Relative induction of aryl hydrocarbon hydroxylase by 2,3,7,8‐TCDD and two coplanar PCBs in rainbow trout (oncorhynchus mykiss)

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Abstract
Relative aryl hydrocarbon hydroxylase (AHH) induction potencies of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) and the two coplanar PCB congeners 3,3′,4,4′‐tetrachlorobiphenyl (IUPAC congener 77) and 3,3′,4,4′,5‐pentachlorobiphenyl (IUPAC congener 126) were determined in rainbow trout (Oncorhynchus mykiss). Trout were injected intraperitoneally with chemicals dissolved in corn oil. The ED50 for AHH induction in TCDD‐injected fish was 0.005 μmol/kg. The ED50s for PCB congeners 126 and 77 were 1.0 μmol/kg and 2.2 μmol/kg, respectively. AHH toxic equivalent factors (AHH‐TEFs), or TCDD equivalents, were determined as the ratio of the TCDD ED50 to the ED50 for each PCB congener. AHH‐TEFs for congeners 126 and 77 were 0.005 and 0.002, respectively. Toxic equivalent quantities for AHH induction (AHH‐TEQs) were calculated by multiplying the TCDD equivalent for each chemical by its concentration in fish at environmental levels. AHH‐TEQs, based on whole‐body TCDD and PCB concentrations in Lake Ontario rainbow trout, were two‐ to threefold greater for the coplanar PCBs than for TCDD. Although these data do not necessarily indicate that coplanar PCBs are highly toxic to fish, the results indicate that these compounds have the potential to contribute to the environmental effects of compounds structurally related to TCDD.