Accounting for an Isobaric Interference Allows Correct Determination of Folate Vitamers in Serum by Isotope Dilution-Liquid Chromatography-Tandem MS

Mild and prolonged oxidative degradation of 5-methyltetrahydrofolate (5-methylTHF) leads to the biologically inactive pyrazino-s-triazine derivative of 4 alpha-hydroxy-5-methylTHF (MeFox). MeFox and the biologically active 5-formyltetrahydrofolate (5-formyITHF) are isobaric compounds that behave similarly during chromatographic and mass separation, making coelution and misidentification likely. Our published routine liquid chromatography-tandem MS (LC-MS/MS) method did not discern between 5-formyITHF and MeFox, measuring the sum of these compounds at a mass to charge ratio (m/z) of 474-327 as 5-formyITHF. We modified this method to separate MeFox and 5-formyITHF by either chromatography or unique mass transitions and then applied the 2 methods to serum specimens to determine typical concentrations of these compounds. The 2 unique transitions (m/z. 5-formyITHF, 474-299; MeFox, 474-284) showed good sensitivity [limit of detection (nmol/L): 5-formyITHF, 0.21; MeFox, 0.34], selectivity (no interfering peaks), spiking recovery (mean +/- SD: 5-formyITHF, 103 +/- 3.4%; MeFox, 94 +/- 10%), and low imprecision (CV: 5-formyITHF, 3.9% at 2.4 nmol/L; MeFox, 5.1% at 2.9 nmol/L). The mass separation method detected 5-formyITHF in the same specimens as the chromatographic separation method. Analysis of several thousand serum specimens showed that the majority (similar to 85%) contained MeFox at <3 nmol/L but no detectable 5-formyITHF concentrations, some (similar to 14%) contained 5-formyITHF at <0.5 nmol/L, and a few specimens contained 5-formyITHF at >1 nmol/L and MeFox at >10 nmol/L. In summary, serum can contain 5-formyITHF high enough to contribute to total folate and contains MeFox that will bias total folate if not appropriately separated. Including measurements of MeFox and 5-formyITHF along with the other folate vitamers will enhance assessments of the association between biologically active folate and health effects. J. Nutr. 143: 108-113, 2013.