Matrix metalloproteinase that plays an essential role in local proteolysis of the extracellular matrix and in leukocyte migration (PubMed:2551898, PubMed:1480034, PubMed:12879005). Could play a role in bone osteoclastic resorption (By similarity). Cleaves KiSS1 at a Gly-|-Leu bond (PubMed:12879005). Cleaves NINJ1 to generate the Secreted ninjurin-1 form (PubMed:32883094). Cleaves type IV and type V collagen into large C-terminal three quarter fragments and shorter N-terminal one quarter fragments (PubMed:1480034). Degrades fibronectin but not laminin or Pz-peptide. Cleavage of gelatin types I and V and collagen types IV and V. Binds 2 Zn(2+) ions per subunit. Binds 3 Ca(2+) ions per subunit. Inhibited by histatin-3 1/24 (histatin-5). Inhibited by ECM1. Exists as monomer or homodimer; disulfide-linked (PubMed:1281792, PubMed:7683678). Exists also as heterodimer with LCN2 (PubMed:1281792, PubMed:7683678). Macrophages and transformed cell lines produce only the monomeric form. Interacts with ECM1 (PubMed:16512877). (Microbial infection) Interacts with Staphylococcus aureus protein SSL5; this interaction inhibits MMP9 activity. Detected in neutrophils (at protein level) (PubMed:7683678). Produced by normal alveolar macrophages and granulocytes. Activated by 4-aminophenylmercuric acetate and phorbol ester. Up-regulated by ARHGEF4, SPATA13 and APC via the JNK signaling pathway in colorectal tumor cells. (Microbial infection) Expression induced by M.bovis MPB83 (at protein level) (PubMed:20800577). The conserved cysteine present in the cysteine-switch motif binds the catalytic zinc ion, thus inhibiting the enzyme. The dissociation of the cysteine from the zinc ion upon the activation-peptide release activates the enzyme. Processing of the precursor yields different active forms of 64, 67 and 82 kDa. Sequentially processing by MMP3 yields the 82 kDa matrix metalloproteinase-9. N- and O-glycosylated. Disease susceptibility is associated with variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. In the arthritis patient this enzyme might contribute to the pathogenesis of joint destruction and might constitute a useful marker of disease status. Belongs to the peptidase M10A family.