An assay for uracil in human DNA at baseline : effect of marginal vitamin B6 deficiency

Improvements are made to our gas chromatography-mass spectrometry based assay for quantifying low levels of DNA-uracil. Folate deficiency leads to increased deoxyuridine monophosphate/thymidylate (dUMP/dTMP) ratios and uracil misincorporation into DNA, which may increase cancer risk. Vitamin B6 (B6) deficiency might also result in increased DNA-uracil because B6 is a cofactor for serine hydroxymethyltransferase (SHMT), which catalyzes the methylation of tetrahydrofolate (THF) to methylene-THF, the folate form that is required to convert dUMP to dTMP. However, the low baseline levels of DNA-uracil in healthy human lymphocytes are difficult to measure accurately. This version of the assay (Uracil assay V3) has an approximately ten-fold increase in signal strength over the previous method, and a ten-fold lower detection limit (0.2 pg uracil). Five micrograms of DNA, the amount in about 1 ml of human blood, is a suitable amount for this assay. Using this improved assay, DNA-uracil was measured in lymphocytes from twelve healthy smoking or nonsmoking young men and women who consumed a B6 restricted diet (0.7 mg B6/day, or ≈ half the RDA) for 28 days. DNA-uracil concentration was not significantly related to B6 status or smoking. More severe and/or prolonged vitamin B6 deficiency may be necessary to detect significant changes in DNA-uracil in humans. The average concentration of DNA-uracil from these subjects was found to be approximately 3,000 uracils per diploid lymphocyte, which is comparable to steady state levels of one of the oxidative adducts of DNA, 8-oxoguanine.