Folic acid from purine nucleotides

In summary, DNA synthesis requires synthesis of dTMP and the purines adenine and guanine. THF, derived from the vitamin folic acid, is required for the biosynthesis of these nucleotides. Treatment with methotrexate blocks the cell s ability to regenerate THF, leading to inhibition of these biosynthetic pathways. The lack of nucleotides prevents DNA synthesis, and these cancer cells cannot divide without DNA synthesis. Unfortunately, the effects of methotrexate are nonspecific and other rapidly dividing cells such as epithelial cells in the oral cavity, intestine, skin, and blood cells are also inhibited. This leads to the side effects associated with methotrexate (and other cancer chemotherapy drugs) such as mouth sores, low white blood cell counts, stomach upset, hair loss, skin rashes, and itching. 

DNA synthesis requires the formation of dTMP and the purines adenine and guanine. THF, derived from the vitamin folic acid, is required for the biosynthesis of these nucleotides. 

In order to systematically investigate the mechanism(s) of fragile chromosome expression in folate deficient media we developed a basic culture medium. The medium is deficient in folic acid and a number of metabolites produced by folate mediated one-carbon transfer reactions (Fig. 1). These include the amino acids, serine, glycine and methionine the purine source hypoxanthine and the thymidylate source thymidine. Previous studies of fragile site expression have used leucocytes isolated in autologous plasma and cultured in medium supplemented with undialyzed serum. In order to eliminate serum sources of folate, amino acids and nucleotide precursors, we have used washed lymphocytes isolated from ficoll-hypaque banding and medium containing dialyzed human AB serum. 

The mechanism of formation of thymine and hydroxymethylcytosine are not yet known, but the addition of a carbon to position 5 appears to involve a transfer from a folic acid derivative. As in the case of the purines, the relation between ribose and deox3nibose nucleotides is obscure. Deoxyribonucleosides can be formed from deoxyribose-l-phosphate by a nucleoside phosphorylase reaction. 

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