REDUCTION OF ARYL-HYDROCARBON RECEPTOR-MEDIATED ACTIVITY OF POLYCHLORINATED BIPHENYL MIXTURES DUE TO ANAEROBIC MICROBIAL DECHLORINATION

Aroclors 1242 and 1254 were anaerobically dechlorinated by microorgani sms eluted from polychlorinated biphenyl (PCB)-contaminated River Rais in (RR) and Silver Lake (SL) sediments. Dechlorination occurred primar ily from the meta positions. H4IIE rat hepatoma cell bioassays for eth oxyresorufin-O-deethylase (EROD)induction potency, a measure of aryl h ydrocarbon receptor-mediated toxic potential, revealed that the potenc ies of the Aroclors were considerably reduced after dechlorination. Th e decreases in EROD induction potencies were dependent on the extent o f dechlorination of the non-ortho-substituted and mono-ortho-substitut ed PCBs. Calculated 2,3.7,8-tetrachlorodibenzo-p-dioxin (TCDD) equival ents (TEqs(calc)) for the PCB mixtures were 5.7, 0.08, and 0.05 pM TEq (calc)/mu mole of PCBs for non-, RR-, and SL-dechlorinated Aroclor 124 2, and 7.8, 1.6, and 1.0 phl TEq(calc)/ mu mole of PCBs for non-, RR-, and SL-dechlorinated. Aroclor 1254, respectively. Correspondence betw een TEqs(assay) determined using the H4IIE bioassay and TEqs(calc) ind icated that no unexpected synergistic interactions occurred among comp onents of the dechlorinated mixtures. These results indicate that the dioxinlike toxicities of PCB mixtures are substantially reduced by mic robial reductive dechlorination, and that a constant ratio of TEq to t otal PCBs cannot be assumed in risk assessments.