In this issue TracingtheFateofSelenium Selenium is both an essential trace nutrient and a toxic metalloid, depending on the amount and chemical form ingested. Dietary Se is primarily derived from plant sources as selenomethionine (SeMet) and methylselenocysteine (MeSeCys). The trans-selenation pathway converts SeMet to selenocysteine (SeCys), which is incorporated into a small number of selenoproteins. It is this role of Se in selenoprotein synthesis that is the basis for its essiential nutrient status. Alternatively, both SeMet and MeSeCys may be converted to methylselenol (CH3SeH) by the enzymes γ-lyase and β-lyase, respectively. Methylselenol has recently become a focus of attention because of its selective toxicity to tumor cells and the resultant suggestion that increased intake of dietary Se may be an effective cancer chemopreventive measure. Any attempt to capitalize on the selective toxicity of methylselenol to tumor cells requires a complete understanding of the metabolic disposition of this compound. Methylselenol is sequentially methylated in vivo to yield dimethylselenide (DMSe) and trimethylselonium ion (TMSe), which can be detected as excretion products in the breath and urine, respectively. Complicating matters is the use of methylselinic acid (MSA) as an artificial source of methylselenol in many laboratory studies. MSA is converted to methylselenol by sequential reductions. Finally, methylselenol may be convertedtoselenide,which is then incorporated into
selenosugars, another urinary excretory form of Se. The toxicity of different forms of Se and their potential usefulness as antitumor agents cannot be evaluated unless their metabolic fate is fully explored. To this end, Ohta et al. (p 1795) have used HPLC- and GCinductively coupled plasma mass spectrometry to monitor the disposition of a mixture of MeSeCys, SeMet, and MSA following oral administration to rats. Their approach exploited the fact that Se occurs as six stable isotopes, allowing each compound to be uniquely labeled. This provided the means to distinguish the source of excreted Se from each compound following the administration of all together to a single animal. Following oral dosing of the three stable isotopelabeled compounds, Ohta et al. collected exhaled breath from each rat over a period of three hours for analysis of the volatile DMSe metabolite. They found the rapid appearance of DMSe from MeSeCys and MSA that peaked at 1 to 2 h after compound administration. Only small amounts of exhaled DMSe were attributable to SeMet. At the end of the 3 h collection period, the proportions of Se excreted as DMA from MSA, MeSeCys, and SeMet were 0.062%, 0.039%, and 0.005%, respectively.
The major urinary metabolite obtained from all three of the exogenous Se compounds was selenosugar,
Published online 11/16/2009 • DOI: 10.1021/tx900349z © 2009 American Chemical Society
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with TMSe also present in lesser quantities. The isotopic distributions of the metabolites indicated that the rank order of urinary excretion was MeSeCys > MSA > SeMet. The efficiencies of conversion of the three compounds to selenosugar were similar, but conversion to TMSe was much less efficient for SeMet than for MeSeCys or MSA. The rapid excretion of Se from MeSeCys and MSA as DMSe and TMSe suggests efficient conversion of these compounds to methylselenol. The fact that SeMet is poorly excreted in these forms is consistent with the utilization of this compound for SeCys formation via the trans-selenation pathway. These results strongly suggest that prior attempts to use SeMet as a source of methylselenolincancerchemotherapy or chemoprevention studies is misguided. Clearly, a full understanding of the complex metabolic interconversion of the different chemical forms of Se is a key to evaluating both its beneficial and its toxic effects in vivo.
Allergy and Reactivity Allergic contact dermatitis (ACD) is a hypersensitivity reaction that causes considerable morbidity across the general population. ACD usually results from exposure of the skin to a small, highly reactive, electrophilic molecule that forms adducts with protein nucleophiles. The consequence is a structural modification to the protein that is recognized as foreign by the immune system, triggering a response. Future exposures to the allergen elicit inflammation of the skin, triggering a range of symptoms including a rash, itching, burning, and blistering. For more on ACD see Radical Skin Sensitization below.
A major cause of occupational ACD is exposure to epoxy resins. These materials are usually supplied as epoxy resin systems (ERS), including, in addition to the epoxy resins, curing agents, modifiers, and diluents. The monomers most commonly
Special Features Chemical Research in Toxicology has long been a major forum for research on the causes and consequences of DNA damage. Now EAB member Kent Gates (p 1747) compiles our wealth of knowledge into a seminal review on the chemical mechanisms of DNA damaging processes. Do not miss this excellent opportunity to develop a greater appreciation and understanding of our progress in this important area of toxicology, and watch for the accompanying Thematic Compilation that highlights important recent articles on this topic. Vol. 22,
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In this issue used in epoxy resins are the diglycidyl ether of bisphenol A (DGEBA) and the diglycidyl ether of bisphenol F (DGEBF).Thesereactivecompounds are believed to be the major sensitizing compounds in ACD resulting from epoxy resin exposure. However, ERS diluents often also include reactive species, among them phenyl glycidyl ether (PGE). PGE is an extreme sensitizer, and it exhibits cross-reactivity in animals sensitized to DGEBA and DGEBF. Potency as an ACD sensitizer depends strongly on chemical reactivity with protein nucleophiles. Therefore, Niklasson et al. (p 1787) proposed that it should be possible to identify diluents that exhibit lower chemical reactivity and would thus be less allergenic. To this end, they synthesized a series of six epoxide-containing structural analogues of PGE for evaluation of their chemical reactivity and their potency as ACD sensitizers.
differences in reactivity among the analogues. Niklasson et al. next evaluated the potency of each compound as a skin sensitizer. Again, although all compounds were active in the in vivo assay, they exhibited differences in potency. Most importantly, the rank order of potency agreed with the relative chemical reactivity of the six epoxides. The results demonstrated that replacement of the phenyl ring with an alkyl moiety resulted in a loss of activity. Similarly, increasing the length of the carbon chain between the phenyl ring and the oxygen or between the oxygen and the epoxide reduced both chemical reactivity and sensitization potency. Niklasson et al. conclude that relatively minor structural changes can lead to markedly reduced potential as an ACD sensitizer. Application of this basic finding could lead to the development of safer epoxy resins.
Reaction of each of the epoxides with the hexapeptide PHCKRM led to the rapid formation of a monoadduct with cysteine and slower formation of a diadduct with cysteine and proline. All of the epoxides formed adducts, but reaction kinetics demonstrated
Radical Skin Sensitization November’s issue of CRT boasts two excellent articles on the chemical mechanisms of skin sensitization that lead to allergic contact dermatitis (ACD). As described in Allergy and Reactivity (see above), ACD results when reactive small molecules form adducts with proteins leading to structural changes that are recognized as foreign by the immune system. Most frequently, skin sensitizing compounds are reactive electrophiles that form adducts through attack by
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protein nucleophiles. However, as noted by Johansson et al. (p 1774), a number of sensitizers are not electrophiles. Among these compounds are hydroperoxides formed by air oxidationofunsaturatedhydrocarbons and alcohol ethoxylates. Such compounds are a leading cause of ACD in response to fragrances. The mechanism by which hydroperoxide sensitizers form protein adducts is not known. One possibility is direct radical coupling between the hapten and protein. Alternatively, the hydroperoxide may undergo rearrangement to form an electrophilic species, which then reacts with the protein via a conventional nucleophilic attack. To distinguish between these possibilities, Johansson et al. performed a detailed investigation of the reaction of limonene-2-hydroperoxide (Lim-2-OOH, formed in the autoxidation of R-limonene) with 5,10,15,20-tetraphenyl21H,23H-prophine iron(III) chloride (Fe(III)TPPCl) in the presence of N-acetyl-cysteine methyl ester (NAc-CysOMe). R-Limonene is a frequently used fragrance terpene. Its oxidation products are a common cause of ACD, and the hydroperoxides are the most potent of these.
The reaction of Lim-2OOH with Fe(III)TPPCl in the presence of NAc-Cys-OMe resulted in a complex assortment of products. Mass
spectral analysis suggested adduct formation between cysteine and carvone, between cysteine and carveol, or between cysteine and carvone with the addition of one or two extra oxygen atoms. Fragmentation patterns and NMR analyses were combined to identify the carvone adducts as the result of NAcCys-OMe addition to the endocyclic or the isoprenoid double bonds of carvone. In the case of the carveol adduct, addition occurred at the endocyclic double bond only.
An adduct formed between NAc-Cys-OMe and theendocyclicdoublebond of carvone could occur by a nucleophile-electrophile reaction mechanism. Indeed, this adduct was also formed by Michael addition of NAc-Cys-OMe to carvone. However, adducts formed at the isoprenoid double bond could only occur by a radical mechanism, likely involving a thiyl radical intermediate. Johansson et al. point out that patients suffering from ACD to oxidized limonene show little reaction to carvone, suggesting that the allergenic species produced by the hydroperoxides in vivo is not the adduct formed at the endocyclic double bond. Together, the data support a radical mechanism for limonene hydroperoxide sensitization in ACD. TX900349Z
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DOI: 10.1021/tx900349z $40.75 © 2009 American Chemical Society
