$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Water-soluble metabolites are low-molecular-weight intermediates and products of metabolism that contribute to essential cellular processes. These evolutionarily conserved processes include the conversion of nutrients into usable energy, synthesis of macromolecules, cellular growth and signaling, cell cycle control, regulation of gene expression, stress response, post-translational regulation of metabolism, maintenance of mitochondrial functionality, vesicular cellular trafficking, autophagy, cellular aging, and regulated cell death1,2,3.
Many of these essential roles of water-soluble metabolites have been discovered by studies in the budding yeast S. cerevisiae1,3,4,7,9,14,15,16,17,18,19,20,21,22. This unicellular eukaryote is a useful model organism for dissecting mechanisms through which water-soluble metabolites contribute to cellular processes due to its amenability to advanced biochemical, genetic, and molecular biological analyses23,24,25,26. Although the LC-MS/MS methods of non-targeted metabolomics have been used for studying the roles of water-soluble metabolites in budding yeast3,18,22,27, this type of analysis requires the improvement of its versatility, robustness, sensitivity, and ability to distinguish between different structural isomers and stereoisomeric forms of these metabolites.
Recent years are marked by significant advances in applying the LC-MS/MS methods of non-targeted metabolomics to the profiling of water-soluble metabolites in vivo. However, many challenges in using this methodology remain2,28,29,30,31,32,33,34,35,36. These challenges include the following. First, the intracellular concentrations of many water-soluble metabolites are below a threshold of sensitivity for the presently used methods. Second, the efficiency of metabolic activity quenching is too low, and the extent of quenching-associated cell leakage of intracellular metabolites is too high for current methods; hence, the presently used methods under-estimate the intracellular concentrations of water-soluble metabolites. Third, the existing methods cannot differentiate the structural isomers (i.e., molecules with the same chemical formula but different atomic connectivity) or stereoisomers (i.e., molecules with the same chemical formula and atomic connectivity, but with the different atomic arrangement in space) of specific metabolites; this prevents the correct annotation of certain metabolites by the presently used methods. Fourth, the existing mass spectral online databases of parent ions (MS1) and secondary ions (MS2) are incomplete; this affects the correct identification and quantitation of specific metabolites using the raw LC-MS/MS data produced with the help of the current methods. Fifth, the existing methods cannot use a single type of metabolite extraction to recover all or most classes of water-soluble metabolites. Sixth, the existing methods cannot use a single type of the LC column to separate from each other all or most classes of water-soluble metabolites.
Here, we optimized conditions for quenching of metabolic activity within S. cerevisiae cells, maintaining most of the water-soluble metabolites within these cells before extraction, and extracting most classes of water-soluble metabolites from yeast cells. We developed a versatile, robust, and sensitive method for the LC-MS/MS-based identification and quantification of more than 370 water-soluble metabolites extracted from S. cerevisiae cells. This method of non-targeted metabolomics enables to assess the intracellular concentrations of various energy carrier molecules, nucleotides, amino acids, monosaccharides, intermediates of glycolysis, and tricarboxylic cycle intermediates. The developed LC-MS/MS method permits the identification and quantification of different structural isomers and stereoisomeric forms of water-soluble metabolites with diverse structural, physical, and chemical properties.