In enzymology, a thiomorpholine-carboxylate dehydrogenase (EC 1.5.1.25) is an enzyme that catalyzes the chemical reaction

thiomorpholine 3-carboxylate + NAD(P)+ 3,4-dehydro-thiomorpholine-3-carboxylate + NAD(P)H + H⁺

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.

• Nardini M, Ricci G, Caccuri AM, Solinas SP, Vesci L, Cavallini D (1988). "Purification and characterization of a ketimine-reducing enzyme". Eur. J. Biochem. 173 (3): 689–94. doi:10.1111/j.1432-1033.1988.tb14053.x. PMID 3371353.