Matrix metalloproteinase-9 (MMP-9) is secreted from cells and, once activated, is thought to degrade collagen in the extracellular matrix. Because collagen is not readily localized where neurons have been shown to produce MMP-9 in the human brain, the ability of this enzyme to degrade bioactive peptides was investigated with representative tachykinin peptides [substance P (SP), neurokinin A, neurokinin B, and kassinin]. Latent MMP-9 (94 kDa) was purified from the human cell line HL-60 and converted to an intermediary active form (84 kDa) with p-aminophenylmercuric acetate. This active form of MMP-9 degraded SP with a kcat/Km of 170 mM-1 min-1, which is 30-fold greater than the previously reported value for a representative collagen-derived peptide. The major digestion products were identified as SP and SP, which were derived from cleavage of the Gln6-Phe7 bond. Minor products were also generated from cleavage of the Gly9-Leu10 bond. The other representative tachykinin peptides were cleaved at rates > 10-fold slower than that of SP. The 84-kDa peptidase was also active as a gelatinase. Longer treatment of MMP-9 with p-aminophenylmercuric acetate caused the conversion of the 84-kDa enzyme to the established 68-kDa active form; however, the rate of SP degradation did not increase. Because MMP-9 is produced by neurons of the CNS, these results suggest a possible regulatory role for the enzyme in interacellular communication by altering the availability of bioactive peptides.