Link protein as a monitor in situ of endogenous proteolysis in adult human articular cartilage.

The link protein components of proteoglycan aggregates in adult human articular cartilage show heterogeneity due to proteolysis. Cleavages near the N-terminus of the intact link proteins, before residues 17, 19 and 24, generate three proteins of slightly diminished size (LP3). Cleavages within the N-terminal disulphide-bonded loop, before residues 66 and 73 of the intact link proteins, generate proteins that yield smaller degradation products upon reduction (LP fragments). In vitro, modified link protein components of a similar size to LP3 can be generated by a variety of proteinases, but of the physiologically relevant enzymes only stromelysin, cathepsin B and cathepsin G have the ability to yield modified link proteins with N-termini identical with those observed in situ. None of the proteolytic agents tested was able to produce LP fragments with N-termini identical with those observed in situ, and the majority of proteinases were not able to cleave within the disulphide-bonded loops. Cathepsin L and hydroxyl radicals can cleave within the N-terminal disulphide-bonded loop, and have the potential of initially opening the loop to allow further proteolytic processing by other agents to generate the native cleavage sites.