Kinin cleavage by human erythrocytes

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Abstract
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.