Fig. 1

The kallikrein-kinin system. Kinins are either formed in tissues by cleavage of low molecular weight (LMW) kininogen into kallidin or in plasma via activity of plasma kallikrein and formation of bradykinin from high molecular weight (HMW) or LMW kininogen. The activity of plasma kallikrein is reciprocally amplified by activation of factor XII after contact with artificial or pathophysiological surfaces, leading to increased generation of bradykinin. While kallidin and bradykinin activate B₂-R, their N-terminal des-Arg metabolites stimulate B₁-R. Kinin amino acid sequences are displayed in the one-letter code. APP: aminopeptidase P, ACE: angiotensin-converting enzyme, ACE 2: angiotensin-converting enzyme 2, B_1/2-R: bradykinin receptor type 1/2, CP: carboxypeptidase, DPP IV: dipeptidyl peptidase IV, ECE: endothelin-converting enzyme, NEP: neprilysin, PRCP: prolyl carboxypeptidase, RNA: ribonucleic acid