Hypochlorous acid oxygenates the cysteine switch domain of pro-matrilysin (MMP-7). A mechanism for matrix metalloproteinase activation and atherosclerotic plaque rupture by myeloperoxidase.

Myeloperoxidase uses hydrogen peroxide (H2O2) to generate hypochlorous acid (HOCl), a potent cytotoxic oxidant. We demonstrate that HOCl regulates the activity of matrix metalloproteinase-7 (MMP-7, matrilysin) in vitro, sugge sting that this oxidant activates MMPs in the artery wall. Indeed, both MMP -7 and myeloperoxidase were colocalized to lipid-laden macrophages in human atherosclerotic lesions. A highly conserved domain called the cysteine swi tch has been proposed to regulate MMP activity. When we exposed a synthetic peptide that mimicked the cysteine switch to HOCl, HPLC analysis showed th at the thiol residue reacted rapidly, generating a near-quantitative yield of products. Tandem mass spectrometric analysis identified the products as sulfinic acid, sulfonic acid, and a dimer containing a disulfide bridge. In contrast, the peptide reacted slowly with H2O2, and the only product was t he disulfide. Moreover, HOCl markedly activated pro-MMP-7, an MMP expressed at high levels in lipid-laden macrophages in vivo. Tandem mass spectrometr ic analysis of trypsin digests revealed that the thiol residue of the enzym e's cysteine switch domain had been converted to sulfinic acid. Thiol oxida tion was associated with autolytic cleavage of pro-MMP-7, strongly suggesti ng that oxygenation activates the latent enzyme. In contrast, H2O2 failed t o oxidize the thiol residue of the protein or activate the enzyme. Thus, HO Cl activates pro-MMP-7 by converting the thiol residue of the cysteine swit ch to sulfinic acid. This activation mechanism is distinct from the well-st udied proteolytic cleavage of MMP pro-enzymes. Our observations raise the p ossibility that HOCl generated by myeloperoxidase contributes to MMP activa tion, and therefore to plaque rupture, in the artery wall. HOCl and other o xidants might regulate MMP activity by the same mechanism in a variety of i nflammatory conditions.