Metalloprotein separation and analysis by directly coupled size exclusion high-performance liquid chromatography inductively coupled plasma mass spectroscopy

Inductively coupled plasma-MS in drug development: bioanalytical aspects and applications

The vast majority of today's modern bioanalytical methods for pharmacokinetic, pharmacodynamic and immunogenicity purposes are based on LC-MS/MS and immunoanalytical approaches. Indeed, these methodologies are suitable for a wide range of molecules from small to large. For a smaller but not insignificant group of compounds, LC-MS/MS is not suitable - or in some cases much less suitable - as a reliable bioanalytical methodology, and inductively coupled plasma (ICP)-MS is a more appropriate methodology. ICP-MS is one of these less widely used techniques in drug development. This methodology is predominantly used for elemental bioanalysis for pharmacokinetics, for imaging purposes, for mass-balance, food-effect and biomarker studies. In addition, in the last couple of years an increasing number of applications has been published, where ICP-MS and its various hyphenations (LC-ICP-MS, CE-ICP-MS) have been used for speciation/metabolism and proteomics studies. Here, the analytical potential, the quantitative bioanalytical aspects, the various modes of operation and the challenges of the application of ICP-MS in life sciences applications are given. This includes an overview of recent applications in this area in scientific literature, the various hyphenation possibilities and their application areas and the analysis of the various sample matrices applicable to these fields. It also provides a brief outlook of where the potential of this technique lies in the future of regulated bioanalysis and drug development.

Metal Detoxification in Freshwater Animals. Roles of Metallothioneins

In laboratory experiments with a variety of aquatic animals, the toxicity of non-essential metals normally exhibits a threshold response. At low exposure concentrations (low internal doses), the organism can detoxify the incoming metal and thus tolerate the exposure, whereas at higher concentrations, i.e., above some threshold, the detoxification mechanism is no longer able to protect the organism completely, the incoming metal binds to metal-sensitive sites within the cell, and deleterious effects begin to occur. Several metal-detoxification strategies have been identified in laboratory experiments, including metal sequestration in insoluble granules and metal complexation by metallothionein or metallothionein-like peptides (MTLP), but the ability of these mechanisms to prevent metals from binding to metal-sensitive sites in the intracellular environment has not been rigorously tested in field situations.

In this chapter we briefly summarize the laboratory evidence supporting the threshold model for metal toxicity and then present the results of our field studies on chronically exposed freshwater animals (Pyganodon grandis, a unionid bivalve mollusc; Perca flavescens, a percid fish), studies that were designed to test the threshold model under field conditions. The two biomonitor species were collected from lakes located along a metal concentration gradient, as defined by ambient dissolved metal concentrations. Subcellular metal partitioning was determined in target organs (bivalve gills and digestive gland; perch liver) by differential centrifugation, and MTLP were measured independently by a mercury saturation assay. From these studies we extract evidence for the inadequacy of the threshold model in the case of chronic/life-long metal exposures (i.e., evidence for incomplete detoxification even at low metal exposures in the field). We speculate that under chronic exposure conditions the animals establish a trade-off between the “cost” of detoxifying non-essential metals and the “cost” of allowing some of these metals to spill over onto metal-sensitive sites. Finally, we discuss the implications of the apparent absence of an exposure threshold below which metal detoxification is complete.

Use of stable isotopically enriched proteins and directly coupled high-performance liquid chromatography inductively coupled plasma mass spectrometry for quantitatively monitoring the transfer of metals between proteins

Studies have shown that metallothionein (MT) may play an important role in modulating the activity of certain Zn-regulated enzymes under various oxidoreductive conditions by either donating or removing Zn. To better determine the role of MT in interprotein metal transfer, we describe a procedure that uses stable isotopically enriched (67)Zn(7) metallothionein 2 ((67)Zn(7)-MT-2) to quantitatively determine the stoichiometry of transfer of Zn from the protein to a recipient apo-metalloenzyme, apo-carbonic anhydrase (apo-CA) by directly coupled ion exchange high-performance liquid chromatography inductively coupled plasma mass spectrometry. Quantitatively, the transfer of (67)Zn was consistent with the enzymatic activation of the apo-enzyme as judged by its esterase activity and ability to cleave p-nitrophenyl acetate. Maximum enzyme activation occurred at an MT-2:apo-CA molar ratio of 1, implying the release of a single atom of Zn from MT-2. Preincubation of (67)Zn(7)-MT-2 with an excess of oxidized glutathione (GSSG) increased metal donation fourfold, whereas reduced glutathione (GSH) inhibited donation by approximately 50%. By using multiple recipient and donor proteins having different stable isotopic signatures, the technique has the potential for quantitatively studying the kinetic and thermodynamic aspects of Zn transfer between numerous competing ligands in vitro, an important first step toward understanding the regulatory role of this metal in protein functioning and cellular metabolism in vivo.

The use of biomarkers in biomonitoring: a 2-tier approach assessing the level of pollutant-induced stress syndrome in sentinel organisms

The paper outlines a 2-tier approach for wide-scale biomonitoring programmes. To obtain a high level of standardization, we suggest the use of caged organisms (mussels or fish). An "early warning", highly sensitive, low-cost biomarker is employed in tier 1 (i.e. lysosomal membrane stability (LMS) and survival rate, a marker for highly polluted sites). Tier 2 is used only for animals sampled at sites in which LMS changes are evident and there is no mortality, with a complete battery of biomarkers assessing the levels of pollutant-induced stress syndrome. Possible approaches for integrating biomarker data in a synthetic index are discussed, along with our proposal to use a recently developed Expert System. The latter system allows a correct selection of biomarkers at different levels of biological organisation (molecular/cellular/tissue/organism) taking into account trends in pollutant-induced biomarker changes (increasing, decreasing, bell-shape). A selection of biomarkers of stress, genotoxicity and exposure usually employed in biomonitoring programmes is presented, together with a brief overview of new biomolecular approaches.

ICP/OES application for assessing cadmium uptake (or toxicity) in glomerular cells: influence of extracellular calcium

The risks of metals for health are highlighted by their chemical stability and their persistence in the environment. Chronic exposure to low cadmium (Cd) concentrations results in renal dysfunction mainly. Cd has been regarded primarily as a renal tubular toxicant, but glomerular structures may also be affected. Since the cellular environment may influence metal toxicity, differences concerning Cd uptake and toxicity were evaluated according to calcium (Ca) medium concentrations. An optimized inductively coupled plasma emission spectrometry method (ICP/OES) was developed under defined conditions, as a selective analytical tool to determine cadmium uptake in glomerular mesangial cells. The performance characteristics of the analytical system were evaluated for both Cd and Ca by calibration (50 to 250 microg/L and 1 to 5 mg/L), linearity (r2 .9968 and .9943), limits of detection (1 microg/L and 0.1 mg/L) and quantitation (3 microg/L and 0.3 mg/L), accuracy with spiking, and repeatability (1.2 and 2.9%) with matrix matched standards. Total intracellular Cd content was significantly threefold lower in 0.175 mM Ca medium (Ca-free Eagle's minimum essential medium [EMEM] medium with 5% fetal bovine serum [FBS]) than in EMEM medium (1.8 mM Ca) with respectively 0.16 and 0.37 microg/mg proteins after 24 h of Cd (1 microM) exposure. Similar differences were obtained in cytotoxicity studies with a fourfold reduction in the mortality index (IC50). Complementary assays using Ca-spiked medium reinforced that Cd cytotoxicity and uptake were significantly dependent on the concentration of extracellular Ca. These findings suggest direct link between Cd uptake and toxicity, underlining the relevance of the analytical method.

Characterization of the StcE protease activity of Escherichia coli O157:H7

The StcE zinc metalloprotease is secreted by enterohemorrhagic Escherichia coli (EHEC) O157:H7 and contributes to intimate adherence of this bacterium to host cells, a process essential for mammalian colonization. StcE has also been shown to localize the inflammatory regulator C1 esterase inhibitor (C1-INH) to cell membranes. We tried to more fully characterize StcE activity to better understand its role in EHEC pathogenesis. StcE was active at pH 6.1 to 9.0, in the presence of NaCl concentrations ranging from 0 to 600 mM, and at 4 degrees C to 55 degrees C. Interestingly, antisera against StcE or C1-INH did not eliminate StcE cleavage of C1-INH. Treatment of StcE with the proteases trypsin, chymotrypsin, human neutrophil elastase, and Pseudomonas aeruginosa elastase did not eliminate StcE activity against C1-INH. After StcE was kept at 23 degrees C for 65 days, it exhibited full proteolytic activity, and it retained 30% of its original activity after incubation for 8 days at 37 degrees C. Together, these results show the StcE protease is a stable enzyme that is probably active in the environment of the colon. Additionally, k(cat)/K(m) data showed that StcE proteolytic activity was 2.5-fold more efficient with the secreted mucin MUC7 than with the complement regulator C1-INH. This evidence supports a model which includes two roles for StcE during infection, in which StcE acts first as a mucinase and then as an anti-inflammatory agent by localizing C1-INH to cell membranes.

The application of inductively coupled plasma mass spectrometry in pharmaceutical and biomedical analysis

With the development of life science, pharmaceutical and biomedical analysis becomes more and more important in medical science. Further studies will be hopefully established if it is possible to use inorganic elemental standards or small organic compounds in the quantitative determination of all kinds of drugs, nucleotides and sulfur or phosphorus containing peptides and proteins at appropriate concentration with an acceptable accuracy. Since 1980, inductively coupled plasma mass spectrometry (ICP-MS) has emerged as a new and powerful analytical technique which is suitable for element and isotope analysis. It offers extremely wide detection range of element and co-analysis of most elements in the periodic table. Also, it can be applied to perform qualitative, semiquantitative, and quantitative analysis and isotopic ratios through mass-to-electric charge ratio. With the help of ICP-MS, the struggle of searching for an excellent quantification technique in, e.g. drugs and proteomics has come appreciably close to an end. This review mainly focuses on the introduction of application of ICP-MS in pharmaceutical and biomedical analysis. Some problems in application and the handling strategies are simply presented at the end.

Potential for Using Isotopically Altered Metalloproteins in Species-Specific Isotope Dilution Analysis of Proteins by HPLC Coupled to Inductively Coupled Plasma Mass Spectrometry

The production and evaluation of an isotopically enriched metalloprotein standard for use as a calibrant in species-specific isotope dilution analysis by HPLC coupled to inductively coupled plasma mass spectrometry is described. Using a model system involving the copper-containing protein rusticyanin (Rc) from the bacterium Acido-thiobacillus ferrooxidans, it was possible to demonstrate the analytical conditions that could be used for the measurement of metalloproteins by on-line IDMS analysis. Rc was chosen because it is a well-characterized protein with an established amino acid sequence and can be produced in suitable quantities using a bacterial recombinant system. Three different forms of the protein were studied by organic and inorganic mass spectrometry: the native form of the protein containing a natural isotopic profile for copper, an isotopically enriched species containing virtually all of its copper as the 65Cu isotope, and the nonmetalated apo form. Incorporation of the copper isotopes into the apo form of the protein was determined using a UV-vis spectrophotometric assay and shown to be complete for each of the copper-containing species. The experimental conditions required to maintain the conformational form of the protein with a nonexchangeable copper center were established using +ve electrospray mass spectrometry. A pH 7.0 buffer was found to afford the most appropriate conditions, and this was then used with HPLC-ICP-MS to verify the stability of the copper center by analysis of mixtures of different isotopic solutions. No exchange of the enriched copper isotope from Rc with an added naturally abundant inorganic copper cation was observed under a neutral pH environment, indicating that species-specific ID-MS analysis of metalloproteins is possible.

Elements in environmental and occupational medicine

Occupational and environmental medicine traditionally dealt with elements, particularly with heavy metals. The interest was justified by the wide exposure in the workplace and in the general environment and by the evidence of their specific biological and toxicological effects. During the last 2 decades of 20th century the availability of indicators of exposure or of internal dose has substantially increased thanks to improvement in AAS-ETAAS techniques and to the entrance of ICP-MS into the field of biological monitoring. There are now more and more demands for controlling pre-analytical and analytical factors, for analysing biological matrices in addition to blood and urine and for setting up methods for elements not yet extensively studied in respect to their possible biological or toxicological role. Finally, deeper knowledge has to be reached in order to evaluate the significance of elements and, possibly, of their species in biological fluids at current doses and in order to face their effects, especially those in the first portion of the dose-response curve, which is going to be the main field of interest of occupational and environmental toxicology for the next few years.

Investigation of metal complexes with metallothionein in rat tissues by hyphenated techniques

The potential of hyphenated techniques based on a combination of microbore reversed-phase (RP) HPLC or capillary zone electrophoresis (CZE) with inductively coupled plasma (ICP) or electrospray (ES) mass spectrometry (MS) was demonstrated for the characterization of metal complexes with metallothionein in rat liver and kidney. The mixture of MT complexes was isolated from the tissues by size-exclusion LC and further characterized in neutral pH conditions (pH 6.8-7.2) by RP-HPLC or CZE. The metal stoichiometry and the molar mass of the eluted complexes was measured by ICP-MS and ES-MS, respectively. An additional dimension to the analysis was achieved by post-column acidification of the chromatographic eluent that allowed the determination of the molecular weight of the demetallated complexes with 10-fold higher sensitivity. The approach allowed the detection of two major metallothionein (MT) isoforms (MT-1 and MT-2) in liver and one MT isoform in kidney. The actual number of peaks in chromatograms and electropherograms was bigger because of the formation of mixed Cd-Cu complexes of the same MT isoform that showed different hydrophobicities.

Separation of selenium-containing proteins in human and mouse plasma using tandem high-performance liquid chromatography columns coupled with inductively coupled plasma-mass spectrometry

An analytical method that uses two different high-performance liquid chromatography (HPLC) columns in tandem has been developed that separates three major selenium-containing proteins (albumin, glutathione peroxidase, and selenoprotein P) found in human blood plasma. The first column was a heparin affinity column and the second was a gel filtration column whose outlet was directly connected to an inductively coupled plasma-mass spectrometer. The method successfully separated plasma selenium into the three selenium-containing proteins and revealed the preferential retention of selenium in the form of selenoprotein P in a selenium-deficient human and in selenium-deficient mice. Our results also confirm the results of previous studies that showed a preference for supplemented selenium to be taken up as selenoprotein P in rats. Advantages of the tandem column method are that it allows rapid and convenient analyses of the distribution of plasma selenium, and that it is suitable for stable isotope tracer studies and metal interaction studies.

Inductively coupled plasma mass spectrometric detection for chromatography and capillary electrophoresis

Inductively coupled plasma mass spectrometry (ICP-MS) is now a well established detection technique for liquid chromatography, gas chromatography, supercritical fluid chromatography and capillary electrophoresis. A review of the literature with particular regard to ICP-MS as a chromatographic and capillary electrophoretic detector is presented. The various modes of chromatography and capillary electrophoresis are discussed and practical descriptions for hyphenating the techniques with the ICP mass spectrometer are given. Sample introduction systems and data acquisition methods are reviewed along with the numerous applications of ICP-MS as a chromatographic detector. In addition, alternative plasma sources, such as the atmospheric and reduced pressure helium microwave-induced plasmas for chromatographic detection are described.

Inhibition, reactivation, and determination of metal ions in membrane metalloproteases of bacterial origin using high-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry

High-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) was used for the characterization of metal ions in several metalloproteases of bacterial origin. The different components of the bacterial extracts were separated on a size-exclusion column. The eluent of the HPLC system was continuously transported to the ICP-MS system for rapid, reproducible, and sensitive analyses of trace elements in the metalloproteases. Two different membrane proteases from Bacillus cereus and Pseudomonas aeruginosa were characterized to be zinc metalloproteases using enzymological methods and HPLC-ICP-MS. The zinc content was determined to be three molecules of zinc per protein molecule for the B. cereus protease and one molecule of zinc per protein molecule for the P. aeruginosa protease. For another purified protease, a periplasmic alanyl aminopeptidase of P. aeruginosa, the lack of protein-bound metal ions could be clearly determined-a confirmation that this main aminopeptidase of P. aeruginosa belongs to the cysteine protease family. The presence of nonionic detergents can influence the distribution of trace elements during the HPLC separation. Therefore, the use of these substances should be avoided during enzyme purification for metal analyses or they should be exchanged later for zwitterionic and ionic detergents with more strongly dissociating properties.

Simultaneous speciation of endogenous and exogenous elements by HPLC/ICP-MS with enriched stable isotopes

High performance liquid chromatography (HPLC)/ inductively coupled argon plasma-mass spectrometry (ICP-MS) was introduced to investigate the distributions of selenium (Se) in biological fluids. The method was to determine both the natural abundance of Se and an enriched stable isotope of Se used as a tracer. The distributions of Se in plasma and in urine specimens were determined in Wistar rats on various Se diets with and without an intravenous injection of 82Se-selenite. Although the distribution of natural abundance Se (endogenous Se) in the plasma was affected little by the nutritional status of Se, that in the urine gave a Se peak depending on the nutritional status of Se, and the peak was identified as methylselenol. When 82Se-selenite was injected in excess into rats given three different Se diets (Se-deficient, Se-adequate, Se-excessive), three Se peaks occurred in the HPLC chromatogram of the urine samples, corresponding to selenite, methylselenol and trimethylselenonium ion in the order of elution, and the intensities of the tracer peaks reflected the nutritional status. These results indicate that the HPLC/ICP-MS method is a powerful analytical tool for specifying Se-containing biological constituents, both natural abundance and enriched stable isotopes. Methylselenol in urine is proposed to be a sensitive and Se-specific biological indicator for diagnosing the nutritional status of Se. Furthermore, it was shown that an enriched stable isotope such as 82Se-selenite was shown to be used for the same purpose, and that 82Se-methylselenol and 82Se-trimethylselenonium ion in urine were more sensitive indicators of the Se status of the rats.

Enriched stable isotopes of elements used as tracers: methods of presenting high-performance liquid chromatographic-inductively coupled argon plasma mass spectrometric data

High-performance liquid chromatography (HPLC) of biological fluids and tissue cytosolic preparations was employed in conjunction with argon-induced inductively coupled plasma mass spectrometry (ICP-MS) to investigate the distribution of stable isotopes as tracers. The common way of presenting the data from the ICP-MS is by plotting the count rates versus the retention time of HPLC fractions. Additional information can be derived, e.g., the composite peaks can be further resolved, and the level of enrichment in various biological components can be expressed by alternative ways of presenting these data. The two additional approaches described here involve presenting the ratios of enriched tracer with a suitable naturally abundant mass number of the same element, and by expressing the extent of enrichment by the tracer isotope in a given fraction to that of the same mass number in the fraction derived from an untreated source. Each method of presentation has different merits and drawbacks. The data therefore may be best presented in more than one way to emphasize the conclusions from a given experiment. Observations are presented after simultaneously injecting stable isotopes of three essential elements, copper, selenium and zinc, into mice. Plasma and liver cytosolic fractions were analysed and data represented in different ways as indicated above.

Determination of zinc-65, copper-64 and sulphur-35 labelled rat hepatoma tissue culture metallothioneins by high-performance liquid chromatography with on-line radioactivity detection

Molecular size exclusion (MSE), reversed-phase (RP), and anion-exchange (AE) high-performance liquid chromatography (HPLC) techniques were employed in combination with on-line radioactivity detection, in a study on the kinetic behaviour of 65Zn-, 64Cu- and [35S]cysteine-labelled metallothionein (MT) in rat hepatoma tissue culture (HTC) cells. MSE-HPLC of [35S]cysteine-labelled HTC cell cytosol resulted in co-eluting MT-I and MT-II isoforms (tR 19.80 min; Ve/Vo: 1.85). AE-HPLC of 65Zn-treated HTC cell cytosol yielded separated 65Zn MT-I (tR 11.5 min; I = 64 mM) and 65Zn MT-II (tR 14.5 min; I = 104 mM). RP-HPLC of 64Cu-treated HTC cytosol resulted in separated 64Cu MT-I (tR 26.4 min) and 64Cu MT-II (tR 23.4 min). Determination of the amino acid composition, apparent molecular mass and cysteine content of HTC MT-I and MT-II isoforms showed the characteristics of class I metallothioneins. The rate of dissociation of Zn2+ from Zn-MT could be determined from the losses of 65Zn from MT during a single AE-HPLC run, showing a Zn-MT dissociation half-life of 0.66 h. RP-HPLC showed a delay in incorporation of newly accumulated 64Cu into MT, possibly owing to the appearance of reduced glutathione as an intracellular copper-transfer compound. Application of compartmental analysis in [35S]-cysteine accumulation experiments permitted the determination of the actual rate of MT degradation; when 200 microM of Zn were applied, the MT degradation half-life was 2.0 +/- 0.8 h. These results indicate the potential of combined HPLC techniques and application of radionuclides in studies on the synthesis and degradation of MT and metal-MT complexes.

Effects of cadmium, copper and metallothionein synthesis inhibiting and stimulating compounds on zinc uptake and accumulation in rat hepatoma HTC cells

Experiments were carried out to investigate the uptake and accumulation of Zn in rat hepatoma HTC cells, as affected by interfering metals (Cd, Cu), metallothionein synthesis inhibiting compounds (Actinomycin D, cycloheximide) and metallothionein synthesis stimulating compounds (dexamethasone, dibu-cAMP). Cell viability was tested under all experimental conditions by the measurement of LDH leakage, K+ uptake and total cell protein. Determinations of Zn were performed by AAS (total Zn) or by gamma-ray spectrometry (65Zn). Metallothionein analysis was carried out by Cd-saturation tests. The results indicate that cellular responses in rat hepatoma HTC cells with respect to the uptake and accumulation of 65Zn are fully comparable with literature data existing for 65Zn accumulation in rat hepatocytes, under all experimental conditions applied. Cu2+ and dibutyryl-cAMP did not significantly affect rates of 65Zn accumulation. Cd2+, Actinomycin D and cycloheximide reduced 65Zn uptake, but dexamethasone additions resulted in increased 65Zn accumulation in the cells. Effects on 65Zn were shown both in cytosolic and in the membranes/organelles cell fractions. HPLC chromatography in control cells suggested that newly accumulated cytosolic 65Zn was predominantly MT-associated. Dexamethasone-induced 65Zn accumulation could not be related to elevated cellular MT levels, nor were the total cytosolic Zn levels significantly affected. Non-specific attenuations in MT levels (Actinomycin D, cycloheximide and dibu-cAMP) yielded linear relations between cytosolic 65Zn and MT levels, without any change in cytosolic Zn (AAS). Combined addition of Cd and dexamethasone yielded elevated MT levels, but severely reduced total cytosolic Zn and 65Zn concentrations. The results further indicate the non-Zn-specific nature of dexamethasone-action and suggest the relatively easy Zn-complexing and Zn-release of MT. The simultaneous determinations of total cytosolic zinc and cytosolic 65Zn levels showed that the application and sole measurement of radiotracers may yield only one-sided views of what is actually present or occurring in the cells.

Elemental speciation by liquid chromatography-inductively coupled plasma mass spectrometry with direct injection nebulization

A new version of the direct injection nebulizer (DIN) is used to interface liquid chromatographic (LC) separations with element-selective detection using inductively coupled plasma mass spectrometry (ICP-MS). The DIN injects all of the sample into the ICP and has a dead volume of less than 1 microliter. Charged species of arsenic and tin are separated as ion pairs on a micro-scale (1 mm i.d.), packed, reversed-phase column. Detection limits are 0.2-0.6 pg for arsenic and 8-10 pg for tin. For methanol + water eluents, the signal is highest at 25% methanol and stays within 25% of this maximum as the methanol fraction is varied from 20 to 80%. Compared with LC-ICP-MS with conventional nebulizers, the absolute detection limits and chromatographic resolution are substantially superior, and the dependence of analyte signal on solvent composition is somewhat less severe with the DIN.

Preliminary study of metals in proteins by high-performance liquid chromatography-inductively coupled plasma mass spectrometry using multi-element time-resolved analysis

The feasibility of multi-element and multi-isotope determinations during the on-line chromatographic separation of protein matrices was investigated using time-resolved software with inductively coupled plasma mass spectrometry. Size-exclusion chromatography was used to separate a mixture of known proteins and the behaviour of associated elements was monitored. It was possible to scan either the entire mass range or a selected number of isotopes. Reversed-phase chromatography was used to separate Zn-containing species in chicken meat that had been intrinsically and extrinsically labelled with stable isotopes of Zn. The ratio obtained for the intrinsic label agreed with that obtained by other workers using thermal ionization mass spectrometry. Time-resolved analysis permitted rapid data acquisition, thus reducing the number of chromatographic runs required. However, at present, data handling can be time consuming. Further software developments will add to the advantages that this technique offers.