In enzymology, a thiomorpholine-carboxylate dehydrogenase is an enzyme that catalyzes the chemical reaction The 3 substrates of this enzyme are thiomorpholine 3-carboxylate, NAD+, and NADP+, whereas its 4 products are 3,4-dehydro-thiomorpholine-3-carboxylate, NADH, NADPH, and H+. 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.