Iron-stimulated ring-opening of benzene in a mouse liver microsomal system: Mechanistic studies and formation of a new metabolite

Abstract

In the present study, we investigated the mechanism(s) of ring-opening of benzene in a mouse liver microsomal system in the presence of Fe 2+. HPLC analysis based on coelution with authentic standards and on-line UV spectra obtained using a diode array detector indicated that benzene is metabolized to phenol, hydroquinone (HQ), trans,trans-muconaldehyde (muconaldehyde, MUC), 6-oxo- trans,trans-2,4-hexadienoic acid (COOHMCHO), 6-hydroxy- trans,trans-2,4-hexadienal (CHOMOH), and 6-hydroxy- trans,trans-2,4-hexadienoic acid (COOHMOH). CHOMOH was confirmed by mass spectrometry. Muconaldehyde was also metabolized to CHOMOH, COOHMCHO and COOHMOH, in the same microsomal system. The inhibition of muconaldehyde metabolism by microsomes in the presence of pyrazole indicates that there is cytosolic alcohol dehydrogenase (ADH) activity in the microsomes. Metabolism by contaminating ADH of muconaldehyde formed during microsomal incubation of benzene could be involved in the formation of CHOMOH and COOHMOH The ring-opening of benzene was stimulated by added Fe 2+. Hydrogen peroxide was produced in the microsomal system and consumed in the presence of added Fe 2+. Addition of catalase inhibited the formation of ring-opened products, while superoxide dismutase increased their formation in the presence of azide. Singlet oxygen scavengers, i.e. histidine, deoxyguanosine, Tris and azide (at concentrations above 1.0 mM), dramatically decreased the ring-opening of benzene. Hydroxyl radical scavengers, DMSO, mannitol and formate, but not ethanol, also decreased the ring-opening of benzene. The data indicate that Fenton chemistry plays an important role in benzene ring-opening by microsomes. An unknown peak with UV absorption maxima at 275 and 345 nm was also detected. Based on pH sensitivity of the UV spectrum, the reactivity with thiobarbituric acid (giving a chromogen with absorption maximum at 532 nm) and the molecular weight (126), this compound was identified tentatively as α- or β-hydroxymuconaldehyde.