5-FORMYLTETRAHYDROFOLATE REGULATES HOMOCYSTEINE REMETHYLATION IN HUMAN NEUROBLASTOMA

The metabolic role of 5-formyltetrahydrofolate is not known; however, it is an inhibitor of several folate-dependent enzymes including serin e hydroxymethyltransferase, Methenyltetrahydrofolate synthetase (MTHFS ) is the only enzyme known to metabolize 5-formyltetrahydrofolate and catalyzes the conversion of 5-formyltetrahydrofolate to 5,10-methenylt etrahydrofolate. In order to address the function of 5-formyltetrahydr ofolate in mammalian cells, intracellular 5-formyltetrahydrofolate lev els were depleted in human 5Y neuroblastoma by overexpressing the huma n cDNA encoding MTHFS (5YMTHFS cells), When cultured with 2 mM exogeno us glycine, the intracellular serine and glycine concentrations in 5YM THFS cells are elevated approximately 3-fold relative to 5Y cells; 5YM THFS cells do not contain measurable levels of free methionine and dis play a 30-40% decrease in cell proliferation rates compared with 5Y ce lls. Medium supplemented with pharmacological levels of exogenous foli nate or methionine ameliorated the glycine induced growth inhibition. Analysis of the folate derivatives demonstrated that 5-methyltetrahydr ofolate accounts for 30% of total cellular folate in 5Y cells when cul tured with 5 mm exogenous glycine. 5YMTHFS cells do not contain detect able levels of 5-methyltetrahydrofolate under the same culture conditi ons. These results suggest that 5-formyltetrahydrofolate inhibits seri ne hydroxymethyltransferase activity in vivo and that serine synthesis and homocysteine remethylation compete for one-carbon units in the cy toplasm.