2-Nitropropane (2-NP) is a genotoxicant and hepatocarcinogen in rodents. Conversion to propane 2-nitronate (P2N), the anion of the tautomeric aci form of 2-NP, seems to be a pivotal part of the mechanism by which 2-NP causes its toxicity. We tested the hypothesis that the tautomeric equilibrium is influenced by enzymes in the liver, the target organ of 2-NP toxicity. Rat or mouse hepatocytes were incubated with 2-NP, P2N or the 2-NP isotopomer 2-deutero 2-nitropropane (2H-2-NP), which equilibrates with P2N much more slowly than 2-NP. Tautomers were analyzed by HPLC. The rates of conversion of 2-NP to P2N expressed as nmol P2N x (10(6) cells/ml)-1 x min-1 were 4.0 and 4.2 in the presence of hepatocytes from rats or mice, respectively, and 2.6 in the absence of cells. Production of 2-NP to P2N expressed as nmol 2-NP x (10(6) cells/ml)-1 x min-1 was increased from 6.1 in the absence of cells to 11.9 or 9.9 in the presence of hepatocytes from rats or mice, respectively. The rate of formation of P2N from 2H-2-NP as compared to 2-NP was characterised by a primary isotope effect of 3.4 and 3.8 in hepatocytes from rats and mice, respectively, contrasting with a value of 9.6 measured in medium omitting cells. When 2-NP was incubated with subfractions of rodent or human liver homogenate, production of P2N by cytosol was between 7.3 (mouse liver) and 28.1 times (human liver) higher than that observed in microsomes. Similarly generation of 2-NP from P2N by cytosol exceeded that in microsomes by a factor of two. Tautomerism in heat-activated cytosol, mitochondria or microsomes was not different from that in buffer only. The results suggest that the nitro-aci tautomerism of secondary nitroalkanes is catalysed by a hepatic enzyme which resides predominantly in the cytosol and may thus contribute to the generation of the toxic species via which 2-NP exerts its toxicity.