MMP-9 (also known as gelatinase B) is secreted as a 92 kDa zymogen. Cleavage of pro-MMP-9 at or near residue 87 results in the active enzyme with a mass of approximately 82 kDa. MMP-9 has three fibronectin type II domains, a hemopexin-like domain and a proline-rich type V collagen-homologous domain.

Pro-MMP-9 can be activated by MMP-3 or by certain bacterial proteinases. MMP-9 is inhibited byα2-macroglobulin or by TIMP-1, which binds to pro-MMP-9 as well as to active MMP-9.Pro-MMP-9 is secreted by monocytes, macrophages, neutrophils, keratinocytes, fibroblasts, osteoclasts, chondrocytes, skeletal muscle satellite cells, endothelial cells, and various tumor cells. Pro-MMP-9 expression is upregulated by TGF-β1, IL-1β, TGF-α, PDGF-AB,