Heterogeneity of proteoglycans extracted before and after collagenase treatment of human articular cartilage. I. Physical properties related to age.

Proteoglycans were isolated from young and mature human articular cartilage 4 different ways: by direct extraction with 4M guanidine hydrochloride (GuHCl); after digestion of the residue from this first extraction with collagenase, by extraction with 4M GuHCl; associatively with 0.5M GuHCl after digestion of the cartilage with collagenase; and dissociatively with 4M GuHCl after digestion of the cartilage with collagenase. The structural properties of these proteoglycans were compared. Proteoglycan aggregates and monomers isolated from second extractions and from young cartilage were of larger hydrodynamic size than proteoglycans isolated from first extractions and mature cartilage, respectively. The same applied to the chondroitin sulfate chain lengths of these proteoglycans. The proteoglycan fraction from second extractions of cartilage contained a larger proportion of monomers than the fraction from first extractions. Associative extraction of mature collagenase-digested cartilage yielded mainly proteoglycan monomers, whereas an appreciable amount of proteoglycan aggregate was also liberated from young collagenase-digested cartilage. Our results indicate that, because of their larger size, proteoglycans from second extractions of cartilage are more entrapped in the collagen network. These large proteoglycans can only be liberated from the matrix after extraction of the smaller proteoglycans, followed by digestion of the residue with collagenase. This indicates that proteoglycans overlap and entangle with the collagen and protect it from degradation by collagenase.