We thank the financial support from Genome Canada and Genome British Columbia for operations (204PRO) and technology development (214PRO) through the Genome Innovations Network (GIN). We thank the Drug Discovery Platform at the Research Institute of the McGill University Health Center for the use of the MALDI-TOF instrument for filming. H.L. is grateful for support from a postdoctoral fellowship from the National Science and Engineering Research Council of Canada (NSERC). C.H.B is grateful for support from the Leading Edge Endowment Fund (LEEF). C.H.B. is grateful for support from the Segal McGill Chair in Molecular Oncology at McGill University (Montreal, Quebec, Canada). M.X.C. and C.H.B. are grateful for support from the Warren Y. Soper Charitable Trust and the Alvin Segal Family Foundation to the Jewish General Hospital (Montreal, Quebec, Canada).

The amounts of the reagents described below are based on the measurement of 20 patient plasma samples. The protocol presented below follows the guidelines of the University of Victoria&#39;s human research ethics committee.
1. Generation of Ang I in Human Plasma
<ol>
	<li>Thaw plasma samples (&#8805;200 &#181;L) in a room temperature water bath for 5 min, and then put the samples on ice until completely thawed.</li>
	<li>Transfer 200 &#181;L of each plasma sample manually to separate wells of a 1.1 mL deep-well plate (sample plate), and centrifuge the plate in a centrifuge for 10 min at 2 &#176;C and 1278 x g.</li>
	<li>Use an automated liquid handling system to serially dilute a 500 fmol/&#181;L Ang I NAT standard solution to prepare six calibrator solutions with chicken egg white albumin (0.1, 0.2, 0.6, 1.9, 5.7, 17.2 fmol/&#181;L).</li>
	<li>Pipette 200 &#181;L of each calibrator to a well, and 125 &#181;L of generation buffer to sample plate. 
	NOTE: The CEWA in phosphate buffered saline (PBS) buffer needs to be freshly prepared on the day of the experiment.</li>
	<li>Using an automated liquid handling system, in a new plate mix 125 &#181;L of plasma supernatant or 125 &#181;L of CEWA in PBS with 25 &#181;L of Ang I generation buffer, which contains 1 M Tris, 0.2 mM ethylenediaminetetraacetic acid (EDTA), and 1 mM phenylmethylsulfonyl fluoride (PMSF). 
	NOTE: Prepare the Ang I generation buffer by mixing a 1 M Tris/0.2 mM EDTA aqueous buffer (adjust to pH 5.5 using acetic acid) and a PMSF solution (100 mM in methanol). Both solutions can be stored up to 1 month at 4 &#176;C. Mix the two solutions on the day of the experiment.</li>
	<li>Automatically transfer the solutions into a 96-well plate, 3 replicates per solution, 34 &#181;L per well. Incubate the plate at 37 &#176;C for 3 h.</li>
</ol>
2. Immuno-enrichment of Ang I Using Antibody-coated Beads
<ol>
	<li>Conjugation of antibody onto magnetic Protein G beads 
	NOTE: Conjugation of the antibody with the beads is performed manually during the 3 h Ang I generation period on the day of the experiment. For other analytes, the conjugated beads might be able to be stored in PBS buffer containing 0.015% CHAPS (PBSC) at 4 &#176;C for three months or longer, depending on the properties and stability of the antibody.
	<ol>
		<li>Transfer 110 &#181;L of bead slurry (enough for measuring 20 samples and making a 6-point standard curve) into a 1.5 mL tube. Wash the beads seven times with 1 mL of 25% acetonitrile/PBSC, and three times with 1 mL of PBSC. Use a magnetic stand to pellet the beads between each washing step. Remove the wash buffer after the last wash. 
		NOTE: Washing 7 times with 25% acetonitrile/PBSC is critical for removing the MS-incompatible additives in the bead slurry, such as Tween-20. If there are no such additives in the selected beads, this extensive washing step might not be necessary.</li>
		<li>Resuspend the beads in 110 &#181;L of PBSC, and add 110 &#181;L of anti-Ang I antibody (final antibody concentration: 100 &#181;g/mL). Mix the beads and solution by pipetting, and then incubate them at room temperature for 1 h, rotating at 8 rpm.</li>
		<li>Wash the beads 3 times with 1 mL of PBSC, and resuspend in 1100 &#181;L of PBSC. 
		NOTE: If any bead solution has flowed into the cap of the tube during incubation, spin down the solution using a benchtop centrifuge at 2680 x g.</li>
		<li>Transfer the bead solution manually to a 96-well plate (bead standard plate). Use the automated liquid handling system to aliquot the beads to the same 96-well plate, 120 &#181;L per well.&#160;</li>
	</ol>
	</li>
	<li>Immuno-enrichment on the beads
	<ol>
		<li>After the 3 h Ang I generation period, place the incubation plate on ice for 10 min to terminate the generation of Ang I.</li>
		<li>Automatically dilute the SIS peptide stock solution (10 pmol/&#956;L) 100-fold with PBS buffer, and further automatically aliquot the stable isoptope standard peptide dilution to a 96-well PCR plate. Transfer 1.5 &#181;L of a stable isotope standards (SIS) peptide solution (containing 100 fmol) into each well of the incubation plate and mix it with the plasma samples or the CEWA in PBS buffers.</li>
		<li>Automatically transfer the contents of the incubation plate to the bead solution, 10 mL per well, mix.</li>
		<li>Incubate the plate at 4 &#176;C for 1 h while rotating at 8 rpm.</li>
		<li>Wash the beads three times automatically with 5 mM ammonium bicarbonate (AmBic) solution, 100 &#181;L per well per wash. After the last wash, resuspend the beads in 7 &#181;L of 5 mM AmBic solution in each well. Use a magnet to pull the beads to the bottom after the last wash.</li>
	</ol>
	</li>
</ol>
3. Transfer of Beads onto a MALDI Target Plate and Adding Matrix Solution
<ol>
	<li>Transfer 7 &#181;L of the bead slurry automatically onto a MALDI target plate with a spot size of 2,600 &#181;m. Let the beads dry out. 
	NOTE: A small USB-powered fan can be used to accelerate the bead drying process.</li>
	<li>Automatically add 2 &#181;L of &#945;-cyano-4-hydroxycinnamic acid (HCCA)-matrix solution (containing 3 mg/mL HCCA, 1.8 mg/mL ammonium citrate, 70% acetonitrile, and 0.1% trifluoroacetic acid) from the matrix well onto each sample spot on the target plate.</li>
</ol>
4. Signal Acquisition by a MALDI-TOF-MS and Data Analysis
<ol>
	<li>Analyze the sample spots with a MALDI-TOF instrument using positive reflector mode. Perform internal calibration, data smoothing, and baseline subtraction automatically with vendor-specific software. 
	NOTE: The mode (positive/negative, linear/reflector) selected for MALDI-TOF measurement depends on the target peptides or proteins.</li>
	<li>Calculate the relative response ratio (Nat/SIS intensity ratio) and compare it to the standard curve to determine of the Ang I concentration in each sample. Calculate PRA using Equation (1), where &#916;tAng I generation represents the time used for the generation of Ang I. 
	PRA = [Ang I]/&#916;tAng I generation (1)</li>
</ol>

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C.H.B holds the patent on the iMALDI technology.

Compared to conventional MS-based protein quantification, iMALDI uses antibodies to enrich the analytes and purify them from complex samples, therefore making it possible to quantify proteins or peptides at low concentrations. A critical step in the iMALDI protocol is the immuno-enrichment of the target peptides. For this purpose, antibodies with high specificity and affinity should be selected. In SISCAPA, it has been reported that antibody affinities at 10-9 M or better would be needed to achieve high sensitivity at low ng/mL21. In addition, beads with high antibody binding capacity are preferred for optimal enrichment.
It is also important to generate relatively &#34;clean&#34; spectra for the target molecules with minimal background levels. High signal-to-noise ratios can be achieved through extensive bead washing after the immuno-enrichment step. Also, we have found that washing the bead spots on the target plate after adding and drying the matrix solution can significantly reduce the noise signals and thus improve the signal-to-noise ratios.
The iMALDI PRA assay demonstrated in this work does not require the proteolysis steps, as the target molecule Ang I is a peptide. To quantify protein molecules in a sample, the protein is typically digested into peptides before immuno-enrichment, although for epitope-containing peptides, the digestion can be performed on the captured protein22,23. In this case, an existing protein digestion protocol can be easily inserted into the iMALDI workflow.&#160; Depending on the digestion buffer, a desalting step might be needed before capture of the peptides on beads, to avoid interference with the antibody-peptide binding.
Similar to other immuno-MS methods, production of anti-peptide antibodies is a major challenge in the development of iMALDI assays. Recently, Triple X Proteomics technology has been developed to produce affinity binders that can target multiple peptides per binder24. This may reduce the cost of anti-peptide antibodies, and would facilitate the development of simultaneous multiplexed assays using a single incubation. Also, antibody fragments25 or synthetic aptamers26 could be alternatives to the use of intact antibodies and would have the advantage of easier production. Synthetic aptamers have been reported as showing lower background levels than intact antibodies when used for affinity-enrichment26.
iMALDI technology combines the advantages of immunoassays with MALDI mass spectrometry, enabling rapid, high-throughput protein/peptide analysis in a large number of complex samples, with high sensitivity and specificity. Unlike other immuno-MS methods, after immuno-capture, the target peptides are not eluted in the tube, but the beads carrying analytes are transferred onto the MALDI target plate for MS-analysis, thus avoiding the loss of peptides due to their adsorption to plastic tubes. Compared to widely used proteomic tools such as western blot or ELISA, only one antibody is required in iMALDI, significantly reducing the assay development time and cost. The lower limit of detection we have obtained in the PRA assay is comparable to conventional ELISA, and well within the physiological range. The antibody and the mass-to-charge ratio provide a double selection process, which ensures the high specificity of the assay. Applications of iMALDI is not limited to PRA assays, but can be applied for quantitation of any protein expression and even quantification of protein modification in complex samples. Notably, the automated high-throughput iMALDI could be a powerful clinical tool for the discovery and validation of protein biomarkers in a variety of diseases, due to the presence of benchtop MALDI instruments currently used in clinics for bacterial identification.

Mass spectrometry has become an indispensable tool in quantitative proteomics. Mass spectrometry can determine the masses of target proteins or peptides, therefore the obtained analyte signals can be highly specific compared to immunoassays. Two ionization methods, electrospray and MALDI, are most commonly used for detecting proteins and peptides1,2,3,4. A major challenge in MS-based protein quantification lies in the detection of low-abundance proteins in complex samples at ng/mL or pg/mL concentrations in the presence of high-abundance proteins, and many candidate protein biomarkers found in human plasma are within this range5. This problem is largely caused by the inherently wide dynamic range and complexity of the human proteome6.
To overcome these detection challenges, immuno-MS methods have been developed to enrich the target proteins or peptides from the sample solutions onto a solid surface, followed by elution of the analytes and MS measurement7,8,9,10. Through immuno-enrichment, the analytes are purified from complex samples and therefore the ion-suppression effects from other molecules are minimized. Among various solid supports, magnetic beads are currently most widely used as they have the advantages of high antibody binding capacity and ease of handling. Magnetic beads with different functionalizations and sizes have been developed and commercialized for immunoprecipitation experiments. To date, immuno-enrichment on beads has been interfaced with both electrospray ionization (ESI) and MALDI-MS for protein and peptide measurement. In stable isotope standards and capture by anti-peptide antibodies (SISCAPA) technology, proteins in the samples are digested, followed by incubation with antibody-coated beads for immuno-enrichment. In &#34;classical&#34; SISCAPA, the captured proteotypic peptides are eluted from the beads, and measured by Liquid Chromatography-ESI-MS (LC-MS), or by direct infusion ESI-Multiple Reaction Monitoring-MS (ESI-MRM-MS)11,12. Immuno-enrichment improved the MRM assay sensitivity by 3-4 orders of magnitude, reaching the low ng/mL range13.
Compared to electrospray-MS, MALDI-MS is faster, and does not involve the cleaning and re-equilibration of LC columns so there are no carryover and contamination issues, making it more suitable for high-throughput studies14. Immuno-MALDI technology has been developed in our laboratory to combine immuno-enrichment with MALDI-MS for sensitive and specific quantification of peptides and proteins (based on quantitation of proteotypic peptides)15,16,17. After immuno-enrichment, the beads are deposited on a MALDI target plate, the matrix solution is added to beads, and the plate is ready for analysis by a MALDI-TOF-MS after drying. Elution of the peptides from the beads is not performed as a separate step, but affinity-bound analytes are eluted by the MALDI matrix solution when it is added to the bead spots, thereby simplifying the sample preparation and minimizing sample loss. The iMALDI technology has been applied in a variety of applications18,19, and recently has been automated and used for measuring Angiotensin I (Ang I) for determining plasma renin activity (PRA)20. This protocol will demonstrate the procedure used to perform an automated iMALDI assay. Taking the PRA assay as an example, inter-day CVs of less than 10% have been achieved through automation, with the capability of measuring 744 samples per day20.
The iMALDI PRA assay demonstrated in this manuscript does not require protein digestion, as the target molecule (Ang I) is a peptide with a molecular weight of 1295.7 Da. In other assays where protein digestion is necessary and a peptide is used as the surrogate for the intact protein, the selected peptide for iMALDI should be unique and specific to the target protein, with a mass over 800 Da so that it can be easily distinguished from the chemical noise from the MALDI matrix solution. Anti-peptide antibodies are required for the immuno-enrichment of the peptides. The protocol for an iMALDI assay measuring PRA consists of four steps: 1) generation of Ang I in human plasma; 2) immuno-enrichment of Ang I using antibody-coated beads; 3) transfer of beads to a MALDI target plate and adding matrix solution; and 4) signal acquisition by a MALDI-TOF-MS and data analysis20.

<table width="892">
	<tbody>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Healthy control human plasma</td>
			<td class="xl63" style="width:134pt" width="178">Bioreclamation</td>
			<td class="xl68" style="width:89pt" width="119">HMPLEDTA2</td>
			<td class="xl66" style="width:242pt" width="323"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Ammonium bicarbonate</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl69">09830</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Ammonium citrate dibasic</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl68" style="width:89pt" width="119">09833</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">CHAPS (&#62;=98%)</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl68" style="width:89pt" width="119">C9426</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Albumin from chicken egg white (&#62;98%)</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl68" style="width:89pt" width="119">A5503</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Ethylenediaminetetraacetic acid</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl68" style="width:89pt" width="119">EDS</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Alpha-cyano-4-hydroxycinnamic acid</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl68" style="width:89pt" width="119">70990</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Phosphate buffered saline</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl68" style="width:89pt" width="119">P4417</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Phenylmethanesulfonyl fluoride</td>
			<td class="xl63" style="width:134pt" width="178">Sigma Aldrich</td>
			<td class="xl68" style="width:89pt" width="119">78830</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Trifluoroacetic acid</td>
			<td class="xl63" style="width:134pt" width="178">Thermo Fisher Scientific</td>
			<td class="xl68" style="width:89pt" width="119">85172</td>
			<td class="xl66">LC-MS grade</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">acetonitrile</td>
			<td class="xl63" style="width:134pt" width="178">Fluka</td>
			<td class="xl68" style="width:89pt" width="119">34967</td>
			<td class="xl66">LC-MS grade</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">water</td>
			<td class="xl63" style="width:134pt" width="178">Fluka</td>
			<td class="xl68" style="width:89pt" width="119">39253</td>
			<td class="xl66">LC-MS grade</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">acetic acid</td>
			<td class="xl63" style="width:134pt" width="178">Fluka</td>
			<td class="xl68" style="width:89pt" width="119">320099</td>
			<td class="xl66">LC-MS grade</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Tris(hydroxymethyl)aminomethane</td>
			<td class="xl63" style="width:134pt" width="178">Roche Diagnostics</td>
			<td class="xl71">3118169001</td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Dynabeads Protein G magnetic beads</td>
			<td class="xl63" style="width:134pt" width="178">Thermo Fisher Scientific</td>
			<td class="xl68" style="width:89pt" width="119">10003D</td>
			<td class="xl66">2.8 &#956;m, 30 mg/mL</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">anti-Ang I goat polyclonal antibody</td>
			<td class="xl63" style="width:134pt" width="178">Santa Cruz Biotechnology</td>
			<td class="xl68" style="width:89pt" width="119">sc-7419</td>
			<td class="xl66"></td>
		</tr>
		<tr height="84" style="height:63.0pt">
			<td class="xl63" height="84" style="width:204pt;height:63.0pt" width="272">Nat and SIS Ang I</td>
			<td class="xl63" style="width:134pt" width="178">synthesized at the University of Victoria-Genome BC Proteomics Centre</td>
			<td class="xl68" style="width:89pt" width="119"></td>
			<td class="xl66"></td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Automated liquid handling system</td>
			<td class="xl63" style="width:134pt" width="178">Agilent</td>
			<td class="xl68" style="width:89pt" width="119">16050-102</td>
			<td class="xl66">Agilent Bravo robotic workstation</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Magnet</td>
			<td class="xl63" style="width:134pt" width="178">Thermo Fisher Scientific</td>
			<td class="xl68" style="width:89pt" width="119">12321D</td>
			<td class="xl66">Invitrogen DynaMag-2 magnet</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Tube rotator</td>
			<td class="xl63" style="width:134pt" width="178">Theromo Scientific</td>
			<td class="xl68" style="width:89pt" width="119">400110Q</td>
			<td class="xl66">Labquake Tube Rotator</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Magnet</td>
			<td class="xl63" style="width:134pt" width="178">Thermo Fisher Scientific</td>
			<td class="xl68" style="width:89pt" width="119">12027</td>
			<td class="xl66">DynaMag-96 side skirted magnet</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">Magnet</td>
			<td class="xl63" style="width:134pt" width="178">VP Scientific</td>
			<td class="xl68" style="width:89pt" width="119">771RM-1</td>
			<td class="xl66">used to pull the beads to the bottom of the well</td>
		</tr>
		<tr height="21" style="height:15.75pt">
			<td class="xl63" height="21" style="width:204pt;height:15.75pt" width="272">MALDI-TOF</td>
			<td class="xl63" style="width:134pt" width="178">Bruker</td>
			<td class="xl68" style="width:89pt" width="119"></td>
			<td class="xl66">Bruker Microflex LRF instrument</td>
		</tr>
	</tbody>
</table>

peptide and protein quantification using automated immuno-maldi (imaldi)

Mass spectrometry (MS) is one of the most commonly used technologies for quantifying proteins in complex samples, with excellent assay specificity as a result of the direct detection of the mass-to-charge ratio of each target molecule. However, MS-based proteomics, like most other analytical techniques, has a bias towards measuring high-abundance analytes, so it is challenging to achieve detection limits of low ng/mL or pg/mL in complex samples, and this is the concentration range for many disease-relevant proteins in biofluids such as human plasma. To assist in the detection of low-abundance analytes, immuno-enrichment has been integrated into the assay to concentrate and purify the analyte before MS measurement, significantly improving assay sensitivity. In this work, the immuno- Matrix-Assisted Laser Desorption/Ionization (iMALDI) technology is presented for the quantification of proteins and peptides in biofluids, based on immuno-enrichment on beads, followed by MALDI-MS measurement without prior elution. The anti-peptide antibodies are functionalized on magnetic beads, and incubated with samples. After washing, the beads are directly transferred onto a MALDI target plate, and the signals are measured by a MALDI-Time of Flight (MALDI-TOF) instrument after the matrix solution has been applied to the beads. The sample preparation procedure is simplified compared to other immuno-MS assays, and the MALDI measurement is fast. The whole sample preparation is automated with a liquid handling system, with improved assay reproducibility and higher throughput. In this article, the iMALDI assay is used for determining the peptide angiotensin I (Ang I) concentration in plasma, which is used clinically as readout of plasma renin activity for the screening of primary aldosteronism (PA).

An automated iMALDI procedure for measuring Ang I is shown in Figure 1. Target peptides (either endogenous peptides or peptides from digested proteins) are enriched on anti-peptide magnetic beads, and then the beads are transferred to a target plate for MALDI measurement. The whole procedure is simplified compared to other immuno-MS technologies that require additional peptide elution steps. Automation of the iMALDI assay allows for high-throughput analysis of a large number of samples with an inter-day coefficient of variation (CV) below 10% 20. Representative spectra obtained by measuring Ang I in human plasma samples are shown in Figure 2. The SIS peptides function as an internal standard for peptide quantitation. Correlation of PRA values from 188 patient samples obtained with the automated iMALDI assay with PRA values obtained by using a clinical LC-MS/MS procedure17 is shown in Figure 3. The two methods have a correlation coefficient of 0.98; the difference between the slopes might be caused by the use of different internal standards in the two methods, or by the use of different antibodies in iMALDI procedure20. The linear range of the assay and the assay precision are shown in Figure 4 and Figure 5.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/55933/55933fig1.jpg" /> 
Figure 1: Process flow schematic of an automated iMALDI PRA assay. Adapted from reference20, with permission. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55933/55933fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/55933/55933fig2.jpg" /> 
Figure 2: Representative spectra of the NAT and SIS Ang I peptides measured from a human plasma sample.
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/55933/55933fig3.jpg" /> 
Figure 3: Correlation of PRA values measured by iMALDI and by LC-MS/MS from 188 patient samples. Adapted from reference20, with permission.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/55933/55933fig4.jpg" /> 
Figure 4: Linear ranges of the iMALDI PRA assay in both (A) reflector and (B) linear mode. Adapted from reference20, with permission. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55933/55933fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/55933/55933fig5.jpg" /> 
Figure 5: Intraday precision (A) and interday precision (B) of iMALDI PRA assays on plasma pools with low, medium, and high PRA values. Adapted from reference20, with permission. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55933/55933fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Peptide and Protein Quantification Using Automated Immuno-MALDI iMALDI at JoVE.com

Peptide and Protein Quantification Using Automated Immuno-MALDI iMALDI

A protocol for the protein quantification in complex biological fluids using automated immuno-MALDI (iMALDI) technology is presented.

Peptide and Protein Quantification Using Automated Immuno-MALDI (iMALDI)

Mass spectrometry (MS) is one of the most commonly used technologies for quantifying proteins in complex samples, with excellent assay specificity as a ...

peptide-and-protein-quantification-using-automated-immuno-maldi-imaldi

Nanyang Technological University

Research

JoVE Journal

Chemistry

7.9K Views.  University of Victoria-Genome BC Proteomics Centre. A protocol for the protein quantification in complex biological fluids using automated immuno-MALDI (iMALDI) technology is presented. 

Video: Peptide and Protein Quantification Using Automated Immuno-MALDI iMALDI

Production of Disulfide-stabilized Transmembrane Peptide Complexes for Structural Studies

Physical interactions among the lipid-embedded alpha-helical domains of membrane proteins play a crucial role in folding and assembly of membrane protein complexes and in dynamic processes such as transmembrane (TM) signaling and regulation of cell-surface protein levels. Understanding the structural features driving the association of particular sequences requires sophisticated biophysical and biochemical analyses of TM peptide complexes. However, the extreme hydrophobicity of TM domains makes them very difficult to manipulate using standard peptide chemistry techniques, and production of suitable study material often proves prohibitively challenging. Identifying conditions under which peptides can adopt stable helical conformations and form complexes spontaneously adds a further level of difficulty. Here we present a procedure for the production of homo- or hetero-dimeric TM peptide complexes from materials that are expressed in E. coli, thus allowing incorporation of stable isotope labels for nuclear magnetic resonance (NMR) or non-natural amino acids for other applications relatively inexpensively. The key innovation in this method is that TM complexes are produced and purified as covalently associated (disulfide-crosslinked) assemblies that can form stable, stoichiometric and homogeneous structures when reconstituted into detergent, lipid or other membrane-mimetic materials. We also present carefully optimized procedures for expression and purification that are equally applicable whether producing single TM domains or crosslinked complexes and provide advice for adapting these methods to new TM sequences.

Biophysical and biochemical studies of interactions among membrane-embedded protein domains face many technical challenges, the first of which is obtaining appropriate study material. This article describes a protocol for producing and purifying disulfide-stabilized transmembrane peptide complexes that are suitable for structural analysis by solution nuclear magnetic resonance (NMR) and other analytical applications.

Physical  interactions among the lipid-embedded alpha-helical domains of membrane  proteins play a crucial role in folding and assembly of membrane ...

Multiplexed Fluorescent Microarray for Human Salivary Protein Analysis Using Polymer Microspheres and Fiber-optic Bundles

Herein, we describe a protocol for simultaneously measuring six proteins in saliva using a fiber-optic microsphere-based antibody array. The immuno-array technology employed combines the advantages of microsphere-based suspension array fabrication with the use of fluorescence microscopy. As described in the video protocol, commercially available 4.5 &#956;m polymer microspheres were encoded into seven different types, differentiated by the concentration of two fluorescent dyes physically trapped inside the microspheres. The encoded microspheres containing surface carboxyl groups were modified with monoclonal capture antibodies through EDC/NHS coupling chemistry. To assemble the protein microarray, the different types of encoded and functionalized microspheres were mixed and randomly deposited in 4.5 &#956;m microwells, which were chemically etched at the proximal end of a fiber-optic bundle. The fiber-optic bundle was used as both a carrier and for imaging the microspheres. Once assembled, the microarray was used to capture proteins in the saliva supernatant collected from the clinic. The detection was based on a sandwich immunoassay using a mixture of biotinylated detection antibodies for different analytes with a streptavidin-conjugated fluorescent probe, R-phycoerythrin. The microarray was imaged by fluorescence microscopy in three different channels, two for microsphere registration and one for the assay signal. The fluorescence micrographs were then decoded and analyzed using a homemade algorithm in MATLAB.

We describe a procedure for profiling salivary proteins using multiplexed microsphere-based antibody arrays. Monoclonal antibodies were covalently linked to fluorescent dye-encoded 4.5 &#x03BC;m polymer microspheres using carbodiimide chemistry. The modified microspheres were deposited in fiber-optic microwells to measure protein levels in saliva using fluorescence sandwich immunoassays.

Herein, we describe a protocol for simultaneously measuring six proteins in saliva using a fiber-optic microsphere-based antibody array. The immuno-array ...

Structure and Coordination Determination of Peptide-metal Complexes Using 1D and 2D 1H NMR

Copper (I) binding by metallochaperone transport proteins prevents copper oxidation and release of the toxic ions that may participate in harmful redox reactions. The Cu (I) complex of the peptide model of a Cu (I) binding metallochaperone protein, which includes the sequence MTCSGCSRPG (underlined is conserved), was determined in solution under inert conditions by NMR spectroscopy.
NMR is a widely accepted technique for the determination of solution structures of proteins and peptides. Due to difficulty in crystallization to provide single crystals suitable for X-ray crystallography, the NMR technique is extremely valuable, especially as it provides information on the solution state rather than the solid state. Herein we describe all steps that are required for full three-dimensional structure determinations by NMR. The protocol includes sample preparation in an NMR tube, 1D and 2D data collection and processing, peak assignment and integration, molecular mechanics calculations, and structure analysis. Importantly, the analysis was first conducted without any preset metal-ligand bonds, to assure a reliable structure determination in an unbiased manner.

Structure and Coordination Determination of Peptide-metal Complexes Using 1D and 2D 1H NMR

The NMR-solution structure of a metallochaperone model peptide with Cu (I) was determined, and a detailed protocol from sample preparation and 1D and 2D data collection to a three-dimensional structure is described.

Copper (I) binding by metallochaperone transport proteins prevents copper oxidation and release of the toxic ions that may participate in harmful redox ...

Analyzing Protein Dynamics Using Hydrogen Exchange Mass Spectrometry

All cellular processes depend on the functionality of proteins. Although the functionality of a given protein is the direct consequence of its unique amino acid sequence, it is only realized by the folding of the polypeptide chain into a single defined three-dimensional arrangement or more commonly into an ensemble of interconverting conformations. Investigating the connection between protein conformation and its function is therefore essential for a complete understanding of how proteins are able to fulfill their great variety of tasks. One possibility to study conformational changes a protein undergoes while progressing through its functional cycle is hydrogen-1H/2H-exchange in combination with high-resolution mass spectrometry (HX-MS). HX-MS is a versatile and robust method that adds a new dimension to structural information obtained by e.g. crystallography. It is used to study protein folding and unfolding, binding of small molecule ligands, protein-protein interactions, conformational changes linked to enzyme catalysis, and allostery. In addition, HX-MS is often used when the amount of protein is very limited or crystallization of the protein is not feasible. Here we provide a general protocol for studying protein dynamics with HX-MS and describe as an example how to reveal the interaction interface of two proteins in a complex. &#160;&#160;

Protein conformation and dynamics are key to understanding the relationship between protein structure and function. Hydrogen exchange coupled with high-resolution mass spectrometry is a versatile method to study the conformational dynamics of proteins as well as characterizing protein-ligand and protein-protein interactions, including contact interfaces and allosteric effects.

All cellular processes depend on the functionality of proteins. Although the functionality of a given protein is the direct consequence of its unique ...

Split-and-pool Synthesis and Characterization of Peptide Tertiary Amide Library

Peptidomimetics are great sources of protein ligands. The oligomeric nature of these compounds enables us to access large synthetic libraries on solid phase by using combinatorial chemistry. One of the most well studied classes of peptidomimetics is peptoids. Peptoids are easy to synthesize and have been shown to be proteolysis-resistant and cell-permeable. Over the past decade, many useful protein ligands have been identified through screening of peptoid libraries. However, most of the ligands identified from peptoid libraries do not display high affinity, with rare exceptions. This may be due, in part, to the lack of chiral centers and conformational constraints in peptoid molecules. Recently, we described a new synthetic route to access peptide tertiary amides (PTAs). PTAs are a superfamily of peptidomimetics that include but are not limited to peptides, peptoids and N-methylated peptides. With side chains on both &#945;-carbon and main chain nitrogen atoms, the conformation of these molecules are greatly constrained by sterical hindrance and allylic 1,3 strain. (Figure 1) Our study suggests that these PTA molecules are highly structured in solution and can be used to identify protein ligands. We believe that these molecules can be a future source of high-affinity protein ligands. Here we describe the synthetic method combining the power of both split-and-pool and sub-monomer strategies to synthesize a sample one-bead one-compound (OBOC) library of PTAs.

Peptide tertiary amides (PTAs) are a superfamily of peptidomimetics that include but are not limited to peptides, peptoids and N-methylated peptides. Here we describe a synthetic method which combines&#160;both split-and-pool and sub-monomer strategies to synthesize a one-bead one-compound library of PTAs.

Peptidomimetics are great sources of protein ligands. The oligomeric nature of these compounds enables us to access large synthetic libraries on solid ...

Development of a Backbone Cyclic Peptide Library as Potential Antiparasitic Therapeutics Using Microwave Irradiation

Protein-protein interactions (PPIs) are intimately involved in almost all biological processes and are linked to many human diseases. Therefore, there is a major effort to target PPIs in basic research and in the pharmaceutical industry. Protein-protein interfaces are usually large, flat, and often lack pockets, complicating the discovery of small molecules that target such sites. Alternative targeting approaches using antibodies have limitations due to poor oral bioavailability, low cell-permeability, and production inefficiency.
Using peptides to target PPI interfaces has several advantages. Peptides have higher conformational flexibility, increased selectivity, and are generally inexpensive. However, peptides have their own limitations including poor stability and inefficiency crossing cell membranes. To overcome such limitations, peptide cyclization can be performed. Cyclization has been demonstrated to improve peptide selectivity, metabolic stability, and bioavailability. However, predicting the bioactive conformation of a cyclic peptide is not trivial. To overcome this challenge, one attractive approach it to screen a focused library to screen in which all backbone cyclic peptides have the same primary sequence, but differ in parameters that influence their conformation, such as ring size and position.
We describe a detailed protocol for synthesizing a library of backbone cyclic peptides targeting specific parasite PPIs. Using a rational design approach, we developed peptides derived from the scaffold protein Leishmania receptor for activated C-kinase (LACK). We hypothesized that sequences in LACK that are conserved in parasites, but not in the mammalian host homolog, may represent interaction sites for proteins that are critical for the parasites&#39; viability. The cyclic peptides were synthesized using microwave irradiation to reduce reaction times and increase efficiency. Developing a library of backbone cyclic peptides with different ring sizes facilitates a systematic screen for the most biological active conformation. This method provides a general, fast, and facile way to synthesize cyclic peptides.

A simple and general method for the synthesis of cyclic peptides using microwave irradiation is outlined. This procedure enables the synthesis of backbone cyclic peptides with a collection of different conformations while retaining the side chains and the pharmacophoric moieties., and therefore, allows to screen for the bioactive conformation.

Protein-protein interactions (PPIs) are intimately involved in almost all biological processes and are linked to many human diseases. Therefore, there is ...

CN-GELFrEE - Clear Native Gel-eluted Liquid Fraction Entrapment Electrophoresis

Protein complexes perform an array of crucial cellular functions. Elucidating their non-covalent interactions and dynamics is paramount for understanding the role of complexes in biological systems. While the direct characterization of biomolecular assemblies has become increasingly important in recent years, native fractionation techniques that are compatible with downstream analysis techniques, including mass spectrometry, are necessary to further expand these studies. Nevertheless, the field lacks a high-throughput, wide-range, high-recovery separation method for native protein assemblies. Here, we present clear native gel-eluted liquid fraction entrapment electrophoresis (CN-GELFrEE), which is a novel separation modality for non-covalent protein assemblies. CN-GELFrEE separation performance was demonstrated by fractionating complexes extracted from mouse heart. Fractions were collected over 2 hr and displayed discrete bands ranging from ~30 to 500 kDa. A consistent pattern of increasing molecular weight bandwidths was observed, each ranging ~100 kDa. Further, subsequent reanalysis of native fractions via SDS-PAGE showed molecular-weight shifts consistent with the denaturation of protein complexes. Therefore, CN-GELFrEE was proved to offer the ability to perform high-resolution and high-recovery native separations on protein complexes from a large molecular weight range, providing fractions that are compatible with downstream protein analyses.

This protocol describes how to prepare and perform clear native gel-eluted liquid fraction entrapment electrophoresis (CN-GELFrEE), a native separation technique for non-covalent biomolecular assemblies and proteins from heterogeneous samples that is compatible with various downstream protein analysis techniques.

Protein complexes perform an array of crucial cellular functions. Elucidating their non-covalent interactions and dynamics is paramount for understanding ...

Characterization, Quantification and Compound-specific Isotopic Analysis of Pyrogenic Carbon Using Benzene Polycarboxylic Acids (BPCA)

Fire-derived, pyrogenic carbon (PyC), sometimes called black carbon (BC), is the carbonaceous solid residue of biomass and fossil fuel combustion, such as char and soot. PyC is ubiquitous in the environment due to its long persistence, and its abundance might even increase with the projected increase in global wildfire activity and the continued burning of fossil fuel. PyC is also increasingly produced from the industrial pyrolysis of organic wastes, which yields charred soil amendments (biochar). Moreover, the emergence of nanotechnology may also result in the release of PyC-like compounds to the environment. It is thus a high priority to reliably detect, characterize and quantify these charred materials in order to investigate their environmental properties and to understand their role in the carbon cycle.
Here, we present the benzene polycarboxylic acid (BPCA) method, which allows the simultaneous assessment of PyC&#39;s characteristics, quantity and isotopic composition (13C and 14C) on a molecular level. The method is applicable to a very wide range of environmental sample materials and detects PyC over a broad range of the combustion continuum, i.e., it is sensitive to slightly charred biomass as well as high temperature chars and soot. The BPCA protocol presented here is simple to employ, highly reproducible, as well as easily extendable and modifiable to specific requirements. It thus provides a versatile tool for the investigation of PyC in various disciplines, ranging from archeology and environmental forensics to biochar and carbon cycling research.

Characterization, Quantification and Compound-specific Isotopic Analysis of Pyrogenic Carbon Using Benzene Polycarboxylic Acids BPCA

We present the benzene polycarboxylic acid (BPCA) method for assessing pyrogenic carbon (PyC) in the environment. The compound-specific approach uniquely provides simultaneous information about the characteristics, quantity and isotopic composition (13C and 14C) of PyC.

Fire-derived, pyrogenic carbon (PyC), sometimes called black carbon (BC), is the carbonaceous solid residue of biomass and fossil fuel combustion, such as ...

Capillary Electrophoresis to Monitor Peptide Grafting onto Chitosan Films in Real Time

Free-solution capillary electrophoresis (CE) separates analytes, generally charged compounds in solution through the application of an electric field. Compared to other analytical separation techniques, such as chromatography, CE is cheap, robust and effectively requires no sample preparation (for a number of complex natural matrices or polymeric samples). CE is fast and can be used to follow the evolution of mixtures in real time (e.g., chemical reaction kinetics), as the signals observed for the separated compounds are directly proportional to their quantity in solution.
Here, the efficiency of CE is demonstrated for monitoring the covalent grafting of peptides onto chitosan films for subsequent biomedical applications. Chitosan&#39;s antimicrobial and biocompatible properties make it an attractive material for biomedical applications such as cell growth substrates. Covalently grafting the peptide RGDS (arginine - glycine - aspartic acid - serine) onto the surface of chitosan films aims at improving cell attachment. Historically, chromatography and amino acid analysis have been used to provide a direct measurement of the amount of grafted peptide. However, the fast separation and absence of sample preparation provided by CE enables equally accurate yet real-time monitoring of the peptide grafting process. CE is able to separate and quantify the different components of the reaction mixture: the (non-grafted) peptide and the chemical coupling agents. In this way the use of CE results in improved films for downstream applications.
The chitosan films were characterized through solid-state NMR (nuclear magnetic resonance) spectroscopy. This technique is more time-consuming and cannot be applied in real time, but yields a direct measurement of the peptide and thus validates the CE technique.

Free solution capillary electrophoresis is a fast, cheap and robust analytical method that enables the quantitative monitoring of chemical reactions in real time. Its utility for rapid, convenient and precise analysis is demonstrated here through analysis of covalent peptide grafting onto chitosan films for improved cell adhesion.

Free-solution capillary electrophoresis (CE) separates analytes, generally charged compounds in solution through the application of an electric field. ...

In Situ Detection and Single Cell Quantification of Metal Oxide Nanoparticles Using Nuclear Microprobe Analysis

Micro-analytical techniques based on chemical element imaging enable the localization and quantification of chemical composition at the cellular level. They offer new possibilities for the characterization of living systems and are particularly appropriate for detecting, localizing and quantifying the presence of metal oxide nanoparticles both in biological specimens and the environment. Indeed, these techniques all meet relevant requirements in terms of (i) sensitivity (from 1 up to 10 &#181;g.g-1 of dry mass), (ii) micrometer range spatial resolution, and (iii) multi-element detection. Given these characteristics, microbeam chemical element imaging can powerfully complement routine imaging techniques such as optical and fluorescence microscopy. This protocol describes how to perform a nuclear microprobe analysis on cultured cells (U2OS) exposed to titanium dioxide nanoparticles. Cells must grow on and be exposed directly in a specially designed sample holder used on the optical microscope and in the nuclear microprobe analysis stages. Plunge-freeze cryogenic fixation of the samples preserves both the cellular organization and the chemical element distribution. Simultaneous nuclear microprobe analysis (scanning transmission ion microscopy, Rutherford backscattering spectrometry and particle induced X-ray emission) performed on the sample provides information about the cellular density, the local distribution of the chemical elements, as well as the cellular content of nanoparticles. There is a growing need for such analytical tools within biology, especially in the emerging context of Nanotoxicology and Nanomedicine for which our comprehension of the interactions between nanoparticles and biological samples must be deepened. In particular, as nuclear microprobe analysis does not require nanoparticles to be labelled, nanoparticle abundances are quantifiable down to the individual cell level in a cell population, independently of their surface state.

In Situ Detection and Single Cell Quantification of Metal Oxide Nanoparticles Using Nuclear Microprobe Analysis

We describe a procedure for the detection of chemical elements present in situ in human cells as well as their in vitro quantification. The method is well-suited to any cell type and is particularly useful for quantitative chemical analyses in single cells following in vitro metal oxide nanoparticles exposure.

Micro-analytical techniques based on chemical element imaging enable the localization and quantification of chemical composition at the cellular level. ...