Identification of a mitochondrial p-dinitrobenzene reductase activity in rat liver

Nitroreductase activity has been known to occur under nitrogen atmosphere in the cytosolic and microsomal fractions of liver homogenate. The present study describes a new localisation for a subcellular nitro reduction activity which occurs in liver mitochondria under aerobic conditions. Mitochondria were isolated from rat liver and assayed for their capacity to reduce certain nitro compounds by measuring spectrophotometrically both the appearance of amino compounds and the consumption of NADH. Intact mitochondria were found to possess a p-dinitrobenzene (p-DNB) reductase activity which was reduced by over 50% upon addition of detergent. The activity was destroyed by heat, and was present at only 20% in the microsomal fraction. It was strictly NADH-dependent, while only little or no activity occurred with NADPH or other oxidative substrates. Moreover, this nitro reduction was protein concentration- and time-dependent reaching a plateau after 20 min, and was also inhibited by thiol reagents. Assayed under the same conditions, rat liver mitochondria showed about 15% activity with o-DNB and m-DNB, while there was less than 5% activity with a series of p-nitro compounds including chloramphenicol. The presence of a nitroreductase activity in liver mitochondria, although shown here to be restricted to p-DNB, may have important implications regarding cytotoxicity from nitro compounds.