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  • Title: Carboxypeptidase N: colorimetric assay using a new substrate.
    Author: Hendriks D, Scharpe S, van Sande M, Lommaert MP, Kasahara Y.
    Journal: Anal Biochem; 1987 Jul; 164(1):90-5. PubMed ID: 3118736.
    Abstract:
    A method has been developed for determining carboxypeptidase N (EC 3.4.17.3) activity by a hippuricase (EC 3.5.1.14)-assisted colorimetric assay. The method is based on the absorbance at 506 nm of a quinoneimine dye, produced by the action of carboxypeptidase N on the new substrates p-hydroxybenzoylglycine-L-Arg and p-hydroxybenzoylglycine-L-Lys. The enzyme acts on the substrates producing p-hydroxybenzoylglycine and L-Arg or L-Lys. The former is then hydrolyzed by hippuricase into p-hydroxybenzoic acid and Gly. Subsequently, oxidative coupling of p-hydroxybenzoic acid with 4-aminoantipyrine by sodium periodate forms a quinoneimine dye. The mean value of carboxypeptidase N activities in sera of 50 normal individuals was 30.8 (SD 5.9) nmol of p-hydroxybenzoylglycine released per milliliter of serum for the p-hydroxybenzoylglycine-L-Arg substrate and 137.8 (SD 28.1) for the p-hydroxybenzoylglycine-L-Lys substrate. The sensitivity of the assay is such that as little as 20 microliters of serum provides reliable and precise results (RSD% ranging from 1.8 to 4.9).
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