Abstract
A methotrexate-resistant subline of the Ehrlich ascites carcinoma contained approximately 14-fold more dihydrofolate reductase activity than did the drug-sensitive parent neoplasm. The activities of a number of tetrahydrofolate-dependent enzymes (i.e., of
N
5-formyltetrahydrofolate synthetase,
N
5,
N
10-methylenetetrahydrofolate dehydrogenase,
N
5-formyltetrahydrofolate isomerase, and serine hydroxymethylase) in cells of the resistant subline were similar to those of the sensitive neoplasm; thus, the increase in dihydrofolate reductase activity in the methotrexate-resistant cells was specific. Several properties of partially purified dihydrofolate reductase preparations from each cell line were similar, suggesting that no quantitative changes in the enzyme accompanied the transition to the resistant state. The elevated dihydrofolate reductase activity in the drug-resistant variant appeared to be sufficient to account for the refractoriness of this neoplasm, since considerable enzyme activity could be detected in these cells after large amounts of methotrexate. The problems of relating
in vitro enzyme assays to events occurring
in vivo are discussed.