L-Thiazolidine-4-carboxylate (T4C, thiaproline) is a sulfur-containing proline analog that stimulates the immune system in aging mice and inhibits urinary tract pathogens such as Escherichia coli. A constitutive NADP+-dependent T4C dehydrogenase activity was detected in the soluble fraction of a putA::Tn5 mutant of E. coli lacking l-proline dehydrogenase and partially purified by ammonium sulfate precipitation, dye-affinity chromatography on Cibacron Blue 3GA agarose, and ion-exchange chromatography on DEAE-cellulose. At each step in the purification, T4C dehydrogenase activity copurified with Delta1-pyrroline-5-carboxylate (P5C) reductase activity. E. coli strains with greatly reduced P5C reductase activity due to a proC mutation had no detectable T4C dehydrogenase activity. Although P5C reductase did not act on proline, it also catalyzed the oxidation of 3,4-dehydroproline. These results suggest that this biosynthetic enzyme may play a role in the degradation of proline analogs and limit the clinical efficacy of these compounds.