QUINONE METHIDE FORMATION FROM PARA ISOMERS OF METHYLPHENOL (CRESOL),ETHYLPHENOL, AND ISOPROPYLPHENOL - RELATIONSHIP TO TOXICITY

The oxidative metabolism and toxicity of the para isomers of methylphe nol (cresol), ethylphenol, and isopropylphenol were studied using male Sprague-Dawley rat liver microsomes and precision-cut liver slices. R eactive intermediates from each compound were trapped using radiolabel ed glutathione and were detected and quantified by HPLC. Conjugates we re collected and their structures determined by fast atom bombardment mass spectrometry and proton nuclear magnetic resonance. During micros omal incubations each test compound formed monoglutathione conjugates with structures which are consistent with the formation of quinone met hide intermediates. In each case the glutathione moiety was attached t o the benzylic carbon on the alkyl side chain of the phenol. With ethy lphenol, which has a prochiral benzylic carbon, two isomeric conjugate s were detected. The rate of formation of the glutathione conjugates i n liver slice incubations was 4-isopropylphenol > 4-ethylphenol > 4-me thylphenol. This correlated with the toxicity of the three compounds i n liver slices. At equimolar concentrations 4-isopropylphenol was the most toxic while 4-methylphenol was the least toxic. Depletion of intr acellular glutathione was observed in the presence of each test compou nd which preceded cell death. Enhancement of cellular thiol levels wit h N-acetylcysteine protected cells from the toxic effects of all three compounds as did inhibition of cytochrome P450 activity with metyrapo ne. These results suggest the formation of quinone methide intermediat es from three alkylphenols during oxidative metabolism and demonstrate a correlation between the amount of reactive intermediate formed and toxicity observed in liver slices.