(D)-amino acid oxidase (DAO) is of considerable practical importance, such as bioconversion and enzymatic assay. In this study, we succeeded in obtaining a thermostable mutant DAO from porcine kidney by a single amino acid substitution. This mutant enzyme, F42C, was stable at 55 degrees C, while the wild-type enzyme was stable only up to 45 degrees C. The K, values of F42C for (D)-amino acids was about half of those of the wild-type enzyme. This mutant DAO with improved stability and affinity for its substrates is advantageous for the determination of (D)-amino acids. (c) 2005 Wiley Periodicals, Inc.