Estimation of the dermal carcinogenic activity of petroleum fractions using a modified Ames assay

The Ames Salmonella/microsomal activation mutagenesis assay has been adapted to improve sensitivity to complex hydrocarbon mixtures produced by the refining of petroleum. Extraction of oil samples with dimethyl sulfoxide produces aqueous-compatible solutions that more easily interact with the tester bacteria. These extracts, therefore, produce higher revertant values than do equivalent volumes of oil delivered neat or dissolved in organic solvent. Parallel increases in the liver microsomal S-9 concentration further improve the sensitivity of the assay, allowing detection of mutagenicity in otherwise inactive samples. The effect of increased microsomal fraction from rodent liver is apparently attributable to the higher levels of activating enzymes rather than to the concomitant increase in the overall hydrophobicity of the test system. The modified assay has been used to rank thirteen petroleum-derived oils and a corn oil control for relative mutagenic activity. This ranking closely correlates (r = 0.97) with potency rankings of the same samples previously determined from dermal carcinogenicity bioassays.