D-Amino acid oxidase (DAAO) was used to study the oxidative deamination of racemic mixturesof
D,
L-methionine in its soluble and immobilized forms and thus obtain the corresponding
-ketoacid. The soluble enzyme form was obtained from a
Trigonopsis variabilis CBS 4095 extract,free of
L-amino acid oxidase, and was co-immobilized with a 200-fold excess of catalase toavoid the undesirable side reaction of H
2O
2 with the
-keto acid, which would otherwise renderits corresponding decarboxylated acid, the 3-methylthiopropionic acid (MTPA). With thisbiocatalyst, quantitative conversion (>98%) of
D-methionine into the
-keto acid 4-methylthio-2-oxobutyric acid (MTOB) and into MTPA was achieved using 5 mg·mL
-1 of biocatalyst at pH8.0, 25
C, and pure oxygen at 3 vvm. A stirred tank reactor with in situ product removal(STR-ISPR) was developed to avoid conversion of MTOB into MTPA. The reaction mediumwas re-circulated through a strong anion exchange column (Amberlite IRA-400). This resultedin the complete removal of MTOB from the reaction medium. After the reaction, the reactionproducts were eluted sequentially with water (
L-methionine), 10 mM HCl (MTPA), and 0.5 MHCl (MTOB). After elution, MTOB was crystallized to its sodium salt.