Mass spectrometry analysis of FRETS-VWF73 cleaved by leukocyte proteases. Commercial FRETS-VWF73 peptide was incubated without (A), and with recombinant ADAMTS13 (rADAMTS13; B), neutrophil elastase (C), PR3 (D), cathepsin G (E), and MMP9 (F) at 30°C for 60 minutes. The digested materials were analyzed by MALDI-TOF mass spectrometry. The masses of the major peaks were compared with the calculated molecular weights of carboxyl terminus fragments of FRETS-VWF73. The mass of the major peak arising from rADAMTS13 cleavage and cathepsin G cleavage corresponds to cleavage at the M¹⁶⁰⁵-Y¹⁶⁰⁶ peptide bond. The major peaks resulting from neutrophil elastase cleavage and PR3 cleavage correspond to cleavage at the V¹⁶⁰⁷-T¹⁶⁰⁸ peptide bond. The major peak resulting from cleavage by MMP9 corresponds with cleavage at M¹⁶⁰⁶-V¹⁶⁰⁷. The mass spectrometry margin of error is 0.2%.