Site-directed mutant of spinach betaine aldehyde dehydrogenase (BADH) was obtained by replacing Glu 103 with Gln 103 and the resulting E 103 Q mutant was expressed in Escherichia coli. The mutant BADH as compared to the wild-type BADH was slightly more sensitive to inhibition by NaCl but less sensitive to inhibition by (NH(4))(2)SO(4). Glycinebetaine (GB) activated the wild-type enzyme but not the mutant enzyme. Stronger inhibition by choline was observed in the mutant enzyme than in the wild-type enzyme whereas the reverse was observed for the inhibition by isovaleraldehyde. The mutant enzyme exhibited a broader temperature optimum than the wild-type enzyme, however, the mutant enzyme appeared to be more heat labile. Both mutant and wild-type enzymes could be protected by NAD(+) against thermal inactivation in a similar manner. However, neither GB nor NaCl could afford protection against thermal inactivation in the mutant enzyme whereas some protection was observed in the wild-type enzyme. Similar pH activity profile was obtained for both mutant and wild-type enzymes. The mutant enzyme was less stable than the wild-type enzyme under the pH range of 5-11. Overall results suggest that the negative charge of Glu 103 at the surface of the spinach BADH plays some roles in the maintenance of the structural integrity of the enzyme.