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We have developed a practical method for the comprehensive analysis of polar anionic metabolites in biological samples with the use of a nano-LC/MS system. A polyamine-bonded polymer-based apHera NH2 column, which is compatible with ammonium carbonate buffer, effectively retained anionic polar metabolites, such as organic acids, sulfates, and phosphates, but multiply phosphorylated or carboxylated compounds showed highly distorted peak shapes on chromatograms. We found that addition of a trace amount of the metal chelating reagent ethylenediaminetetraacetic acid (EDTA) to the sample solution dramatically improved peak shapes of multiply charged anionic compounds, even though the mass spectra showed no trace of adduct ions in the absence of EDTA. The detection limits of typical polar anionic metabolites in the full-scan mode were from 0.19 to 2.81 pmol. After optimization of all the procedures from sample preparation to nano-LC/MS analysis, we applied our method to real biological samples: Hela cells, mouse brain, human cerebrospinal fluid (CSF), and human plasma. Our results indicated that phosphorylated metabolites were abundant in Hela cells and brain, while plasma and cerebrospinal fluid (CSF) mostly contained organic acids. Phosphorylated compounds might not be secreted into CSF/plasma or might be unstable in CSF/plasma. Finally, the method was used to examine the mode of action of the anticancer drug methotrexate (MTX), which inhibits purine de novo biosynthesis and thymidine biosynthesis. In addition of the expected changes of metabolite levels, we found that a previously unreported metabolite, probably a methylated uridine 5'-triphosphate (UTP), was produced by MTX-treated Hela cells.
Myint et al. (Mon,) studied this question.