The focus of studies examining EROD inductionin fish by PCBs has shifted from using commercialPCB formulations to specific PCB congenersthat are most abundant in fish and that havethe highest toxic potential. This shift occurred afterit was demonstrated that the PCB congener compositionin fish tissue does not correspond to the compositionof commercial PCB standards (Schwartzet al., 1987). In addition, profiles of PCB congenersin fish tissue vary due to differences in congener-specific bioaccumulation factors and susceptibilityto metabolism and elimination (Oliverand Niimi, 1988). As with PCDDs and PCDFs,the potential for PCBs to induce EROD (and otherAhR-mediated effects) depends on AhR affinity.PCB congeners with lateral chlorine substitutionsin both 4 and 4 positions and at least one chlorinein the 3 position are most potent if there are nochlorines on the 2 or 6 ortho positions. The lackof chlorine substitution at the ortho position (nonorthoPCBs) leads to a planar configuration of thephenyl rings at the lowest energy state (McKinneyand Singh, 1981; Tsuzuki and Tanabe, 1991). Aplanar conformation is requisite for AhR bindingand as a result planar PCB congeners demonstrateEROD-inducing potential in fish (Metcalfe andHaffner, 1995). Three congeners in this categorythat have received great attention are 3,3,4,4-tetrachlorobiphenyl (PCB 77), 3,3,4,4,5-pentachlorobiphenyl(PCB 126) and 3,3´,4,4´,5,5´-hexachlorobiphenyl(PCB 169). These three congenersare strong AhR agonists and are routinely detectedin environmental samples (Metcalfe, 1994). Theyare generally moderate to strong inducers of ERODin fish (Table 1). Melancon and Lech (1983) concludedthat induction of EROD activity in rainbowtrout and common carp (Cyprinus carpio) byplanar PCBs was sufficiently great to account forthe inducing potency of Aroclor 1254 in these species.Strong induction of rainbow trout hepaticEROD over controls occurred at i.p. injection dosesof 0.01 mg/kg, 0.005 mg/kg and 0.025 mg/kgb.w. for PCBs 77, 126, and 169, respectively(Melancon and Lech, 1983; Newsted et al., 1995).The planar 3,4,4,5-tetrachlorobiphenyl (PCB 81)(Harris et al., 1994) strongly induces CYP1A activity,measured as AHH, in fish (Harris et al.,1994), but at present its EROD-inducing potencyin fish has not been examined. Based on the AHHinduction results, this congener may contribute toEROD induction in fish exposed to PCB mixturescontaining this congener.
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