Metabolic activation of 1-nitropyrene and 1,6-dinitropyrene by nitroreductases from Bacteroides fragilis and distribution of nitroreductase activity in rats.

Abstract

Nitrated polycyclic aromatic compounds, 1-nitropyrene (1-NP) and 1,6-dinitropyrene (1,6-diNP), are environmental mutagens and carcinogens. Nitroreductases purified from an anaerobic bacterium, Bacteroides fragilis, catalyzed the metabolic activation of these compounds to produce DNA- and tRNA-bound adducts in vitro. Formation of the adducts was inhibited by p-chloromercuribenzoic acid, which is an inhibitor of nitroreductases from B. fragilis. The enzyme and coenzyme (NADPH) were essential for the adduct formation. These results suggest that nitroreduction is a necessary step in the metabolic activation of nitropyrenes. 1-NP bound specifically to poly(G) and poly(dG), and 1,6-diNP bound to poly(G), poly(dG), and poly(X). The other purine polynucleotides were weak acceptors. However, the reactive products of nitropyrenes formed by nitroreductases could not bind to pyrimidine polynucleotides. Enzymatic hydrolysis of 1-NP-bound DNA and subsequent analysis by high-performance liquid chromatography showed one major and two minor adducts in the hydrolysate. The peak of the major adduct corresponded to that of N-(deoxyguanosin-8-y1)-1-aminopyrene, which is the same as an adduct formed by xanthine oxidase, a mammalian nitroreductase. Nitroreductase activity in the various organs and intestinal contents of Sprague-Dawley rats was assayed in the presence of NADPH or NADH under nitrogen gas. Nitroreductase activity was widely distributed in the organs of the rats; in particular, that of the liver and of the small intestine was relatively high, but that of the respiratory organs such as lung and alveolar macrophages was very low. Intestinal contents had high nitroreductase activity, which was proportional to the number of bacteria, especially anaerobic bacteria, in the intestine. These results suggest that the nitroreductase activity of the normal bacterial flora is very high in rats and that the intestinal bacteria play a major role in the metabolism of nitropyrenes in vivo.