Oxidation of 3,4-dehydro-D-proline and other D-amino acid analogues by D-alanine dehydrogenase from Escherichia coli

3,4-Dehydro-DL-proline is a toxic analogue of L-proline which has been useful in studying the uptake and metabolism of this key amino acid. When membrane fractions from Escherichia coli strain UMM5 (putA1::Tn5 proC24) lacking both L-proline dehydrogenase and L-Delta(1)-pyrroline-5-carboxylate reductase were incubated with 3,4-dehydro-DL-proline, pyrrole-2-carboxylate was formed. There was no enzyme activity with 3,4-dehydro-L-proline, but activity was restored after racemization of the substrate. Oxidation of 3,4-dehydro-DL-proline by membrane fractions from strain UMM5 was induced by growth in minimal medium containing D- or L-alanine, had a pH optimum of 9, and was competitively inhibited by D-alanine. An E. coli strain with no D-alanine dehydrogenase activity due to the dadA237 mutation was unable to oxidize either 3,4-dehydro-D-proline or D-alanine, as were spontaneous Dad(-) mutants of E. coli strain UMM5. Membrane fractions containing D-alanine dehydrogenase also catalyzed the oxidation of D-2-aminobutyrate, D-norvaline, D-norleucine, cis-4-hydroxy-D-proline, and DL-ethionine. These results indicate that d-alanine dehydrogenase is responsible for the residual 3,4-dehydro-DL-proline oxidation activity in putA proC mutants of E. coli and provide further evidence that this enzyme plays a general role in the metabolism of D-amino acids and their analogues.