Analysis by MALDI-TOF mass spectrometry of the cleavage of the TR4 peptide by HLE, Cat G, and PR3. The lower tracing (Control) is the mass spectrum of the untreated TR4 peptide corresponding to the Glu⁶⁰-Asn⁷⁵ domain of the thrombin receptor (m/z, observed mass charge = 1853.1). The three upper tracings are mass spectra of the peptides obtained after a 10-minute digestion with each of the neutrophil proteinases (arrows). Tracings are representative of two distinct experiments.