The degradation of bradykinin (BK) and of des-Arg9-BK in plasma

Abstract

The metabolism of bradykinin (BK), des-Arg9-BK, and two analogues of the latter was studied in human, rat, and rabbit plasma, using bioassays and high pressure liquid chromatography (HPLC) for measuring the concentrations and characterizing the types of metabolites. A solution of BK containing 100 μg/mL was added to 20% (v/v) plasma and incubated for up to 75 min at 37 °C. The concentration of BK, measured by the rabbit isolated jugular vein assay, decreased progressively whereas that of the fragment des-Arg9-BK, measured on the rabbit mesenteric vein, increased. This was taken as an indication that a carboxypeptidase is present in the plasma of the three species and transforms BK into des-Arg9-BK. Solutions of BK (3 mg/mL) containing 20% human plasma were incubated for 1 – 18 h and then analyzed by HPLC in order to identify the metabolic fragments resulting from the action of peptidases in human plasma. This assay confirmed that des-Arg9-BK is the principal metabolite and the first fragment to appear; this compound is further transformed into des-(Phe8, Arg9)-BK. No other fragments were found to be present in significant amounts. The addition of Captopril (100 ng/mL), an inhibitor of the converting enzyme, did not change the pattern of appearance or the quantities of metabolites.The degradation of synthetic des-Arg9-BK was measured by bioassay after incubation with the plasma of the three species. The fragment is inactivated, although more slowly than BK, and is presumably transformed into des-(Phe8, Arg9)-BK by the action of a carboxypeptidase. This conclusion is supported by the finding that the two analogues, [Phe-OMe8]-des-Arg9-BK and [D-Phe8]-des-Arg9-BK, are more resistant to degradation than des-Arg9-BK. As des-Arg9-BK is a strong stimulant of the B1 receptor for kinins, the generation and degradation of des-Arg9-BK is discussed in view of a possible involvement of this receptor in pathological states.