Quinone Methide Formation from Para Isomers of Methylphenol (Cresol), Ethylphenol, and Isopropylphenol: Relationship to Toxicity

Abstract

The oxidative metabolism and toxicity of the para isomers of methylphenol (cresol), ethylphenol, and isopropylphenol were studied using male Sprague-Dawley rat liver microsomes and precision-cut liver slices. Reactive intermediates from each compound were trapped using radiolabeled glutathione and were detected and quantified by HPLC. Conjugates were collected and their structures determined by fast atom bombardment mass spectrometry and proton nuclear magnetic resonance. During microsomal incubations each test compound formed monoglutathione conjugates with structures which are consistent with the formation of quinone methide intermediates. In each case the glutathione moiety was attached to the benzylic carbon on the alkyl side chain of the phenol. With ethylphenol, which has a prochiral benzylic carbon, two isomeric conjugates were detected. The rate of formation of the glutathione conjugates in liver slice incubations was 4-isopropylphenol > 4-ethylphenol > 4-methylphenol. This correlated with the toxicity of the three compounds in liver slices. At equimolar concentrations 4-isopropylphenol was the most toxic while 4-methylphenol was the least toxic. Depletion of intracellular glutathione was observed in the presence of each test compound which preceded cell death. Enhancement of cellular thiol levels with N-acetylcysteine protected cells from the toxic effects of all three compounds as did inhibition of cytochrome P450 activity with metyrapone. These results suggest the formation of quinone methide intermediates from three alkylphenols during oxidative metabolism and demonstrate a correlation between the amount of reactive intermediate formed and toxicity observed in liver slices.

Pharmacometabonomic identification of a significant host-microbiome metabolic interaction affecting human drug metabolism

“…We envisage that, by depleting hepatic sulfonation capacity, continual exposure to colonically produced p-cresol would leave the liver more vulnerable to acetaminophen-induced damage and that markedly increased p-cresol production could potentially explain the reported association between fasting and an increased likelihood of hepatotoxicity from acetaminophen (42,43). However, in principle, sustained prior exposure to colonically produced p-cresol could also potentially increase acetaminophen hepatotoxicity by other means, such as by enzyme induction or glutathione depletion (44)(45)(46), and preliminary data (SI Text) suggest that high p-cresol exposure might lead to a more generalized impairment of sulfurdependent reactive metabolite detoxification, with PAPS depletion possibly leading to depletion of both taurine and glutathione. However, it remains to be investigated if our present finding has any significance for adverse reactions to acetaminophen.…”

Scientific Opinion on Flavouring Group Evaluation 22, Revision 1 (FGE.22Rev1): Ring‐substituted phenolic substances from chemical groups 21 and 25

“…In both cases this is reflected in lower cytotoxicity of the respective parent phenol (Thompson et al, 1995b). If concentrations of the quinone methide intermediates are insufficient to deplete intracellular GSH, they may be detoxicated via conjugation with this scavenger (Thompson et al, 1994;Thompson et al, 1995a;Thompson et al, 1995b). No data have been submitted to describe or support the metabolism of 2,6-dimethoxy- .061] with respect to the vinyl ring substituent.…”

Opinion of the Scientific Panel on food additives, flavourings, processing aids and materials in contact with food (AFC) related to Flavouring Group Evaluation 22 (FGE.22): Ring‐substituted phenolic substances from chemical groups 21 and 25 (Commission Regulation (EC) No 1565/2000 of 18 July 2000)

“…Outside this range, the quinone methide either reacts with the solvent water rather than the target biomacromolecules, or has only low reactivity towards cellular nucleophiles. In both cases this is reflected in lower cytotoxicity of the respective parent phenol (Thompson et al, 1995b). If concentrations of the quinone methide intermediates are insufficient to deplete intracellular GSH, they may be detoxicated via conjugation with this scavenger (Thompson et al, 1994;Thompson et al, 1995a;Thompson et al, 1995b).…”