Specifically catalyzes the reduction of imine bonds in brain substrates that may include cystathionine ketimine (CysK) and lanthionine ketimine (LK). Binds thyroid hormone which is a strong reversible inhibitor. Presumably involved in the regulation of the free intracellular concentration of triiodothyronine and access to its nuclear receptors (By similarity). (3R)-1,4-thiomorpholine-3-carboxylate + NAD(+) = 3,4-dehydrothiomorpholine-3-carboxylate + 2 H(+) + NADH (3R)-1,4-thiomorpholine-3-carboxylate + NADP(+) = 3,4-dehydrothiomorpholine-3-carboxylate + 2 H(+) + NADPH Homodimer. Belongs to the ornithine cyclodeaminase/mu-crystallin family.