A qualitative and quantitative assay of dihydrofolate reductase in carrot cells resistant to methotrexate

The cytochemical pattern and the quantitative assay of DHFR on Daucus carota cell lines sensitive and resistant to methotrexate (MTX) are described. Cytoplasmatic enzyme activity is evidenced as granules of formazan, final reaction product, in the majority of the cells. DHFR activity appears low in Daucus carota cells used as control while in carrot cells resistant to MTX is very high, an observation which parallels previous biochemical studies. These results are supported by quantitative data of DHFR content in a random cell population. A possible correlation between overproduction of DHFR and gene amplification in MTX-resistant cells is discussed but the solution of this problem will depend on the availability of a dhfr plant probe.