Multitechnique mass-spectrometric approach for the detection of bovine glutathione peroxidase selenoprotein: focus on the selenopeptide

Selenoproteome Expression Studied by Non-Radioactive Isotopic Selenium-Labeling in Human Cell Lines

Selenoproteins, in which the selenium atom is present in the rare amino acid selenocysteine, are vital components of cell homeostasis, antioxidant defense, and cell signaling in mammals. The expression of the selenoproteome, composed of 25 selenoprotein genes, is strongly controlled by the selenium status of the body, which is a corollary of selenium availability in the food diet. Here, we present an alternative strategy for the use of the radioactive ⁷⁵Se isotope in order to characterize the selenoproteome regulation based on (i) the selective labeling of the cellular selenocompounds with non-radioactive selenium isotopes (⁷⁶Se, ⁷⁷Se) and (ii) the detection of the isotopic enrichment of the selenoproteins using size-exclusion chromatography followed by inductively coupled plasma mass spectrometry detection. The reliability of our strategy is further confirmed by western blots with distinct selenoprotein-specific antibodies. Using our strategy, we characterized the hierarchy of the selenoproteome regulation in dose–response and kinetic experiments.

Glutathione Peroxidase-Activatable Two-Photon Ratiometric Fluorescent Probe for Redox Mechanism Research in Aging and Mercury Exposure Mice Models

Solid evidence confirms that glutathione peroxidase (GPx) is a kind of vital protease in the first-line antioxidant defense system and participates in regulation of redox homeostasis as well as the pentose phosphate pathway. However, the current methods cannot achieve real-time and in situ visualization studies of GPx. In addition, GPx is highly reactive and susceptible to external interference, and there is rare research for exploring the roles of GPx under environmental factor exposure. Herein, we report a novel two-photon ratiometric fluorescent probe (TP-SS) for GPx detection for the first time. Using TP-SS, we explore the reversible catalytic cycle and the antioxidant mechanisms of GPx/GSH redox pool in aging and mercury exposure models. We detect the concentration fluctuation of GPx in aging and mercury exposure mice models. Also, we perform GPx detection in deep brain tissue and the imaging depth up to 100 μm. We believe that the novel two-photon ratiometric fluorescent probe TP-SS can facilitate the development of GPx-targeting tools and offer great advances in exploring the physiological/pathological functions of GPx.

Comparison of analytical methods using enzymatic activity, immunoaffinity and selenium-specific mass spectrometric detection for the quantitation of glutathione peroxidase 1

Glutathione peroxidase 1 (Gpx1), one of the most responsive selenoproteins to the variation of selenium concentration, is often used to evaluate "selenium status" at a cellular or organismal level. The four major types of analytical methodologies to quantify Gpx1 were revisited. They include (i) an enzymatic assay, (ii, iii) polyacrylamide gel electrophoresis (PAGE) with (ii) western blot detection of protein or (iii) inductively coupled plasma mass spectrometry (ICP MS) detection of selenium, and (iv) size-exclusion chromatography with ICP MS detection. Each of the four methods was optimized for the quantification of Gpx1 with maximum sensitivity. The methods based on the enzymatic and immunodetection offer a much higher sensitivity but their accuracy is compromised by the limited selectivity and limited dynamic range. The advantages, drawbacks and sources of error of each technique are critically discussed and the need for the cross-validation of the results using the different techniques to assure the quality assurance of quantitative analysis is emphasized.

Selenoproteins: molecular pathways and physiological roles

Selenium is an essential micronutrient with important functions in human health and relevance to several pathophysiological conditions. The biological effects of selenium are largely mediated by selenium-containing proteins (selenoproteins) that are present in all three domains of life. Although selenoproteins represent diverse molecular pathways and biological functions, all these proteins contain at least one selenocysteine (Sec), a selenium-containing amino acid, and most serve oxidoreductase functions. Sec is cotranslationally inserted into nascent polypeptide chains in response to the UGA codon, whose normal function is to terminate translation. To decode UGA as Sec, organisms evolved the Sec insertion machinery that allows incorporation of this amino acid at specific UGA codons in a process requiring a cis-acting Sec insertion sequence (SECIS) element. Although the basic mechanisms of Sec synthesis and insertion into proteins in both prokaryotes and eukaryotes have been studied in great detail, the identity and functions of many selenoproteins remain largely unknown. In the last decade, there has been significant progress in characterizing selenoproteins and selenoproteomes and understanding their physiological functions. We discuss current knowledge about how these unique proteins perform their functions at the molecular level and highlight new insights into the roles that selenoproteins play in human health.

Characterization of selenium incorporation into wheat proteins by two-dimensional gel electrophoresis-laser ablation ICP MS followed by capillary HPLC-ICP MS and electrospray linear trap quadrupole Orbitrap MS

A method has been developed for a rapid and precise location of selenium-containing proteins in large two-dimensional (2D) electrophoresis gels. A sample was divided into four aliquots which were analyzed in parallel by 1D isoelectric focusing electrophoresis (IEF)-laser ablation (LA) inductively coupled plasma mass spectrometry (ICP MS), 1D sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE)-LA ICP MS, and, in duplicate, by 2D IEF-PAGE. On the basis of the 1 D electropherograms obtained, areas supposed to contain the largest concentrations of Se were subjected to LA ICP MS imaging to locate precisely the position of Se-containing proteins which were then identified in the parallel 2D gel by electrospray Orbitrap MS/MS. The method was applied to the identification and semiquantitative determination of selenium storage proteins in wheat. MS evidence is presented for the Se-S substitution in plants not only in methionine but also in cysteine.

Proteome changes driven by phosphorus deficiency and recovery in the brown tide-forming alga Aureococcus anophagefferens

Shotgun mass spectrometry was used to detect proteins in the harmful alga, Aureococcus anophagefferens, and monitor their relative abundance across nutrient replete (control), phosphate-deficient (-P) and -P refed with phosphate (P-refed) conditions. Spectral counting techniques identified differentially abundant proteins and demonstrated that under phosphate deficiency, A. anophagefferens increases proteins involved in both inorganic and organic phosphorus (P) scavenging, including a phosphate transporter, 5'-nucleotidase, and alkaline phosphatase. Additionally, an increase in abundance of a sulfolipid biosynthesis protein was detected in -P and P-refed conditions. Analysis of the polar membrane lipids showed that cellular concentrations of the sulfolipid sulphoquinovosyldiacylglycerol (SQDG) were nearly two-fold greater in the -P condition versus the control condition, while cellular phospholipids were approximately 8-fold less. Transcript and protein abundances were more tightly coupled for gene products involved in P metabolism compared to those involved in a range of other metabolic functions. Comparison of protein abundances between the -P and P-refed conditions identified differences in the timing of protein degradation and turnover. This suggests that culture studies examining nutrient starvation responses will be valuable in interpreting protein abundance patterns for cellular nutritional status and history in metaproteomic datasets.

High-throughput quantification of selenium in individual serum proteins from a healthy human population using HPLC on-line with isotope dilution inductively coupled plasma-MS

In this study, a method, based on dual column affinity chromatography hyphenated to isotope dilution inductively coupled plasma-quadrupole MS, was developed for selenium determination in selenoprotein P, glutathione peroxidase, and selenoalbumin in human serum samples from a group of healthy volunteers (n=399). Method improvement was achieved using methanol-enhanced isotope dilution which resulted in improved sensitivity and removal of isobaric interferences. Although no human serum reference materials are currently certified for their selenium species levels, method development was conducted using human serum reference material BCR 637 and 639 as their Se species content has been reported in the previous studies, and thus comparisons were possible. The mean selenium concentrations determined for the 399 healthy volunteer serum samples were 23 ± 10 ng Se mL(-1) for glutathione peroxidase, 49 ± 15 ng Se mL(-1) for selenoprotein P and 11 ± 4 ng Se mL(-1) for selenoalbumin. These values are found to be in close agreement with published values for a limited number of healthy volunteer samples, and to establish baseline Se levels in serum proteins for an apparently healthy group of individuals, thus allowing for subsequent comparisons with respective values determined for groups of individuals with selenium related health issues, as well as assist in the discovery of potential selenium biomarkers. Also, the relationship between Se serum protein levels and some anthropometric characteristics of the volunteer population were investigated. Additionally, further development of the analytical method used in this study was achieved by adding a size exclusion chromatography column after the two affinity columns via a switching valve. This allowed for the separation of small selenium-containing molecules from glutathione peroxidase and thus enhanced the overall confidence in its identification.

Progress and possible applications of miniaturised separation techniques and elemental mass spectrometry for quantitative, heteroatom-tagged proteomics

The application of miniaturised separation techniques such as capillary LC, nano LC or capillary electrophoresis offers a number of advantages in terms of analytical performance, solvent consumption and the ability to analyse very small sample amounts. These features make them attractive for various bioanalytical tasks, in particular those related to the analysis of proteins and peptides. The skillful combination of such techniques with inductively coupled plasma mass spectrometry (ICP-MS) has recently permitted the design of combined analytical approaches utilising either elemental or molecule-specific detection techniques such as electrospray ionisation (ESI) or matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry in a highly complementary manner for, as an example, proteomics-orientated research (heteroatom-tagged proteomics). Such hybrid approaches are, in particular, providing promising new options for the fast screening of complex samples for specific metal-containing or--more generally speaking--heteroatom-containing biomolecules, as well as the accurate absolute quantification of biomolecules, which is still an unsolved problem in bioanalysis. Here, progress in as well as the potential and the special requirements of hyphenating miniaturised separation techniques with ICP-MS are reviewed and critically discussed. In addition, selected applications are highlighted to indicate current and possible future trends within this emerging area of research.

In vivo screening of proteins likely to bind uranium in exposed rat kidney

Uranium is a naturally abundant element which has been used in several industries. Internal exposure could occur via three main pathways that are ingestion, inhalation and wounds. It has been recently shown that chronic ingestion of uranium in drinking water induces an important uranium accumulation in kidney with a perturbation of iron metabolism in this organ.

Whereas uranium speciation is a key parameter to elucidate the chemical reactivity and the mobility of an element, it remains poorly documented in most of environmental and biological media. A few examples of uranium complexation with biomolecules have been published recently but most of them are in vitro studies whereas in vivo experiments remain poorly investigated.

In order to better understand possible competition of uranium towards metals involved in the metal-protein binding, i.e. iron, copper, calcium, a study on uranium speciation was investigated by doing an in vivo screening of target proteins likely to bind it in kidneys of exposed rats. Rats were chronically exposed via contaminated drinking water at 40 mg L-1 and killed 9 months after the beginning of exposure. Kidneys were dissected out and protein extract was prepared. Then, separation of renal proteins by isoelectric focusing gel electrophoresis (IEF) and two-dimensional gel electrophoresis (2-DE) followed by LA-ICPMS analysis were performed.

IEF-LA-ICP MS showed that uranium could specifically bind few proteins in kidney whereas 2-DE-LA-ICP MS could indicate that uranium is not covalently bound to proteins in this organ. The results suggested that even at moderate concentrations of exposure, uranium can be observed chelated with some renal proteins that is very encouraging to understand the entry, storage and elimination of this element in kidneys.

Quantification of protein phosphorylation by microLC-ICP-MS

Determination of the protein amount and of the extent of protein phosphorylation is crucial for a variety of research fields, but is not always straightforward. We describe the application of capillary LC-ICP-MS (liquid chromatography-inductively coupled plasma-mass spectrometry) for quantification of phospho-proteins and their phosphorylation degree. Element mass spectrometry is ideally suited for monitor ing and quantification of compounds with heteroelements such as phosphorus and sulphur, particularly because the ICP-MS response is virtually independent from the chemical form of the element. Determination of the phosphorylation stoichiometry, i.e. the relative abundance of the phosphorylated isoforms, can be assessed by the relative abundance of phosphorus compared with sulphur as a marker for the protein amount. Moreover, isotope dilution analysis by post-column addition of a 34S-Spike provides absolute protein quantification with exceptionally high accuracy. Phosphoprotein analysis by capillary LC-ICP-MS may be applied to isolated proteins or protein digests and may include separation of impurities by 1D-SDS-PAGE followed by enzymatic digestion. Alternatively, digestion of complex protein mixtures such as cellular protein extracts allows determination of global, tissue-specific phosphorylation degrees.