Studies of the digestion of bradykinin, Lys-bradykinin, and des-Arg9-bradykinin by angiotensin converting enzyme.

We have studied the degradation of bradykinin, lysyl bradykinin and des-Arg9-bradykinin by the angiotensin converting enzyme. Bradykinin was cleaved at two sites to produce the pentapeptide Arg-Pro-Pro-Gly-Phe plus dipeptides Ser-Pro and Phe-Arg. Lysyl bradykinin was cleaved similarly to release the same dipeptides plus the hexapeptide Lys-Arg-Pro-Pro-Gly-Phe. The tripeptidase activity of ACE was observed when des-Arg9-bradykinin was digested. A single cleavage yielded the above pentapeptide plus Ser-Pro-Phe. Although des-Arg9-bradykinin was the most rapidly digested, when mixtures of des-Arg9-bradykinin and bradykinin or lysyl bradykinin were tested, virtually all of the bradykinin and most of the lysyl bradykinin was digested prior to the onset of digestion of des-Arg9-bradykinin. This was shown to be due to inhibition of des-Arg9-bradykinin cleavage by kinins and kinin-degradation products. The order in terms of potency was bradykinin greater than lysyl bradykinin greater than Ser-Pro much greater than Phe-Arg greater than Arg-Pro-Pro-Gly-Phe. The concentration of chloride ion was an important parameter which affected the rate of digestion of each substrate examined. des-Arg9-bradykinin was not digested by ACE in the absence of sodium chloride and the rate of digestion increased as the chloride concentration was increased to 100-150 mM. On the other hand, increasing NaCl concentration was inhibitory for bradykinin digestion. The rate of Lys-bradykinin digestion was increased from 0 to 1 mM NaCl and decreased thereafter up to physiologic concentration. A half-maximal rate was seen at 100-150 mM NaCl compared to no salt. Of the divalent cations examined, cupric ion inhibited further digestion of des-Arg9-bradykinin at physiologic concentrations. Our data indicate that the rate of degradation of kinins and the nature of the stable final cleavage products in plasma or serum (studied in vitro) are dependent upon the effects of chloride ion, metal ions, and the kinetic effects of multiple metabolites produced by at least two kininases.