Molecular Pharmacology, Vol 8, 353-361, Copyright © 1972 by the American Society for Pharmacology and Experimental Therapeutics

Lack of Intracellular Conversion of Dihydrohomofolate to Tetrahydrohomofolate in Leukemia Cells with High Levels of Dihydrofolate Reductase

¹ Department of Pharmacology, Tufts University School of Medicine, Boston, Massachusetts 02111

The uptake of tritium-labeled dihydrohomofolate was investigated with intact leukemia cells, known to be either sensitive or resistant to amethopterin. Comparable experiments were carried out with H₂-folate. Chromatography to DEAE-cellulose was used to determine possible intracellular conversion of H₂-homofolate to H₄-homofolate, whereas microbiological assays were utilized for uptake studies of H₂-folate and conversion to H₄-folate. The amethopterin-resistant cells showed a higher uptake of H₂-folate and a higher intracellular conversion of H₂-folate to H₄-folate than the sensitive cells, consistent with high H₂-folate reductase activity in lysates of resistant cells. The situation with H₂-homofolate was quite different. Uptake of H₂-homofolate by sensitive and resistant cells was almost the same, with no detectable intracellular conversion of H₂-homofolate to H₄-homofolate, despite the fact that H₂-homofolate is an excellent substrate of H₂-folate reductase.

Note:
ACKNOWLEDGMENT We wish to thank Dr. Roy L. Kisliuk for valuable advice about microbiological assays as well as for critical evaluation of the design of the experiments described herein.