Evaluation of S-benzyl- L -cysteine-4'-nitroanilide as a substrate for serum cystine aminopeptidase.

Under appropriate conditions, S-benzyl-L-cysteine-49-nitroanilide is rapidly hydrolyzed by serum cystine aminopeptidase. The enzyme may be estimated kinetically by determining the rate of liberation of 4-nitroaniline. A simple automated procedure has been devised, and 50 samples of pregnancy sera can be analyzed per hour by use of the LKB 8600 Reaction Rate Analyzer. The correlation coefficient between results of this method and those obtained with L-cystine-di-49-nitroanilide as substrate was 0.97.

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