Calpain, originally considered to be an intracellular proteinase, was demonstrated to be present extracellularly in growth cartilage of Wistar rat and also in synovial fluids of osteoarthritic patients. Proteoglycan-degrading proteolytic activities of calpain were demonstrated in vitro and in osteoarthritic synovial fluid. These data plus some recent reports of other investigators suggest that calpain-calpastatin system may have an important role in the metabolism of the extracellular matrix, although the pathways of externalization of these intracellular proteinases remain unclarified.
Biomedical Reviews 1992; 1: 69-72.
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