Ochemical validation of MMP-2 and MMP-9 substrates. Recombinant proteins identified by iTRAQ-TAILS as high confidence or potential substrates were incubated for 18 h at 37 with MMP-2, MMP-9, or buffer alone, or MMP-2 and MMP-9 were incubated alone. A, digestion products of recombinant human Dkk-3 were separated by 10 SDS-PAGE. Cleavage fragments of recombinant human peptidyl-prolyl cis-trans is