Assay of whole blood (6S)-5-CH3-H4folate using ultra performance liquid chromatography tandem mass spectrometry

Folates act as essential coenzymes in many biological pathways, including the synthesis and methylation of DNA. Low folate concentration in serum and whole blood (WB) is associated with several disease conditions. We describe a stable-isotope-dilution ultra performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) method for the quantification of (6S)-5-CH(3)-H(4)folate (where H(4)folate is tetrahydrofolate) and non-CH(3)-H(4)folate [sum of HCO-H(4)folate, (6R)-5,10-CH(+)-H(4)folate, (6R)-5,10-CH(2)-H(4)folate, (6S)-H(4)folate, dihydrofolate, and folic acid] in WB. The assay includes a solid-phase extraction procedure after the hemolysis and deconjugation. The method was linear over the concentration range from 0.2 to 200 nmol/L. The limits of detection were 0.40 nmol/L or lower for the folate forms. The interassay coefficients of variation were 7.4% for (6S)-5-CH(3)-H(4)folate and 15.4% for non-CH(3)-H(4)folate. For the folate forms, the recoveries were between 97.1% and 102.7%. Sample preparation caused the generation of artificial folic acid in WB and serum in a dose-dependent manner, which can lead to misinterpretation of the results. The use of antioxidants could not prevent the formation of folic acid. The median fasting WB folate concentrations from 42 nonsupplemented and nonfortified adults were 576 nmol/L (6S)-5-CH(3)-H(4)folate and 73.6 nmol/L non-CH(3)-H(4)folate, and 1,206 nmol/L (6S)-5-CH(3)-H(4)folate and 155 nmol/L non-CH(3)-H(4)folate for 35 adults who had taken 500 μg of folic acid, 50 mg of vitamin B(6), and 500 μg of vitamin B(12) per day orally for 6 months. In conclusion, the UPLC-MS/MS method is fast and has a good sensitivity and selectivity for WB folates. We observed a dose-dependent oxidation of (6S)-H(4)folate, which resulted in the formation of artificial folic acid in serum and WB. To minimize this effect, we recommend a fast sample preparation.