Macrophage metalloelastase (MMP-12) is implicated in the pathology of many diseases such as emphysema, aortic lesions and cancer. Recently, MMP-12 was cloned and purified from mouse and human macrophages. We report here the expression of the full-length and catalytic domain of rat MMP-12 in Escherichia coli and characterization of the purified enzyme. Inclusion bodies of expressed rat MMP-12 catalytic domain were denatured and refolded using a new method, and then affinity purified to near homogeneity with zinc chelating Sepharose, The purified rat MMP-18 catalytic domain was highly active in digesting substrates, having a K-m of 12 muM and optimal pH of 7.5-8.5. During investigation of natural substrate specificity, we found that rat MMP 12 catalytic domain was able to completely degrade collagen-V, partially degrade collagen-I, but it was unable to digest collagen-IV. The enzyme could also degrade osteonectin, vitronectin, and fibronectin, but not laminin and albumin. The catalytic properties and natural substrate specificity of rat MMP-12 catalytic domain differed from those of human MMP-12 catalytic domain,