The chondroitin sulphate (CS) linkage regions have been isolated from human articular cartilage aggrecan (from 10- to 72-year-olds) by chondroitin ABC endolyase digestion and size-exclusion chromatography. Linkage region hexasaccharides have been characterized and their abundance estimated by high-pH anion-exchange chromatography. The basic structure for the CS linkage region oligosaccharides identified from human aggrecan is as follows: ΔUA(β1–3)GalNAc[0S/4S/6S](β1–4)GlcA(β1–3)Gal[0S/6S](β1–3)Gal(β1–4)Xyl, where ΔUA represents 4,5-unsaturated hexuronic acid, 4S and 6S represent an O-ester sulphate group on C-4 and C-6 respectively, and 0S represents zero sulphation. There are significant age-related changes in the abundance of the various N-acetylgalactosamine (GalNAc) sulphation forms identified, occurring up to approx. 20 years old. During the period from 10 to 20 years old the level of GalNAc 6-sulphation at the linkage region increases from approx. 43% to approx. 75%, while there is a corresponding reduction in unsulphated (approx. 30% to approx. 20%) and 4-sulphated (approx. 25% to approx. 6%) GalNAc residues. There is also an increase in the incidence of linkage region galactose 6-sulphation (approx. 2% to approx. 10%) which was only observed in linkage regions with GalNAc 6-sulphation. Beyond 20 years old there are few changes in the relative abundance of these GalNAc sulphation variants; however, there is a slight increase in the abundance of 6-sulphation between approx. 20 years old and approx. 40 years old and a slight decrease in its abundance beyond approx. 40 years old. Our data show that in the majority of chains from tissues of all ages the GalNAc residue closest to the linkage region is 6-sulphated, but the level of GalNAc 6-sulphation within the linkage region is lower than the average level observed within the repeat region.
Age-related changes in the composition and structure of human articular-cartilage proteoglycans