Detection of proteolysis. Percent sequence coverage for fibronectin in the NaCl (A) and guanidine extracts (B) is shown. “R” denotes RePlay analysis. Note the increase of fibronectin products in the NaCl extracts of the fourth aortic specimen. Western immunoblotting of fibronectin in the NaCl (C) and guanidine (D) extracts confirmed the laddering of fibronectin. The product ion spectrum of the doubly charged tryptic peptide SLGPALLLLQK was identified as MMP-9 (E) (peptide probability, 95%; SEQUEST XCorr score, 2.67; SEQUEST ΔCn score, 0.40; observed m/z, 576.87; actual peptide mass, 1,151.73 Da; peptide charge, 2; delta amu, 0.0049; delta ppm, 4.3; peptide start index, 66; peptide stop index, 76). MMP-9 was only detected by LC-MS/MS in the NaCl extract of the fourth aortic specimen. Gelatin zymography confirmed higher levels of MMP-9 in this sample (C, bottom panel).