Kinin cleavage by human erythrocytes

An aminopeptidase‐P has been purified 230‐fold from human erythrocytes. The purified enzyme cleaved arginine from des‐(Arg9)‐bradykinin (Arg‐Pro‐Pro‐Gly‐Phe‐Ser‐Pro‐Phe) had a molecular weight in nondenaturing buffers of 155,000 ± 6,900 daltons, was not inactivated by chelating agents, had a pH optimum of 7.2, and was stimulated by manganous ions. The aminopeptidase‐P was stable in intact erythrocytes for at least 21 days. Extensively washed and intact human erythrocytes cleaved arginine from exogenously supplied des‐(Arg9)‐bradykinin; arginine was the earliest‐appearing reaction product. Purified aminopeptidase‐P also cleaved a group of X‐proline dipeptides including leucyl‐proline, methionyl‐proline, phenylalanyl‐proline, arginyl‐proline, and alanyl‐proline. The total intra‐erythrocytic aminopeptidase‐P activity of the “average 70‐kg man” was 2,600 units, approximately five times the amount of activity in the total lung mass. The human erythrocyte aminopeptidase‐P activity was not tightly bound to the erythrocyte membrane. Intact erythrocytes also exhibited some kinin‐converting enzyme activity.