A comparison of folic acid and 5-methyltetrahydrofolate for prevention of DNA damage and cell death in human lymphocytes in vitro

Abstract

Folic acid (FA), the most oxidized and stable form of folate, is commonly used as a dietary supplement and in culture media. FA must be reduced and methylated to become the metabolically active form found in blood and utilized by tissues, i.e. 5-methyltetrahydrofolate (5-MeTHF). 5-MeTHF is the methyl group donor required for the conversion of homocysteine to methionine catalyzed by vitamin B(12)-dependent methionine synthase. It is hypothesized that 5-MeTHF may be more effective than FA in reducing spontaneous DNA damage and improving cell proliferation because, unlike FA, it can donate a methyl group for methionine synthesis, which is required for cell division via polyamine production and for maintenance methylation of DNA after its conversion to S-adenosylmethionine. We aimed to determine whether FA and 5-MeTHF differed in their capacity to prevent genetic damage and cell proliferation of human lymphocytes in vitro. Lymphocytes from eight female volunteers (40-48 years) were cultured in RPMI 1640 medium containing 12-120 nM FA or 5-MeTHF for 9 days. Mitogenesis was stimulated with phytohemagglutinin and the medium changed on days 3 and 6. Cytokinesis was inhibited by adding cytochalasin B on day 8 and cells were harvested and transferred to microscope slides on day 9. Chromosome damage, cell death and cytostasis was measured using the cytokinesis-block micronucleus assay in its comprehensive mode. The results showed that the frequency of micronucleated binucleate cells was significantly lower at 120 nM FA compared with 120 nM 5-MeTHF (P < 0.05), however, at 12 nM concentration both forms of folate were associated with increased frequency of micronuclei and nuclear buds relative to 120 nM (P < 0.05). Apoptosis tended to be significantly higher in 5-MeTHF cultures compared with FA cultures, however, necrosis and nuclear division were similar between cultures. We conclude that 5-MeTHF is not more efficient than FA in preventing human lymphocyte genomic instability in this in vitro system. Further research is needed to clarify the role of choline and methionine concentration and the importance of the reduced folate carrier and the folate receptor in determining the relative bioavailability of 5-MeTHF and FA with regard to genome stability.