Hydrolysis of Bradykinin by Angiotensin-Converting Enzyme

Two dipeptides, phenylalanylarginine (Phe-Arg) and serylproline (Ser-Pro), are released sequentially from bradykinin by angiotensin-converting enzyme purified from hog lungs; chloride increases the rate of release of both dipeptides. Using an automated ninhydrin-reagent method, we studied the kinetics of bradykinin hydrolysis. The reaction proceeded in the absence of chloride; however, the addition of chloride increased the rate of hydrolysis by decreasing K m and increasing V m . The K m values for bradykinin were 3.9 x 10 -6 M in the absence of chloride and 0.85 x 10 -6 M in the presence of 0.01m NaCl (optimal concentration). Both of these K m values were well below the value of 30 x 10 -6 M determined for angiotensin I at its optimal chloride concentration of 0.1M. Hydrolysis of bradykinin had a pH optimum of 7 and was inhibited by low concentrations (10 -6 M) of ethylenediaminetetraacetic acid or the nonapeptide pyroglutamyl (Pyr)-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro. It is concluded that one enzyme, acting as a dipeptidyl carboxypeptidase, catalyzes both the conversion of angiotensin I to angiotensin II and the hydrolysis of bradykinin.