Reductions of Nitro and 9-Oxo Groups of Environmental Nitrofluorenes by the Rat Mammary Gland in Vitro

Nitrofluorenes and C-9-oxidized nitrofluorenes are widespread environmental genotoxins which may be relevant for breast cancer on the basis of their carcinogenicities, particularly of 2,7-dinitrofluorene (2,7-diNF), for the rat mammary gland. Since their metabolism to active carcinogens may involve nitroreduction, this study examined the reduction of 2-nitrofluorene (2-NF) and 2,7-diNF and their 9-oxo- and 9-hydroxy (OH) derivatives by the rat mammary gland. Cytosolic fractions catalyze NADH- and NADPH-dependent reductions of the 2-nitro and 9-oxo to the respective 2-amino and 9-OH compounds at rates 4- and ≥10-fold greater than those with microsomes. Rates of amine formation catalyzed by cytosol from 2,7-diNF are greater than the rate from 2-NF and increase for C-9-oxidized derivatives:  9-oxo-2-NF > 9-OH-2-NF > 2-NF and 9-OH-2,7-diNF ≫ 9-oxo-2,7-diNF > 2,7-diNF. Nitroreduction is inhibited by O<INF>2</INF> or allopurinol (20 μM), dicoumarol (100 μM), and rutin (50 μM). 9-Oxoreduction is inhibited by rutin, dicoumarol, and indomethacin (100 μM), but not by O<INF>2</INF> or allopurinol. Pyrazole or menadione does not inhibit nitro or 9-oxoreduction. Xanthine, hypoxanthine, 2-hydroxypyrimidine, and N‘-methylnicotinamide support cytosol-catalyzed nitro, but not 9-oxo, reduction. The data suggest that the nitroreduction is catalyzed largely by a xanthine oxidase and partially by a diaphorase and 9-oxoreduction by a carbonyl reductase. The extents of the nitro and carbonyl reductions of the nitrofluorenes may determine their reactivities with DNA, and thus genotoxicities for the mammary gland.