Two- and one-electron dependent in vitro reductive metabolism of nitroaromatics by Mytilus edulis, Carcinus maenas and Asterias rubens

The subcellular and tissue distribution of 2- and 1-electron NAD(P)H-dependent nitroreductase activity was investigated in marine invertebrates from three phyla. The metabolites of 2-electron nitroreduction were characterised by reverse phase, isocratic HPLC. One-electron nitroreduction, resulting in the formation of reactive oxygen species, was assayed by the iron/EDTA-mediated oxidation of the hydroxyl radical scavenging agent 2-keto-methiolbutyric acid. NADPH dependent 2-electron nitroreduction occurred only under anaerobic conditions, with the amine end-product, but not the nitroso- or hydroxylamine intermediates, being detected. Activity was present in cytosolic (p-nitrobenzoic acid) or both microsomal and cytosolic (1-nitropyrene) fractions of the major digestive tissues of mussel Mytilus edulis (digestive gland) and crab Carcinus maenas (hepatopancreas), but not in the gills of either species. No activity was detectable in the pyloric caeca or stomach region of the starfish Asterias rubens. A wider subcellular distribution was evident for NAD(P)H-dependent 1-electron nitroreduction, with activity towards nitrofurantoin, p-nitrobenzoic acid and 1-nitropyrene being present in all subcellular (mitochondrial, microsomal and cytosolic) fractions of the major digestive tissues of the three species. Activity was higher for NADH than NADPH, and highest for the model redox cycling compound nitrofurantoin. NAD(P)H-dependent nitrofurantoin-mediated activity was present in several tissues (digestive, respiratory, gonads, other) of all three species, but was highest in the digestive tissues. The results indicate a widespread potential for nitroaromatic metabolism and toxicity, with oxygen availability being a factor in determining the metabolic fate of nitroaromatic xenobiotics.