Normal human synovial fluid and articular cartilage contain similar intact proteoglycans

This work, directed to characterization of proteoglycans present in normal human synovial fluid by Western blotting techniques, revealed an intimate relationship of these proteoglycans to those of articular cartilage. Analyses were performed on samples subjected to digestion with chondroitinase ABC, in the presence or absence of keratanase, yielding products containing core proteins with glycosaminoglycan side chain stubs. The proteoglycan core proteins contained epitopes reactive with monoclonal antibodies that distinguish between chondroitin sulfate-4 and chondroitin sulfate-6. Additionally, these products reacted with monoclonal antibody to keratan sulfate when keratanase was omitted from the digestion. The analysis of synovial fluid revealed that the proteoglycan core proteins expressed predominantly the chondroitin sulfate-6 epitope, with expression of the chondroitin sulfate-4 epitope demonstrable only in prepubertal individuals. There was coexpression of both chondroitin sulfate epitopes in all proteoglycan core proteins of prepubertal individuals. Coexpression of chondroitin sulfate and keratan sulfate epitopes occurred in all proteoglycan core proteins. Proteoglycan core proteins had M(r) similar to those obtained from articular cartilage. Hence, in individuals free of joint disease, most proteoglycans seem to be transferred from articular cartilage to the synovial fluid without major alteration in the apparent size of the proteoglycan core protein. Only a minor set of proteoglycan core proteins had no direct articular cartilage equivalent. As this set also contained keratan sulfate, it is likely to be of articular cartilage origin, but probably modified by proteolysis.