Metabolism of the genotoxicant 2-nitropropane to a nitric oxide species

The mechanisms by which the paint constituent 2-nitropropane (2-NP) exerts genotoxicity and hepatocarcinogenicity are poorly understood. The hypothesis was tested that nitric oxide (NO) is a hepatic metabolic intermediate generated from 2-NP and/or its anionic tautomer propane 2-nitronate (P2N). Incubations of liver microsomes from phenobarbital-pretreated rats or mice with 2-NP or P2N gave spectra with Soret maxima at 448 nm which indicated the presence of a ferrous-NO complex. Levels of 3':5'-cyclic guanosine monophosphate (cGMP) and nitrite were measured by ELISA assay and HPLC, respectively, in freshly isolated mouse hepatocytes. Levels of cGMP generated within 3 h in cells by 2-NP, P2N (5 mM each) or the diethylamine/NO complex [Et2NNO(N==O)]Na (0.6 mM), an NO precursor, were 6, 15 and 34 times, respectively, those seen in control hepatocytes. Production of cGMP following treatment with 2-NP was linear with time of incubation; cGMP generation from P2N reached its peak already after 1 h. cGMP levels observed in incubates with 1-nitropropane and 2-deutero 2-nitropropane (5 mM), 2-NP isomers devoid of genotoxic properties, were significantly lower than those seen in the presence of 2-NP. Inclusion in the incubate of methylene blue, which inhibits NO-mediated reactions, decreased cGMP formation in hepatocytes with [Et2NNO(N==O)]Na, but increased it in cells with 2-NP or P2N. The production of nitrite from 2-NP, P2N or [Et2NNO(N==O)]Na mirrored cGMP formation. The results suggest that 2-NP and its nitronate generate an NO species in cells which may mediate, or contribute to, 2-NP genotoxicity.