Serine hydroxymethyltransferase: Evidence for its presence in human, monkey and rat lenses

Serine hydroxymethyltransferase (SHMT) is present in cultured rat, monkey and human lenses as shown by ¹⁵N-serine or ¹⁵N-glycine labeling studies. Following incubation with ¹⁵N-serine, the percent enrichment of ¹⁵N in glycine increases with time, and vice versa, demonstrating the presence of the enzyme, and the reversibility of the reaction in intact cultured lenses. Similar patterns of ¹⁵N enrichment were found in all three species, but lenses from young rats showed a higher percent enrichment than lenses obtained from older animals. Label from ¹⁵N-serine or ¹⁵N-glycine was also incorporated into a number of other amino acids, including aspartate, alanine, glutamate and proline. Conclusive evidence for the presence of SHMT in rat lens homogenates has been obtained by direct enzyme assay. The specific activity of rat lens SHMT was age dependent; approximately 2·4 units per mg protein in day old rats, declining to about 0·15 units per mg in adult animals. The higher specific activity observed in younger animals is consistent with the ¹⁵N labeling results obtained with cultured lenses. Lens SHMT has been partially characterized. In the presence of excess tetrahydrofolate the assay was essentially linear with increasing time. With serine as the substrate, the enzyme requires tetrahydrofolate for activity, the pH optimum is between pH 7·5 and 8·3, the K_m for serine is about 0·25 mm, and the enzyme is inhibited by cycloserine. In conclusion, this study demonstrates the existence of SHMT in rat, monkey and human lenses. Rat lens specific activity has been shown to decrease with increasing age, and the enzyme has been partially characterized. Activity was detected by measuring the direct formation of the product, N⁵,N¹⁰-methylenetetrahydrofolate, and by following the reversible interconversion of ¹⁵N-labeled serine and glycine in intact cultured lenses.