Effects of modifying structure on electrophilic reactions with biological nucleophiles

RNA Adduction Resulting from the Metabolic Activation of Myristicin by P450 Enzymes and Sulfotransferases

Myristicin (MYR) mainly occurs in nutmeg and belongs to alkoxy-substituted allylbenzenes, a class of potentially toxic natural chemicals. RNA interaction with MYR metabolites in vitro and in vivo has been investigated in order to gain a better understanding of MYR toxicities. We detected two guanosine adducts (GA1 and GA2), two adenosine adducts (AA1 and AA2), and two cytosine adducts (CA1 and CA2) by LC-MS/MS analysis of total RNA extracts from cultured primary mouse hepatocytes and liver tissues of mice after exposure to MYR. An order of nucleoside adductions was found to be GAs > AAs > CAs, and the result of density functional theory calculations was in agreement with that detected by the LC-MS/MS-based approach. In vitro and in vivo studies have shown that MYR was oxidized by cytochrome P450 enzymes to 1'-hydroxyl and 3'-hydroxyl metabolites, which were then sulfated by sulfotransferases (SULTs) to form sulfate esters. The resulting sulfates would react with the nucleosides by SN1 and/or SN2 reactions, resulting in RNA adduction. The modification may alter the biochemical properties of RNA and disrupt RNA functions, perhaps partially contributing to the toxicities of MYR.

The expanding repertoire of covalent warheads for drug discovery

The reactive functionalities of drugs that engage in covalent interactions with the enzyme/receptor residue in either a reversible or an irreversible manner are called 'warheads'. Covalent warheads that were previously neglected because of safety concerns have recently gained center stage as a result of their various advantages over noncovalent drugs, including increased selectivity, increased residence time, and higher potency. With the approval of several covalent inhibitors over the past decade, research in this area has accelerated. Various strategies are being continuously developed to tune the characteristics of warheads to improve their potency and mitigate toxicity. Here, we review research progress in warhead discovery over the past 5 years to provide valuable insights for future drug discovery.

Formation of RNA adducts resulting from metabolic activation of spice ingredient safrole mediated by P450 enzymes and sulfotransferases

Safrole (SFL) is an IARC class 2B carcinogen. To better understand the mechanism involved in SFL toxicity, we explored the potential interactions between SFL metabolites and RNA. Three guanosine adducts (G1-G3), two adenosine adducts (A1-A2), and two cytosine adducts (C1-C2) were detected by LC-MS/MS in mouse liver S9 incubations, cultured mouse primary hepatocytes, and liver tissues of mice after exposure to SFL. These adducts were chemically synthesized, and one of the guanosine adducts was structurally characterized by ¹H-NMR. Studies in vitro and in vivo showed that SFL was oxidized by cytochrome P450 enzymes to the corresponding 1'-hydroxyl metabolite which was further metabolized by sulfotransferases to form allylic sulfate esters. The formed reactive intermediate(s) subsequently reacted with bases of RNA, leading to RNA adduction, which could play a partial role in the toxicities of SFL through the alteration of RNA biochemical properties and interruption of RNA functions.

Kinetics and Reaction Mechanism of Biothiols Involved in SNAr Reactions: An Experimental Study

Few kinetic parameters, or reaction rates, are known up to date in detail about 1-chloro and 1-fluoro-2,4-dinitrobenzene (ClDNB and FDNB, respectively) with a series of biothiols in aqueous media. These biological nucleophiles with thiol groups have been widely used as a reference in nucleophile reactivity assays due to their prevalence and cellular abundance. The main aim of this study was to elucidate the reaction mechanism based on Brönsted-type plots and reactivity patterns of the electrophile/nucleophile pairs. A complete kinetic study was performed in terms of the comparison of Brönsted-type slope parameters (β_nuc) for the reactions and was used for assigning the mechanism and the rate-determining step associated with the reaction route. A mass spectrometry analysis demonstrated that the nucleophilic center of the biothiols is the -SH group and there is only one kinetic product. The kinetic study suggests that the reaction mechanism might be the borderline between concerted and stepwise pathways. An amine–enol equilibrium for the most reactive nucleophiles appears to be the main determining factor controlling the nucleophilic attack in the nucleophilic aromatic substitution reactions investigated, highlighting the anionic form for these nucleophiles. This amine–enol equilibrium involves a hydrogen bond which stabilizes the intermediate species in the reaction pathway. Thus, intramolecular bonds are formed and enhance the nucleophilic strength through the contribution of the solvent surrounding the electrophile/nucleophile pairs. Finally, we highlight the importance of the formation of electrophile/nucleophile adducts that could modify structures and/or functions of biological systems with potential toxic effects. Therefore, it is essential to know all these kinetic and reactivity patterns and their incidence on other studies.

Does the food processing contaminant acrylamide cause developmental neurotoxicity? A review and identification of knowledge gaps

There is a worldwide concern on adverse health effects of dietary exposure to acrylamide (AA) due to its presence in commonly consumed foods. AA is formed when carbohydrate rich foods containing asparagine and reducing sugars are prepared at high temperatures and low moisture conditions. Upon oral intake, AA is rapidly absorbed and distributed to all organs. AA is a known human neurotoxicant that can reach the developing foetus via placental transfer and breast milk. Although adverse neurodevelopmental effects have been observed after prenatal AA exposure in rodents, adverse effects of AA on the developing brain has so far not been studied in humans. However, epidemiological studies indicate that gestational exposure to AA impair foetal growth and AA exposure has been associated with reduced head circumference of the neonate. Thus, there is an urgent need for further research to elucidate whether pre- and perinatal AA exposure in humans might impair neurodevelopment and adversely affect neuronal function postnatally. Here, we review the literature with emphasis on the identification of critical knowledge gaps in relation to neurodevelopmental toxicity of AA and its mode of action and we suggest research strategies to close these gaps to better protect the unborn child.

In Vitro and In Vivo Evidence for RNA Adduction Resulting from Metabolic Activation of Methyleugenol

Methyleugenol (ME) is a ubiquitous component in spices and other culinary herbal products. A prevailing theory in ME toxicity is its ability to be metabolically activated by P450 enzymes and sulfotransferases, which initiates sequential reactions of the resulting metabolites with functional biomolecules. The present study aimed at a potential interaction between the reactive metabolites of ME and RNA. Cultured mouse primary hepatocytes were incubated with ME followed by RNA extraction and NaOH and alkaline phosphatase-based RNA hydrolysis. Three adenosine adducts were detected in the hydrolytic mixture by LC-MS/MS. The same adenosine adducts were also detected in hepatic tissues harvested from ME-treated mice. These three adducts were chemically synthesized and structurally characterized by ¹H NMR. Additionally, two guanosine adducts and one cytidine adduct were detected in the in vivo samples. These results provided solid evidence that the reactive metabolites of ME attacked RNA, resulting in RNA adduction.

NRF2 and the Ambiguous Consequences of Its Activation during Initiation and the Subsequent Stages of Tumourigenesis

NF-E2 p45-related factor 2 (NRF2, encoded in the human by NFE2L2) mediates short-term adaptation to thiol-reactive stressors. In normal cells, activation of NRF2 by a thiol-reactive stressor helps prevent, for a limited period of time, the initiation of cancer by chemical carcinogens through induction of genes encoding drug-metabolising enzymes. However, in many tumour types, NRF2 is permanently upregulated. In such cases, its overexpressed target genes support the promotion and progression of cancer by suppressing oxidative stress, because they constitutively increase the capacity to scavenge reactive oxygen species (ROS), and they support cell proliferation by increasing ribonucleotide synthesis, serine biosynthesis and autophagy. Herein, we describe cancer chemoprevention and the discovery of the essential role played by NRF2 in orchestrating protection against chemical carcinogenesis. We similarly describe the discoveries of somatic mutations in NFE2L2 and the gene encoding the principal NRF2 repressor, Kelch-like ECH-associated protein 1 (KEAP1) along with that encoding a component of the E3 ubiquitin-ligase complex Cullin 3 (CUL3), which result in permanent activation of NRF2, and the recognition that such mutations occur frequently in many types of cancer. Notably, mutations in NFE2L2, KEAP1 and CUL3 that cause persistent upregulation of NRF2 often co-exist with mutations that activate KRAS and the PI3K-PKB/Akt pathway, suggesting NRF2 supports growth of tumours in which KRAS or PKB/Akt are hyperactive. Besides somatic mutations, NRF2 activation in human tumours can occur by other means, such as alternative splicing that results in a NRF2 protein which lacks the KEAP1-binding domain or overexpression of other KEAP1-binding partners that compete with NRF2. Lastly, as NRF2 upregulation is associated with resistance to cancer chemotherapy and radiotherapy, we describe strategies that might be employed to suppress growth and overcome drug resistance in tumours with overactive NRF2.

Structural basis for expanding the application of bioligand in metal bioremediation: A review

Bioligands (BL) present in plant and microbes are primarily responsible for their use in metal decontamination. Both primary (proteins and amino acid) and secondary (proliferated) response in the form of BL is possible in plants and microbes toward metal bioremediation. Structure of these BL have specific requirement for preferential binding towards a particular metal in biomass. The aim of this review is to explore various templates from BL (as metal host) for the metal detoxification/decontamination and associated bioremediation. Mechanistic explanation for bioremediation may involve the various processes like: (i) electron transfer; (ii) translocation; and (iii) coordination number variation. HSAB (hard and soft acid and base) concept can act as guiding principle for many such processes. It is possible to investigate various structural homolog of BL (similar to secondary response in living stage) for the possible improvement in bioremediation process.

Urinary Methyleugenol-deoxyadenosine Adduct as a Potential Biomarker of Methyleugenol Exposure in Rats

Methyleugenol (ME), a natural ingredient of several herbs and spices used in the human diet, is hepatocarcinogenic in rodents. Following metabolic activation to the reactive carbocation intermediate, ME can bind covalently to DNA, which is directly associated with its carcinogenicity. In this work, a non-invasive approach to determine ME exposure was established by monitoring the urinary N⁶-(methylisoeugenol-3'-yl)-2'-deoxyadenosine (ME-dA) adduct. The developed method entails liquid-liquid extraction enrichment of urinary ME-dA, incorporation of deuterated ME-dA as an internal standard, and analysis by liquid chromatography coupled tandem mass spectrometry. Male rats (10-12 weeks, 180-200 g) were treated (p.o.) with ME, and ME-dA was excreted in urine in a dose- and time-dependent manner. The non-invasive approach enabled us to successfully determine exposure to ME-containing herbs and spices. These results suggest that ME-dA can potentially serve as an effective biomarker of ME exposure in rats. It is expected that the developed approach of detecting urinary ME-dA will facilitate the investigation of ME carcinogenesis.

When alcohol is the answer: Trapping, identifying and quantifying simple alkylating species in aqueous environments

Alkylating agents are a significant class of environmental carcinogens as well as commonly used anticancer therapeutics. Traditional alkylating activity assays have utilized the colorimetric reagent 4-(4-nitrobenzyl)pyridine (4NBP). However, 4NBP based assays have a relatively low sensitivity towards harder, more oxophilic alkylating species and are not well suited for the identification of the trapped alkyl moiety due to adduct instability. Herein we describe a method using water as the trapping agent which permits the trapping of simple alkylating electrophiles with a comparatively wide range of softness/hardness and permits the identification of donated simple alkyl moieties.

Inactivation of CYP2A6 by the Dietary Phenylpropanoid trans-Cinnamic Aldehyde (Cinnamaldehyde) and Estimation of Interactions with Nicotine and Letrozole

Human exposure to trans-cinnamic aldehyde [t-CA; cinnamaldehyde; cinnamal; (E)-3-phenylprop-2-enal] is common through diet and through the use of cinnamon powder for diabetes and to provide flavor and scent in commercial products. We evaluated the likelihood of t-CA to influence metabolism by inhibition of P450 enzymes. IC50 values from recombinant enzymes indicated that an interaction is most probable for CYP2A6 (IC50 = 6.1 µM). t-CA was 10.5-fold more selective for human CYP2A6 than for CYP2E1; IC50 values for P450s 1A2, 2B6, 2C9, 2C19, 2D6, and 3A4 were 15.8-fold higher or more. t-CA is a type I ligand for CYP2A6 (KS = 14.9 µM). Inhibition of CYP2A6 by t-CA was metabolism-dependent; inhibition required NADPH and increased with time. Glutathione lessened the extent of inhibition modestly and statistically significantly. The carbon monoxide binding spectrum was dramatically diminished after exposure to NADPH and t-CA, suggesting degradation of the heme or CYP2A6 apoprotein. Using a static model and mechanism-based inhibition parameters (K(I) = 18.0 µM; k(inact) = 0.056 minute(-1)), changes in the area under the concentration-time curve (AUC) for nicotine and letrozole were predicted in the presence of t-CA (0.1 and 1 µM). The AUC fold-change ranged from 1.1 to 3.6. In summary, t-CA is a potential source of pharmacokinetic variability for CYP2A6 substrates due to metabolism-dependent inhibition, especially in scenarios when exposure to t-CA is elevated due to high dietary exposure, or when cinnamon is used as a treatment of specific disease states (e.g., diabetes).

Antitumor sulfonylhydrazines: design, structure-activity relationships, resistance mechanisms, and strategies for improving therapeutic utility

1,2-Bis(sulfonyl)-1-alkylhydrazines (BSHs) were conceived as more specific DNA guanine O-6 methylating and chloroethylating agents lacking many of the undesirable toxicophores contained in antitumor nitrosoureas. O(6)-Alkylguanine-DNA alkyltransferase (MGMT) is the sole repair protein for O(6)-alkylguanine lesions in DNA and has been reported to be absent in 5-20% of most tumor types. Many BSHs exhibit highly selective cytotoxicity toward cells deficient in MGMT activity. The development of clinically useful MGMT assays should permit the identification of tumors with this vulnerability and allow for the preselection of patient subpopulations with a high probability of responding. The BSH system is highly versatile, permitting the synthesis of many prodrug types with the ability to incorporate an additional level of tumor-targeting due to preferential activation by tumor cells. Furthermore, it may be possible to expand the spectrum of activity of these agents to include tumors with MGMT activity by combining them with tumor-targeted MGMT inhibitors.

Influence of glutathione and glutathione S-transferases on DNA interstrand cross-link formation by 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine, the active anticancer moiety generated by laromustine

Prodrugs of 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE) are promising anticancer agents. The 90CE moiety is a readily latentiated, short-lived (t_1/2 ∼ 30 s) chloroethylating agent that can generate high yields of oxophilic electrophiles responsible for the chloroethylation of the O-6 position of guanine in DNA. These guanine O-6 alkylations are believed to be responsible for the therapeutic effects of 90CE and its prodrugs. Thus, 90CE demonstrates high selectivity toward tumors with diminished levels of O⁶-alkylguanine-DNA alkyltransferase (MGMT), the resistance protein responsible for O⁶-alkylguanine repair. The formation of O⁶-(2-chloroethyl)guanine lesions ultimately leads to the generation of highly cytotoxic 1-(N³-cytosinyl),-2-(N¹-guaninyl)ethane DNA interstrand cross-links via N¹,O⁶-ethanoguanine intermediates. The anticancer activity arising from this sequence of reactions is thus identical to this component of the anticancer activity of the clinically used chloroethylnitrosoureas. Herein, we evaluate the ability of glutathione (GSH) and other low molecular weight thiols, as well as GSH coupled with various glutathione S-transferase enzymes (GSTs) to attenuate the final yields of cross-links generated by 90CE when added prior to or immediately following the initial chloroethylation step to determine the major point(s) of interaction. In contrast to studies utilizing BCNU as a chloroethylating agent by others, GSH (or GSH/GST) did not appreciably quench DNA interstrand cross-link precursors. While thiols alone offered little protection at either alkylation step, the GSH/GST couple was able to diminish the initial yields of cross-link precursors. 90CE exhibited a very different GST isoenzyme susceptibility to that reported for BCNU, this could have important implications in the relative resistance of tumor cells to these agents. The protection afforded by GSH/GST was compared to that produced by MGMT.

Influence of phosphate and phosphoesters on the decomposition pathway of 1,2-bis(methylsulfonyl)-1-(2-chloroethyhydrazine (90CE), the active anticancer moiety generated by Laromustine, KS119, and KS119W

Prodrugs of the short-lived chloroethylating agent 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE) and its methylating analogue 1,2-bis(methylsulfonyl)-1-(methyl)hydrazine (KS90) are potentially useful anticancer agents. This class of agents frequently yields higher ratios of therapeutically active oxophilic electrophiles responsible for DNA O⁶-guanine alkylations to other electrophiles with lower therapeutic relevance than the nitrosoureas. This results in improved selectivity toward tumors with diminished levels of O⁶-alkylguanine-DNA alkyltransferase (MGMT), the resistance protein responsible for O⁶-alkylguanine repair. The formation of O⁶-(2-chloroethyl)guanine, which leads to the formation of a DNA–DNA interstrand cross-link, accounts for the bulk of the anticancer activity of 90CE prodrugs. Herein, we describe a new decomposition pathway that is available to 90CE but not to its methylating counterpart. This pathway appears to be subject to general/acid base catalysis with phosphate (Pi), phosphomonoesters, and phosphodiesters, being particularly effective. This pathway does not yield a chloroethylating species and results in a major change in nucleophile preference since thiophilic rather than oxophilic electrophiles are produced. Thus, a Pi concentration dependent decrease in DNA–DNA interstand cross-link formation was observed. Changes in 90CE decomposition products but not alkylation kinetics occurred in the presence of Pi since the prebranch point elimination of the N-1 methanesulfinate moiety remained the rate-limiting step. The Pi catalyzed route is expected to dominate at Pi and phosphoester concentrations totaling >25–35 mM. In view of the abundance of Pi and phosphoesters in cells, this pathway may have important effects on agent toxicity, tumor selectivity, and resistance to prodrugs of 90CE. Furthermore, it may be possible to design analogues that diminish this thiophile-generating pathway, which is likely superfluous at best and potentially detrimental to the targeting of hypoxic regions where Pi concentrations can be significantly elevated.

Molecular mechanism of acrylamide neurotoxicity: lessons learned from organic chemistry

BACKGROUND

Acrylamide (ACR) produces cumulative neurotoxicity in exposed humans and laboratory animals through a direct inhibitory effect on presynaptic function.

OBJECTIVES

In this review, we delineate how knowledge of chemistry provided an unprecedented understanding of the ACR neurotoxic mechanism. We also show how application of the hard and soft, acids and bases (HSAB) theory led to the recognition that the α,β-unsaturated carbonyl structure of ACR is a soft electrophile that preferentially forms covalent bonds with soft nucleophiles.

METHODS

In vivo proteomic and in chemico studies demonstrated that ACR formed covalent adducts with highly nucleophilic cysteine thiolate groups located within active sites of presynaptic proteins. Additional research showed that resulting protein inactivation disrupted nerve terminal processes and impaired neurotransmission.

DISCUSSION

ACR is a type-2 alkene, a chemical class that includes structurally related electrophilic environmental pollutants (e.g., acrolein) and endogenous mediators of cellular oxidative stress (e.g., 4-hydroxy-2-nonenal). Members of this chemical family produce toxicity via a common molecular mechanism. Although individual environmental concentrations might not be toxicologically relevant, exposure to an ambient mixture of type-2 alkene pollutants could pose a significant risk to human health. Furthermore, environmentally derived type-2 alkenes might act synergistically with endogenously generated unsaturated aldehydes to amplify cellular damage and thereby accelerate human disease/injury processes that involve oxidative stress.

CONCLUSIONS

These possibilities have substantial implications for environmental risk assessment and were realized through an understanding of ACR adduct chemistry. The approach delineated here can be broadly applied because many toxicants of different chemical classes are electrophiles that produce toxicity by interacting with cellular proteins.

Characterization of model peptide adducts with reactive metabolites of naphthalene by mass spectrometry

Naphthalene is a volatile polycyclic aromatic hydrocarbon generated during combustion and is a ubiquitous chemical in the environment. Short term exposures of rodents to air concentrations less than the current OSHA standard yielded necrotic lesions in the airways and nasal epithelium of the mouse, and in the nasal epithelium of the rat. The cytotoxic effects of naphthalene have been correlated with the formation of covalent protein adducts after the generation of reactive metabolites, but there is little information about the specific sites of adduction or on the amino acid targets of these metabolites. To better understand the chemical species produced when naphthalene metabolites react with proteins and peptides, we studied the formation and structure of the resulting adducts from the incubation of model peptides with naphthalene epoxide, naphthalene diol epoxide, 1,2-naphthoquinone, and 1,4-naphthoquinone using high resolution mass spectrometry. Identification of the binding sites, relative rates of depletion of the unadducted peptide, and selectivity of binding to amino acid residues were determined. Adduction occurred on the cysteine, lysine, and histidine residues, and on the N-terminus. Monoadduct formation occurred in 39 of the 48 reactions. In reactions with the naphthoquinones, diadducts were observed, and in one case, a triadduct was detected. The results from this model peptide study will assist in data interpretation from ongoing work to detect peptide adducts in vivo as markers of biologic effect.

Application of the Hard and Soft, Acids and Bases (HSAB) theory to toxicant--target interactions

Many chemical toxicants and/or their active metabolites are electrophiles that cause cell injury by forming covalent bonds with nucleophilic targets on biological macromolecules. Covalent reactions between nucleophilic and electrophilic reagents are, however, discriminatory since there is a significant degree of selectivity associated with these interactions. Over the course of the past few decades, the theory of Hard and Soft, Acids and Bases (HSAB) has proven to be a useful tool in predicting the outcome of such reactions. This concept utilizes the inherent electronic characteristic of polarizability to define, for example, reacting electrophiles and nucleophiles as either hard or soft. These HSAB definitions have been successfully applied to chemical-induced toxicity in biological systems. Thus, according to this principle, a toxic electrophile reacts preferentially with biological targets of similar hardness or softness. The soft/hard classification of a xenobiotic electrophile has obvious utility in discerning plausible biological targets and molecular mechanisms of toxicity. The purpose of this perspective is to discuss the HSAB theory of electrophiles and nucleophiles within a toxicological framework. In principle, covalent bond formation can be described by using the properties of their outermost or frontier orbitals. Because these orbital energies for most chemicals can be calculated using quantum mechanical models, it is possible to quantify the relative softness (σ) or hardness (η) of electrophiles or nucleophiles and to subsequently convert this information into useful indices of reactivity. This atomic level information can provide insight into the design of corroborative laboratory research and thereby help investigators discern corresponding molecular sites and mechanisms of toxicant action. The use of HSAB parameters has also been instrumental in the development and identification of potential nucleophilic cytoprotectants that can scavenge toxic electrophiles. Clearly, the difficult task of delineating molecular sites and mechanisms of toxicant action can be facilitated by the application of this quantitative approach.

Molecular mechanisms underlying chemical liver injury

The liver is necessary for survival. Its strategic localisation, blood flow and prominent role in the metabolism of xenobiotics render this organ particularly susceptible to injury by chemicals to which we are ubiquitously exposed. The pathogenesis of most chemical-induced liver injuries is initiated by the metabolic conversion of chemicals into reactive intermediate species, such as electrophilic compounds or free radicals, which can potentially alter the structure and function of cellular macromolecules. Many reactive intermediate species can produce oxidative stress, which can be equally detrimental to the cell. When protective defences are overwhelmed by excess toxicant insult, the effects of reactive intermediate species lead to deregulation of cell signalling pathways and dysfunction of biomolecules, leading to failure of target organelles and eventual cell death. A myriad of genetic factors determine the susceptibility of specific individuals to chemical-induced liver injury. Environmental factors, lifestyle choices and pre-existing pathological conditions also have roles in the pathogenesis of chemical liver injury. Research aimed at elucidating the molecular mechanism of the pathogenesis of chemical-induced liver diseases is fundamental for preventing or devising new modalities of treatment for liver injury by chemicals.

Assessing the structure-activity relationships of fluorotelomer unsaturated acids and aldehydes with glutathione. Reactivity of glutathione with fluorotelomer unsaturated acids and aldehydes

Fluorotelomer alcohols (FTOHs) have been shown to degrade via abiotic and biotic mechanisms to perfluorocarboxylates (PFCAs) which are environmentally persistent and bioaccumulate in humans and wildlife depending on their chain length. Fluorotelomer unsaturated aldehydes (FTUALs) and acids (FTUCAs) are intermediate metabolites that form from the degradation of FTOHs. Their potential for toxicity is not yet defined and may be more significant compared to PFCAs. Past studies have shown that these intermediates form adducts with glutathione (GSH). The purpose of this study was to further assess the reactivity of these intermediate compounds. In vitro experiments were carried out in an aqueous buffer system (pH 7.4) where FTUCAs and FTUALs of varying chain lengths were reacted with GSH. To quantify the reactivity of FTUCAs and FTUALs, unreacted free GSH was derivatized with 5,5'-dithiobis(2-nitrobenzoic acid), its absorbance measured at 412 nm, and the percentage of unconjugated free GSH evaluated over time. EC50 values were obtained for the reactions of GSH with acrolein and methyl methacrylate to assess the accuracy of the method, as well as for acrylic acid, FTUCAs, and FTUALs. The results of this study indicated that α,β-unsaturated aldehydes are comparatively the most reactive and reaction with GSH may be influenced by the length of the fluorinated tail. This is the first study to examine the relationship of FTUCAs and FTUALs with biological nucleophiles by quantifying their intrinsic reactivity.

Assessment of reactive metabolites in drug-induced liver injury

The aim of the current review is to summarize present methods used for the determination of reactive metabolites, which can predict drug-induced liver injury (DILI) in drug discovery and development. DILI is one of the most frequent reasons for the withdrawal of an approved drug from the market, and it accounts for up to 50% of acute liver failure cases. This review is structured into three sections. The first section is a general overview of the relationship between drug metabolism and liver injury. The second section introduces in vitro methods for the assessment of reactive metabolites for drug discovery and development. In the third section, limitations and future directions for the development of methods for predicting DILI are described.

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Molecular mechanisms of 4-hydroxy-2-nonenal and acrolein toxicity: nucleophilic targets and adduct formation

Acrolein and 4-hydroxy-2-nonenal (HNE) are byproducts of lipid peroxidation and are thought to play central roles in various traumatic injuries and disease states that involve cellular oxidative stress, for example, spinal cord trauma, diabetes, and Alzheimer's disease. In this review, we will discuss the chemical attributes of acrolein and HNE that determine their toxicities. Specifically, these aldehydes are classified as type 2 alkenes and are characterized by an alpha,beta-unsaturated carbonyl structure. This structure is a conjugated system that contains mobile pi-electrons. The carbonyl oxygen atom is electronegative and can promote the withdrawal of mobile electron density from the beta-carbon atom causing regional electron deficiency. On the basis of this type of electron polarizability, both acrolein and HNE are considered to be soft electrophiles that preferentially form 1,4-Michael type adducts with soft nucleophiles. Proteomic, quantum mechanical, and kinetic data will be presented, indicating that cysteine sulfhydryl groups are the primary soft nucleophilic targets of acrolein and HNE. This is in contrast to nitrogen groups on harder biological nucleophiles such as lysine or histidine residues. The toxicological outcome of adduct formation is not only dependent upon residue selectivity but also the importance of the targeted amino acid in protein function or structure. In attempting to discern the toxicological significance of a given adduct, we will consider the normal roles of cysteine, lysine, and histidine residues in proteins and the relative merits of corresponding adducts in the manifestations of diseases or toxic states. Understanding the molecular actions of acrolein and HNE could provide insight into many pathogenic conditions that involve initial cellular oxidative stress and could, thereby, offer new efficacious avenues of pharmacological defense.

Molecular mechanisms of the conjugated alpha,beta-unsaturated carbonyl derivatives: relevance to neurotoxicity and neurodegenerative diseases

Conjugated alpha,beta-unsaturated carbonyl derivatives such acrylamide, acrolein, and 4-hydroxy-2-nonenal (HNE) are members of a large class of chemicals known as the type-2 alkenes. Human exposure through diet, occupation, and pollution is pervasive and has been linked to toxicity in most major organs. Evidence suggests that these soft electrophiles produce toxicity by a common mechanism involving the formation of Michael-type adducts with nucleophilic sulfhydryl groups. In this commentary, the adduct chemistry of the alpha,beta-unsaturated carbonyls and possible protein targets will be reviewed. We also consider how differences in electrophilic reactivity among the type-2 alkenes impact corresponding toxicokinetics and toxicological expression. Whereas these concepts have mechanistic implications for the general toxicity of type-2 alkenes, this commentary will focus on the ability of these chemicals to produce presynaptic damage via protein adduct formation. Given the ubiquitous environmental presence of the conjugated alkenes, discussions of molecular mechanisms and possible neurotoxicological risks could be important. Understanding the neurotoxicodynamic of the type-2 alkenes might also provide mechanistic insight into neurodegenerative conditions where neuronal oxidative stress and presynaptic dysfunction are presumed initiating events. This is particularly germane to a recent proposal that lipid peroxidation and the subsequent liberation of acrolein and HNE in oxidatively stressed neurons mediate synaptotoxicity in brains of Alzheimer's disease patients. This endogenous neuropathogenic process could be accelerated by environmental type-2 alkene exposure because common nerve terminal proteins are targeted by alpha,beta-unsaturated carbonyl derivatives. Thus, the protein adduct chemistry of the conjugated type-2 alkenes offers a mechanistic explanation for the environmental toxicity induced by these chemicals and might provide insight into the pathogenesis of certain human neurodegenerative diseases.

Time dependence in mixture toxicity with soft electrophiles: 1. Combined effects of selected SN2- and SNAr-reactive agents with a nonpolar narcotic

Frequently the toxicity of an organic chemical mixture is close to dose-additive, even when the agents are thought to induce toxicity at different molecular sites of action. These findings appear to conflict with the hypothesis that a strictly dose-additive combined effect will be observed for agents sharing a single molecular site of toxic action within the organism. In this study, several SN2-reactive (alpha-halogen) or S(N)Ar-reactive (halogenated dinitrobenzene) soft electrophiles were tested with a model nonpolar narcotic (NPN) to determine the toxicity of the combinations. A sham combination of the model NPN (3-methyl-2-butanone) was also tested as a positive control. The study design incorporated time-dependent toxicity (TDT) determinations at 15, 30, and 45 minutes using a Microtox (Vibrio fischeri) protocol that included testing seven duplicated concentrations for each single agent and mixture per combination. Additionally, in chemico reactivity was determined for each compound using thiol in glutathione as a model nucleophile. The model NPN alone lacked reactivity and TDT. The SN2-reactive agents individually showed varying levels of both reactivity and TDT alone, while the SNAr-reactive chemicals alone were reactive and had toxicity that was fully time-dependent between 15 and 45 minutes of exposure. Data analyses indicated that the sham combination was dose additive, as expected, whereas three of four SN2:NPN combinations showed effects close to that predicted for dose addition but with some differences. The fourth SN2:NPN combination, which included an alpha-halogen with full TDT, showed a less-than-dose-additive combined effect as did both of the SNAr:NPN pairings. By incorporating TDT values, shapes of the dose-response curves, chemical reactivity data with thiol, reactive mechanisms for the soft electrophiles, and quantitative structure activity relationship information on whether the toxicity of the individual soft electrophiles did or did not exceeded that predicted for baseline narcosis, the results suggested that the alpha-halogens elicited two toxic effects at the concentrations tested (reactivity and narcotizing effects), whereas toxicity induced by the halogenated dinitrobenzenes was essentially limited to reactive effects. Collectively, these results provide experimental evidence consistent with previous explanations as to why binary mixtures of industrial organic chemicals often show combined effects that are close to dose additive, even when the chemicals are thought to induce toxicity at different molecular sites of action.

Structure-Toxicity Analysis of Type-2 Alkenes: In Vitro Neurotoxicity

Acrylamide (ACR) is a conjugated type-2 alkene that produces synaptic toxicity presumably by sulfhydryl adduction. The alpha,beta-unsaturated carbonyl of ACR is a soft electrophile and, therefore, adduction of nucleophilic thiol groups could occur through a conjugate (Michael) addition reaction. To address the mechanism of thiol adduct formation and corresponding neurotoxicological importance, we defined structure-toxicity relationships among a series of conjugated type-2 alkenes (1 microM-10mM), which included acrolein and methylvinyl ketone. Results show that exposure of rat striatal synaptosomes to these chemicals produced parallel, concentration-dependent neurotoxic effects that were correlated to loss of free sulfhydryl groups. Although differences in relative potency were evident, all conjugated analogs tested were equiefficacious with respect to maximal neurotoxicity achieved. In contrast, nonconjugated alkene or aldehyde congeners did not cause synaptosomal dysfunction or sulfhydryl loss. Acrolein and other alpha,beta-unsaturated carbonyls are bifunctional (electrophilic reactivity at the C-1 and C-3 positions) and could produce in vitro neurotoxicity by forming protein cross-links rather than thiol monoadducts. Immunoblot analysis detected slower migrating, presumably derivatized, synaptosomal proteins only at very high acrolein concentrations (>or= 25 mM). Exposure of synaptosomes to high concentrations of ACR (1M), N-ethylmaleimide (10mM), and methyl vinyl ketone (MVK) (100mM) did not alter the gel migration of synaptosomal proteins. Furthermore, hydralazine (1mM), which blocks the formation of protein cross-links, did not affect in vitro acrolein neurotoxicity. Thus, type-2-conjugated alkenes produced synaptosomal toxicity that was linked to a loss of thiol content. This is consistent with our hypothesis that the mechanism of ACR neurotoxicity involves formation of Michael adducts with protein sulfhydryl groups.

Chemical mixture toxicity testing with Vibrio fischeri: combined effects of binary mixtures for ten soft electrophiles

The toxicity of 30 binary combinations of 10 soft electrophiles was examined in Microtox using dose-response curve (DRC) analysis. Chemicals from three groups of soft electrophiles-vinyl Michael acceptors (I--react with a thiol group), dicarbonyl reactive agents (II--react with a primary amine), and alpha-haloactivation compounds (III--react with a thiol group)--were selected for testing to evaluate the relationship between molecular site of chemical action and combined toxic effect. For each combination tested, each single agent was tested alone at six duplicated concentrations and three 1:1 mixtures of the agents were also tested, each at six duplicated concentrations. Exposure duration was 15 min for each single agent and mixture test. Sigmoid DRCs for each single chemical and mixture were constructed and the single chemical curves were used to develop a theoretical dose-addition DRC for the combination. Additivity quotient (AQ) values for slope and EC50 were calculated by dividing the actual mixture slope or EC50 for a given combination by the predicted slope or EC50, respectively, from the theoretical dose-addition DRC. Three criteria were selected for value in determining the combined effect obtained for each combination: (1) slope AQ 95% confidence interval (CI) overlap with 1.0 (1.0=dose addition), (2) EC50 AQ 95% CI overlap with 1.0, and (3) mean mixture data point 95% and 99% CI overlap with the theoretical dose-addition DRC. Each of three sham combinations showed combined effects consistent with dose addition for each criterion. Dose addition was expected for 15 nonsham combinations (nine within-group combinations and six group I:III combinations) and a nondose-additive effect was expected for 12 combinations (all I:II and II:III combinations). Actual combined effects obtained by incorporating all three criteria (noted above) showed only six instances of dose addition. Therefore, time-dependent toxicity (TDT) tests of each soft electrophile alone and for three nonpolar narcotic chemicals alone were conducted, using 15-, 30-, and 45-min exposure durations, to assess the time-dependent nature of the toxicity. Results of the TDT tests suggested that five had fully (or nearly fully) TDT (interpreted as an irreversible effect representing one molecular site of action), five of the soft electrophiles had partially TDT (i.e., representing two or more molecular sites of action for the agents, one irreversible and one reversible), and the three nonpolar narcotics had no TDT (i.e., a fully reversible toxic effect). With this TDT information, the combined effects for 25 of the 27 mixtures, although rather complex, could be explained. It is noteworthy that all combined effects obtained, whether concluded to be dose-additive or not, were close to dose-additive for hazard assessment purposes.

A conceptual framework for predicting the toxicity of reactive chemicals: modeling soft electrophilicity

Although the literature is replete with QSAR models developed for many toxic effects caused by reversible chemical interactions, the development of QSARs for the toxic effects of reactive chemicals lacks a consistent approach. While limitations exit, an appropriate starting-point for modeling reactive toxicity is the applicability of the general rules of organic chemical reactions and the association of these reactions to cellular targets of importance in toxicology. The identification of plausible "molecular initiating events" based on covalent reactions with nucleophiles in proteins and DNA provides the unifying concept for a framework for reactive toxicity. This paper outlines the proposed framework for reactive toxicity. Empirical measures of the chemical reactivity of xenobiotics with a model nucleophile (thiol) are used to simulate the relative rates at which a reactive chemical is likely to bind irreversibly to cellular targets. These measures of intrinsic reactivity serve as correlates to a variety of toxic effects; what's more they appear to be more appropriate endpoints for QSAR modeling than the toxicity endpoints themselves.

Monocrotaline pyrrole targets proteins with and without cysteine residues in the cytosol and membranes of human pulmonary artery endothelial cells

A single injection of monocrotaline produces a pulmonary insult in rats with a phenotype similar to human primary pulmonary hypertension. Although extensively used as a model, the mechanism(s) by which this chemical insult mimics a condition with genetic and environmental links remains an enigma, although formation of protein adducts has been implicated. Monocrotaline (MCT) is non-toxic and must undergo hepatic dehydrogenation to the soft electrophile monocrotaline pyrrole as prerequisite to damaging endothelial cells lining arterioles at remote pulmonary sites. In this report we extend our earlier investigation (J. Biol. Chem. 2000, 275, 29091-29099) by examining protein adducts to lower abundance adducts, a pI range not covered before, and subcellular localization of adduct-forming proteins associated with plasma membranes. Human pulmonary artery endothelial cells were exposed to [(14)C]MCT pyrrole (MCTP) and protein targets were identified using 2-DE with IPG 4-11. Adducted proteins were identified by pI, apparent molecular weight, and PMF using MALDI-TOF MS. Results of this study show that the majority of adducts form on proteins that contain reactive thiols in a CXXC motif, such as protein disulfide isomerase A(3) (ERp57), protein disulfide isomerase (PDI), and endothelial PDI. These same proteins were the major adduct-forming proteins associated with the plasma membrane. Other proteins found to be targets were thioredoxin, galectin-1, reticulocalbin 1 and 3, cytoskeletal tropomyosin, mitochondrial ATP synthase beta-chain, annexin A2 and cofilin-1. For the first time, MCTP adducts were observed on proteins not known to contain cysteine residues. However, known reactive proteins including nucleophosmin did not form detectable adducts, potentially indicating that MCTP did not reach the interior of nucleus to the same extent as other cellular sites. These findings suggest that molecular events underlying MCTP toxicity are initiated at the plasma membrane or readily accessible subcellular regions including the cytosol and membranes of the endoplasmic reticulum and mitochondria.

Non-enzymatic glutathione reactivity and in vitro toxicity: A non-animal approach to skin sensitization

The development of non-animal methods to predict the potential of chemicals to cause skin sensitization is of great importance. On the basis of many published studies into the underlying chemical mechanisms skin sensitization, the immunological priming which leads to the disease allergic contact dermatitis, is recognized as a reactive chemistry endpoint. Consequently, the combination of chemical assays with in vitro techniques may provide a useful surrogate to animal testing for skin sensitization. This study attempts to investigate the relationship between skin sensitization assessed in the local lymph node assay (LLNA) initially and a thiol reactivity index based on glutathione (GSH), pEC(50) thiol (EC(50) being defined as the concentration of the test substance which gives 50% depletion of free thiol under standard conditions) in combination with a measure of cytotoxicity (pIGC(50)) to Tetrahymena pyriformis (TETRATOX). The pEC(50) thiol values and the pIGC(50) values were determined for twenty-four compounds for which LLNA test data were available. Thiol reactivity was found to discriminate sensitizers from non-sensitizers according to the rule: pEC(50) thiol>-0.55 indicates that the compound will be a skin sensitizer. However, because of metabolic activation a pEC(50) thiol<-0.55 does not necessarily mean that the compound will be a non-sensitizer. Excess toxicity to T. pyriformis (i.e. the extent of toxic potency over that expected by non-polar narcosis) was determined in order to assess biological reactivity. The best discrimination based on excess toxicity in the TETRATOX assay was given by the "rule": excess toxicity>0.50 indicates that the compound will be a skin sensitizer. These approaches become more powerful when combined. When taken together, the thiol and TETRATOX assays predict the sensitization potential of 23 of the 24 compounds correctly. alpha-Hexylcinnamic aldehyde is incorrectly predicted to be a non-sensitizer, whereas LLNA results suggest it may be a weak sensitizer, this inaccuracy being rationalized in terms of its high hydrophobicity. Due to the selectivity of electro(nucleo)philic reactions some sensitizing compounds will not be identified using a single nucleophile such as thiol.

Identification of proteins adducted by reactive naphthalene metabolitesin vitro

Metabolic activation of inert chemicals to electrophilic intermediates has been correlated with the incidence and severity of cytotoxicity. The current studies have identified several proteins adducted by reactive metabolites of the lung toxicant, naphthalene. Proteins isolated from microsomal incubations of (14)C-naphthalene were separated by 2-DE, proteins were blotted to PVDF membranes and radioactive proteins were localized by storage phosphor analysis. Adducted proteins were isolated from complimentary gels and identified by peptide mass mapping. A total of 18 adducted proteins were identified including: protein disulfide isomerase precursor, ER-60 protease, alpha actin, mouse urinary proteins, and cytochrome b5 reductase. In supernatant fractions, protein disulfide isomerase, heat shock protein 70, and alpha-actin were key proteins to which reactive naphthalene metabolites were bound. All of the proteins adducted, with the exception of cytochrome b5 reductase were sulfhydryl rich. Although several of the proteins found to be adducted in these studies have also been shown to be adducted by other electrophiles, several others have not been reported as common targets of reactive metabolites. These studies provide a basis for both in situ and in vivo work designed to follow the fate and formation of reactive metabolite protein adducts.

Toxicity to T etrahymena and abiotic thiol reactivity of aromatic isothiocyanates

Toxicity (1/IGC(50)) in the Tetrahymena population growth assay and reactivity (1/EC(50)) with the thiol moiety of the cysteine residue of glutathione (GSH) were determined for a series of aromatic isothiocyanates (NCSs). Comparison of both toxicity and reactivity between the analogues revealed that derivatives with the NCS-moiety attached directly to an aromatic ring (e.g., phenyl derivatives) are less toxic and less reactive than those with the NCS attached to an aliphatic carbon (e.g., benzyl derivatives). These differences in potency are hypothesized to relate to difference in the ease of the Michael reaction, the proposed molecular mechanism. 1,4-Phenylene diisothiocyanate is more toxic and more reactive than its mono-NCS homologue. While there is good predictivity for the phenyl and naphthyl derivatives with the model log(1/IGC(50)) = 0.545(log K(ow)) + 16.21A(max) - 5.91, based on the 1-octanol/water partition coefficient (K(ow)) and maximum acceptor superdelocalizability (A(max)), toxicity of the other derivatives, which are outside the structural domain of the model training set, are poorly fitted. Owing to hydrolysis, the benzoyl, and cinnamyl analogues are less toxic than predicted by their thiol reactivity; however, the toxicity of the remaining compounds is modeled by the relationship log(1/IGC(50)) = 1.77 [log (1/EC(50))] + 0.60; n = 12, s = 0.34, r(2) = 0.718, q(2) = 0.629, F = 26.

Detoxification and transcriptome response in Arabidopsis seedlings exposed to the allelochemical benzoxazolin-2(3H)-one

Benzoxazolin-2(3H)-one (BOA) is an allelochemical most commonly associated with monocot species, formed from the O-glucoside of 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one by a two-step degradation process. The capacity of Arabidopsis to detoxify exogenously supplied BOA was analyzed by quantification of the major known metabolites BOA-6-OH, BOA-6-O-glucoside, and glucoside carbamate, revealing that detoxification occurs predominantly through O-glucosylation of the intermediate BOA-6-OH, most likely requiring the sequential action of as-yet-unidentified cytochrome P450 and UDP-glucosyltransferase activities. Transcriptional profiling experiments were also performed with Arabidopsis seedlings exposed to BOA concentrations, representing I(50) and I(80) levels based on root elongation inhibition assays. One of the largest functional categories observed for BOA-responsive genes corresponded to protein families known to participate in cell rescue and defense, with the majority of these genes potentially associated with chemical detoxification pathways. Further experiments using a subset of these genes revealed that many are also transcriptionally induced by a variety of structurally diverse xenobiotic compounds, suggesting they comprise components of a coordinately regulated, broad specificity xenobiotic defense response. The data significantly expand upon previous studies examining plant transcriptional responses to allelochemicals and other environmental toxins and provide novel insights into xenobiotic detoxification mechanisms in plants.

Protein targets of monocrotaline pyrrole in pulmonary artery endothelial cells

A single administration of monocrotaline to rats results in pathologic alterations in the lung and heart similar to human pulmonary hypertension. In order to produce these lesions, monocrotaline is oxidized to monocrotaline pyrrole in the liver followed by hematogenous transport to the lung where it injures pulmonary endothelium. In this study, we determined specific endothelial targets for (14)C-monocrotaline pyrrole using two-dimensional gel electrophoresis and autoradiographic detection of protein metabolite adducts. Selective labeling of specific proteins was observed. Labeled proteins were digested with trypsin, and the resulting peptides were analyzed using matrix-assisted laser desorption ionization mass spectrometry. The results were searched against sequence data bases to identify the adducted proteins. Five abundant adducted proteins were identified as galectin-1, protein-disulfide isomerase, probable protein-disulfide isomerase (ER60), beta- or gamma-cytoplasmic actin, and cytoskeletal tropomyosin (TM30-NM). With the exception of actin, the proteins identified in this study have never been identified as potential targets for pyrroles, and the majority of these proteins have either received no or minimal attention as targets for other electrophilic compounds. The known functions of these proteins are discussed in terms of their potential for explaining the pulmonary toxicity of monocrotaline.

Determination of glutathione S-transferase mu and theta polymorphisms in neurological disease

1. Correlations between deletions in two glutathione S-transferase (GST) genes, GSTM1 and GSTT1 and susceptibility to Alzheimer's disease (AD), motor neuron disease (MND) and Parkinson's disease (PD) have been investigated by PCR, using primers specific for both genes. 2. It was found that males with a deletion of the GSTM1 gene were more susceptible to PD and males with a deletion of the GSTT1 gene more susceptible to MND and PD, possibly implying that environmental factors which specifically target men may be involved. Furthermore, subjects with a deletion of the GSTT1 gene were more susceptible to AD.

Sodium cyanate alters glutathione homeostasis in rodent brain: relationship to neurodegenerative diseases in protein-deficient malnourished populations in Africa

Sodium cyanate, a neurotoxic chemical in rodents, primates and humans, is implicated in neurodegenerative disorders in protein-deficient populations subsisting in parts of Africa on the cyanogenic plant cassava. The molecular and cellular mechanisms of cyanate neurotoxicity are not understood. This study investigates the effect of sodium cyanate on glutathione (GSH) homeostasis in rodent brain and liver in vitro and in vivo. GSH levels in mouse brain were rapidly, time- and dose-dependently decreased following intraperitoneal administration of 100, 200 or 300 mg/kg sodium cyanate. By contrast, GSH disulfide (GSSG) levels were increased and GSH/GSSG ratios were decreased in a dose-dependent manner in rat brain. Sodium cyanate depleted GSH levels in all regions of mouse brain. Brain glutathione reductase activity was dose-dependently inhibited, while glutathione peroxidase activity was not affected by sodium cyanate. The disruption of GSH homeotasis, as evidenced by reduced tissue GSH/GSSG ratios, likely results from cyanate-induced inhibition of glutathione reductase activity. The results of this study suggest that cyanate neurotoxicity, and perhaps cassava-associated neurodegenerative diseases, are mediated in part by disruption of glutathione homeostasis in neural tissue.

Alpha-hydroxytamoxifen, a genotoxic metabolite of tamoxifen in the rat: identification and quantification in vivo and in vitro

The metabolic formation of a-hydroxytamoxifen, a reactive metabolite of tamoxifen in rat liver, was characterized and quantified in vitro (hepatic microsomal incubations) and in vivo (bile-duct cannulated animals). This minor metabolite was identified by chromatographic and mass spectral comparisons with the authentic compound. The rates of formation of alpha-hydroxytamoxifen in incubations (30 min) of tamoxifen (25 microM) with liver microsomal preparations from women (pool of six), female CD1 mice or female Sprague-Dawley rats, as quantified by liquid chromatography-mass spectrometry (LC-MS), were 1.15+/-0.03, 0.30+/-0.05 and 2.70+/-0.35 pmol/min/mg protein, respectively. Selective inhibition of microsomal P450 indicated that alpha-hydroxylation was catalysed predominantly by CYP3A in humans. Bile-duct cannulated and anaesthetized female rats and mice given [14C]tamoxifen (43 micromol/kg, i.v.) excreted, respectively, 24 and 21% of the administered radioactivity in bile over 5 and 3.5 h. The major radiolabelled biliary metabolite in rats, characterized by LC-MS after enzymic hydrolysis of conjugates, was the glucuronide of 4-hydroxytamoxifen (10% of dose) and only 0.1% of the dose was recovered as alpha-hydroxytamoxifen. After administration of alpha-hydroxytamoxifen (43 micromol/kg, i.v.) to rats, only 1.19% of the administered compound was recovered from a glucuronide metabolite in bile, indicating a possible 0.84% alpha-hydroxylation of tamoxifen in vivo. There was, however, no indication of the presence in bile of either O-sulphonate or glutathione conjugates derived from alpha-hydroxytamoxifen. This study shows for the first time that alpha-hydroxytamoxifen can be glucuronylated in rat liver. Whereas sulphonation results in electrophilic genotoxic intermediates, glucuronidation may represent a means of detoxifying alpha-hydroxytamoxifen.

Influence of quinone methide reactivity on the alkylation of thiol and amino groups in proteins: studies utilizing amino acid and peptide models

Quinone methides (QMs) are electrophiles formed in several biological processes including direct oxidations of 4-alkylphenols by cytochromes P450. These species may be responsible for the adverse effects of certain phenolic compounds through protein alkylation, but little information is available concerning specific targets or the resulting mechanisms of cell injury. The present goal was to determine the most likely sites of adduct formation among competing protein nucleophiles utilizing QMs of varying electrophilicity. Reactions of poorly reactive, moderately reactive, and highly reactive QMs, 2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone (BHT-QM), 6-tert-butyl-2-(2'-hydroxyl-1',1'-dimethylethyl)-4-methylene- 2,5-cyclohexadienone (BHTOH-QM), and 2-tert-butyl-6-methyl-4-methylene-2,5-cyclohexadienone (BDMP-QM), respectively, were investigated in aqueous solutions with nucleophilic amino acids. Each QM rapidly formed a thioether derivative of cysteine with little or no competition from the addition of water (hydration). The alpha-amino groups were the primary sites of alkylation for all other amino acids examined including lysine, histidine, tyrosine, and serine, and the pseudo-first order rates were 5 to 8-fold greater than the rates of hydration. Alkylation of the side chain nitrogens of lysine and histidine occurred at about one-fourth the rate of hydration for BDMP-QM, but no reaction was detectable for BHT-QM and no reactions occurred between QMs and amino acid hydroxyl groups. The results indicate that, based on chemical reactivity, peptide alkylation should occur in the order cysteine thiol > N-terminal amino > N epsilon-lysine = NIm-histidine, with side chain modifications occurring only with the more electrophilic QMs. Reactions of QMs with the tripeptide Gly-His-Lys confirmed the results with amino acids as N alpha-glycine alkylation predominated, but side chain adducts also formed with BHTOH-QM and BDMP-QM. Human hemoglobin was treated with QMs, hydrolyzed, and assayed by HPLC-thermospray mass spectrometry. This work revealed that N epsilon-lysine was the main alkylation site, emphasizing the importance of factors, in addition to chemical reactivity, which influence protein modification by electrophiles.

Intracellular glutathione content of urothelial cancer in correlation to chemotherapy response

We evaluated the possible correlation between intracellular glutathione (GSH) and drug sensitivity of urothelial cancer. Tissue GSH content of surgical specimens from 20 patients with urothelial cancer was assayed with high performance liquid chromatography (HPLC). GSH levels of cancer tissue (7.887 +/- 6.176 microM/mg protein) were significantly higher than GSH levels of normal mucosa (1.345 +/- 1.252 microM/mg). All patients having measurable lesions were then treated with methotrexate, epirubicin and cisplatin (MEC). These patients were classified into three groups according to clinical response criteria. GSH content in cancer tissue from four patients with complete response was 0.804 +/- 1.183 microM/mg protein. However, the cancer cells from patients with partial response and non-response contained a significantly higher level of GSH (6.295 +/- 2.459 (n = 8) and 12.955 +/- 6.141 microM/mg protein (n = 8), respectively). Intracellular glutathione content may play an important role in intrinsic resistance of urothelial cancer to MEC chemotherapy. It might be potentially used to predict drug sensitivity in urothelial cancer patients before starting chemotherapy.

Alkylation of 2'-deoxynucleosides and DNA by quinone methides derived from 2,6-di-tert-butyl-4-methylphenol

4-Alkylphenols, such as the antioxidant 2, 6-di-tert-butyl-4-methylphenol (BHT), exhibit toxicities that appear to be mediated by their oxidative metabolism to electrophilic quinone methides. Reactions of these Michael acceptors with simple nucleophiles and proteins have been reported, but little information is available on quinone methide binding to the competing nucleophilic sites in DNA. In the present investigation, 2'-deoxynucleoside adducts generated in vitro with two BHT-derived quinone methides, 2,6-di-tert-butyl-4-methylenecyclohexa-2,5-dienone and 6-tert-butyl-2- (2'-hydroxy-1',1'-dimethylethyl)-4-methylenecyclohexa-2,5-dieno ne (BHTOH-QM) were isolated and identified. Both quinone methides produced adducts at the 1- and N2-positions of deoxyguanosine (dG) and the N6-position of deoxyadenosine (dA). In addition, a labile adduct formed at the 7-position of dG, which degraded to the corresponding 7-alkylguanine derivative. Additional work was conducted with BHTOH-QM, the more reactive of the two quinone methides. This species also formed stable adducts at the N4-position of deoxycytosine (dC) and the 3-position of thymidine and formed a labile adduct at the 3-position of dC that underwent hydrolytic cleavage to regenerate dC. In mixtures of deoxynucleosides treated with [14C]BHTOH-QM, alkylation occurred primarily at the N2- and 7-positions of dG and the N6-position of dA and occurred secondarily at the 1-position of dG. Treatment of calf thymus DNA with this quinone methide yielded N6-dA and N2-dG adducts with the former predominating. The unstable 7-dG adduct was detected by analysis of the 7-alkylguanine product from depurination. These results demonstrate that quinone methides are most likely to damage DNA through alkylation of the exocyclic amino groups of purine residues and possibly also by attack at the 7-position of dG followed by depurination.

Roles of electrophilic sulfuric acid ester metabolites in mutagenesis and carcinogenesis by some polynuclear aromatic hydrocarbons

Hydroxylation of meso-methyl groups with subsequent formation of reactive benzylic esters bearing a good leaving group (e.g. sulfate) was proposed as a possible biochemical mechanism of activation and tumorigenicity of methyl-substituted polycyclic aromatic hydrocarbons (PAHs). In support of this postulation, recent studies have demonstrated the formation by rodent hepatic sulfotransferase activity of electrophilic, mutagenic, and carcinogenic sulfuric acid esters of several hydroxymethyl aromatic hydrocarbons including hydroxymethyl derivatives of benz[a]anthracene, 6-hydroxymethylbenzo[a]pyrene, 5-hydroxymethylchrysene, 9-hydroxymethyl-10-methylanthracene, and 1-hydroxymethylpyrene. Besides these hydroxymethyl PAHs containing a primary benzylic alcoholic group, some aromatic hydrocarbons with secondary benzylic hydroxyl functional group(s) are also metabolically activated through sulfuric acid esterification.

Prediction and monitoring of the carcinogenicity of polycyclic aromatic compounds (PACs)

Chemical carcinogenesis is a multistage process that includes initiation, promotion, and progression. Some carcinogenic PACs have been shown to activate proto-oncogenes and deactivate tumor-suppression genes in the carcinogenic process. The function of DNA repair processes appears to be changed in some cases by PACs. Many PACs are well known for their carcinogenic activity, but for this activity to be exerted, metabolic activation by microsomal enzymes must occur. The enzyme system responsible for PAC activation is the mixed-function oxidase system and, in particular, cytochrome P-450. In the case of PAHs, oxidation predominantly produces reactive diol-epoxides that can then be converted to carbonium ions as the reactive electrophiles that can then covalently bind to DNA. Regions of high activity exist in PAHs, namely, the "bay," "K," and "L" regions which are associated with pi electron distribution. The diol-epoxides can exist in either syn or anti forms, each of which has two enantiomers producing four stereoisomers in all. Energy considerations favor the formation of the anti form. Nitrogen-containing PACs can be metabolically activated in a manner similar to that for PAHs, or the nitrogen atom can be oxidized to form hydroxylamines. These reactive electrophiles can then form covalently bound DNA adducts. The monitoring of DNA adducts has been used in risk assessment for human exposure to PACs. This form of biomonitoring has advantages over the monitoring of external exposure or body levels of the chemicals in question. In the case of PACs, binding to DNA is an important step in the multistage carcinogenic process. The estimation of DNA adducts has been used in the monitoring of humans exposed to PAHs in a wide range of industrial situations. Recent research has shown a dose-response relationship between PAH adduct levels and human cancer, thus developing molecular epidemiology as a relevant science for the field of risk assessment. Techniques have been developed for the determination of DNA adducts and these include immunochemical, fluorescence spectroscopic, GC-MS, and 32P-postlabeling methods. The 32P-postlabeling assay is by far the most sensitive, with limits of detection being of the order of one adduct in 10(10) normal nucleotides. The use of HPLC for separation of adducted nucleotides in this postlabeling assay is becoming more common and gives better resolution of adducts than does the TLC technique used in the traditional assay. The detection of adducts on hemoglobin and other proteins has been used as a surrogate for DNA adduct estimation.(ABSTRACT TRUNCATED AT 400 WORDS)