A comparison of dihydrofolate reductase activity in intact leukemia cells and squamous cell carcinoma

This report examines the intracellular activity of dihydrofolate reductase using an in situ assay designed to measure enzymatic activity in intact cells. The rate of uptake of folic acid exceeded the rate of in situ dihydrofolate reductase activity suggesting that the reduction of folate to dihydrofolate, rather than transport, was the rate limiting step. In situ dihydrofolate reductase activity varied linearly with cell number. A comparison of the in situ activity revealed that a squamous cell carcinoma selected for methotrexate (MTX) resistant (SCC-15R) had 100 times greater dihydrofolate reductase (DHFR) activity than L1210 leukemia. In agreement with this finding, the in situ DHFR activity in SCC-15R cells was 50-fold less sensitive to the inhibitory effects of MTX than the L1210 in situ DHFR activity (IC50 = 1.1 x 10(-5) M and 2.4 x 10.7(-7) M respectively). The inhibition of in situ dihydrofolate reductase activity by MTX was found to correlate with the inhibition of growth, DNA synthesis (CdR incorporation) and in situ thymidylate synthase activity.