Effect of excess methionine on methotrexate cell-killing action in vitro.

: Methionine at the concentrations 10, 15 and 20 times higher than in complete Fischer's medium produced a transient impairment or a transient block in the growth of L 5178 Y cells in culture without influencing their viability. Autoradiographic data revealed that methionine-induced inhibition of DNA synthesis was followed by a partial synchronization of cells in the phase of DNA synthesis. Methionine at a high concentration (1.5 mg/ml) enhanced the cell-killing action of methotrexate under conditions of delayed drug addition and did not diminish it when added with the drug simultaneously.