Mass spectrometry. Mass spectrometry (MALDI) analysis of kinins generated by cleavage of WT HK/LK-Met³⁷⁹ and variant HK/LK-Lys³⁷⁹ kininogens (200 nM) by PKa or plasmin. The top 2 elution profiles show results for synthetic bradykinin and kallidin control peptides. For bradykinin, the peak at m/z = 1060.5 is unmodified bradykinin. Additional peaks are likely monosodated and disodated forms of bradykinin. For kallidin, the peak at m/z = 1189.8 is unmodified kallidin. When HK-Met³⁷⁹ and HK-Lys³⁷⁹ are digested with 2 nM kallikrein, peaks with m/z = 1076 were obtained, corresponding to an oxidized form of bradykinin (supplemental Figure 5). Similarly, when HK-Met³⁷⁹ and LK-Met³⁷⁹ were incubated with 40 nM plasmin, peaks with m/z = 1077.2 and m/z = 1076.9, respectively, representing oxidized bradykinin, were obtained. The peak with m/z = 1060.9 for LK-Met³⁷⁹ with plasmin represents unoxidized bradykinin. Incubating HK-Lys³⁷⁹ or LK-Lys³⁷⁹ with 40 nM plasmin generated peaks with m/z = 1205.5 and m/z = 1204.8, respectively, representing oxidized Lys-bradykinin. The peak for the reaction with LK-Lys³⁷⁹ and plasmin with m/z = 1189.8 is nonoxidized Lys-bradykinin.