The authors describe a simplified and rapid method for determining L-cystineaminopeptidase ( CAP) in serum with the aid of S-benzyl-L-cysteine-p-nitroanilide (BCN). The liberated p-nitroaniline reacts with p-dimethylaminocinnamaldehyde to give ared dye. The method requires 15min incubation at 37°, followed by 10min colour development.
The proposed method is more sensitive than that with using L-cystine-di-P-nitroaniiinde (CDNl) and using L-cystine-di-naphthylamide (Wada's method). The correlation coefficient between results of this method and those obtained with L-cystine-di-naphthylamide (CDNA) as substrate was 0.97.