A new mechanism for activation of the proactivator of procollagenase [Vater, Nagase & Harris (1983) J. Biol. Chem. 258, 9374-9382] has been found. Collagenolytic and other proteolytic enzyme activities in the medium of cultured rabbit synovial fibroblasts were found to be activated by a new mechanism: short-term incubation at 37 degrees C performed in the presence of EGTA followed by replacement of Ca2+ during enzyme assay. The crucial event in procollagenase activation is the production of a functional activator enzyme. Activation of procollagenase in the culture medium did not occur when proactivator was removed by immunoprecipitation. Proteolytic activity of proactivator was fully activated, whereas procollagenase alone could not be activated by the same sequence. EGTA treatment of the culture medium at 0 degrees C did not result in enzyme activation if Ca2+ was replaced before incubation at 37 degrees C. Certain other bivalent metal ions (e.g. Sn2+, Cd2+, Zn2+ and Mn2+) could substitute for Ca2+ to stabilize the proactivator as a zymogen and therefore prevent the appearance of proteolytic activity in culture medium. Isolation of proactivator and procollagenase from EGTA-treated radiolabelled culture medium by immunoprecipitation and subsequent analyses by fluorography revealed that a time-dependent proteolysis of both molecules occurred after replacement of Ca2+ and incubation at 37 degrees C. However, comparison of enzyme activity with fluorographic analyses showed that the maximal activation of both enzymes was achieved before any detectable decrease in Mr. The results suggest that the activation of proactivator and the subsequent activation of procollagenase may be initiated by conformational changes in structure of the proactivator molecule produced by removal of stabilizing bivalent metal ions.

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