The function of CRYM (crystallin mu, Ensembl gene identifier ENSG00000103316) is as follows. Catalyzes the NAD(P)H-dependent reduction of imine double bonds of a number of cyclic ketimine substrates, including sulfur- containing cyclic ketimines (PubMed:21332720, PubMed:25931162). Under physiological conditions, it efficiently catalyzes delta(1)- piperideine-2-carboxylate (P2C) and delta(1)-pyrroline-2-carboxylate (Pyr2C) reduction, suggesting a central role in lysine and glutamate metabolism (PubMed:25931162). Additional substrates are delta(2)- thiazoline-2-carboxylate (T2C), 3,4-dehydrothiomorpholine-3-carboxylate (AECK), and (R)-lanthionine ketimine (LK) that is reduced at very low rate compared to other substrates (PubMed:25931162). Also catalyzes the NAD(P)H-dependent reduction of (S)-cystathionine ketimine (CysK) (By similarity). {ECO:0000250|UniProtKB:Q2KHX6, ECO:0000269|PubMed:21332720, ECO:0000269|PubMed:25931162, ECO:0000269|PubMed:9328354}.