Chemical and Immunological Assay of the Nonreducing Terminal Residues of Chondroitin Sulfate from Human Aggrecan

Samples of aggrecan chondroitin sulfate, isolated from normal human knee cartilages of individuals from fetal to 72 years of age, were digested with chondroitin lyases. The products were analyzed by fluorescence-based anion exchange high performance liquid chromatography to separate and quantitate nonreducing terminal structures, in addition to internal unsaturated disaccharide products. The predominant terminal structures were the monosaccharides, GalNAc4S and GalNAc4,6S as they were present on 85–90% of all chains. The remaining chains terminated with the disaccharides GlcAβ1,3GalNAc4S and GlcAβ1,3GalNAc6S. Marked changes in the relative abundance of these terminals were identified in the transition from growth cartilage to adult articular cartilage. First, terminal GalNAc residues were almost exclusively 4-sulfated in aggrecan from fetal through 15 years of age, but were ∼50% 4,6-disulfated in aggrecans from adults (22–72 years of age). Second, the terminal disaccharide GlcAβ1,3GalNAc4S was on ∼7% of chains on aggrecan from fetal through 15 years of age, but on only ∼3% of chains on adult aggrecan. In contrast, the proportion of chains terminating in GlcAβ1,3GalNAc6S, ∼9%, was unchanged from fetal to 72 years of age. This terminal disaccharide is proposed to be recognized by the widely used monoclonal antibody 3B3. However, chemical quantitation of the structure together with solid phase 3B3(−) immunoassay of fetal and adult aggrecans showed that the content of the terminal disaccharide does not necessarily correlate with immunoreactivity of the proteoglycan, as chain density and presentation on the solid phase are critical factors for recognition of chain terminals by 3B3. The quantitative results obtained from chemical analyses of all nonreducing termini of aggrecan chondroitin sulfate chains revealed important changes in chain termination that occur when cellular activities are altered as adult articular cartilage is formed after removal of growth cartilage. These findings are discussed in relation to specific enzymatic steps that generate the nonreducing termini of chains in the biosynthesis pathway of chondroitin sulfate proteoglycans and their modulation in tissue development and pathology.