Determination of carboxypeptidase A using N-acetyl-phenylalanyl-3-thiaphenylalanine as substrate: Application to a direct serum assay☆

Abstract N -Acetyl- l -phenylalanyl- l -3-thiaphenylalanine has been shown to be a substrate for carboxypeptidase A. Hydrolysis of the compound obeys Michaelis-Menten kinetics with a K M of 0.22 m m and a k cat of 6720 min −1 at 22°C. A colorimetric assay, employing Ellman's reagent to detect the thiophenol released upon cleavage of the peptide, has been developed. The assay can be used for the direct determination of carboxypeptidase A in serum.