d-Amino acid oxidase and presence of d-proline in Xenopus laevis

We purified d-amino acid oxidase (EC 1.4.3.3, DAO) from Xenopus laevis tadpoles. The optimal temperature and pH for enzyme activity were 35–40 °C and 8.3–9.0, respectively, depending on the substrate amino acids available to the enzyme; the highest activity was observed with d-proline followed by d-phenylalanine. Activity was significantly inhibited by p-hydroxymercuribenzoate, but only moderately by p-chloromercuribenzoate or benzoate. Enzyme activity was increased until the final tadpole stage, but was reduced to one-third in the adult and was localized primarily in the kidney. The tadpoles contained high concentrations of d-proline close to the final developmental stage and nearly no d-amino acids were detected in the adult frog, indicating that d-amino acid oxidase functions in metamorphosis.