In enzymology, 4-oxoproline reductase () is an enzyme that catalyzes the chemical reaction
The substrates of this enzyme are cis-4-hydroxyproline, and oxidised nicotinamide adenine dinucleotide (NAD<sup>+</sup>). Its products are 4-oxo-L-proline and reduced NADH.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD<sup>+</sup> or NADP<sup>+</sup> as acceptor. The systematic name of this enzyme class is 4-hydroxy-L-proline:NAD<sup>+</sup> oxidoreductase. This enzyme is also called hydroxy-L-proline oxidase. This enzyme was originally thought to participate in the metabolism of arginine and proline. However, recent data show that it is unlikely since neither 4-oxo-L-proline nor cis-4-hydroxy-L-proline are metabolites of these metabolic pathways.
The gene encoding 4-oxo-L-proline reductase was identified as 3-hydroxybutyrate dehydrogenase 2 (BDH2) by Sebastian Kwiatkowski and co-workers in 2022. The enzyme is a member of the Short-chain Dehydrogenases/Reductases (SDR) family of enzymes.