Matrix metalloproteinases (MMPs) are a family of extracellular proteinases defined by the presence of two conserved motifs. One motif is a cysteine-containing prodomain, which functions, in part, to restrain catalytic activity. The other conserved motif is a histine-rich catalytic domain responsible for the endopeptidase activity.
Most MMPs possess other domains, which are thought to function in substrate recognition. In the lung, essentially all cell types, including epithelial, interstitial, vascular, and inflammatory cells, produce MMPs; however, both the pattern and levels of MMPs expressed vary among cell types and situations. Furthermore, the function of a given MMP produced by one cell type is likely distinct from the function of the same MMP produced by another cell. Although long thought to be responsible for the turnover and degradation of the extracellular matrix, matrix degradation per se is neither the sole nor the predominant function of these proteinases.Findings indicate that MMPs act on a variety of extracellular proteins, such as cytokines, chemokines, antimicrobial peptides, and other proteins, that regulate varied aspects of inflammation and immunity.