Modulation of quinol/quinone-thioether toxicity by intramolecular detoxication

The chemical biology of naphthoquinones and its environmental implications

Quinones are a group of highly reactive organic chemical species that interact with biological systems to promote inflammatory, anti-inflammatory, and anticancer actions and to induce toxicities. This review describes the chemistry, biochemistry, and cellular effects of 1,2- and 1,4-naphthoquinones and their derivatives. The naphthoquinones are of particular interest because of their prevalence as natural products and as environmental chemicals, present in the atmosphere as products of fuel and tobacco combustion. 1,2- and 1,4-naphthoquinones are also toxic metabolites of naphthalene, the major polynuclear aromatic hydrocarbon present in ambient air. Quinones exert their actions through two reactions: as prooxidants, reducing oxygen to reactive oxygen species; and as electrophiles, forming covalent bonds with tissue nucleophiles. The targets for these reactions include regulatory proteins such as protein tyrosine phosphatases; Kelch-like ECH-associated protein 1, the regulatory protein for NF-E2-related factor 2; and the glycolysis enzyme glyceraldehyde-3-phosphate dehydrogenase. Through their actions on regulatory proteins, quinones affect various cell signaling pathways that promote and protect against inflammatory responses and cell damage. These actions vary with the specific quinone and its concentration. Effects of exposure to naphthoquinones as environmental chemicals can vary with the physical state, i.e., whether the quinone is particle bound or is in the vapor state. The exacerbation of pulmonary diseases by air pollutants can, in part, be attributed to quinone action.

The cytoprotective effect of N-acetyl-L-cysteine against ROS-induced cytotoxicity is independent of its ability to enhance glutathione synthesis

2,3,5-Tris(glutathion-S-yl)-hydroquinone (TGHQ), a metabolite of hydroquinone, is toxic to renal proximal tubule epithelial cells. TGHQ retains the ability to redox cycle and create an oxidative stress. To assist in elucidating the contribution of reactive oxygen species (ROS) to TGHQ-induced toxicity, we determined whether the antioxidant, N-acetyl-L-cysteine (NAC), could protect human kidney proximal tubule epithelial cells (HK-2 cell line) against TGHQ-induced toxicity. NAC provided remarkable protection against TGHQ-induced toxicity to HK-2 cells. NAC almost completely inhibited TGHQ-induced cell death, mitochondrial membrane potential collapse, as well as ROS production. NAC also attenuated TGHQ-induced DNA damage and the subsequent activation of poly (ADP-ribose) polymerase and ATP depletion. Moreover, NAC significantly attenuated c-Jun NH2-terminal kinase and p38 mitogen-activated protein kinase phosphorylation induced by TGHQ. In contrast, NAC itself markedly increased extracellular regulated kinase1/2 (ERK1/2) activation, and the upstream mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor, PD-98059, only partially inhibited this activation, suggesting that NAC can directly activate ERK1/2 activity. However, although NAC is frequently utilized as a glutathione (GSH) precursor, the cytoprotection afforded by NAC in HK-2 cells was not a consequence of increased GSH levels. We speculate that NAC exerts its protective effect in part by directly scavenging ROS and in part via ERK1/2 activation.

Metabolic activation of a pentafluorophenylethylamine derivative: Formation of glutathione conjugatesin vitroin the rat

The aim was to investigate the metabolic activation potential of a pentafluorophenylethylamine derivative (compound I) in vitro in the rat and to identify the cytochrome P450 (CYP) enzymes that catalyse these metabolic activation processes. Reduced glutathione (GSH) was fortified in rat hepatocytes and liver microsomes to trap possible reactive intermediates. Four glutathione conjugates (M1-4) were identified by LC-MS(n) following incubation of compound I in GSH-enriched rat hepatocytes and liver microsomes. Three of these conjugates (M2-4) have not been reported previously for pentafluorophenyl derivatives. Elemental composition analysis of these conjugates was obtained using high-resolution quadrupole time-of-flight mass spectrometry. The formation of GSH conjugate M1 was rationalized as a direct nucleophilic replacement of fluoride by glutathione, whereas the formation of the GSH conjugates M2-4 was proposed to occur by NADPH-dependent metabolic activation of the pentafluorophenyl ring via arene oxide, quinone and/or quinoneimine reactive intermediates. Formation of these conjugates was enhanced three- to five-fold in liver microsomes obtained from phenobarbital- and dexamethasone-treated rats. In incubations with pooled rat liver microsomes and recombinant rat CYP3A1 and CYP3A2, troleandomycin (TAO) reduced the formation of GSH conjugates M2-4 by 80-90%, but it had no effect on the formation of M1. Incubation of compound I with rat supersomes indicated that only CYP3A1 and CYP3A2 were capable of mediating these metabolic activation processes.

Characterization of Glutathione Conjugates of the Remoxipride Hydroquinone Metabolite NCQ-344 Formed in Vitro and Detection following Oxidation by Human Neutrophils

Remoxipride is an atypical antipsychotic displaying selective binding to the dopamine D2 receptor. Several cases of aplastic anemia led to the withdrawal of remoxipride from the market in December 1993. The remoxipride metabolite NCQ-344 is a hydroquinone while the structural isomer NCQ-436 is a catechol, both of which have been suggested to be capable of forming a reactive para- and ortho-quinone, respectively. Recently, these two remoxipride metabolites were shown to induce apoptosis in human bone marrow progenitor cells. Furthermore, NCQ-344 also caused necrosis of these cells unlike NCQ-436. Although NCQ-344 has been detected in plasma of humans dosed with remoxipride, to date, no experimental evidence for the formation of the corresponding para-quinone has been obtained. Here, we report the detection of three glutathione (GSH) conjugates of NCQ-344 in vitro that were formed following a chemical reaction and characterized by tandem mass spectrometry and for a cyclized conjugate additionally with derivatization and deuterium exchange. In contrast, NCQ-436 did not form a GSH conjugate. Hypochlorous acid oxidized NCQ-344 to the para-quinone while NCQ-436 was resistant to oxidation. Upon incubation with NCQ-344, stimulated human neutrophils produced from 2- to 5-fold greater amounts of glutathione conjugates than unstimulated neutrophils. Ab initio calculations on these remoxipride metabolites indicated that the reaction leading to the respective quinone was spontaneous for the para-quinone (e.g., from NCQ-344) while ortho-quinone (e.g., from NCQ-436) formation was not. These results demonstrate that NCQ-344 is capable of facile formation of a reactive para-quinone capable of reacting with GSH and may rationalize previous findings regarding the biological effects observed in vitro with these two remoxipride metabolites.

Quenching of quercetin quinone/quinone methides by different thiolate scavengers: stability and reversibility of conjugate formation

Oxidation of flavonoids with a catechol structural motif in their B ring leads to formation of flavonoid quinone/quinone methides, which rapidly react with GSH to give reversible glutathionyl flavonoid adducts. Results of the present study demonstrate that as a thiol-scavenging agent for this reaction Cys is preferred over GSH and N-acetylcysteine. The preferential scavenging by Cys over GSH reported in the present study appeared not to provide a basis for detection of thiol-based flavonoid conjugates in biological systems. This is because physiological concentrations of GSH are substantially higher than those of Cys, which was shown to shift the balance of thiol conjugate formation in favor of glutathionyl adduct formation. Furthermore, the cysteinyl quercetin adducts, although not showing the reversible nature of the glutathionyl conjugates, appeared nevertheless to be unstable. Thus, as a biomarker for formation of reactive quercetin quinone/quinone methides in biological systems, detection of the glutathionyl conjugates or the N-acetylcysteinyl conjugates derived from them should still be the method of choice. At GSH levels that dominate the level of other cellular thiol groups, covalent addition of the quinone to other cellular thiol groups may be efficiently prevented. However, various tissues are known to contain higher levels of protein-bound sulfhydryl moieties than of nonprotein sulfhydryl groups, the latter consisting of especially GSH. Thus, the results of the present study indicate that in biological systems covalent addition of quercetin quinone methide to tissue protein sulfhydryl groups can be expected. The transient nature of these adducts, as shown for all three types of thiol quercetin adducts in the present study, will, however, also result in a transient nature of the protein-bound quercetin adducts to be expected. Because stability of the various thiol quercetin adducts appeared a matter of minutes to hours instead of days, this rapid transient nature of possible quercetin quinone methide adducts may also restrict the ultimate toxicity to be expected from the quercetin quinone/quinone methides.

Ring addition of the alpha-amino group of glutathione increases the reactivity of benzoquinone thioethers

2-(Glutathion-S-yl)-1,4-benzoquinone was found to be remarkably unstable in phosphate buffer (pH 7.4) even in the absence of oxygen. Intramolecular addition of the alpha-amino group of the glutamate residue to the quinone ring yielded ultimately 2,3-(glutathion-N, S-yl)-1,4-benzoquinone and 2,6-(glutathion-N,S-yl)-1,4-benzoquinone in a 3:1 ratio along with 2-(glutathion-S-yl)-1,4-hydroquinone. Kinetic studies indicated that the cyclization reactions proceeded at a rate k1 of 0.093 min-1, while intermolecular reactions followed a second-order kinetics with a k2 of 94 M-1 min-1 (pH 7.4, 37 degreesC), resulting in multiple polymerization products. Both intramolecular amino adducts of 2-(glutathion-S-yl)-1,4-benzoquinone are prone to hydrolysis, leading to the insertion of an additional OH group in the ring. These S-substituted trihydroxybenzene derivatives are particularly susceptible to autoxidation. The model compound 6-(N-acetylcystein-S-yl)-2-hydroxy-1,4-hydroquinone was shown to form readily two atropoisomeric biphenyls upon autoxidation: 2,4'-bis(N-acetylcystein-S-yl)-2',3,3',4,6, 6'-hexahydroxybiphenyl, indicating C-C coupling, presumably via semiquinone radical intermediates. Thus, the sequence of glutathione S-addition, followed by oxidation, N-addition, oxidation, and hydrolysis, constitutes a novel and very effective activation pathway of quinones for eliciting oxidative stress. These data underline the fact that glutathione conjugates of autoxidizable aromatics are no obligatory stable end products of a detoxication reaction. The possible toxicological impacts of intra- and intermolecular addition reactions of quinoid thiol conjugates are discussed.

Dialkylquinonimines validated as in vivo metabolites of alachlor, acetochlor, and metolachlor herbicides in rats

The oncogenicity of chloroacetanilide herbicides (1-5) is proposed to involve bioactivation to 2,6-dialkylbenzoquinonimines (quinonimines, 9) based on two earlier observations: (1) in vitro conversion of the alachlor (1) metabolite diethylaniline (7Et2) to 2, 6-diethylquinonimine (9Et2) which reacts readily with GSH and (2) induction of sister chromatid exchanges in human lymphocytes for the parent herbicides and their purported 9 metabolites. This hypothesis lacks in vivo evidence for 9 formation which might be provided by analysis of urine and tissue for thiol adducts of 9. Accordingly, two mercapturates (10Et2 and 10Me2) and a cysteine conjugate (11Me2) were prepared by addition of N-acetylcysteine or cysteine to 9Et2 and the 2,6-dimethyl homologue (9Me2). Mercapturate 10Et2 was characterized by HPLC using the urine of rats treated ip with hydroxyaniline 8Et2, and both mercapturate 10Me2 and cysteine conjugate 11Me2 were found in the urine of mice administered hydroxyaniline 8Me2. The mercapturates were then converted to the N, N-dimethyl-2,6-dialkyl-4-methoxy-3-(methylthio)anilines (12Et2, 12EtMe, and 12Me2) by alkaline permethylation, thereby providing a method for analysis of 9-derived thiol adducts in urine and liver as the 12 derivatives by GC/MS with selected ion monitoring. The urine of rats 0-6 h after ip treatment with 1, butachlor (2), acetochlor (3), metolachlor (4), and dimethachlor (5) at 0.74 mmol/kg yields permethylated derivatives which are definitively diagnostic for the 9 intermediates from each of the herbicides in amounts of 3-24-fold above the minimum detectable limit, as well as 1 and 2 orders of magnitude higher from the corresponding anilines (7) and hydroxyanilines (8), respectively. Similar liver analyses reveal tissue thiol adducts of 9 6 h after treatment with 7 and 8 but not with the parent herbicides. The yields of urinary 9 derivatives from the parent herbicides are higher from the 2,6-diethyl series (1 and 2) and the 2-ethyl-6-methyl derivatives (3 and 4) than from the 2, 6-dimethyl analogue (5). These findings provide direct evidence in vivo that quinonimines are metabolites of 1-5 in rats.