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An excessive amount of matrix metalloproteinase-9 (MMP-9) has been well documented in inflammatory diseases, including chronic wounds and cancers. Secreted as a zymogen, proMMP-9 can be irreversibly converted to a mature form through cleavage of the N-terminal propeptide domain. Although the converting enzyme for proMMP-9 in human tissues is unknown, we previously found that tumor necrosis factor-α (TNF-α) promotes activation of proMMP-9 in human skin, and characterized the converting activities as tissue-associated chymotrypsin-like proteinases. On the other hand, the pathophysiologic inhibitor to prevent proMMP-9 maturation also remains elusive. In this regard, we observed the presence of the inhibitory property in burn blister fluid that abrogates the skin extract–mediated activation of proMMP-9. Then we determined that α-1-antichymotrypsin (α-ACT), an acute-phase factor abundantly present in the blister, effectively inhibited proMMP-9 activation in human and rodent skin. In contrast, the aminophenylmercuric acetate-induced “cysteine switch” and activation of proMMP-9 were not affected by α-ACT. TNF-α-induced activation of proMMP-9 by the explants of human skin was inhibited by α-ACT but not by related α-1-antitrypsin. α-ACT specifically attenuated maturation of proMMP-9 but not proMMP-2 or proMMP-13. Furthermore, short peptides that mimic the reactive center loop (RCL) of α-ACT were sufficient to inhibit the conversion. Mutation analysis demonstrated that a conserved leucine within the RCL was critical for α-ACT-exerted inhibition. In chronic wounds, a large amount of mature MMP-9 was associated with fragmentation and inactivation of α-ACT. Taken together, these results demonstrate that, to the best of our knowledge, α-ACT is a previously unreported pathophysiologic inhibitor that controls proMMP-9 activation in skin tissue.Journal of Investigative Dermatology (2008) 128, 2334–2342; doi:10.1038/jid.2008.77; published online 10 April 2008 [ABSTRACT FROM AUTHOR]