Matrix metalloproteases (MMPs) are Zn-containing endopeptidases involved in the degradation of extracellular matrix components and are typically secreted in a latent (pro-MMP) form and activated either by proteolytic or oxidative disruption of a conserved cysteine switch. Several recent studies have suggested that nitric oxide (NO) can contribute to the activation of MMPs, but the mechanisms involved are incompletely understood. We investigated the ability of NO to regulate the activation of (pro)MMP-9 using a variety of NO-donor compounds and characterized modifications of the cysteine switch using a synthetic peptide (PRCGVPDLGR) representing the cysteine switch domain of MMP-9. Among the NO-donors used, only S-nitrosocysteine (SNOC) was found to be capable of modest activation of proMMP-9, but S-nitrosoglutathione (GSNO) or the NONOates, DEA-NO, SPER-NO, or DETA-NO, were ineffective. In fact, high concentrations of DETA-NO were found to inhibit MMP-9 activity, presumably by direct interaction with the active-site Zn2+. Analysis of chemical modifications within the Cys-containing peptide, PRCGVPDLGR, revealed rapid and transient S-nitrosylation by SNOC and GSNO, and formation of mixed disulfides and dimerized peptide as major final products. Similarly, NONOates induced transient S-nitrosylation and primarily peptide dimerization. Coordination of the peptide Cys with a synthetic Zn2+ complex, to more closely mimic the structure of the active site in proMMP-9, reduced peptide nitrosylation and oxidation by NONOates, but enhanced peptide nitrosylation by SNOC and GSNO. Collectively, our results demonstrate that NO is incapable of directly activating proMMP-9 and that S-nitrosylation of MMP-9 propeptide by NO-donors is unrelated to their ability to regulate MMP-9 activity.