Folic acid, a double-edged sword? Influence of folic acid on intracellular folate and dihydrofolate reductase activity

Objective Synthetic folic acid is widely used, amongst others in the prevention of neural tube defects and reducing homocysteine, a risk factor for cardiovascular disease. Its effect on intracellular folate and the activity of essential cellular folate-converting enzymes is poorly understood. Methods Six lymphoblast cell lines were cultured for 15 days with medium containing either FA, 5-methyltetrahydrofolate or 5-formyltetrahydrofolate at physiological levels of 20 nmol/L or at supra-physiological concentrations of 2.3 µmol/L (standard concentration in cell culture medium). Subsequently, intracellular concentrations of folate, homocysteine, and activities of dihydrofolate reductase and methylenetetrahydrofolate reductase were measured. Results Unmetabolised FA was present inside lymphoblasts cultured on 2.3 µmol/L FA. Under these conditions, methylenetetrahydrofolate reductase activity remained unchanged, but dihydrofolate reductase activity decreased 20% compared to cells cultured on the same amount of 5-methyltetrahydrofolate. For both physiological and supra-physiological folate culture conditions, intracellular total folate concentrations were reduced more than 80% when cells were cultured on FA instead of 5-methyltetrahydrofolate. Intracellular folate concentrations were similar for cells cultured on 5-methyltetrahydrofolate and 5-formyltetrahydrofolate. In addition, FA inhibited the conversion of dihydrobiopterin to tetrahydrobiopterin, which may lead to endothelial nitric oxide synthase uncoupling. Conclusions Lymphoblasts cultured on standard medium containing FA show a remarkably lower intracellular folate content and perturbed DHFR activity than cells cultured on 5-methyltetrahydrofolate. Apparently, FA is a much less efficient substrate to boost intracellular folate levels than natural folates, and may even disturb activity of essential enzymes in one-carbon metabolism.