Adaptation of the salmonella/mammalian microsome test to the determination of the mutagenic properties of mineral oils.

Two techniques allowing the determination of the mutagenicity of lipophilic compounds such as mineral oils with the Ames test have been developed by using benzo[a]pyrene (BP) dissolved in white oil as a synthetic reference oil. The first technique involves prior extraction of polynuclear aromatic hydrocarbons (PAH) with dimethyl sulfoxide. In the second method, which proved simpler and of more general use, the compounds to be tested are directly dispersed in aqueous medium with Tween 80. The use of these techniques made possible the study of mutagenicity of various kinds of mineral oil. Mutagenic activity was found in used crankcase oils, and also in petroleum distillates but much less in solvent-refined oils. A good correlation was observed between mutagenic activity and PAH content but not BP content of oils. Because of their peculiar response to the test, petroleum distillates were studied in more detail. When added in low amounts to pure PAH compounds such as BP, they enhanced its mutagenic activity (enhancement). When added in higher amounts, on the contrary, these oils completely inhibited BP mutagenic activity (inhibition effect). Both effects correlated well with the PAH content and the mutagenic activity of the petroleum distillates tested. These results explain the abnormal dose-response curves curves obtained with these petroleum distillates and the negative results regarding their mutagenic activity reported in earlier studies. A likely explanation is discussed for the enhancement and inhibition effects.