LC-method development for the quantification of neuromedin-like peptides. Emphasis on column choice and mobile phase composition

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

Improving the LC-MS/MS analysis of neuromedin U-8 and neuromedin S by minimizing their adsorption behavior and optimizing UHPLC and MS parameters

Neuromedin U (NmU) and neuromedin S (NmS) are two closely related neuropeptides belonging to the neuromedin family. NmU usually occurs either as a truncated eight amino acid long peptide (NmU-8) or as an 25 amino acid long peptide, although other molecular forms exist depending on the species considered. NmS, on the other hand, is a 36 amino acid long peptide, sharing the same amidated C-terminal heptapeptide with NmU. Nowadays, liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) is the preferred analytical technique for peptide quantification, because of its excellent sensitivity and selectivity. However, reaching the required quantification limits for these compounds in biological samples remains an extremely challenging task, especially because of their nonspecific binding (NSB). This study highlights the difficulties that are faced when quantifying larger neuropeptides (23-36 amino acids) compared to smaller ones (< 15 amino acids). The first part of this work aims to solve the adsorption problem for NmU-8 and NmS, by investigating the different steps involved in the sample preparation, i.e. the different solvents applied and the pipetting protocol. The addition of 0.05% plasma as an adsorption competitor was found to be primordial to avoid peptide loss due to NSB. The second part of this work focusses on further improving the sensitivity of the LC-MS/MS method for NmU-8 and NmS, by evaluating some UHPLC-parameters, including the stationary phase, the column temperature and the trapping conditions. For both peptides of interest, the best results were achieved when combining a C₁₈ trap column with a C₁₈ iKey separation device containing a positively charged surface. Column temperatures of 35 and 45 °C for NmU-8 and NmS respectively, resulted in the highest peak areas and S/N ratios, while applying higher column temperatures substantially decreased sensitivity. Moreover, a gradient starting at 20% organic modifier instead of 5% significantly improved the peak shape of both peptides. Finally, some compound-specific MS parameters, i.e. the capillary and the cone voltages, were evaluated. The peak areas increased with a factor 2 and 7 for NmU-8 and NmS respectively and peptide detection in the low picomolar range is now feasible.

Simultaneous Determination of the Residues of Isopyrazam Isomers and Their Metabolites in Soil and Tomatoes by Ultraperformance Liquid Chromatography-Tandem Mass Spectrometry

An effective and sensitive method for the determination of isopyrazam (IZM) isomers (syn-IZM and anti-IZM) and their metabolites (syn545364 and syn545449) in tomato and soil by ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was developed in the present study. The method showed excellent linearities (R² = 0.999) at 0.005-5 mg/L. The recoveries were 92.0-107%, and the relative standard deviation (RSD) values were lower than 9.40% in tomato and soil matrices at 0.01, 0.1, and 10 mg/kg. The limits of detection (LODs) of the four compounds ranged from 6.88 × 10^-5 to 2.70 × 10^-4 mg/kg, while the limits of quantification (LOQs) ranged from 2.20 × 10^-4 to 9.20 × 10^-4 mg/kg. The storage stability test results showed that syn-IZM, anti-IZM, syn545449, and syn545364 were stable in tomato at -20 °C within 36 weeks, and the maximum degradation rates were 16.0, 12.0, 7.10, and 12.0%, respectively. The field dissipation test results showed that the half-lives of syn-IZM in tomato and soil were 2.60-10.2 and 13.6-33.0 days, respectively, while the half-lives of anti-IZM in soil were 21.7-46.2 days, and no residues of anti-IZM were detected in tomato. The terminal residue test results showed that the residue of syn-IZM and anti-IZM in tomato ranged from <0.0100-0.490 to <0.0100-0.0850 mg/kg. The present results showed that anti-IZM degraded faster than syn-IZM in tomato and soil, and had a lower residue level in tomato.

Validated mass spectrometric assay for the quantification of substance P and human hemokinin-1 in plasma samples: A design of experiments concept for comprehensive method development

INTRODUCTION

Elevated plasma concentrations of the inflammatory neurokinins Substance P (SP) and human Hemokinin-1 (hHK-1) were found in infectious diseases. SP and hHK-1 plasma levels in diseased subjects are determined by immunoassays although not recommended by most immunoassay suppliers owing to their limitations to differentiate accurately between both peptides. A selective and reliable alternative (e.g. mass spectrometric (MS) assay) was missing because of a lack of sensitivity for the determination of endogenous plasma levels.

METHOD

Using a Design of Experiments (DoE) concept, a highly sensitive MS assay was developed for the quantification of SP, its inactive analog as the free acid, and hHK-1 in human plasma. Critical method aspects as the plasma extraction, peptide separation, and the method sensitivity were comprehensively optimized. The method was validated according to international bioanalytical guidelines and its applicability was evaluated in plasma of volunteers.

RESULTS

Within 106 experiments utilizing the DoE concept, the sensitivity of the assay was substantially improved to achieve limits of detection of 5.8 pg/mL for SP, 6.2 pg/mL for its free acid, and 5.3 pg/mL for hHK-1 in plasma. The lean method development was followed by the successful validation according to the regulatory guidelines resulting in a wide quantification range of 7.8-2000 pg/mL. In the volunteers' plasma, no SP and hHK-1 were detectable. Instead, the free acid of SP was quantified in individually distinct levels (202.5-1024.1 pg/mL).

CONCLUSION

An accurate and precise MS assay for the quantification of SP, its free acid, and hHK-1 in plasma was established. The mass spectrometric quantification of the free acid of SP in human plasma samples lead to the question about possible cross-reactivity of the immunoassays in former determinations.