Discovering Nature's Fingerprints: Isotope Ratio Analysis on Bioanalytical Mass Spectrometers

For a generation or more, the mass spectrometry that developed at the frontier of molecular biology was worlds apart from isotope ratio mass spectrometry, a label-free approach done on optimized gas-source magnetic sector instruments. Recent studies show that electrospray-ionization Orbitraps and other mass spectrometers widely used in the life sciences can be fine-tuned for high-precision isotope ratio analysis. Since isotope patterns form everywhere in nature based on well-understood principles, intramolecular isotope measurements allow unique insights into a fascinating range of research topics. This Perspective introduces a wider readership to current topics in stable isotope research with the aim of discussing how soft-ionization mass spectrometry coupled with ultrahigh mass resolution can enable long-envisioned progress. We highlight novel prospects of observing isotopes in intact polar compounds and speculate on future directions of this adventure into the overlapping realms of biology, chemistry, and geology.

150 - La cinétique des D-dimères pendant l'hospitalisation est un facteur pronostique majeur de décès chez les patients hospitalisés pour COVID-19

Contexte

La tendance évolutive des D-dimères pourrait être un facteur prédictif de la gravité de cette maladie mais est très peu étudié. Cette étude avait pour objectif d'identifier des phénotypes biologiques d’évolution des D-dimères et leur valeur pronostique sur le risque de décès ou de thrombose chez les patients hospitalisés pour COVID-19.

Méthodes

Étude monocentrique incluant 270 patients hospitalisés pour COVID-19 au CHRU de Nancy du 21/02/20 au 12/05/20. Les données biologiques et cliniques ont été́ recueillies à l'aide des dossiers médicaux informatisés des patients. Le dosage des D-dimères plasmatiques mesurés par méthode immuno-enzymatique. Le critère de jugement principal était la mortalité toutes causes et le critère secondaire était la survenue d’événements thrombotiques. Nous avons utilisé le modèle conjoint à classes latentes qui permet de modéliser simultanément les trajectoires latentes de D-dimères et le risque de décès ou de thrombose.

Résultats

Trois types de trajectoires de D-dimères ont été identifiés : trajectoire 1) « Niveau initial élevé puis décroisant » (n=65, 24,1 %); trajectoire 2) « Niveau initial bas puis augmentation rapide » (n=18, 6,6 %); trajectoire 3) « Niveau initial modéré puis stable » (n=187, 69,3 %). Dans l'ensemble de l’échantillon les taux de décès et de thromboses étaient de 14,4 % (38/270) et 12,2 % (32/263) respectivement. Comparée à la trajectoire 3, les patients des trajectoires 1 et 2 avaient un risque de décès 3,6 (HR=3,6 [1,1–12,3], p=0,037) et 18,1 (HR=18,1 [4,3–76,2], p<0,0001) fois plus élevées respectivement. Le risque de thrombose n’était pas significativement différent entre les trois trajectoires.

Conclusion

Cette étude a montré qu'un niveau très élevé de D-dimères à l'admission ou un niveau bas mais avec une augmentation rapide durant l'hospitalisation sont des facteurs pronostiques important de la sévérité de la COVID-19. Un suivi rapproché de la cinétique des D-dimères, notamment dès les premiers jours d'hospitalisation devrait être renforcé chez ces patients.

Déclaration de liens d'intérêts

Les auteurs déclarent ne pas avoir de liens d'intérêts.

Intramolecular non-covalent isotope effects at natural abundance associated with the migration of paracetamol in solid matrices during liquid chromatography

Position-specific isotope analysis by Nuclear Magnetic Resonance spectrometry was employed to study the ¹³C intramolecular isotopic fractionation associated with the migration of organic substrates through different stationary phases chromatography columns. Liquid chromatography is often used to isolate compounds prior to their isotope analysis and this purification step potentially alters the isotopic composition of target compounds introducing a bias in the later measured data. Moreover, results from liquid chromatography can yield the sorption parameters needed in reactive transport models that predict the transport and fate of organic contaminants to in the environment. The aim of this study was to use intramolecular isotope analysis to study both ¹³C and ¹⁵N isotope effects associated with the elution of paracetamol (acetaminophen) through different stationary phases and to compare them to effects observed previously for vanillin. Results showed very different intramolecular isotope fractionation profiles depending on the chemical structure of the stationary phase. The data also demonstrate that both the amplitude and the distribution of measured isotope effects depend on the nature of the non-covalent interactions involved in the migration process. Results provided by theoretical calculation performed during this study also confirmed the direct link between observed intramolecular isotope fractionation and the nature of involved intermolecular interactions. It is concluded that the nature of the stationary phase through which the substrate passes has a major impact on the intramolecular isotopic composition of organic compounds isolated by chromatography methods..

Intramolecular isotope effects during permanganate oxidation and acid hydrolysis of methyl tert-butyl ether

Stable isotopes have been widely used to monitor remediation of environmental contaminants over the last decades. This approach gives a good mechanistic description of natural or assisted degradation of organic pollutants, such as methyl tert-butyl ether (MTBE). Since abiotic degradation seems to be the most promising assisted attenuation method, the isotopic fractionation associated with oxidation and hydrolysis processes need to be further investigated in order to understand better these processes and make their monitoring more efficient. In this study, position-specific isotope effects (PSIEs) associated with permanganate oxidation and acid hydrolysis of MTBE were determined using isotope ratio monitoring by ¹³C Nuclear Magnetic Resonance Spectrometry (irm-¹³C NMR) combined with isotope ratio monitoring by Mass Spectrometry (irm-MS). The use of this Position-Specific Isotopic Analysis (PSIA) method makes it possible to observe a specific normal isotope effect (IE) associated with each of these two abiotic degradation mechanisms. The present work demonstrates that the ¹³C isotope pattern of the main degradation product, tert-butyl alcohol (TBA), depends on the chemical reaction by which it is produced. Furthermore, this study also demonstrates that PSIA at natural abundance can give new insights into reaction mechanisms and that this methodology is very promising for the future of modeling the remediation of organic contaminants.

Difficulties in Differentiating Natural from Synthetic Alkaloids by Isotope Ratio Monitoring using 13C Nuclear Magnetic Resonance Spectrometry

Within the food and pharmaceutical industries, there is an increasing legislative requirement for the accurate labeling of the product's origin. A key feature of this is to indicate whether the product is of natural or synthetic origin. With reference to this context, we have investigated three alkaloids commonly exploited for human use: nicotine, atropine, and caffeine. We have measured by ¹³C nuclear magnetic resonance spectrometry the position-specific distribution of ¹³C at natural abundance within several samples of each of these target molecules. This technique is well suited to distinguishing between origins, as the distribution of the ¹³C isotope reflects the primary source of the carbon atoms and the process by which the molecule was (bio)synthesized. Our findings indicate that labeling can be misleading, especially in relation to a supplied compound being labeled as "synthetic" even though its ¹³C profile indicates a natural origin.

Simulating Stable Isotope Ratios in Plumes of Groundwater Pollutants with BIOSCREEN-AT-ISO

BIOSCREEN is a well-known simple tool for evaluating the transport of dissolved contaminants in groundwater, ideal for rapid screening and teaching. This work extends the BIOSCREEN model for the calculation of stable isotope ratios in contaminants. A three-dimensional exact solution of the reactive transport from a patch source, accounting for fractionation by first-order decay and/or sorption, is used. The results match those from a previously published isotope model but are much simpler to obtain. Two different isotopes may be computed, and dual isotope plots can be viewed. The dual isotope assessment is a rapidly emerging new approach for identifying process mechanisms in aquifers. Furthermore, deviations of isotope ratios at specific reactive positions with respect to "bulk" ratios in the whole compound can be simulated. This model is named BIOSCREEN-AT-ISO and will be downloadable from the journal homepage.

Safety, tolerability, pharmacokinetics and pharmacodynamics of AZP-531, a first-in-class analogue of unacylated ghrelin, in healthy and overweight/obese subjects and subjects with type 2 diabetes

AIM

To explore the safety, pharmacokinetics and pharmacodynamics in humans of the unacylated ghrelin analogue AZP-531, designed to improve glycaemic control and reduce weight.

METHODS

Assessments, including glucose measurements, were performed in a three-part randomized study. In Part A, healthy subjects [n = 44, age 18-50 years, body mass index (BMI) 20-28 kg/m(2) ] received a single subcutaneous dose of 0.3, 3, 15, 30, 60 or 120 µg/kg AZP-531 or placebo. In Part B, overweight/obese subjects (n = 32, age 18-65 years, BMI 28-38 kg/m(2) ) and in Part C, patients with type 2 diabetes [T2D; n = 36, age 18-65 years, BMI 20-40 kg/m(2) , glycated haemoglobin (HbA1c) 7-10%] received AZP-531 or placebo for 14 days (daily doses of 3, 15, 30 or 60 µg/kg and 15, 2 × 30 or 60 µg/kg, respectively).

RESULTS

AZP-531 was well tolerated. Single- and multiple-dose pharmokinetic variables were similar. Maximum AZP-531 concentrations were typically reached at 1 h post-dose. Observed maximum concentration (Cmax ) and area under the curve were dose-proportional. The mean terminal half-life (t1/2 ) was 2-3 h. In Part B, AZP-531 doses of ≥15 µg/kg significantly improved glucose concentrations, without increasing insulin levels, suggesting an insulin-sensitizing effect. AZP-531 decreased mean body weight by 2.6 kg (vs 0.8 kg for placebo). In Part C, glucose variables improved in all groups, including placebo, suggesting a study effect in uncontrolled patients at baseline. Notwithstanding, AZP-531 60 µg/kg reduced HbA1c by 0.4% (vs 0.2% for placebo) and body weight by 2.1 kg (vs 1.3 kg for placebo).

CONCLUSIONS

AZP-531 was well tolerated in this first-in-human study. Its pharmacokinetic profile, suitable for once-daily dosing, and metabolic effects support further clinical development for T2D.

Elevated ratio of acylated to unacylated ghrelin in children and young adults with Prader-Willi syndrome

Prader-Willi syndrome (PWS) is characterized by a switch from failure to thrive to excessive weight gain and hyperphagia in early childhood. Hyperghrelinemia may be involved in the underlying mechanisms of the switch. The purpose of this study is to evaluate acylated ghrelin (AG) and unacylated ghrelin (UAG) levels in PWS and investigate their associations with hyperphagia. This is a cross-sectional clinical study conducted in three PWS expert centers in the Netherlands and France. Levels of AG and UAG and the AG/UAG ratio were determined in 138 patients with PWS (0.2-29.4 years) and compared with 50 age-matched obese subjects (4.3-16.9 years) and 39 healthy controls (0.8-28.6 years). AEBSF was used to inhibit deacylation of AG. As a group, PWS patients had higher AG but similar UAG levels as healthy controls (AG 129.1 vs 82.4 pg/ml, p = 0.016; UAG 135.3 vs 157.3 pg/ml, resp.), resulting in a significantly higher AG/UAG ratio (1.00 vs 0.61, p = 0.001, resp.). Obese subjects had significantly lower AG and UAG levels than PWS and controls (40.3 and 35.3 pg/ml, resp.), but also a high AG/UAG ratio (1.16). The reason for the higher AG/UAG ratio in PWS and obese was, however, completely different, as PWS had a high AG and obese a very low UAG. PWS patients without weight gain or hyperphagia had a similar AG/UAG ratio as age-matched controls, in contrast to those with weight gain and/or hyperphagia who had an elevated AG/UAG ratio. The switch to excessive weight gain in PWS seems to coincide with an increase in the AG/UAG ratio, even prior to the start of hyperphagia.

Insights into Mechanistic Models for Evaporation of Organic Liquids in the Environment Obtained by Position-Specific Carbon Isotope Analysis

Position-specific isotope effects (PSIEs) have been measured by isotope ratio monitoring (13)C nuclear magnetic resonance spectrometry during the evaporation of 10 liquids of different polarities under 4 evaporation modes (passive evaporation, air-vented evaporation, low pressure evaporation, distillation). The observed effects are used to assess the validity of the Craig-Gordon isotope model for organic liquids. For seven liquids the overall isotope effect (IE) includes a vapor-liquid contribution that is strongly position-specific in polar compounds but less so in apolar compounds and a diffusive IE that is not position-specific, except in the alcohols, ethanol and propan-1-ol. The diffusive IE is diminished under forced evaporation. The position-specific isotope pattern created by liquid-vapor IEs is manifest in five liquids, which have an air-side limitation for volatilization. For the alcohols, undefined processes in the liquid phase create additional PSIEs. Three other liquids with limitations on the liquid side have a lower, highly position-specific, bulk diffusive IE. It is concluded that evaporation of organic pollutants creates unique position-specific isotope patterns that may be used to assess the progress of remediation or natural attenuation of pollution and that the Craig-Gordon isotope model is valid for the volatilization of nonpolar organic liquids with air-side limitation of the volatilization rate.

Enhanced forensic discrimination of pollutants by position-specific isotope analysis using isotope ratio monitoring by (13)C nuclear magnetic resonance spectrometry

In forensic environmental investigations the main issue concerns the inference of the original source of the pollutant for determining the liable party. Isotope measurements in geochemistry, combined with complimentary techniques for contaminant identification, have contributed significantly to source determination at polluted sites. In this work we have determined the intramolecular (13)C profiles of several molecules well-known as pollutants. By giving additional analytical parameters, position-specific isotope analysis performed by isotope ratio monitoring by (13)C nuclear magnetic resonance (irm-(13)C NMR) spectrometry gives new information to help in answering the major question: what is the origin of the detected contaminant? We have shown that isotope profiling of the core of a molecule reveals both the raw materials and the process used in its manufacture. It also can reveal processes occurring between the contamination site 'source' and the sampling site. Thus, irm-(13)C NMR is shown to be a very good complement to compound-specific isotope analysis currently performed by mass spectrometry for assessing polluted sites involving substantial spills of pollutant.

Fractionation in position-specific isotope composition during vaporization of environmental pollutants measured with isotope ratio monitoring by ¹³C nuclear magnetic resonance spectrometry

Isotopic fractionation of pollutants in terrestrial or aqueous environments is a well-recognized means by which to track different processes during remediation. As a complement to the common practice of measuring the change in isotope ratio for the whole molecule using isotope ratio monitoring by mass spectrometry (irm-MS), position-specific isotope analysis (PSIA) can provide further information that can be exploited to investigate source and remediation of soil and water pollutants. Position-specific fractionation originates from either degradative or partitioning processes. We show that isotope ratio monitoring by (13)C NMR (irm-(13)C NMR) spectrometry can be effectively applied to methyl tert-butylether, toluene, ethanol and trichloroethene to obtain this position-specific data for partitioning. It is found that each compound exhibits characteristic position-specific isotope fractionation patterns, and that these are modulated by the type of evaporative process occurring. Such data should help refine models of how remediation is taking place, hence back-tracking to identify pollutant sources.

Suppression of radiation damping for high precision quantitative NMR

True quantitative analysis of concentrated samples by (1)H NMR is made very difficult by Radiation Damping. A novel NMR sequence (inspired by the WET NMR sequence and by Outer Volume Saturation methods) is therefore proposed to suppress this phenomenon by reducing the spatial area and consequently the number of spins contributing to the signal detected. The size of the detected volume can be easily chosen in a large range and line shape distortions are avoided thanks to a uniform signal suppression of the outer volume. Composition of a mixture can as a result be determined with very high accuracy (precision and trueness) at the per mille level whatever the concentrations and without hardware modification.

Predicting equilibrium vapour pressure isotope effects by using artificial neural networks or multi-linear regression - A quantitative structure property relationship approach

We aim at predicting the effect of structure and isotopic substitutions on the equilibrium vapour pressure isotope effect of various organic compounds (alcohols, acids, alkanes, alkenes and aromatics) at intermediate temperatures. We attempt to explore quantitative structure property relationships by using artificial neural networks (ANN); the multi-layer perceptron (MLP) and compare the performances of it with multi-linear regression (MLR). These approaches are based on the relationship between the molecular structure (organic chain, polar functions, type of functions, type of isotope involved) of the organic compounds, and their equilibrium vapour pressure. A data set of 130 equilibrium vapour pressure isotope effects was used: 112 were used in the training set and the remaining 18 were used for the test/validation dataset. Two sets of descriptors were tested, a set with all the descriptors: number of(12)C, (13)C, (16)O, (18)O, (1)H, (2)H, OH functions, OD functions, CO functions, Connolly Solvent Accessible Surface Area (CSA) and temperature and a reduced set of descriptors. The dependent variable (the output) is the natural logarithm of the ratios of vapour pressures (ln R), expressed as light/heavy as in classical literature. Since the database is rather small, the leave-one-out procedure was used to validate both models. Considering higher determination coefficients and lower error values, it is concluded that the multi-layer perceptron provided better results compared to multi-linear regression. The stepwise regression procedure is a useful tool to reduce the number of descriptors. To our knowledge, a Quantitative Structure Property Relationship (QSPR) approach for isotopic studies is novel.

Simultaneous determination of natural-abundance δ15N values and quantities of individual amino acids in proteins from milk of lactating women and from infant hair using gas chromatography/isotope ratio mass spectrometry

RATIONALE

In isotope tracer experiments used in nutritional studies, it is frequently desirable both to determine the (15)N/(14)N ratios of target compounds and to quantify these compounds. This report shows how this can be achieved in a single chromatographic run for protein amino acids using an isotope ratio mass spectrometer.

METHODS

Protein hydrolysis by acidic digestion was used to release amino acids, which were then derivatized as their N-pivaloyl-O-isopropyl esters. Suitable conditions for sample preparation were established for both hair and milk proteins. The N-pivaloyl-O-isopropyl esters of amino acids were separated by gas chromatography (GC) on a 60 m ZB-WAX column linked via a combustion interface to an isotope ratio mass spectrometer. The (15)N/(14)N ratios were obtained from the m/z 28, 29 and 30 peak intensities and the quantities from the Area All (Vs) integrated peak areas.

RESULTS

It is shown from a five-point calibration curve that both parameters can be measured reliably within the range of 1.0 to 2.0 mg/mL for the major amino acids derived from the hydrolysis of human maternal milk or hair samples. The method was validated in terms of inter-day and inter-user repeatability for both parameters for 14 amino acids. The amino acid percentage composition showed a good correlation with literature values. The method was applied to determine the variability in a population of lactating mothers and their infants.

CONCLUSIONS

It has been established that δ(15)N values can be simultaneously determined for a complex mixture of amino acids at variable concentrations. It is shown that the percentage composition obtained correlates well with that obtained by calculation from the protein composition or from literature values. This procedure should provide a significant saving in analysis time, especially in those cases where the GC run-time is necessarily long. It allows the satisfactory determination of the variation within a sample population.

Phosphate-dependent stimulation of MGP and OPN expression in osteoblasts via the ERK1/2 pathway is modulated by calcium

Inorganic phosphate (Pi) acts as a signaling molecule in bone-forming cells, affecting cell functions and gene expression. Particularly, Pi stimulates the expression of mineralization-associated genes such as matrix gla protein (MGP) and osteopontin (OPN) through the ERK1/2 pathway. With respect to the presence of elevated extracellular calcium and Pi levels during bone remodeling, we questioned whether calcium might play a role in the Pi-dependent effects in osteoblasts. We first showed by Western blot and real-time PCR that the concomitant presence of 10 mM Pi and 1.8 mM calcium is required to stimulate ERK1/2 phosphorylation and MGP/OPN genes expression. The mechanisms involved in the cellular effects of calcium in the presence of Pi were subsequently examined. Firstly, the use of the calcium-sensing receptor (CaSR) agonist gadolinium and the G-protein inhibitor pertussis toxin enabled us to determine that a CaSR mechanism is not involved in the Pi and calcium mediated cellular effects. By transmission electron microscopy, we next demonstrated that adding 10mM Pi to the culture medium containing 1.8mM calcium led to the formation calcium phosphate precipitates (CaPp). Moreover, treatment of osteoblasts with exogenous pre-synthesized CaPp stimulated ERK1/2 phosphorylation and MGP/OPN genes expression. In spite of high extracellular calcium and Pi concentrations, this stimulation was blunted in the presence of phosphocitrate, an inhibitor of crystal formation. Finally, we showed that despite that CaPp are not endocytosed, their effect on ERK1/2 phosphorylation and MGP/OPN genes expression were dependent on lipid rafts integrity. In summary, we showed that calcium is required for Pi-dependent ERK1/2 phosphorylation and regulation of mineralization-associated genes in osteoblasts and that its effect could originate from extracellular-related effects of CaPp that are dependent on the integrity of lipid rafts.