Oxidation of L-thiazolidine-4-carboxylate by L-proline dehydrogenase in Escherichia coli.

L-Thiazolidine-4-carboxylate (T4C, gamma-thioproline) is a toxic analogue of L-proline. T4C can be oxidized by Escherichia coli to form N-formylcysteine, which is hydrolysed to yield formate and cysteine. To determine if L-proline dehydrogenase (EC 1.5.99.8) catalyses T4C degradation, membrane fractions from E. coli were tested for T4C and proline oxidation activity. The specific activity for T4C oxidation in membranes from bacteria grown with 10 mM-proline was similar to the specific activity for proline oxidation and about 100 times that in membranes from bacteria grown without proline. Both oxidation activities were inactivated at 45 degrees C at the same rate. Membranes from a strain with a deletion of the putA gene encoding L-proline dehydrogenase or a strain with a putA::Tn5 insertion mutation had no detectable activity with either substrate. Although T4C was a simple competitive inhibitor of proline oxidation, proline inhibited T4C oxidation in a way that gave competitive but sigmoidal kinetics. At low concentrations, T4C induced proline dehydrogenase synthesis. Cysteine auxotrophs containing the putA::Tn5 mutation could still use T4C as a cysteine source, and bacteria with this mutation consumed oxygen in the presence of T4C at half the control rate. These results indicate that T4C is a substrate and an inducer of L-proline dehydrogenase but suggest that E. coli also contains a second enzyme catalysing T4C degradation.