Electrophilic metabolites

Sun, Y. Dwyer-Nield, L. D. Malkinson, A. M. Zhang, Y. L. Thompson,J. A. Responses of tumorigenic and non-tumorigenic mouse lung epithelial cell lines to electrophilic metabolites ofthe tumor promoter butylated hydroxytoluene. Chem.-Biol. Interact. 2003, 145, 41-51. 

Meier, B. W. Gomez, J. D. Kirichenko, O. V. Thompson, J. A. Mechanistic basis for inflammation and tumor promotion in lungs of 2,6-di-tert-butyl-4-methylphenol-treated mice electrophilic metabolites alkylate and inactivate antioxidant enzymes. Chem. Res. Toxicol. 2007, 20, 199-207. 

In this review, the chemical properties of these electrophilic metabolites (i-XIIl) are discussed in terms of their metabolic formation and reactivity with nucleophiles, solvolysis and redox characteristics, reaction mechanisms, and their role as ultimate carcinogenic metabolites. 

Nocerini MR, Carlson JR, Yost GS. Electrophilic metabolites of 3-methylindole as toxic intermediates in pulmonary oedema. Xenobiotica 1984 14(7) 561—564. 

A rare but serious event that can result from irreversible CYP inhibition is the development of a hypersensitivity reaction. The bioactivation of a drug and the formation of a covalent adduct between the activated substrate and the enzyme can lead to hapten formation and eventually to an idiosyncratic autoimmune response (usually in the form of autoimmune hepatitis) [14]. The hapten formation is the first key step toward the autoimmune response. The CYP macromolecule is made immunogenic ( foreign ) by the covalent binding of the electrophilic metabolites, and the immune reaction follows with the production of autoantibodies against the target molecule (not necessarily alkylated). 

In conclusion, it is clear that a variety of stereoelectronic (internal) factors and external conditions favor a substantial positive charge in the transition state of diol epoxides as they undergo hydration or react with nucleophiles [115-118], Interpreting the reactivity of diol epoxides (or of numerous other electrophilic metabolites) in terms of toxification vs. detoxification is particularly difficult since toxicity depends as much on the nature of the endogenous nucleophile as on the intrinsic reactivity of the metabolites. 

Fig. 8.4 Schematic showing relative softness and hardness of nucleophiles and electrophiles as an indicator of sites of reaction of electrophilic metabolites.

Since biological systems are rich in nucleophiles (DNA, proteins, etc.) the possibility that electrophilic metabolites may become irreversibly bound to cellular macromolecules exists. Electrophiles and nucleophiles are classified as hard or soft depending on the electron density, with hard electrophiles generally having more intense charge localization than soft electrophiles in which the charge is more diffuse. Hard electrophiles tend to react preferentially with hard nucleophiles and soft electrophiles with soft nucleophiles. 

As well as the formation of radicals, oxidative stress may also contribute to the toxicity of electrophilic metabolites. This may result from depletion by electrophiles of the glutathione pool, one of the major defenses against damage by reactive oxygen species. 

Metabolism of the anti-malarial amodiaquine provides quinone-imine, which is an electrophilic metabolite responsible for hepatotoxicity and agranulocytosis. These side effects have severely restricted the clinical use of amodiaquine. The replacement of the phenolic hydroxyl by a fluorine prevents from oxidation process. Then, the A/-dealkylation becomes the major process. This has led to further refinements, with the preparation of the A/-f-butyl analogue, a compound which resists towards metabolic side-chain cleavage and has an excellent in vitro and in vivo profile (Fig. 15) [56]. 

The metabolism of oleftnic or acetylenic substances to electrophilic metabolites is greatly reduced when the oleftnic or acetylenic moieties are not terminal, or are terminal but contain alkyl substituents on the allylic and propargylic positions. 1-Heptene, for example, is bioactivated to electrophilic metabolites whereas 3-hexene, 2-methyl-l-hepteneand3,3-dimethyl-l-hexenearenot [36,37]. Similarly, 1-decyneismetabolizedto electrophilic metabolites whereas 3- and 5-decynes are essentially not [36, 37]. 

Isosteric substitution of the C-2 hydrogen atom of valproic acid (12) with a fluorine atom affords 2-fluorovalproic acid (22), which causes significantly less hepatoxicity than valproic acid, although a reduction in anticonvulsant properties is also observed [59, 60]. The hepatoxicity of 12 involves hepatic cytochrome P450-mediated metabolism to its 4-ene metabolite (14), which undergoes further metabolism, specifically mitchondrial (3-oxidation, to provide ( )-2-propyl-2,4-pentadienoic add (23), a reactive electrophilic metabolite [59, 60]. 

For compounds that require metabolic activation, resonance stabilization of the electrophilic metabolites is important. This is because resonance stabilization... 

Introduction of substituent(s) adjacent to the electrophilic group, or of bulky substituent(s) on the molecule of attachment, will minimize its carcinogenic/ mutagenic potential. This is because the nature and position of substituents may influence electronic or steric effects on the bioactivation and reactivity/stability of many ultimate electrophilic metabolites. 

For compounds that require metabolic activation, avoiding structural features that may provide resonance stabilization of electrophilic metabolites (e.g., conjugated double bonds, or conjugated system/aryl moiety) will decrease the lifetime of the reactive intermediates. 

Barbin, A., Planche, G, Croisy, A., Malaveille, C. Bartsch, H. (1977) Detection of electrophilic metabolites of halogenated olefins with 4-(4-nitrobenzyl)pyridine (NBP) or with Salmonella typhimurium (Abstract), In 2nd International Conference on Environmental Mutagens, Edinburgh, July 1977,p. 59... 

Dowsley, T.F., Forkcrl, P.G, Benesch, L.A. Bolton, J.L. (1995) Reaction of glutathione with the electrophilic metabolites of 1,1-dichloroethylene. Chem.-biol. Interact., 95, 227-244... 

Research over the past three decades in the field of biological reactive intermediates has provided a wealth of information on the functional groups that may be converted by either phase I or phase II enzymes to electrophilic metabolites... 

Heme modification by the products of peroxidase catalysis has been observed with peroxidases other than HRP, but it does not occur with all peroxidases. Some peroxidases are resistant to these types of reactions. In particular, the mammalian peroxidases are resistant to heme modification by both the free radical and electrophilic metabolites [63]. This resistance is due, at least in part, to the covalent bonds that link the heme to the mature protein. A similar resistance to modification by the HOBr produced by HRP is observed when the reaction is carried out with the F41E mutant in which a covalent bond to the heme has been introduced [65]. However, resistance to radical products can occur even without the presence of covalent links between the heme and the protein. Thus, LiP has a heme that is resistant to modification by phenylhydrazine or azide, although the protein is apparently inactivated by modifications of the protein [66]. 

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