Figure 2.

Formation of bradykinin and kallidin peptides. In humans, plasma kallikrein cleaves high molecular weight kininogen to produce bradykinin, whereas tissue kallikrein cleaves both high and low molecular weight kininogens to produce kallidin (Lys⁰-bradykinin). Bradykinin can also be generated by aminopeptidase-mediated cleavage of kallidin. A proportion of high molecular weight kininogen is hydroxylated on the third proline (Hyp³) of the bradykinin sequence, leading to the formation of both hydroxylated and non-hydroxylated bradykinin and kallidin peptides. In the rat, both plasma and tissue kallikrein produce bradykinin, which is not hydroxylated (44–47).