Microsomal nitroreductase activity of rabbit kidney and bladder: implications in 5-nitrofuran-induced toxicity

Reductive metabolism of the aromatic nitro group of 5-nitrofurans is thought to be an important step in the mechanism of their toxicity. Microsomal nitroreductase activity with p-nitrobenzoic acid and N-[4-(5-nitro-2-furyl)-2-thiazolyl[formamide (FANFT) as substrates was assessed in the renal cortex, outer medulla, inner medulla, bladder transitional epithelial and non-epithelial bladder tissue. Cortex and transitional epithelial tissue contained the most p-nitrobenzoic acid reductase activity. However, FANFT reductase activity was similar in all areas tested except nonepithelial bladder tissue, which was 10% of the others. FANFT reduction was inhibited by oxygen, but not by carbon monoxide, allopurinol or aspirin and required NADPH. These results are consistent with NADPH-cytochrome c reductase catalyzed FANFT reduction. In medullary microsomes, the apparent Km and Vmax were 0.125 mM and 0.84 nmol/mg of protein per min, respectively. Transitional epithelial microsomes incorporated approximately 1 and 10% of the total [2-14C]FANFT metabolized into t-RNA and trichloroacetic acid-precipitable material, respectively. Two products of FANFT reduction were demonstrated by high-pressure liquid chromatography. One product was reversibly oxidized to FANFT and the other was tentatively identified by mass spectral analysis as an open chain nitrile. In view of the relatively low oxygen tension in the renal inner medulla and bladder mucosa, these results suggest that medullary and transitional epithelial nitro-reductases may be involved in the pathogenesis of 5-nitrofuran toxicity.