On-column formation of arsenic-glutathione species detected by size-exclusion chromatography in conjunction with arsenic-specific detectors

The emerging application of LC-based metallomics techniques to unravel the bioinorganic chemistry of toxic metal(loid)s

The on-going anthropogenic emission of toxic metal(loid) species into the environment contaminates the food supply and drinking water resources in various parts of the world. Given that inorganic pollutants cannot be degraded, their increased influx into the bloodstream of babies, children and pregnant women is inevitable. Since the ramifications of the ensuing environmental exposure on human health remain poorly defined, fundamentally new insight into their bioinorganic chemistry in organisms is urgently needed. Based on the flow of dietary constituents through organisms, the interaction of toxic metal(loid) species with biomolecules in the bloodstream deserve particular attention as they play an integral role in the mechanisms of their chronic toxicity. Gaining insight into these bioinorganic processes is hampered by the biological complexity of plasma/red blood cells and the low concentrations of the metal(loid) species of interest, but can be overcome by employing LC techniques hyphenated to atomic spectroscopic detectors (i.e. metallomics techniques). This perspective aims to highlight the potential of unconventional hyphenated separation modes to advance our understanding of the bioinorganic chemistry of toxic metal(loid) species in the bloodstream-organ system. Four examples are illustrated. The application of anion-exchange (AEX) and size-exclusion chromatography (SEC) provided new insight into the blood-based bioinorganic mechanisms that direct Cd2+ and MeHg+ to target organs. AEX chromatography also allowed to observe the formation of complexes between Hg2+ and MeHg+ with L-cysteine at pH 7.4, that are implicated in their organ uptake. Lastly, the application of reversed phase (RP) chromatography revealed a possible cytosolic mechanism by which N-acetyl-L-cysteine binds to MeHg+ in the presence of cytosolic glutathione (GSH). New insight into other bioinorganic processes may advance the regulatory framework to better protect public health.

Recent developments of P-glycoprotein inhibitors and its structure-activity relationship (SAR) studies

P-glycoprotein (P-gp) over-expression is a key factor in multi-drug resistance (MDR), which is a major factor in the failure of cancer treatment. P-gp inhibitors have been demonstrated to have powerful pharmacological properties and may be used as a therapeutic approach to overcome the MDR in cancer cells. Combining clinical investigations with biochemical and computational research may potentially lead to a clearer understanding of the pharmacological properties and the mechanisms of action of these P-gp inhibitors. The task of turning these discoveries into effective therapeutic candidates for a variety of malignancies, including resistant and metastatic kinds, falls on medicinal chemists. A variety of P-gp inhibitors with great potency, high selectivity, and minimal toxicity have been identified in recent years. The latest advances in drug design, characterization, structure-activity relationship (SAR) research, and modes of action of newly synthesized, powerful small molecules P-gp inhibitors over the previous ten years are highlighted in this review. P-gp transporter over-expression has been linked to MDR, therefore the development of P-gp inhibitors will expand our understanding of the processes and functions of P-gp-mediated drug efflux, which will be helpful for drug discovery and clinical cancer therapies.

Physiologically relevant hCys concentrations mobilize MeHg from rabbit serum albumin to form MeHg-hCys complexes

Methylmercury (MeHg) is one of the most potent neurotoxins to which humans are exposed via the consumption of fish from which it is effectively absorbed via the gastrointestinal tract into the bloodstream. Its interactions with plasma proteins, small molecular weight (SMW) molecules, and red blood cells, however, are incompletely understood, but critical as they determine if and how much MeHg reaches target organs. To better define the role that SMW thiols play in the delivery of MeHg to known transporters located at the placental and blood-brain barrier, we have employed size exclusion chromatography-inductively coupled plasma-atomic emission spectroscopy to analyze MeHg-spiked rabbit plasma in the absence and presence of SMW thiols dissolved in the PBS-buffer mobile phase. While 300 µM L-methionine did not affect the binding of MeHg to rabbit serum albumin (RSA), cysteine (Cys), homocysteine (hCys) and glutathione (GSH) resulted in the elution of the main Hg-peak in the SMW elution range. In addition, 50 µM of hCys or Cys in the mobile phase resulted in the mobilization of MeHg from RSA in rabbit plasma and from pure RSA in solution. The Hg-peak that eluted in the SMW elution range (50 µM of hCys) was identified by electrospray ionization-mass spectrometry as a MeHg-hCys complex. Since L-type amino acid transporters are present at the blood brain barrier (BBB) which facilitate the uptake of MeHg-Cys species into the brain, our results contribute to establish the bioinorganic mechanisms that deliver MeHg to the BBB, which is critical to predict organ-based adverse health effects.

Assessment of toxicity of selenium and cadmium selenium quantum dots: A review

This paper reviews the current understanding of the toxicity of selenium (Se) to terrestrial mammalian and aquatic organisms. Adverse biological effects occur in the case of Se deficiencies, associated with this element having essential biological functions and a narrow window between essentiality and toxicity. Several inorganic species of Se (-2, 0, +4, and +6) and organic species (monomethylated and dimethylated) have been reported in aquatic systems. The toxicity of Se in any given sample depends not only on its speciation and concentration, but also on the concomitant presence of other compounds that may have synergistic or antagonistic effects, affecting the target organism as well, usually spanning 2 or 3 orders of magnitude for inorganic Se species. In aquatic ecosystems, indirect toxic effects, linked to the trophic transfer of excess Se, are usually of much more concern than direct Se toxicity. Studies on the toxicity of selenium nanoparticles indicate the greater toxicity of chemically generated selenium nanoparticles relative to selenium oxyanions for fish and fish embryos while oxyanions of selenium have been found to be more highly toxic to rats as compared to nano-Se. Studies on polymer coated Cd/Se quantum dots suggest significant differences in toxicity of weathered vs. non-weathered QD's as well as a significant role for cadmium with respect to toxicity.

Remarkable differences in the biochemical fate of Cd2+, Hg2+, CH3Hg+ and thimerosal in red blood cell lysate

The application of a metallomics method revealed that all investigated Hg species bound to hemoglobin and that these interactions are of toxicological significance.

Nrf2 activation ameliorates cytotoxic effects of arsenic trioxide in acute promyelocytic leukemia cells through increased glutathione levels and arsenic efflux from cells

Carnosic acid (CA), a phenolic diterpene isolated from Rosmarinus officinalis, has been shown to activate nuclear transcription factor E2-related factor 2 (Nrf2), which plays a central role in cytoprotective responses to oxidative and electrophilic stress. Recently, the Nrf2-Kelch ECH associating protein 1 (Keap1) pathway has been associated with cancer drug resistance attributable to modulation of the expression and activation of antioxidant and detoxification enzymes. However, the exact mechanisms by which Nrf2 activation results in chemoresistance are insufficiently understood to date. This study investigated the mechanisms by which the cytotoxic effects of arsenic trioxide (ATO), an anticancer drug, were decreased in acute promyelocytic leukemia cells treated with CA, a typical activator of Nrf2 used to stimulate the Nrf2/Keap1 system. Our findings suggest that arsenic is non-enzymatically incorporated into NB4 cells and forms complexes that are dependent on intracellular glutathione (GSH) concentrations. In addition, the arsenic complexes are recognized as substrates by multidrug resistance proteins and subsequently excreted from the cells. Therefore, Nrf2-associated activation of the GSH biosynthetic pathway, followed by increased levels of intracellular GSH, are key mechanisms underlying accelerated arsenic efflux and attenuation of the cytotoxic effects of ATO.

Arsenic and selenium toxicity and their interactive effects in humans

Arsenic (As) and selenium (Se) are unusual metalloids as they both induce and cure cancer. They both cause carcinogenesis, pathology, cytotoxicity, and genotoxicity in humans, with reactive oxygen species playing an important role. While As induces adverse effects by decreasing DNA methylation and affecting protein 53 expression, Se induces adverse effects by modifying thioredoxin reductase. However, they can react with glutathione and S-adenosylmethionine by forming an As-Se complex, which can be secreted extracellularly. We hypothesize that there are two types of interactions between As and Se. At low concentration, Se can decrease As toxicity via excretion of As-Se compound [(GS3)2AsSe](-), but at high concentration, excessive Se can enhance As toxicity by reacting with S-adenosylmethionine and glutathione, and modifying the structure and activity of arsenite methyltransferase. This review is to summarize their toxicity mechanisms and the interaction between As and Se toxicity, and to provide suggestions for future investigations.

Metal Species in Biology: Bottom-Up and Top-Down LC Approaches in Applied Toxicological Research

Since the inception of liquid chromatography (LC) more than 100 years ago this separation technique has been developed into a powerful analytical tool that is frequently applied in life science research. To this end, unique insights into the interaction of metal species (throughout this manuscript “metal species” refers to “toxic metals, metalloid compounds, and metal-based drugs” and “toxic metals” to “toxic metals and metalloid compounds”) with endogenous ligands can be obtained by using LC approaches that involve their hyphenation with inductively coupled plasma-based element specific detectors. This review aims to provide a synopsis of the different LC approaches which may be employed to advance our understanding of these interactions either in a “bottom-up” or a “top-down” manner. In the “bottom-up” LC-configuration, endogenous ligands are introduced into a physiologically relevant mobile phase buffer, and the metal species of interest is injected. Subsequent “interrogation” of the on-column formed complex(es) by employing a suitable separation mechanism (e.g., size exclusion chromatography or reversed-phase LC) while changing the ligand concentration(s), the column temperature or the pH can provide valuable insight into the formation of complexes under near physiological conditions. This approach allows to establish the relative stability and hydrophobicity of metal-ligand complexes as well as the dynamic coordination of a metal species (injected) to two ligands (dissolved in the mobile phase). Conversely, the “top-down” analysis of a biological fluid (e.g., blood plasma) by LC (e.g., using size exclusion chromatography) can be used to determine the size distribution of endogenous metalloproteins which are collectively referred to as the “metalloproteome”. This approach can provide unique insight into the metabolism and the plasma protein binding of metal species, and can simultaneously visualize the dose-dependent perturbation of the metalloproteome by a particular metal species. The concerted application of these LC approaches is destined to provide new insight into biochemical processes which represent an important starting point to advance human health in the 21st century.

Influence of glutathione chemical effectors in the response of maize to arsenic exposure

To support the key role of glutathione (GSH) in the mechanisms of tolerance and accumulation of arsenic in plants, this work examines the impact of several effectors of GSH synthesis or action in the response of maize (Zea mays L.) to arsenic. Maize was exposed in hydroponics to iso-toxic rates of 150 μM arsenate or 75 μM arsenite for 9 days and GSH effectors, flurazole (an herbicide safener), l-buthionine-sulfoximine (BSO, a known inhibitor of GSH biosynthesis), and dimercaptosuccinate (DMS) and dimercaptopropanesulfonate (DMPS) (two thiols able to displace GSH from arsenite-GSH complexes) were assayed. The main responses of plants to arsenic exposure consisted of a biomass reduction (fresh weight basis) of about 50%, an increase of non-protein thiol (NPTs) levels (especially in the GSH precursor γ-glutamylcysteine and the phytochelatins PC₂ and PC₃) in roots, with little effect in shoots, and an accumulation of between 600 and 1000 ppm of As (dry weight basis) in roots with very little translocation to shoots. Growth inhibition caused by arsenic was partially or completely reversed in plants co-treated with flurazole and arsenate or arsenite, respectively, highly exacerbated in plants co-treated with BSO, and not modified in plants co-treated with DMS or DMPS. These responses correlated well with an increase of both NPTs levels in roots and glutathione transferase activity in roots and shoots due to flurazole treatment, the decrease of NPTs levels in roots caused by BSO and the lack of effect on NPT levels caused by both DMS and DMPS. Regarding to arsenic accumulation in roots, it was not modified by flurazole, highly reduced by BSO, and increased between 2.5- and 4.0-fold by DMS and DMPS. Therefore, tolerance and accumulation of arsenic by maize could be manipulated pharmacologically by chemical effectors of GSH.

Arsenic-glutathione conjugate transport by the human multidrug resistance proteins (MRPs/ABCCs)

Millions of people world-wide are chronically exposed to inorganic forms of the environmental toxicant arsenic in drinking water. This has led to a public health crisis because arsenic is a human carcinogen, and causes a myriad of other adverse health effects. In order to prevent and treat arsenic-induced toxicity it is critical to understand the cellular handling of this metalloid. A large body of literature describes the importance of the cellular tripeptide glutathione (γ-Glu-Cys-Gly,GSH/GS) in the excretion of arsenic. The triglutathione conjugate of arsenite [As(III)(GS)(3)] and the diglutathione conjugate of monomethylarsonous acid [MMA(III)(GS)(2)] have been isolated from rat bile and mouse urine, and account for the majority of excreted arsenic, suggesting these are important transportable forms. The ATP-binding cassette (ABC) transporter proteins, multidrug resistance protein 1 (MRP1/ABCC1) and the related protein MRP2 (ABCC2), are thought to play an important role in arsenic detoxification through the cellular efflux of arsenic-GSH conjugates. Current knowledge on the cellular handling of arsenic with a special emphasis on the transport pathways of the arsenic-GSH conjugates As(III)(GS)(3), MMA(III)(GS)(2), and dimethylarsenic glutathione DMA(III)(GS), as well as, the seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-) are reviewed.

Extraction tool and matrix effects on arsenic speciation analysis in cell lines

Arsenic glutathione (As-GSH) complexes have been suggested as possible metabolites in arsenic (As) metabolism. Extensive research has been performed on the toxicological and apoptotic effects of As, while few reports exist on its metabolism at the cellular level due to the analytical challenges. In this study, an efficient extraction method for arsenicals from cell lines was developed. Evaluation of extraction tools; vortex, ultrasonic bath and ultrasonic probe and solvents; water, chemicals (methanol and trifluoroacetic acid), and enzymes (pepsin, trypsin and protease) was performed. GSH effect on the stability of As-GSH complexes was studied. Arsenic metabolites in dimethylarsino glutathione (DMA(GS)) incubated multiple myeloma cell lines were identified following extraction. Intracellular GSH concentrations of myeloma cell lines were imitated in the extraction media and its corresponding effect on the stability and distribution of As metabolites was studied. An enhancement in both extraction recoveries and time efficiency with the use of the ultrasonic probe was observed. Higher stabilities for the As species in water, pepsin and trypsin were obtained. The presence of 0.5mM GSH in the extraction media (PBS, pH 7.4) could not stabilize the As-GSH complexes compared to the 5mM GSH, where high stabilization of the complexes was observed over a 5 day storage study. Finally, the speciation analysis of the DMA(GS) culture incubated cell lines in the presence or absence of GSH revealed the important role GSH plays in the preservation of DMA(GS) identity. Hence, caution is required during the extraction of arsenicals especially the As-GSH complexes, since their identification is highly dependent on GSH concentration.

Comparative hydrolysis and plasma protein binding of cis-platin and carboplatin in human plasma in vitro

Platinum-based anti-cancer drugs are widely used to treat cancer in patients, but they also exhibit severe toxic side-effects. Considering that cis-platin and carboplatin are intravenously administered, their biotransformations in the bloodstream are likely to be directly involved in determining their toxic side-effects, but they are poorly understood. We added pharmacologically relevant doses of cis-platin or carboplatin to human plasma from healthy male or female volunteers in vitro at 37 °C and determined the platinum-distribution in plasma after 5 min, 3 h and 24 h using size exclusion chromatography-inductively coupled plasma atomic emission spectrometry (SEC-ICP-AES). The results revealed a negligible inter-individual variation of the platinum-distribution between males and females and faster hydrolysis of cis-platin than carboplatin. Related to this, 95% of platinum was protein-bound 24 h after the addition of cis-platin to plasma, whereas 40% of platinum was protein-bound in the case of carboplatin. Interestingly, cis-platin and carboplatin-derived platinum species appeared to bind to the same 3 plasma proteins at the 3 h time point and thereafter. The analysis of cis-platin and carboplatin-spiked phosphate buffered saline (PBS) revealed a common platinum-containing hydrolysis product that was also detected in plasma. Since cis-platin is associated with more toxic side-effects in patients than carboplatin (even though it is administered at lower doses), our in vitro data suggest that the toxic side-effects of the investigated platinum-drugs may be predominantly determined by the indiscriminate translocation of the parent drugs to malignant and healthy cells. This information may help to mitigate the toxic side-effects of platinum-containing drugs by devising strategies to delay the influx of the parent drugs into non-target tissues.

Methylated trivalent arsenic-glutathione complexes are more stable than their arsenite analog

The trivalent arsenic glutathione complexes arsenic triglutathione, methylarsonous diglutathione, and dimethylarsinous glutathione are key intermediates in the mammalian metabolism of arsenite and possibly represent the arsenic species that are transported from the liver to the kidney for urinary excretion. Despite this, the comparative stability of the arsenic-sulfur bonds in these complexes has not been investigated under physiological conditions resembling hepatocyte cytosol. Using size-exclusion chromatography and a glutathione-containing phosphate buffered saline mobile phase (5 or 10 mM glutathione, pH 7.4) in conjunction with an arsenic-specific detector, we chromatographed arsenite, monomethylarsonous acid, and dimethylarsinous acid. The on-column formation of the corresponding arsenic-glutathione complexes between 4 and 37°C revealed that methylated arsenic-glutathione complexes are more stable than arsenic triglutathione. The relevance of these results with regard to the metabolic fate of arsenite in mammals is discussed.

Tolerance of arsenate-induced stress in Aspergillus niger, a possible candidate for bioremediation

The arsenate tolerance limit in wild-type Aspergillus niger was determined. Because of its high tolerance, toxic effects of arsenate concentrations ranging from 25 to 100mg/L were investigated in regard to growth, intracellular thiols, proline and malondialdehyde (MDA) contents of wild-type A. niger. Cellular arsenate uptake was analyzed. Activities of catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR) and succinate dehydrogenase (SDH) were assayed. Growth of A. niger increased at 25mg/L arsenate, and it survived up to 100mg/L. MDA, intracellular thiol and proline contents increased up to a certain level. Activities of GR, SOD and CAT declined following a rise at low concentration(s); SDH activity decreased gradually with increased arsenate stress. Results indicated that A. niger had high arsenate uptake potential and could tolerate oxidative stress by manipulating its anti-oxidative defense mechanism, a property that may be exploited for removal of arsenate from contaminated aqua-environment.

Probing bioinorganic chemistry processes in the bloodstream to gain new insights into the origin of human diseases

In the context of elucidating the origin of human diseases, past poisoning epidemics have revealed that exceedingly small doses of inorganic environmental pollutants can result in dramatic effects on human health. Today, numerous organic and inorganic pollutants have been quantified in human blood, but the interpretation of these concentrations remains--from a public health point of view--problematic. Conversely, the biomolecular origin for several grievous human diseases is essentially unknown. Taken together and viewed in the context of recent bioinorganic research findings, the established human blood concentrations of toxic metals and metalloids may be functionally connected with the etiology of specific human diseases. To unravel the underlying biomolecular mechanisms, and taking into account the basic flow of dietary matter through mammalian organisms, a better understanding of the bioinorganic chemistry of toxic metals and metalloid compounds in the bloodstream is emerging as a promising avenue for future research. To this end, the concerted application of modern proteomic methodologies, synchrotron-based X-ray absorption spectroscopy and established spectroscopic techniques will contribute to better define the role that blood-based bioinorganic chemistry-related processes play in the origin of human diseases. The application of this and other modern proteomic methodologies could contribute to a better understanding of the role that blood-based bioinorganic chemistry-related processes play in the origin and etiology of human diseases.

Speciation, formation, stability and analytical challenges of human arsenic metabolites

Human arsenic metabolism produces a number of species with varying toxicities; the presence of some has been identified while the existence of others has been postulated through indirect evidence. Speciation methods for the analysis of arsenite (AsIII), monomethylarsonous acid (MMAIII), dimethylarsinous acid (DMAIII), arsenate (AsV), monomethylarsonic acid (MMAV), dimethylarsinic acid (DMAV), arsino-glutathione (As(GS)3), monomethylarsino-glutathione (MMA(GS)2) and dimethylarsino-glutathione (DMA(GS)) were developed in this study through the use of cation exchange and reverse phase chromatography in a complementary manner. Electrospray ionization mass spectrometry (ESI-MS) was used for molecular identification of the arsenicals while inductively coupled plasma mass spectrometry (ICP-MS) was employed for quantitation purposes. Validation of the developed methods against each other for the quantitation of trivalent and pentavalent arsenicals was performed. The effect of reduced glutathione (GSH) concentration on the formation of arsenic-glutathione (As-GSH) complexes was studied. In the presence of glutathione, the occurrence of chromatographic artifacts on the cation exchange column was observed. The stability of trivalent arsenicals and As-GSH complexes was studied at various pH conditions. The results shed light on the importance of sample preparation, storage and proper choice of analytical column for the accurate identification of the As species. Reinvestigation of some of the previously reported As speciation studies of glutathione-rich biological samples needs to be performed for the verification of occurrence of As-GSH complexes and DMAIII.

Glutathione-dependent interaction of heavy metal compounds with multidrug resistance proteins MRP1 and MRP2

The interactions of three heavy metal-containing compounds, cisplatin (CDDP), arsenic trioxide (As(2)O(3)), and mercury dichloride (HgCl(2)), with the multidrug resistance transporters MRP1 and MRP2 and the involvement of glutathione (GSH)-related processes herein were investigated. In Madin-Darby canine kidney cells stably expressing MRP1 or MRP2, viability, GSH content, calcein efflux and polarized GSH efflux were measured as a function of exposure to CDDP, As(2)O(3) and HgCl(2). In isolated Sf9-MRP1 and Sf9-MRP2 membrane vesicles, the interaction with MRP-associated ATPase activity was measured. In the latter model system adduct formation with GSH is not an issue. The data show that (1) CDDP interacts with both MRP1 and MRP2, and GSH appears to play no major role in this process, (2) As(2)O(3) interacts with MRP1 and MRP2 in which process GSH seems to be essential, and (3) HgCl(2) interacts with MRP1 and MRP2, either alone and/or as a metal-GSH complex.

The seleno bis(S-glutathionyl) arsinium ion is assembled in erythrocyte lysate

Approximately 75 million people are currently exposed to arsenic concentrations in drinking water, which is associated with the development of internal cancers. One way to ameliorate this undesirable situation is to remove arsenic (arsenite and arsenate) from drinking water. An alternative approach is the development of an inexpensive palliative dietary supplement that promotes the excretion of intestinally absorbed arsenite from the body. To this end, the simultaneous administration of New Zealand white rabbits with arsenite and selenite resulted in the biliary excretion of the seleno-bis (S-glutathionyl) arsinium ion, [(GS)2AsSe]-. This apparent detoxification mechanism has been recently extended to environmentally relevant doses [Gailer, J., Ruprecht, L., Reitmeir, P., Benker, B., and Schramel, P. (2004) Appl. Organometal. Chem. 18, 670-675]. The site of formation of this excretory product in the organism, however, is unknown. To investigate if [(GS)2AsSe]- is formed in rabbit blood, we added arsenite and selenite and analyzed blood aliquots using arsenic and selenium X-ray absorption spectroscopy. The characteristic arsenic and selenium X-ray absorption spectra of [(GS)2AsSe]- were detected within 2 min after addition and comprised 95% of the blood selenium 30 min after addition. To elucidate if erythrocytes are involved in the biosynthesis of [(GS)2AsSe]- in blood, arsenite and 77Se-selenite were added to rabbit erythrocyte lysate and the obtained solution was analyzed by 77Se NMR spectroscopy (273 K). This resulted in a 77Se NMR signal with a chemical shift identical to that of synthetic [(GS)2AsSe]- added to lysate. Combined, these results demonstrate that [(GS)2AsSe]- is rapidly formed in blood and that erythrocytes are an important site for the in vivo formation of this toxicologically important metabolite.

Stability of arsenic metabolites, arsenic triglutathione [As(GS)3] and methylarsenic diglutathione [CH3As(GS)2], in rat bile

Inorganic arsenicals such as arsenite (iAs(III)) and arsenate (iAs(V)) are well-known human carcinogens. Arsenic is metabolized by repetitive reduction and oxidative methylation, and is excreted mainly in urine as monomethylated arsenicals (MMAs) and dimethylated arsenicals (DMAs). Recently, it has been shown that iAs(III) administered intravenously or orally is excreted into bile as arsenic-glutathione (As-GSH) complexes such as arsenic triglutathione [As(GS)(3)] and methylarsenic diglutathione [CH(3)As(GS)(2)]. In order to carry out the speciation of As-GSH complexes, it is important to understand their stability. The present study was designed to clarify the stability of As-GSH complexes in rat bile, and the role of GSH in stabilizing these complexes. Arsenic species were separated on an anion-exchange column and were analyzed by high-performance liquid chromatography-inductively coupled argon plasma mass spectrometry (HPLC-ICP MS). As(GS)(3) and CH(3)As(GS)(2) were unstable in bile and were hydrolyzed to iAs(III) and monomethylarsonous acid (MMA(III)) in the absence of GSH. As(GS)(3) appeared to be stable in the presence of 10mM GSH. Exogenously added GSH also stabilized CH(3)As(GS)(2) in bile at the concentrations of 5mM or higher. It has been suggested that trivalent arsenicals, especially MMA(III), are more toxic than corresponding pentavalent ones. These results suggest that GSH plays an important role in preventing hydrolysis of As-GSH complexes and the generation of well-known toxic trivalent arsenicals.

Role of glutathione in reduction of arsenate and of γ-glutamyltranspeptidase in disposition of arsenite in rats

Arsenate (AsV), the environmentally prevalent form of arsenic, is converted sequentially in the body to arsenite (AsIII), monomethylarsonic acid (MMAsV), monomethylarsonous acid (MMAsIII), and dimethylarsinic acid (DMAsV) and some trimethylated metabolites. Although the biliary excretion of arsenic in rats is known to be glutathione (GSH)-dependent, involving transport of arsenic-GSH conjugates, the role of GSH in the reduction of AsV to the more toxic AsIII in vivo has not been defined. Therefore, we studied how the fate of AsV is influenced by buthionine sulfoximine (BSO), which depletes GSH in tissues. Control and BSO-treated rats were given AsV (50 micromol/kg, i.v.) and arsenic metabolites in bile, urine, blood and tissues were analysed by HPLC-HG-AFS. BSO increased retention of AsV in blood and tissues and decreased appearance of AsIII in blood, bile (by 96%) and urine (by 63%). The biliary excretion of MMAsIII was also nearly abolished, the appearance of MMAsIII and MMAsV in the blood was delayed and the renal concentrations of these monomethylated arsenicals were decreased by BSO. Interestingly, appearance of DMAsV in blood and urine remained unchanged and the concentrations of this metabolite in the kidneys and muscle were even increased in response to BSO. To test the role of gamma-glutamyltranspeptidase (GGT) in arsenic disposition, the effect of the of the GGT inhibitor acivicin was investigated in rats injected with AsIII (50 micromol/kg, i.v.). Acivicin lowered the hepatic and renal GGT activities and increased the biliary as well as urinary excretion of GSH, but failed to alter the disposition (i.e. blood and tissue concentrations, biliary and urinary excretion) of AsIII and its metabolites. In conclusion, shortage of GSH decreases not only the hepatobiliary transport of arsenic, but also reduction of AsV and the formation of monomethylated arsenic, while not hindering the production of dimethylated arsenic. While GSH plays an important role in the disposition and toxicity of arsenic, GGT, which hydrolyses GSH and GSH conjugates, apparently does not influence the fate of the GSH-reactive trivalent arsenicals in rats.

Thiol synthesis and arsenic hyperaccumulation in Pteris vittata (Chinese brake fern)

Pteris vittata (Chinese brake fern) has potential for phytoremediation of As-contaminated sites. In this study, the synthesis of total thiols and acid-soluble thiols in P. vittata was investigated under arsenic exposure. The strong and positive correlation between As concentration and acid-soluble thiols in plant leaflets suggests that acid-soluble thiols may play a role in As detoxification. A major As-induced thiol was purified and characterized. A molecular ion (M + 1) of 540 m/z suggests that the thiol was a phytochelatin (PC) with two base units (PC(2)). However, the ratios of acid-soluble thiols to As in leaflets exposed to As ranged from 0.012 to 0.026, suggesting that only a very small part of As is complexed by PC(2). PCs could play a minor detoxification role in this hyperaccumulator. A PC-independent mechanism appears to be mainly involved in As tolerance, while PC-dependent detoxification seems to be a supplement.

Arsenic complexes in the arsenic hyperaccumulator Pteris vittata (Chinese brake fern)

Pteris vittata (Chinese brake fern), the first reported arsenic (As) hyperaccumulating plant, can be potentially applied in the phytoremediation As-contaminated sites. Understanding the mechanisms of As tolerance and detoxification in this plant is critical to further enhance its capability of As hyperaccumulation. In this study, an unknown As species, other than arsenite (AsIII) or arsenate (AsV) was found in leaflets by using anion-exchange chromatography-hydride generation-atomic fluorescence spectroscopy and size-exclusion chromatography-atomic fluorescence spectrometry. The chromatographic behavior of this unknown As species and its stability suggest that it is likely an As complex. Although phytochelatin with two subunits (PC2) was the only major thiol in P. vittata under As exposure, this unknown As complex was unlikely to be an AsIII-PC2 complex by comparison of their chromatographic behaviors, stability at different pHs and charge states. The complex is sensitive to temperature and metal ions, but relatively insensitive to pH. In buffer solution of pH 5.9, it is present in a neutral form.

Purification and Characterization of Thiols in an Arsenic Hyperaccumulator under Arsenic Exposure

Pteris vittata (Chinese brake fern) is the first reported arsenic hyperaccumulator. To investigate the arsenic tolerance mechanism in this plant, reversed-phase HPLC with postcolumn derivatization was used to analyze the thiols induced under arsenic exposure. A major thiol in the plant leaflets was found to be responsive to arsenic exposure. The arsenic-induced compound was purified on a large scale by combining covalent chromatography and preparative reversed-phase HPLC. About 2 mg of this compound was isolated from 1 kg of fresh leaflets. The purified arsenic-induced compound was characterized using electrospray ionization mass spectrometry. A molecular ion (M + 1) of 540 and fragments were obtained, which indicated that the arsenic-induced thiol was a phytochelatin with two subunits (PC(2)). Compared to the classical methods for purification of phytochelatins, this new method is more specific, simple, and rapid and is suitable for purification of PCs in a large scale as well as sample preparation for mass spectrometry analysis.

Biliary excretion of [(GS)(2)AsSe](-) after intravenous injection of rabbits with arsenite and selenate

It has been shown that the seleno-bis (S-glutathionyl) arsinium ion, [(GS)(2)AsSe](-), is the major arsenic and selenium excretory product in bile of rabbits treated with arsenite and selenite [Gailer, J., Madden, S., Buttigieg, G. A., Denton, M. B., and Younis, H. S. (2002) Appl. Organomet. Chem. 16, 72-75]. To investigate the in vivo interaction between the other environmentally common oxy-anions of arsenic and selenium in mammals, we have intravenously injected rabbits with different combinations of the arsenic and selenium oxo-anions (arsenite + selenate, arsenate + selenite, and arsenate + selenate) and analyzed the collected bile and whole blood samples by X-ray absorption spectroscopy. Only the injection of arsenite and selenate led to the biliary excretion of [(GS)(2)AsSe](-) within 25 min. Whole blood collected from these animals (25 min postinjection) contained predominantly unchanged selenate, which suggests the presence of a mammalian selenate reductase in the liver. The lack of any significant biliary excretion of [(GS)(2)AsSe](-) in the other treatment groups implies that arsenate was not reduced in the liver on the time scale of our experiments. The relevance of these results for the human toxicology of arsenic and selenium is discussed.

Synthesis, purification, and structural characterization of the dimethyldiselenoarsinate anion

A novel arsenic-selenium solution species was synthesized by reacting equimolar sodium selenite and sodium dimethylarsinate with 10 mol equiv of glutathione (pH 7.5) in aqueous solution. The solution species showed a single (77)Se NMR resonance at 112.8 ppm. Size-exclusion chromatography (SEC) using an inductively coupled plasma atomic emission spectrometer (ICP-AES) as the simultaneous arsenic-, selenium-, sulfur-, and carbon-specific detector revealed an arsenic-selenium moiety with an As:Se molar ratio of 1:2. Electrospray ionization mass spectrometry (ESI-MS) of the chromatographically purified compound showed a molecular mass peak at m/z 263 in the negative ion mode. Fragmentation of the parent ion (ESI-MS-MS) produced (CH(3))(2)As(-) and Se(2)(-) fragments. Arsenic and selenium extended X-ray absorption fine structure spectroscopy (EXAFS) of the purified species revealed two As-C interactions at 1.943 A and two As-Se interactions at 2.279 A. On the basis of these results this novel solution species is identified as the dimethyldiselenoarsinate anion.

Role of glutathione in the biliary excretion of the arsenical drugs trimelarsan and melarsoprol

After administration of the inorganic sodium arsenite or arsenate to rats, the biliary excretion of arsenic is rapid, is accompanied by the biliary output of large amounts of GSH, and is completely arrested by the GSH depletor diethyl maleate (DEM). We studied the biliary excretion of trimelarsan (TMA) and melarsoprol (MAP) in rats in order to determine whether biliary excretion is also significant in the disposition of these trivalent organic arsenicals that are used as therapeutic agents and whether GSH is also involved in their hepatobiliary transport. After injection of either drug (100 micromol/kg, i.v.), arsenic was rapidly excreted in bile (up to 1 micromol/kg. min, approximately 55% of dose/100 min). Concurrently, TMA and MAP increased the biliary output of GSH 3- and 6 fold, and lowered the hepatic GSH content by 24% and 27%, respectively. In TMA-injected rats, pretreatment with DEM or buthionine sulfoximine decreased the initial biliary excretion of arsenic by 75% and 40%, respectively, whereas in MAP-injected rats these GSH depletors diminished arsenic output by 45% and 20%. Both arsenicals reacted with GSH in vitro, giving rise to the same product, which was also shown by HPLC analysis to be a major biliary metabolite of both TMA and MAP. This metabolite was sensitive to gamma-glutamyltranspeptidase in vitro and its biliary excretion was virtually prevented by the GSH depletors, confirming that it is a GSH conjugate (purportedly melarsen-diglutathione). Some TMA was excreted in the bile unchanged, whereas a significant amount of MAP also appeared there as two glucuronides. The biliary excretion of unchanged TMA and MAP glucuronides was increased by experimental depletion of GSH. These studies indicate that the biliary excretion of TMA and MAP (1) is very significant in their disposition, (2) is partially dependent on the hepatic availability of GSH, as these arsenicals are excreted in part as a GSH conjugate, and (3) is concomitant with the increased appearance of GSH in bile, probably originating from dissociation of the unstable GSH conjugate of these arsenicals. Thus, conjugation with GSH is important in the elimination of both TMA and MAP, although glucuronidation is also involved in the fate of MAP.

Detoxification of arsenic by phytochelatins in plants

As is a ubiquitous element present in the atmosphere as well as in the aquatic and terrestrial environments. Arsenite and arsenate are the major forms of As intoxication, and these anions are readily taken up by plants. Both anions efficiently induce the biosynthesis of phytochelatins (PCs) ([gamma-glutamate-cysteine](n)-glycine) in vivo and in vitro. The rapid induction of the metal-binding PCs has been observed in cell suspension cultures of Rauvolfia serpentina, in seedlings of Arabidopsis, and in enzyme preparations of Silene vulgaris upon challenge to arsenicals. The rate of PC formation in enzyme preparations was lower compared with Cd-induced biosynthesis, but was accompanied by a prolonged induction phase that resulted finally in higher peptide levels. An approximately 3:1 ratio of the sulfhydryl groups from PCs to As is compatible with reported As-glutathione complexes. The identity of the As-induced PCs and of reconstituted metal-peptide complexes has unequivocally been demonstrated by electrospray ionization mass spectroscopy. Gel filtration experiments and inhibitor studies also indicate a complexation and detoxification of As by the induced PCs.