Characterization of oxidative and reductive metabolism in vitro of nitrofluoranthenes by rat liver enzymes

Abstract

Nitrofluoranthenes (NFs) are mutagenic and carcinogenic environmental pollutants found in incomplete combustion products and urban air particulate. We have studied both oxidative and reductive metabolism in vitro of different NF isomers mediated by subcellular rat liver fractions. Under aerobic conditions only ring hydroxylation of NFs by rat liver microsomes occurred and the isomeric position of the nitro group affected both the amount and the type of phenolic metabolites formed. Liver microsomes from 3-methylcholanthrene-induced rats were most effective in giving ring hydroxylated 7- and 8-nitrofluoranthene, whereas liver microsomes from phenobarbital-pretreated rats were the most active in metabolizing 1- and 3-nitrofluoranthene. Under anaerobic conditions, only reduction of NFs mediated by both cytosolic and microsomal rat liver enzymes occurred. Cofactor requirements and inhibition experiments indicated that the reductase activity in rat liver cytosolic fractions could be ascribed to DT-diaphorase, aldehyde oxidase and/or other unknown enzymes. The microsomal reductase activity was inhibited by oxygen, carbon monoxide, 2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride and n-octylamine, and slightly by cytochrome c; flavin mononucleotide greatly enhanced this activity. 3-Nitrofluoranthene microsomal nitroreductase activity was increased by phenobarbital rat pretreatment and this increment correlated well with the content of cytochrome P450. These results indicate a participation of cytochrome P450 in the reductive metabolism of NFs by rat liver microsomes.