1. Inhibition of collagenase from rabbit bone cultures by the chelating agents 1,10-phenanthroline and EDTA is almost completely reversed by Zn2+; other metal cations are less effective in reversing the inhibition. Optimal restoration of activity is achieved at Zn2+ concentrations below that of the chelator, but excess of Zn2+ is inhibitory. 2. Prolonged incubation of collagenase with either chelator causes irreversible inactivation. This inactivation is prevented by Zn2+ at the same concentrations needed to reverse the primary inhibition. 3. Collagenase incorporates 65Zn by exchange when incubated with 1,10-phenanthroline and Zn2+ containing this radioactive isotope. The 65Zn2+ can be removed from its binding site in collagenase by 1,10-phenanthroline or EDTA. Irreversible inactivation of collagenase by chelators destroys its ability to incorporate 65Zn2+. 4. Latent collagenase, the inhibited form in which collagenase first appears in culture, behaves similarly to the active enzyme in 65Zn2+-exchange experiments, but is resistant to irreversible inactivation by chelators. 5. It is concluded that collagenase is a zinc metalloenzyme that forms an inactive and unstable apoenzyme on treatment with chelators. The bound inhibitor component of latent collagenase evidently stabilizes the apoenzyme.

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