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. (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 Optimum pH is 4.5. Homodimer. Expressed in neural tissue, muscle and kidney. The disease is caused by variants affecting the gene represented in this entry. Belongs to the ornithine cyclodeaminase/mu-crystallin family.