Metabolism of Bradykinin In Vivo in Humans: Identification of BK1-5 As a Stable Plasma Peptide Metabolite

Studies investigating the role of bradykinin in disease states such as hypertension, sepsis, and asthma have been confounded by difficulties in measuring the concentration of this short-lived peptide. The purpose of this study was to determine a stable metabolite of bradykinin in the systemic circulation of humans. Bradykinin (containing trace concentrations of [(3)H]bradykinin) was administered i.v. into three human volunteers in increasing amounts up to a maintenance rate of 200 ng/kg/min until a total dose of 1 mg was given. Metabolic products were purified and identified by HPLC and by electrospray ionization mass spectrometry. Infused bradykinin was rapidly degraded, such that no exogenous bradykinin was detected in venous plasma sampled during infusion. BK1-5 (Arg-Pro-Pro-Gly-Phe), the 1-to-5 amino acid fragment of bradykinin, was identified as a major stable plasma metabolite of bradykinin. Plasma concentrations of BK1-5 correlated with dose of bradykinin infused and concentrations at the end of bradykinin infusion were 1510 to 4600 fmol/ml of blood. BK1-5 was cleared from blood with a terminal half-life of 86 to 101 min. Thus, in humans, bradykinin is rapidly degraded in vivo to BK1-5, a stable metabolite. Measurement of this metabolite could provide a tool to assess pathophysiologic and pharmacologic alterations in systemic bradykinin generation associated with human disease.