In enzymology, D-nopaline dehydrogenase () is an enzyme that catalyzes the chemical reaction
The three substrates of this enzyme are nopaline, oxidised nicotinamide adenine dinucleotide phosphate (NADP<sup>+</sup>), and water. Its products are L-arginine, reduced NADPH, ñ-ketoglutaric acid and a proton. Nopaline is a member of the class of compounds called opines.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-NH group of donors with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is N2-(D-1,3-dicarboxypropyl)-L-arginine:NADP+ oxidoreductase (L-arginine-forming). Other names in common use include D-nopaline synthase, nopaline dehydrogenase, nopaline synthase, NOS, 2-N-(D-1,3-dicarboxypropyl)-L-arginine:NADP+ oxidoreductase, and (L-arginine-forming). This enzyme participates in arginine and proline metabolism.