta 1-4GlcNAc(S)beta 1-3Gal(S)beta 1-4GlcNAc(S)-ol, Galalpha 1-3Galbeta 1-4GlcNAc(S)beta 1-3Gal(S)beta 1-4GlcNAc(S)-ol, GlcNAc(S)beta 1-3Gal(S)beta 1-4GlcNAc(S)-ol, and GalNAc(S)-beta 1-3Gal(S)beta 1-4GlcNAc(S)-ol. These structures represent seven families of capping residues, whose relative molar proportions are given in parentheses: NeuAcalpha (2-3)- (12%), NeuAcalpha (2-6)- (41%), NeuGcalpha (2-3)- and NeuGcalpha (2-6)- families (12%), Galalpha (1-3)- (26%), GalNAc(S)beta (1-3)- (5%), and GlcNAc(S)beta (1-3)(4%). It is not clear, at present, where each of these structures occurs on the bi-antennary N-linked corneal keratan sulfate chains, which themselves occur within three keratan sulfate proteoglycan species. However, examination of the relative proportions of the capping to the repeat structures and knowledge of the average molecular size suggests that the sum of these non-reducing termini represents the caps of two antennae.