In enzymology, thiomorpholine-carboxylate dehydrogenase () is an enzyme that catalyzes the chemical reaction
The two substrates of this enzyme are thiomorpholine 3-carboxylate and oxidised nicotinamide adenine dinucleotide (NAD<sup>+</sup>). Its products are 3,4-dehydro-thiomorpholine-3-carboxylate, reduced NADH, and a proton. Nicotinamide adenine dinucleotide phosphate can be used as an alternative cofactor.
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 thiomorpholine-3-carboxylate:NAD(P)+ 5,6-oxidoreductase. Other names in common use include ketimine reductase, and ketimine-reducing enzyme.
CRYM, a taxon-specific crystallin protein that also binds thyroid hormones has thiomorpholine-carboxylate dehydrogenase activity.