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Development of internally quenched fluorogenic substrates for sensitive and continuous assays of angiotensin I-converting enzyme (ACE).We synthesized internally quenched fluorogenic bradykinin-related peptides introducing Abz (ortho-aminobenzoic acid) and EDDnp (N-[2,4-dinrtrophenyl]-ethylenediamine) at their N- and C-terminal groups, respectively, and these were assayed as ACE substrates. We examined two series of peptides, Abz-GFSPFRXEDDnp and Abz-GFSPFXQ-EDDnp (X, various amino acids).Hydrolysis of the fluorogenic substrates by ACE was followed by continuous recording of the rising fluorescence (λem = 420 nm and λex = 320 nm). The peptides were obtained by solid-phase synthesis or by classical solution methods.Despite of the blocked C-terminal sequences, the internally quenched bradykinin-related peptides were hydrolysed by ACE. The best substrates for plasma guinea pig ACE were Abz-GFSPFRA-EDDnp and Abz-GFSPFFQEDDnp, in which the fluorescence appeared after the first cleavage that occurred at R-A and F-Q bond, respectively. This ACE activity was sensitive to NaCl concentration and the optimum pH is greater than 8.0. Measurements of ACE activity with Hip-His-Leu and Abz-QFSPFFQ-EDDnp in the serum of 20 healthy patients correlated closely r = 0.959). Complete inhibition of the hydrolysis of Abz-GFSPFFQ-EDDnp by human serum was observed with captopril and lisinopril.We describe internally quenched fluorogenic substrates for ACE devoid of free C-terminal carboxyl group. They are convenient tools for ACE studies as they permit continuous fluorimetric measurements of the enzymatic activity, even in human serum.