Abstract
The metabolically active folate co-factors that exist in nature are reduced forms which are unstable chemically but readily convert to the corresponding oxidated forms both in vivo and in vitro. These oxidized derivatives are always seen during the analysis of natural materials. This study investigates the utilization of these derivatives and some common dietary folates by human lymphocytes in culture. They were assessed directly by their ability to suppress the incorporation of [3H]-thymidine into folate deficient human peripheral blood lymphocytes. The order of effectiveness was as follows: 5-formyltetrahydrofolate> 5-methyltetrahydrofolate> 5-methyldihydrofolate>folic acid itself. 10-formylfolate, which is a potent inhibitor of dihydrofolate reductase was as effective as folic acid whereas 4a-hydroxy-5-methyltetrahydrofolate was completely inactive.
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