Intracellular metabolism of 5-methyltetrahydrofolate and 5-formyltetrahydrofolate in a human breast-cancer cell line

This report describes the intracellular metabolism of 5-methyltetrahydrofolate (5-methyl-H₄PteGlu) and 5-formyltetrahydrofolate (5-formyl-H₄PteGlu) to the various folate forms and their respective polyglutamated states in the MCF-7 human breast-cancer cell line. The intracellular folate distribution observed in MCF-7 cells treated with 5-methyl-H₄PteGlu was similar to that seen in cells treated with 5-formyl-H₄PteGlu. In cells exposed to 5-formyl-H₄PteGlu for 24 h, the folate pool consisted of 103±10 pmol/mg 10-formyl-H₄PteGlu, 120±18 pmol/mg H₄PteGlu, and 71±18 pmol/mg 5-methyl-H₄PteGlu versus 88±5, 54±20 and 87±10 pmol/mg, respectively, for cells exposed to 5-methyl-H₄PteGlu. Only the difference seen in H₄PteGlu levels between cells exposed to either 5-methyl-H₄PteGlu or 5-formyl-H₄PteGlu reached statistical significance (P<0.05). In the absence of vitamin B12, exposure to 5-methyl-H₄PteGlu resulted in 154±17 pmol/mg 5-methyl-H₄PteGlu along with only 8±5 pmol/mg 10-formyl-H₄PteGlu and 4±2 pmol/mg H₄PteGlu, thus demonstrating the marked dependence on vitamin B12 for the metabolism of 5-methyl-H₄PteGlu to the other intracellular folates. 5–10-Methylene-H₄PteGlu (2±1.3 pmol/mg) was detected only in cells exposed to 5-formyl-H₄PteGlu for 24 h, not in cells treated with 5-methyl-H₄PteGlu. The profile of polyglutamates detected in cells treated with either 5-formyl-H₄PteGlu or 5-methyl-H₄PteGlu for 24 h was not significantly different, although cells treated with 5-methyl-H₄PteGlu tended to have less conversion to the higher polyglutamates (Glu3–Glu5) as compared with those treated with 5-formyl-H₄PteGlu. In 5-methyl-H₄PteGlu-treated cells grown in the absence of vitamin B12, the pentaglutamate was the only polyglutamate form detected, accounting for only 11% of the total folate pool. Since there does not appear to be a greater formation of the optimal reduced-folate forms necessary to achieve enhanced thymidylate synthase (TS) inhibition through ternary-complex formation in cells exposed to 5-methyl-H₄PteGlu versus 5-formyl-H₄PteGlu, these studies suggest that the use of 5-methyl-H₄PteGlu would not be advantageous over that of 5-formyl-H₄PteGlu in combination regimens with the fluoropyrimidines.