Continuous fluorogenic substrates for atrial dipeptidyl carboxyhydrolase. Importance of Ser in the P1 position

Several N-acyltetrapeptides of the general structure 2-aminobenzoyl-Gly-X-Phe(4-nitro)-Arg were synthesized and tested as substrates for atrial dipeptidyl carboxyhydrolase, an enzyme associated with atrial granules that converts one active atrial natriuretic peptide, atriopeptin II, to another, atriopeptin I. Hydrolysis of the X-Phe(4-nitro) bond generates the 2-aminobenzoyl fluorophore and the increasing fluorescence can be monitored in a continuous assay. Based on the ratio of Vmax/Km as an indication of substrate specificity, peptides containing X = Ser greater than Ala approximately equal to Lys- greater than Asn much greater than Thr approximately equal to Asp. With the exception of the Asn substrate, the Km determined for all the substrates was about the same. Thus, the effect of the P1 residue substitution shows up almost exclusively in Vmax.