An automated assay for the clinical measurement of plasma renin activity by immuno-MALDI (iMALDI)

Immuno-MALDI-MS for Accurate Quantitation of Targeted Peptides from Volume-Restricted Samples

The immuno-MALDI-MS method can be used to quantify low-abundance proteins from clinical samples that offer only a limited amount of material for analysis. An internal standard, in the form of a stable isotope-labeled peptide, is used to ensure reproducible and absolute quantitation. The protocol described here was optimized for the quantitation of AKT1 and AKT2, but we offer instructions on how to adapt the method to target other proteins. The described workflow is compatible with automation via a liquid handling robot for high-throughput applications.

A liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based assay for simultaneous quantification of aldosterone, renin activity, and angiotensin II in human plasma

Accurate quantification of plasma aldosterone (ALD) and renin activity (PRA)is critical for the diagnosis of primary aldosteronism (PA). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is considered the "gold standard" method for the determination of ALDand PRA. The aim of this study is to develop a new LC-MRM/MS assay for quantifying plasma ALD, PRA, and angiotensin II (Ang II) simultaneously and validate its effectiveness. To be more specific, plasmasamples were prepared by solid-phase extraction and separated in an ultra-performance reversed-phase column. MS detection was performed via a triple quadrupole mass spectrometer containing both positive and negative ion monitoring modes. The developed assay was then validated according to the standard guidelines and the influence of sample incubation on ALD and Ang II concentration was evaluated. In addition, the variation of endogenous Ang I was explored. The proposed LC-MRM/MS method was compared another LC-MS/MS method, which detects ALD, Ang I, and Ang II separately. Analyteswere separated and quantified within 5 min. The assay wasvalidated to be linear up to 5000 pg/ml for ALD and Ang II and 33.3 ng/ml/h for PRA.The lower limit of quantification (LLOQ) was 15 pg/ml, 15 pg/ml, and 0.1 ng/ml/hfor ALD, Ang II, and PRArespectively. Specificity, precision, accuracy, and stability were tested to meet the requirements of the guidelines. Significant changes were not found in ALD and Ang II concentrations over the 3 h-incubation. In addition, it was demonstratedthat the resultof PRA was not stronglyinfluenced by the endogenous Ang I. Comparison with another LC-MS/MS method was performed using the same apparatusand the proposed method was proved to be in good coincidence with the correlation coefficients rangingfrom 0.955to0.996. A sensitive and reliable method for simultaneousquantification of ALD, PRA, and Ang II has been developed and this study will significantly promote laboratory workflow efficiency and throughput.

Cost-Effective Automated Preparation of Serum Samples for Reproducible Quantitative Clinical Proteomics

Reproducible sample preparation remains a significant challenge in large-scale clinical research using selected reaction monitoring-mass spectrometry (SRM-MS), which enables a highly sensitive multiplexed assay. Although automated liquid-handling platforms have tremendous potential for addressing this issue, the high cost of their consumables is a drawback that renders routine operation expensive. Here we evaluated the performance of a liquid-handling platform in preparing serum samples compared with a standard experiment while reducing the outlay for consumables, such as tips, wasted reagents, and reagent stock plates. A total of 26 multiplex assays were quantified by SRM-MS using four sets of 24 pooled human serum aliquots; the four sets used a fixed number (1, 4, 8, or 24) of tips to dispense digestion reagents. This study demonstrated that the use of 4 or 8 tips is comparable to 24 tips (standard experiment), as evidenced by their coefficients of variation: 13.5% (for 4 and 8 tips) versus 12.0% (24 tips). Thus we can save 37% of the total experimental cost compared with the standard experiment, maintaining nearly equivalent reproducibility. The routine operation of cost-effective liquid-handling platforms can enable researchers to process large-scale samples with high throughput, adding credibility to their findings by minimizing human error.

How iMALDI can improve clinical diagnostics

Protein mass spectrometry (MS) is an indispensable tool to detect molecular signatures that can be associated with cellular dysregulation and disease.

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).

Assuring Consistent Performance of an Insulin-Like Growth Factor 1 MALDImmunoassay by Monitoring Measurement Quality Indicators

Analytical methods based on mass spectrometry (MS) have been successfully applied in biomarker discovery studies, while the role of MS in translating biomarker candidates to clinical diagnostics is less pronounced. MALDImmunoassays—methods that combine immunoaffinity enrichment with matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometric detection—are attractive analytical approaches for large-scale sample analysis by virtue of their ease of operation and high-throughput capabilities. Despite this fact, MALDImmunoassays are not widely used in clinical diagnostics, which is mainly due to the limited availability of internal standards that can adequately correct for variability in sample preparation and the MALDI process itself. Here we present a novel MALDImmunoassay for quantification of insulin-like growth factor 1 (IGF1) in human plasma. Reliable IGF1 quantification in the range of 10–1000 ng/mL was achieved by employing ¹⁵N-IGF1 as internal standard, which proved to be an essential feature of the IGF1 MALDImmunoassay. The method was validated according to U.S. Food and Drug Administration (FDA) guidelines, which included demonstrating the effectiveness of IGF1/IGF binding protein (IGF1/IGFBP) complex dissociation using sodium dodecyl sulfate (SDS). Furthermore, the MALDImmunoassay compared well with the IDS-iSYS IGF1 immunoassay with high correlation (R² = 0.99), although substantially lower levels were reported by the MALDImmunoassay. The method was tested on >1000 samples from a cohort of renal transplant recipients to assess its performance in a clinical setting. On the basis of this study, we identified readouts to monitor the quality of the measurements. Our work shows that MALDI-TOF mass spectrometry is suitable for quantitative biomarker analysis provided that an appropriate internal standard is used and that readouts are monitored to assess the quality of the measurements.

Current trends in quantitative proteomics - an update

Proteins can provide insights into biological processes at the functional level, so they are very promising biomarker candidates. The quantification of proteins in biological samples has been routinely used for the diagnosis of diseases and monitoring the treatment. Although large-scale protein quantification in complex samples is still a challenging task, a great amount of effort has been made to advance the technologies that enable quantitative proteomics. Seven years ago, in 2009, we wrote an article about the current trends in quantitative proteomics. In writing this current paper, we realized that, today, we have an even wider selection of potential tools for quantitative proteomics. These tools include new derivatization reagents, novel sampling formats, new types of analyzers and scanning techniques, and recently developed software to assist in assay development and data analysis. In this review article, we will discuss these innovative methods, and their current and potential applications in proteomics. Copyright © 2017 John Wiley & Sons, Ltd.

Quantitation of Plasma Renin Activity in Plasma Using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Accurate determination of plasma renin activity (PRA) is essential for the development and maintenance of an effective screening program for primary aldosteronism (PA). PRA measurement can also be useful in the investigation of renal artery stenosis, syndrome of mineralocorticoid excess, Addison's disease, congenital adrenal hyperplasia, Bartter and Gitelman syndromes, and for inherited defects in the renin angiotensin aldosterone system (RAAS). We describe a semi-automated and simple method for the accurate and precise measurement of PRA from 500 μL of plasma (250 μL if blank subtraction is omitted, as discussed) using a liquid chromatography and tandem mass spectrometry (LC-MS/MS) method for angiotensin I (AngI) in 96-well format. After a 3 h AngI generation step at 37 °C in buffering conditions at pH 6, the reaction is quenched with 10 % formic acid containing AngI internal standard. Sample preparation then proceeds with offline solid phase extraction, two wash steps, and methanol elution followed by injection into the LC-MS/MS system. Quantitation is performed against a 7-point calibration linear curve prepared in buffer. The assay calibration range is 0.34-30.0 ng/mL which corresponds to PRA values of 0.11-10.0 ng/mL/h: much wider than was possible using traditional competitive antibody-based methods. Total precision in clinical production has been observed to be 5.8 to 5.0 % for Bio-Rad Hypertension Control materials having nominal PRA values ranging from 1.73 to 12.43 ng/mL/h. At AngI concentrations of 0.06 ng/L (corresponding to a PRA of 0.02 ng/mL/h), signal to noise ratios are 50:1 indicating that the limit of quantitation is well below the level required for clinical use.

Antibodies as means for selective mass spectrometry

For protein analysis of biological samples, two major strategies are used today; mass spectrometry (MS) and antibody-based methods. Each strategy offers advantages and drawbacks. However, combining the two using an immunoenrichment step with MS analysis brings together the benefits of each method resulting in increased sensitivity, faster analysis and possibility of higher degrees of multiplexing. The immunoenrichment can be performed either on protein or peptide level and quantification standards can be added in order to enable determination of the absolute protein concentration in the sample. The combination of immunoenrichment and MS holds great promise for the future in both proteomics and clinical diagnostics. This review describes different setups of immunoenrichment coupled to mass spectrometry and how these can be utilized in various applications.