Not all lubricin isoforms are substituted with a glycosaminoglycan chain.

Lubricin, also referred to as superficial zone protein, has been reported to be a proteoglycan. However, the structure of its glycosaminoglycan chain has not been well characterized, and this study was undertaken to investigate the structure of the glycosaminoglycan chain that decorated lubricin in human synovial fluid to provide insight into its biological role. Lubricin was detected as a major band at approximately 360 kDa which co-migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a chondroitin sulfate (CS)-containing proteoglycan that was detected by both monoclonal antibodies (MAb) 2-B-6 and MAb 3-B-3 after chondroitinase ABC treatment and keratan sulfate (KS) that was detected by MAb 5-D-4. Further analysis of lubricin-containing fractions that eluted from an anion exchange column indicated that the major population of lubricin could be separated from the CS and KS stubs which indicated that this fraction of lubricin was not decorated with glycosaminoglycan chain and was the glycoprotein form of lubricin. Lubricin present in fractions that also contained CS was found to be decorated with CS structures which were reactive with MAb 3-B-3 after chondroitinase ABC digestion using a sandwich enzyme-linked immunosorbent assay approach. Aggrecan was not found to form complexes with lubricin in synovial fluid which confirmed that the MAb 3-B-3 CS and MAb 5-D-4 KS structures decorated lubricin. These data demonstrate that lubricin present in human synovial fluid was a heterogeneous population with both glycoprotein and proteoglycan forms.