RESUMEN
By using ligand-binding radioassay for the enzyme dihydrofolate reductase [DHFR], a new form of this enzyme has been identified in L1210 leukemia cells. This form of DHFR displays very low affinity for the anticancer drug, methotrexate [MTX]. Further enzyme kinetic studies using a sensitive radioenzymatic assay for DHFR revealed that this form of DHFR had catalytic activity for the reduction of dihydrofolic acid to tetrahydrofolic acid. Studies using gel filtration chromatography with Sephadex G-75 at pH 5.0 and pH 7.2 on crude L1210 leukemia cell lysate and purified L1210 DHFR, both complexed with radiolabelled MTX, provided additional evidence that the MTX binding results were not due to some experimental artifacts. This low affinity form of enzyme was also found in two human colon tumor tissues. We suggest that, presence of a low affinity form of the enzyme in certain cancer cells may be one of the underlying causes of resistance to MTX therapy in these cells