A Simple Spectrophotometric Assay of Carboxypeptidase N (Kininase I) in Human Serum

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Abstract
Kininase I (carboxypeptidase N) consists of carboxypeptidase N1 (CN1) and carboxypeptidase N2 (CN2); these 2 enzymes can be differentiated by their activities towards hippuryl-L-arginine and hippuryl-L-lysine, respectively. A spectrophotometric assay for both carboxypeptidases in human serum is described and the biochemical behavior of these enzymes investigated. The pH optima are 8.4 for CN1 and CN2. The Km are: CN1 4.59 .+-. 0.03 mmol/l; CN2 37.26 .+-. 3.49 mmol/l. CN2 can be inhibited by EDTA (76%), dimercaprolum (97%) and phenanthroline (98%). DFP has no influence on both enzymes. Elevated Hb only interferes with CN1 measurements, and high bilirubin concentrations slightly alter the activity of both enzymes. High CN1 activities were found in sera of patients with sarcoidosis and elevated CN2 activities were found in lung cancer.