Extracellular appearance of calpain and calpastatin in the synovial fluid of the knee joint.

Extracellular location of calpain and calpastatin was demonstrated in the cell-free synovial fluid obtained from the knee joint of healthy adult humans and several patients with rheumatoid arthritis. Calpains I and II and a few molecular species of calpastatin were identified by chromatographies on DEAE-cellulose and on Ultrogel AcA 34 columns as well as by immunoelectrophoretic blot analysis. Calpains I and II in the synovial fluid of the patients increased 6.7 times and 3.5 times, respectively, compared with those of the control subjects. With the patients, shortening of the heavy subunits of calpains was noted. Calpastatin also increased in the patients, but it showed rather extensive fragmentation.