NAD(H)-dependent dehydrogenase/reductase with a preference for cyclic substrates (PubMed:35150746) (By similarity). Catalyzes stereoselective conversion of 4-oxo-L-proline to cis-4-hydroxy-L-proline, likely a detoxification mechanism for ketoprolines (PubMed:35150746). Mediates the formation of 2,5-dihydroxybenzoate (2,5-DHBA), a siderophore that chelates free cytoplasmic iron and associates with LCN2, thereby regulating iron transport and homeostasis while protecting cells against free radical-induced oxidative stress. The iron-siderophore complex is imported into mitochondria, providing an iron source for mitochondrial metabolic processes in particular heme synthesis (By similarity). May act as a 3-hydroxybutyrate dehydrogenase (PubMed:16380372). cis-4-hydroxy-L-proline + NAD(+) = 4-oxo-L-proline + H(+) + NADH (R)-3-hydroxybutanoate + NAD(+) = acetoacetate + H(+) + NADH kcat is 19 sec(-1) for the NADH-dependent reduction of 4-oxo-L-proline. kcat is 0.02 sec(-1) for the NAD(+)-dependent oxidation of (R)-3-hydroxybutanoate. Optimum pH is 7.5-9.0 with (R)-3-hydroxybutanoate and 6.5-7.0 with 4-oxo-L-proline. Amino-acid metabolism. Siderophore biosynthesis. Homotetramer. Detected in liver (at protein level). Induced by low iron levels, and down-regulated by elevated iron levels. Belongs to the short-chain dehydrogenases/reductases (SDR) family. Postulated to act as a 3-hydroxybutyrate dehydrogenase, however its contribution to ketone body formation appears to be physiologically irrelevant since it has very low affinity for the substrate.