MALDI-TOF mass spectrometric analysis of thrombin cleavage of synthetic R, M, and H GPV peptides. Synthetic peptides corresponding to region 473-486 of R, M, and H GPV, respectively (see Materials and Methods), were incubated with saline or human α-thrombin. Thrombin treatment of rat 473-486(Y) caused loss of the nonnatural COOH-terminal tyrosine giving rise to the 473-486 peptide, which generated a thrombin cleaved 477-486 peak. M and H 473-486(Y) peptides were cleaved at position 476/477 to give peptide 477-486(Y) and a decrease of the 473-486(Y) peak. In the M spectra, the original peptide contained contaminants indicated by stars, which generated a thrombin digest peak indicated by a triangle. M/Z: observed mass/charge.