Recombinant active MMP-7 (EMD Biosciences, Gibbstown, NJ) (7.2 mg, 4 ml) was added to a solution of AAT (Sigma Aldrich, St. Louis, MO) (100 ml, 2.9 mg ml 21 ) in 100 mM ammonium bicarbonate buffer and the mixture was incubated at 37uC. Aliquots (5 ml) were removed for immunoblotting at 0, 5, 20, 60 and 120 min, mixed with RSB (5 ml) and retained for analysis. Immunoblotting was carried out following gel electrophoresis on 4-12% Bis-Tris or 16% Tris-Tricine gels (Invitrogen, Carlsbad, CA) for increased resolution of low molecular weight species. LC-MS/MS analysis was carried out with the 5 min and 120 min time point samples following desalting and concentration by methanol chloroform precipitation [48] .