Methionine regulation of N-5-methyltetrahydrofolate: homocysteine methyltransferase and its influence on the growth and protein synthesis in normal, neoplastic, and transformed cells in culture.

Normal human embryonic fibroblasts (CLV-58) and normal embryonic rat fibroblasts (REF) revealed equal growth in media containing 0.2 mM DL-homocysteine thiolactone (HOM) or in methionine (METH) enriched with 1.5 microM cyanocobalamin and 0.1 mM folic acid, whereas L 5178Y lymphoblasts and transformed rat fibroblasts (REF-S) showed more or less acute need for exogenous METH. In normal cells general protein synthesis remained unimpaired during 24 hours after substitution of METH by HOM. In L 5178Y and REF-S cells, however, protein synthesis was significantly inhibited. Measurements of N-5-methyltetrahydrofolate:homocysteine methyltransferase revealed a two- to threefold increase of enzyme activity in normal cells grown on HOM and in L 5178Y and REF-S cells grown on HOM with added METH. HeLa cells showed variability in growth pattern, in general protein synthesis response, and in inducibility of methyltransferase upon substitution of METH by HOM.