NAD(H)-dependent dehydrogenase/reductase with a preference for cyclic substrates (By similarity). Catalyzes stereoselective conversion of 4-oxo-L-proline to cis-4-hydroxy-L-proline, likely a detoxification mechanism for ketoprolines (By similarity). 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 (PubMed:20550936, PubMed:24863067, PubMed:24777603). May act as a 3-hydroxybutyrate dehydrogenase (By similarity). (Microbial infection) May play a role in susceptibility to bacterial infection by providing an assimilable source of iron that is exploited by pathogenic bacteria. Host iron-siderophore complexes can be used by bacteria to promote their own growth and pathogenicity. cis-4-hydroxy-L-proline + NAD(+) = 4-oxo-L-proline + H(+) + NADH (R)-3-hydroxybutanoate + NAD(+) = acetoacetate + H(+) + NADH Amino-acid metabolism. Siderophore biosynthesis. Homotetramer. Detected in liver, spleen and macrophages (PubMed:24863067). Widely expressed. Down-regulated upon E.coli infection to limit access to host iron pool. Down-regulated in macrophages by exposure to bacterial lipopolysaccharide (LPS). Mutant mice are born at the expected Mendelian frequency. They display microcytic and hypochromic anemia and iron overload in spleen and liver associated with heme toxicity when kept on high-iron diet. The plasma levels of ketone bodies is normal. 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.