Mass spectroscopy. (A) Mass spectrometry (MALDI) analysis of cleavage of HK (200 nM) by PKa (2 nM), Plm- Lys³¹¹ (40 nM), Plm-Glu³¹¹ (40 nM), or vehicle. BK and kallidin standards were used as controls. Mass peak at m/z = 1060 confirms the release of BK from cleavage of HK by PKa and Plm-Glu³¹¹. A BK peak was not observed during incubation with Plm-Lys³¹¹. (B) Comparison of mass spectra (MALDI) of time course reactions of HK incubated with PKa or Plm-Glu³¹¹. Both reactions show accumulation of bradykinin (m/z = 1060) and absence of kallidin (m/z = 1188). The absence of a kallidin peak or other peaks suggests that BK is not formed as a mass fragmentation product during the MALDI process or an intermediate of kallidin generated during the 30-minute incubation of HK with Plm-Glu³¹¹.