Toxicology and Applied Pharmacology

Effect of Ascotbate on Covalent Binding of Benzene and Phenol Metabohtes to Isolated Tissue Preparations. SMART, R. C. and ZANNONI, V. G. (1985). Toxicol. Appl. Pharmacol. 77, 334-343. [‘4C]Phenol and [‘4C]benzene are metabolized in the presence of NADPH and hepatic microsomes isolated from phenobarbitalor benzene-pretreated or untreated guinea pigs to intermediates capable of covalently binding to microsomal protein. When 1 mM ascorbate was included in the incubation mixture containing benzene as the substrate, covalent binding was inhibited by 55%. Increasing the ascorbate concentration to 5 mM inhibited binding by only an additional 17%. In contrast, when phenol was used as the substrate, 1 mM ascorbate inhibited binding by 95%. When DTdiaphorase was included in the incubation mixture containing benzene as the substrate, binding was inhibited by only 18%. This degree of inhibition is in contrast to 70% inhibition with phenol. These results indicate that different metabolites are responsible for a portion of the covalent binding depending upon the substrate employed. GSH inhibited covalent binding greater than 95% with either substrate. The metabolism of phenol to hydroquinone was unaffected by the addition of ascorbate or GSH. The metabolism of benzene to phenol was unaffected by the addition of GSH; however. the addition of ascorbate decreased the formation of phenol by 35%. Tissue ascorbate could be modulated by placing guinea pigs on different dietary intakes of ascorbate. Bone marrow ascorbate concentrations could be modulated IO-fold without any significant change in the GSH concentrations. Bone marrow isolated from guinea pigs on different dietary intakes of ascorbate were incubated with Hz01 and phenol. Bone marrow with low ascorbate concentrations displayed 4-fold more covalent binding of phenol equivalents than those with high ascorbate concentrations. This is an example of how the dietary intake of ascorbate can result in a differential response to a pOtentially toxic event in Vitro. Q 1985 Academsc press. Inc.