Properties of a methotrexate-insensitive variant of dihydrofolate reductase derived from methotrexate-resistant L5178Y cells.

Abstract

We have established the presence of two distinct forms of dihydrofolate reductase in methotrexate-resistant L5178Y lymphoma cells. It was previously found that one of the reductases (Form II) was approximately 100,000-fold more resistant to methotrexate inhibition than Form I. In this report, the two forms of dihydrofolate reductase have been partially purified and their properties have been compared. The two forms were found to differ markedly in their heat stability, isoelectric points, and sensitivity to p-chloromercuriphenyl sulfonate. There were, in addition, minor differences in their Km and Vmax for folic acid, their pH optima, and their molecular weights. The loss of sensitivity to methotrexate inhibition by the Form II reductase is associated with significant alteration in the physical and chemical properties of the enzyme.