Thymidylate synthetase of human lymphocytes augmented in vitro by methotrexate

The addition of methotrexate to the assay system of thymidylate synthetase caused a reduction in the activity of the enzyme but addition of methotrexate to the culture of phytohemagglutinin stimulated normal human lymphocytes caused an increase in the activity of the enzyme which was abolished by the addition of actinomycin D or cycloheximide. These studies suggest that the antimetabolite augmented the enzyme activity by modulating the gene for the enzyme. This modulation of the gene could have been achieved by the thymineless state brought about by methotrexate or the antimetabolite could have affected gene reodont or brought about amplification of the gene. The results of the nucleoside incorporation were consistent with a thymidylate synthetase block; however, other explanations are offered.