A fluorimetric assay for angiotensin-I converting enzyme in human serum

A fluorimetric method is described for a simple, sensitive and reproducible assay for angiotensin-I converting enzyme in human and guinea pig sera. The very weak fluorescent substrate o-aminobenzoylglycyl-p-nitro-L-phenylalanyl-L-proline is enzymatically hydrolyzed, producing the highly fluorescent o-aminobenzoylglycine that is quantitatively determined by spectrofluorimetry. Dependence of activity on substrate concentration, amount of serum, time of incubation and pH were investigated. The KM value for the substrate is 0.1 and 0.032 mM for the human and guinea pig serum enzyme, respectively. The mean value of serum angiotensin-I converting enzyme for 16 normal adult persons was 2.56 +/- 0.10 (S.E.) with a standard deviation of 0.81 nmol/min/ml serum.