Name: absolute quantification of cell-free protein synthesis metabolism by reversed-phase liquid chromatography-mass spectrometry
Uploaded: 2019-10-25T14:00:00
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The work described was supported by the Center on the Physics of Cancer Metabolism through Award Number 1U54CA210184-01 from the National Cancer Institute ( https://www.cancer.gov/ ). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

1. Preparation of reagents for aniline tagging
<ol>
	<li>Prepare a 6 M aniline solution at pH 4.5. Working in a hood, combine 550 &#181;L of aniline with 337.5 &#181;L of LCMS grade water and 112.5 &#181;L of 12 M hydrochloric acid (HCl) in a centrifuge tube. Vortex well and store at 4 &#176;C. 
	NOTE: Aniline can be stored at 4 &#176;C for 2 months. 
	CAUTION: Aniline is highly toxic and should be worked with in a fume hood. Hydrochloric acid is highly corrosive</li>
	<li>Prepare a 6 M 13C aniline...

<ol>
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Cell-free systems have no cell wall, thus there is direct access to metabolites and the biosynthetic machinery without the need for complex sample preparation. However, very little work has been done to develop thorough and robust protocols to quantitatively interrogate cell-free reaction systems. In this study, we developed a fast, robust method to quantify metabolites in cell-free reaction mixtures and potentially in whole-cell extracts. Individual quantification of metabolites in complex mixtures, such as those found ...

Cell-free protein synthesis (CFPS) is a promising platform for manufacturing of proteins and chemicals, an application that has traditionally been reserved for living cells. Cell-free systems are derived from crude cell extracts and eliminate the complications associated with cell growth1. In addition, CFPS allows for direct access to metabolites and the biosynthetic machinery without the interference of a cell wall. However, a fundamental understanding of the performance limits of cell-free processes has been lacking. High-throughput methods for metabolite quantification are valuable for the characterization of metabolism and are critical for the construction of metabolic computational models2,3,4. Common methods used to determine metabolite concentrations include nuclear magnetic resonance (NMR), Fourier transform-infrared spectroscopy (FT-IR), enzyme-based assays, and mass spectrometry (MS)5,6,7,8. However, these methods are often limited by their inability to efficiently measure multiple compounds at once and often require a sample size greater than typical cell-free reactions. For example, enzyme-based assays can often only be used to quantify a single compound in a run, and are limited when the sample size is small, such as in cell-free protein synthesis reactions (typically run on a 10-15 &#956;L scale). Meanwhile, NMR requires a high abundance of metabolites for detection and quantification5.&#160; Toward these shortcomings, chromatography methods in tandem with mass spectrometry (LC/MS) provide several advantages, including high sensitivity and the capability of measuring multiple species simultaneously9; however, the analytical complexity increases considerably with the number and diversity of species being measured. It is important, therefore, to develop methods that fully realize the high-throughput potential of LC/MS systems. Compounds in a sample are separated by liquid chromatography and identified through mass spectrometry. The signal of the compound depends on its concentration and ionization efficiency, where the ionization can vary between compounds and may also depend on the sample matrix. &#160;
Achieving the same ionization efficiency between the sample and standards is a challenge when using LC/MS to quantify analytes. Further, quantification becomes more challenging with metabolite diversity due to signal splitting and heterogeneity in proton affinity and polarity10. Lastly, the co-eluting matrix of the sample can also affect the ionization efficiencies of the compounds. To address these issues, metabolites can be chemically derivatized, increasing the separation resolution and sensitivity by LC/MS systems, while simultaneously decreasing signal splitting in some cases10,11. Chemical derivatization works by tagging specific functional groups of metabolites to adjust their physical properties like charge or hydrophobicity to increase ionization efficiency11. Various tagging agents can be used to target different functional groups (e.g., amines, hydroxyls, phosphates, carboxylic acids, etc.). Aniline, one such derivatization agent, targets multiple functional groups at once, and adds a hydrophobic component into hydrophilic molecules, thereby increasing their separation resolution and signal12. To address the co-eluting matrix ion suppression effect, Yang and coworkers developed a technique based on Group Specific Internal Standard Technology (GSIST) labeling where standards are tagged with 13C aniline isotopes and mixed with the sample12,13. The metabolite and corresponding internal standard have the same ionization efficiency since they co-elute, and their intensity ratio can be used to quantify the concentration in the experimental sample.
In this study, we developed a protocol to detect and quantify 40 compounds involved in glycolysis, the pentose phosphate pathway, the tricarboxylic acid cycle, energy metabolism and cofactor regeneration in CFPS reactions. The method is based on the GSIST approach, where we used 12C-aniline and 13C-aniline to tag, detect, and quantify metabolites using reversed-phase LC/MS. The linear range of all compounds spanned 2-3 orders of magnitude with an average correlation coefficient of 0.988. Thus, the method is a robust and accurate approach to interrogate cell-free metabolism, and possibly whole-cell extracts.&#160;

<table border="0" cellpadding="0" cellspacing="0" style="width:665px;" width="665">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:216px;">12C Aniline</td>
			<td style="width:121px;">Sigma-Aldrich</td>
			<td style="width:161px;">242284</td>
			<td style="width:167px;">Aniline 12C</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">13C labeled aniline</td>
			<td>Sigma-Aldrich</td>
			<td>485797</td>
			<td>Aniline 13C6</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">3-Phosphoglyceric acid</td>
			<td>Sigma-Aldrich</td>
			<td>P8877</td>
			<td>3PG</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Acetic Acid</td>
			<td>FisherScientific</td>
			<td>AC222140010</td>
			<td>ACE</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Acetonitrile, LCMS</td>
			<td>JT BAKER</td>
			<td>9829-03</td>
			<td>ACN</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Acetyl-coenzyme A</td>
			<td>Sigma-Aldrich</td>
			<td>A2056</td>
			<td>ACA</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Acquity UPLC BEH C18 1.7 &#956;M, 2.1 x 150 mm Column</td>
			<td>Waters</td>
			<td>186002353</td>
			<td>Column</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Adenosine diphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>A2754</td>
			<td>ADP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Adenosine monophosphate</td>
			<td>Sigma-Aldrich</td>
			<td>A1752</td>
			<td>AMP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Adenosine triphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>A2383</td>
			<td>ATP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Alpha-ketoglutarate</td>
			<td>Sigma-Aldrich</td>
			<td>K1128</td>
			<td>aKG</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Citrate</td>
			<td>Sigma-Aldrich</td>
			<td>251275</td>
			<td>CIT</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Cytidine diphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>C9755</td>
			<td>CDP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Cytidine monophosphate</td>
			<td>Sigma-Aldrich</td>
			<td>C1006</td>
			<td>CMP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Cytidine triphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>C9274</td>
			<td>CTP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">D-glyceraldehyde 3-phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>39705</td>
			<td>GAP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Erythrose 4-phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>E0377</td>
			<td>E4P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ethanol</td>
			<td>Sigma-Aldrich</td>
			<td>EX0276</td>
			<td>EtOH</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Fisher Scientific accuSpin Micro 17 Centrifuge</td>
			<td>FisherScientific</td>
			<td></td>
			<td>Centrifuge</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Flavin adenine dinucleotide</td>
			<td>Sigma-Aldrich</td>
			<td>F6625</td>
			<td>FAD</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Fructose 1,6-bisphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>F6803</td>
			<td>F16P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Fructose 6-phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>F3627</td>
			<td>F6P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Fumarate</td>
			<td>Sigma-Aldrich</td>
			<td>F8509</td>
			<td>FUM</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Gluconate 6-phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>P7877</td>
			<td>6PG</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Glucose</td>
			<td>Sigma-Aldrich</td>
			<td>G8270</td>
			<td>GLC</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Glucose 6-phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>G7879</td>
			<td>G6P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Glycerol 3-phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>G7886</td>
			<td>Gly3P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Guanosine diphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>G7127</td>
			<td>GDP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Guanosine monophosphate</td>
			<td>Sigma-Aldrich</td>
			<td>G8377</td>
			<td>GMP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Guanosine triphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>G8877</td>
			<td>GTP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Hydrochloric acid</td>
			<td>Sigma-Aldrich</td>
			<td>258148</td>
			<td>HCl</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Isocitrate</td>
			<td>Sigma-Aldrich</td>
			<td>I1252</td>
			<td>ICIT</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Lactate</td>
			<td>Sigma-Aldrich</td>
			<td>L1750</td>
			<td>LAC</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Malate</td>
			<td>Sigma-Aldrich</td>
			<td>02288</td>
			<td>MAL</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">myTXTL - Sigma 70 Master Mix Kit</td>
			<td>ArborBiosciences</td>
			<td>507024</td>
			<td>Cell-free protein synthesis</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">N-(3-dimethylaminopropyl)-N&#8242;-ethylcarbodiimide hydrochloride</td>
			<td>Sigma-Aldrich</td>
			<td>03449</td>
			<td>EDC</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Nicotinamide adenine dinucleotide</td>
			<td>Sigma-Aldrich</td>
			<td>43410</td>
			<td>NAD</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Nicotinamide adenine dinucleotide phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>N5755</td>
			<td>NADP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Nicotinamide adenine dinucleotide phosphate reduced</td>
			<td>Sigma-Aldrich</td>
			<td>481973</td>
			<td>NADPH</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Nicotinamide adenine dinucleotide reduced</td>
			<td>Sigma-Aldrich</td>
			<td>N8129</td>
			<td>NADH</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Oxalacetate</td>
			<td>Sigma-Aldrich</td>
			<td>O4126</td>
			<td>OAA</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Phosphoenolpyruvate</td>
			<td>Sigma-Aldrich</td>
			<td>P0564</td>
			<td>PEP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Pyruvate</td>
			<td>Sigma-Aldrich</td>
			<td>P5280</td>
			<td>PYR</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ribose 5-phosphate</td>
			<td>Sigma-Aldrich</td>
			<td>R7750</td>
			<td>R5P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ribulose 5-phosphate</td>
			<td>CarboSynth</td>
			<td>MR45852</td>
			<td>RL5P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Sedoheptulose 7-phosphate</td>
			<td>CarboSynth</td>
			<td>MS07457</td>
			<td>S7P</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Succinate</td>
			<td>Sigma-Aldrich</td>
			<td>S3674</td>
			<td>SUCC</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Tributylamine</td>
			<td>Sigma-Aldrich</td>
			<td>90780</td>
			<td>TBA</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Triethylamine</td>
			<td>FisherScientific</td>
			<td>O4884</td>
			<td>TEA</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ultrapure water</td>
			<td>FisherScientific</td>
			<td>10977-015</td>
			<td>water</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Uridine diphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>U4125</td>
			<td>UDP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Uridine monophosphate</td>
			<td>Sigma-Aldrich</td>
			<td>U6375</td>
			<td>UMP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Uridine triphosphate</td>
			<td>Sigma-Aldrich</td>
			<td>U6625</td>
			<td>UTP</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">VWR Heavy Duty Vortex</td>
			<td>VWR</td>
			<td></td>
			<td>Vortex</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Water, LCMS</td>
			<td>JT BAKER</td>
			<td>9831-03</td>
			<td>WATER</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Waters Acquity H UPLC Class Quaternary Solvent Manager</td>
			<td>Waters</td>
			<td></td>
			<td>LCMS</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Waters Acquity H UPLC Class Sample Manager FTN</td>
			<td>Waters</td>
			<td></td>
			<td>LCMS</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Waters Acquity Qda detector</td>
			<td>Waters</td>
			<td></td>
			<td>LCMS</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Waters Empower 3</td>
			<td>Waters</td>
			<td></td>
			<td>Software</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Waters LCMS Total Recovery Vial</td>
			<td>Waters</td>
			<td>186000384c</td>
			<td>LCMS Vial</td>
		</tr>
	</tbody>
</table>

absolute quantification of cell-free protein synthesis metabolism by reversed-phase liquid chromatography-mass spectrometry

Cell-free protein synthesis (CFPS) is an emerging technology in systems and synthetic biology for the in vitro production of proteins. However, if CFPS is going to move beyond the laboratory and become a widespread and standard just in time manufacturing technology, we must understand the performance limits of these systems. Toward this question, we developed a robust protocol to quantify 40 compounds involved in glycolysis, the pentose phosphate pathway, the tricarboxylic acid cycle, energy metabolism and cofactor regeneration in CFPS reactions. The method uses internal standards tagged with 13C-aniline, while compounds in the sample are derivatized with 12C-aniline. The internal standards and sample were mixed and analyzed by reversed-phase liquid chromatography-mass spectrometry (LC/MS). The co-elution of compounds eliminated ion suppression, allowing the accurate quantification of metabolite concentrations over 2-3 orders of magnitude where the average correlation coefficient was 0.988. Five of the forty compounds were untagged with aniline, however, they were still detected in the CFPS sample and quantified with a standard curve method. The chromatographic run takes approximately 10 min to complete. Taken together, we developed a fast, robust method to separate and accurately quantify 40 compounds involved in CFPS in a single LC/MS run. The method is a comprehensive and accurate approach to characterize cell-free metabolism, so that ultimately, we can understand and improve the yield, productivity and energy efficiency of cell-free systems.

As a proof-of-concept, we used the protocol to quantify metabolites in an E. coli based CFPS system expressing green fluorescent protein (GFP). &#160;The CFPS reaction (14 &#956;L) was quenched and deproteinized with ethanol. The CFPS sample was then tagged with 12C-aniline, while standards were tagged with 13C-aniline. The tagged sample and standards were then combined and injected into the LC/MS (Figure 1). The protocol detected and quantified 40 metabolites ...

Watch this Scientific Journal Video about Absolute Quantification of Cell-Free Protein Synthesis Metabolism by Reversed-Phase Liquid Chromatography-Mass Spectrometry at JoVE.com

Absolute Quantification of Cell-Free Protein Synthesis Metabolism by Reversed-Phase Liquid Chromatography-Mass Spectrometry

Here, we present a robust protocol to quantify 40 compounds involved in central carbon and energy metabolism in cell-free protein synthesis reactions. The cell-free synthesis mixture is derivatized with aniline for effective separation using reversed-phase liquid chromatography and then quantified by mass spectrometry using isotopically labelled internal standards.

Cell-free protein synthesis (CFPS) is an emerging technology in systems and synthetic biology for the in vitro production of proteins. However, if CFPS is ...

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Biochemistry

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