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). Major component of eye lens in marsupials. (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. Expressed at high abundance in lens, but outside the lens it is preferentially expressed in neural tissues, retina and brain. Belongs to the ornithine cyclodeaminase/mu-crystallin family.