Serine hydroxymethyltransferase: evidence for its existence in rat lens

Previous studies by Jernigan using cultured adult rat lens incubated with N15-glu or gln indicated that the enzyme hydroxymethyltransferase (SHMT) was present in lens. SHMT catalyzes the reversible formation of 5,10-methylene tetrahydrofolate and glycine from serine and tetrahydrofolate (THFA) and is the major point of entry of carbons into the one-carbon folate pool. Using a combination of direct enzyme assay and mass spectroscopy, conclusive evidence for the presence of the enzyme has been obtained. After incubation of cultured adult rat lenses for up to 24 hrs with N15-labeled serine substantial amounts of label were found in glycine. In contrast, after a 24 hr incubation with N15-glycine, lesser amounts of label were found in serine. The data confirm the reversibility of the SHMT reaction in vivo. SHMT activity has been unequivocally demonstrated in rat lens using a newly developed assay based on the binding of 5,10-methylene THFA to DEAE-cellulose paper. SHMT activity was highest in lenses from young animals and decreased with increasing age; for example, the specific activity was 3.98 units/mg on day one and 0.23 units/mg at day 30. The assay requires THFA, is linear with respect to the time and protein. The properties of the lenticular cystosolic SHMT have been studied