Jan C. van Dam

Quantitative evaluation of intracellular metabolite extraction techniques for yeast metabolomics.

Accurate determination of intracellular metabolite levels requires well-validated procedures for sampling and sample treatment. Several methods exist for metabolite extraction, but the literature is contradictory regarding the adequacy and performance of each technique. Using a strictly quantitative approach, we have re-evaluated five methods (hot water, HW; boiling ethanol, BE; chloroform-methanol, CM; freezing-thawing in methanol, FTM; acidic acetonitrile-methanol, AANM) for the extraction of 44 intracellular metabolites (phosphorylated intermediates, amino acids, organic acids, nucleotides) from S. cerevisiae cells. Two culture modes were investigated (batch and chemostat) to check for growth condition dependency, and three targeted platforms were employed (two LC-MS and one GC/MS) to exclude analytical bias. Additionally, for the determination of metabolite recoveries, we applied a novel approach based on addition of (13)C-labeled internal standards at different stages of sample processing. We found that the choice of extraction method can drastically affect measured metabolite levels, to an extent that for some metabolites even the direction of changes between growth conditions can be inverted. The best performances, in terms of efficacy and metabolite recoveries, were achieved with BE and CM, which yielded nearly identical levels for the metabolites analyzed. According to our results, AANM performs poorly in yeast and FTM cannot be considered adequate as an extraction method, as it does not ensure inactivation of enzymatic activity.

Development and application of a differential method for reliable metabolome analysis in Escherichia coli

Quantitative metabolomics of microbial cultures requires well-designed sampling and quenching procedures. We successfully developed and applied a differential method to obtain a reliable set of metabolome data for Escherichia coli K12 MG1655 grown in steady-state, aerobic, glucose-limited chemostat cultures. From a rigorous analysis of the commonly applied quenching procedure based on cold aqueous methanol, it was concluded that it was not applicable because of release of a major part of the metabolites from the cells. No positive effect of buffering or increasing the ionic strength of the quenching solution was observed. Application of a differential method in principle requires metabolite measurements in total broth and filtrate for each measurement. Different methods for sampling of culture filtrate were examined, and it was found that direct filtration without cooling of the sample was the most appropriate. Analysis of culture filtrates revealed that most of the central metabolites and amino acids were present in significant amounts outside the cells. Because the turnover time of the pools of extracellular metabolites is much larger than that of the intracellular pools, the differential method should also be applicable to short-term pulse response experiments without requiring measurement of metabolites in the supernatant during the dynamic period.

Simultaneous quantification of free nucleotides in complex biological samples using ion pair reversed phase liquid chromatography isotope dilution tandem mass spectrometry.

A new sensitive and accurate analytical method has been developed for quantification of intracellular nucleotides in complex biological samples from cultured cells of different microorganisms such as Saccharomyces cerevisiae, Escherichia coli, and Penicillium chrysogenum. This method is based on ion pair reversed phase liquid chromatography electrospray ionization isotope dilution tandem mass spectrometry (IP-LC-ESI-ID-MS/MS. A good separation and low detection limits were observed for these compounds using dibutylamine as volatile ion pair reagent in the mobile phase of the LC. Uniformly (13)C-labeled isotopes of nucleotides were used as internal standards for both extraction and quantification of intracellular nucleotides. The method was validated by determining the linearity, sensitivity, and repeatability.

A comprehensive method for the quantification of the non-oxidative pentose phosphate pathway intermediates in Saccharomyces cerevisiae by GC–IDMS

A gas chromatography isotope dilution mass spectrometry (GC-IDMS) method was developed for the quantification of the metabolites of the non-oxidative part of pentose phosphate pathway (PPP). A mid-polar GC column (Zebron ZB-AAA, 10m, film composition 50% phenyl 50% dimethyl polysiloxane) was used for the chromatographic separation of the intermediates. The optimized GC-MS procedure resulted in improved separation performances and higher sensitivities compared to previous methods. Furthermore, the use of (13)C-labeled cell extracts as internal standards improved the data quality and eliminated the need to perform a recovery check for each metabolite. The applicability of the new method was demonstrated by analyzing intracellular metabolite levels in samples derived from aerobic glucose-limited chemostat cultures of Saccharomyces cerevisiae at steady state as well as following a short-term glucose pulse. The major achievements of the proposed quantitative method are the independent quantification of the epimers ribulose-5-phosphate and xylulose-5-posphate and the measurement of compounds present at very low concentrations in biological samples such as erythrose-4-phosphate and glyceraldehyde-3-phosphate.

Quantitative Metabolomics Using ID-MS

Quantitative intracellular metabolite measurements are essential for systems biology and modeling of cellular metabolism. The MS-based quantification is error prone because (1) several sampling processing steps have to be performed, (2) the sample contains a complex mixture of partly compounds with the same mass and similar retention time, and (3) especially salts influence the ionization efficiency. Therefore internal standards are required, best for each measured compound. The use of labeled biomass, (13)C extract, is a valuable tool, reducing the standard deviations of intracellular concentration measurements significantly (especially regarding technical reproducibility). Using different platforms, i.e., LC-MS and GC-MS, a large number of different metabolites can be quantified (currently about 110).

Analysis of Glycolytic Intermediates with Ion Chromatography- and Gas Chromatography-Mass Spectrometry

In this chapter, we describe a method for the quantitative analysis of glycolytic intermediates using ion chromatography-mass spectrometry (IC-MS) and gas chromatography (GC)-MS as complementary methods. With IC-MS-MS, pyruvate, glucose-6-phosphate, fructuse-6-phosphate, fructose-1,6-bisphosphate, phosphoenolpyruvate, and the sum of 2-phosphoglyceraldehyde + 3-phosphoglyceraldehyde can be quantified. With GC-MS using selected ion monitoring, glyceraldehyde-3-phosphate, dihydroxyacetonephosphate, 2-phosphoglyceraldehyde, and 3-phosphoglyceraldehyde can be analyzed. The derivatization for GC-MS is performed in two steps. In the first step, the keto and the aldehyde groups are oximated. In the next step, a silylation with N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA) is performed, giving TMS-MOX derivatives of the glycolytic intermediates. The derivatives are separated on a GC column and detected with MS in SIM mode.