In enzymology, a 5,6-dihydroxy-3-methyl-2-oxo-1,2,5,6-tetrahydroquinoline dehydrogenase (EC 1.3.1.65) is an enzyme that catalyzes the chemical reaction

5,6-dihydroxy-3-methyl-2-oxo-1,2,5,6-tetrahydroquinoline + NAD⁺ ${\displaystyle \rightleftharpoons }$ 5,6-dihydroxy-3-methyl-2-oxo-1,2-dihydroquinoline + NADH + H⁺

Thus, the two substrates of this enzyme are 5,6-dihydroxy-3-methyl-2-oxo-1,2,5,6-tetrahydroquinoline and NAD⁺, whereas its 3 products are 5,6-dihydroxy-3-methyl-2-oxo-1,2-dihydroquinoline, NADH, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-CH group of donor with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is 5,6-dihydroxy-3-methyl-2-oxo-1,2,5,6-tetrahydroquinoline:NAD+ oxidoreductase.

• Schach S, Schwarz G, Fetzner S, Lingens F (1993). "Microbial metabolism of quinoline and related compounds. XVII Degradation of 3-methylquinoline by Comamonas testosteroni 63". Biol. Chem. Hoppe-Seyler. 374 (3): 175–81. doi:10.1515/bchm3.1993.374.1-6.175. PMID 8489738.