Cytosolic serine hydroxymethyltransferase tetramer (SHMT1) catalyzes the reaction of (3S)-3-hydroxy-N(6),N(6),N(6)-trimethyl-L-lysine(1+) (NTMLYS) to form glycine (Gly) and 4-trimethylammoniobutanal (TEABL). Ogata & Fujioka (1981) and Masuda et al. (1987) have each purified to apparent homogeneity a cytosolic enzyme that in the presence of tetrahydrofolate catalyzes the conversion of serine to glycine but that in its absence catalyzes the cleavage of L-allothreonine to glycine and an aldehyde. The native enzyme appears to be a tetramer. This rat enzyme and its human ortholog are inferred to mediate the cleavage of HTMLYS to yield TEABL and Gly in vivo (Vaz & Wanders 2002).