Identification of plasma kallikrein as an activator of latent collagenase in rheumatoid synovial fluid.

Rheumatoid synovial fluid contains an activator of latent collagenase from culture medium of pig synovium. The activator was purified by gel chromatography on Ultrogel AcA 44 and affinity chromatography on soybean trypsin inhibitor coupled to Sepharose 4B. The purified material was homogeneous on SDS-polyacrylamide gel electrophoresis with Mr 88 000. The activator had limited proteolytic activity against azo-casein, but showed amidase activity on Pro-Phe-Arg-NMec, Z-Phe-Arg-NMec, D-Val-Leu-Arg-NPhNO2 and D-Pro-Phe-Arg-NPhNO2, with an optimum at pH 8.0. Activity was completely inhibited by diisopropyl fluorophosphate, soybean trypsin inhibitor, leupeptin and Pro-Phe-Arg-CH2Cl, whereas lima bean trypsin inhibitor, Tos-Lys-CH2Cl, a specific inhibitor of factor XIIa from maize, EDTA and iodoacetate were not inhibitory. These properties of the activator suggested that it might be plasma kallikrein (EC 3.4.21.34), and the possibility was further examined. The activator was treated with [3H]diisopropyl fluorophosphate, and run in SDS-polyacrylamide gel electrophoresis with reduction; a radioautograph of the gel showed a pair of [3H]diisopropyl phosphoryl-labelled bands (Mr 36 000 and 34 000) identical to those obtained with authentic plasma kallikrein. Double immunodiffusion with monospecific antiserum against human plasma kallikrein confirmed the identification. This is the first demonstration of collagenase-activating activity of plasma kallikrein, and raises the possibility that activation of prokallikrein in the inflamed joint space may contribute to the disease process not only by the production of bradykinin, but also by activating latent collagenase.