Matrix metalloproteinase-9 (MMP-9) is a member of the MMP family that has been associated with degradation of the extracellular matrix in normal and pathological conditions. A unique characteristic of MMP-9 is its ability to exist in a monomeric and a disulfide-bonded dimeric form. However, there exists a paucity of information on the properties of the latent (pro-MMP-9) and active MMP-9 dimer. Here we report the purification to homogeneity of the monomer and dimer forms of pro-MMP-9 and the characterization of their biochemical properties and interactions with tissue inhibitor of metalloproteinase (TIMP)-1 and TIMP-2. Gel filtration and surface plasmon resonance analyses demonstrated that the pro-MMP-9 monomeric and dimeric forms bind TIMP-1 with similar affinities. In contrast, TIMP-2 binds only to the active forms. After activation, the two enzyme forms exhibited equal catalytic competence in the turnover of a synthetic peptide substrate with comparable kinetic parameters for the onset of inhibition with TIMPs and for dissociation of the inhibited complexes. Kinetic analyses of the activation of monomeric and dimeric pro-MMP-9 by stromelysin 1 revealed K m values in the nanomolar range and relative low k cat values (1.9 ؋ 10 ؊3 and 4.1 ؋ 10 ؊4 s
؊1, for the monomer and dimer, respectively) consistent with a faster rate (1 order of magnitude) of activation of the monomeric form by stromelysin 1. This suggests that the rate-limiting event in the activation of pro-MMP-9 may be a requisite slow unfolding of pro-MMP-9 near the site of the hydrolytic cleavage by stromelysin 1.Matrix metalloproteinase-9 (MMP-9), 1 also known as gelatinase B, is a member of the MMP family of zinc-dependent endopeptidases known for their ability to degrade many extracellular matrix (ECM) components (1, 2). MMP-9 is secreted in a latent form (pro-MMP-9) by a variety of normal and transformed cells and has been implicated in the pathogenesis of several human diseases including arthritis (3), cardiovascular disease (4, 5), and cancer metastasis (6, 7). MMP-9 has also been suggested to play a role in the degradation of ECM during inflammation (8), wound healing (9, 10), trophoblast implantation (11), and angiogenesis (12). Structurally, pro-MMP-9 is closely related to pro-MMP-2 (gelatinase A) with both enzymes containing a fibronectin-like type II module (gelatin-binding domain) inserted into the catalytic domain that is thought to facilitate interaction of the enzymes with collagen molecules (13,14). The zymogenic forms of both enzymes interact, via their C-terminal domain (hemopexin-like domain), with tissue inhibitors of metalloproteinases (TIMPs), a family of specific endogenous MMP inhibitors (15, 16). Pro-MMP-9 binds to TIMP-1 (1), whereas pro-MMP-2 binds to TIMP-2 (17) and to TIMP-4 (18). After activation, any TIMP molecule efficiently inhibits the enzymatic activity by binding to the catalytic domain of the MMP (16).Despite the similarities that exist between pro-MMP-9 and pro-MMP-2, the former is unique in several aspects includi...