Ethoxyresorufin-O-deethylase induction in trout exposed to mixtures of polycyclic aromatic hydrocarbons.

This study investigated whether ethoxyresorufin-O-deethylase (EROD) activity in rainbow trout exposed to mixtures of polycyclic aromatic hydrocarbons (PAHs) could be predicted from induction equivalency factors (IEF). The test PAHs were classified into strong and weak inducers on the basis of similar exposure-response curves. Induction equivalency factors of strong inducers, based on benzo[k]fluoranthene (BkF) as the reference compound, ranged from 0.03 to 0.16. Trout exposed to mixtures of strong inducers (2, 4, and 6 equipotent parts) at 0.32-, 1.0-, or 3.2-nM BkF-equivalents showed exposure-dependent increases in EROD activity, consistent with an additive interaction. The extent of activity did not vary greatly among mixtures and single PAHs at a given induction equivalent quantity (IEQ). Induction equivalency factors could not be calculated for weak inducers because the range of induction was too low. Hence, each weak inducer was added to mixtures at concentrations that induced EROD activity fivefold. These mixtures appeared additive because binary and quaternary mixtures caused about 10- and 20-fold induction, respectively. Strong inducers mixed the same way also showed additivity. In contrast, EROD induction by mixtures containing both strong and weak PAHs was 800 to 900% greater than expected, suggesting synergistic interactions. Therefore, if mixtures are composed of PAHs that behave similarly, IEFs may be a suitable approach for assessing risk. However, for mixtures that contain PAHs of differing potency and efficacy, bioassays will be a more reliable measure of risk than IEFs.