The phytohemagglutinin (PHAI-stimulated human lymphocytes demonstrated trace or no activity of (dihydro) folate acid reductase using three methods including a radioassay, but demonstrated ample activity of thymidylate synthetase. This was true regardless of the day of harvest, (first through seventh) of the stimulated lymphocyte. The lymphocyte extracts revealed no inhibitor to the reductase enzyme when these extracts were added before the liver extracts to the assay system. When methotrexate (MTX) was added to the culture media of the lymphocytes, there was, as expected, an increase in the synthetase activity, but the expected rise in the reductase activity did not occur, it remained nil. On the other hand, MTX did influence the incorporation of nucleosides by the stimulated lymphocytes in a fashion similar to its action on the incorporation of the same nucleosides by other cells.
Several of the folate-related enzymes have been investigated in human tissues, particularly in the blood and bone marrow cells. The effects of antimetabolites, methotrexate (MTX) in particular, on the DNA synthesis of normal or leukemic cells have also been the subject of various studies (Bertino et al., '65;