Toxicokinetics of 2,2′,4,4′- and 3,3′,4,4′-Tetrachlorobiphenyl in House Flies Following Topical Administration

Abstract

The effect of chlorine position on the absorption, distribution, metabolism, and excretion of two tetrachlorobiphenyls was studied in the house fly. Adult female house flies (Musca domestica L.) were dosed topically with 0.5, 5, and 10 μg of 2,2′,4,4′-[14C]- or 3,3′,4,4′-[14C]tetrachlorobiphenyls (TCBs) in 1-2 μl of acetone. Total radioactivity in acetone rinses of intact flies, extracts of flies, extracts of excrements, and unextractable fly tissues and/or excrement was determined at 11 time points over a 96-hr period to provide data for the development of toxicokinetic models. Within 96 hr, 97-99% of the applied 2,2′,4,4′-TCB was absorbed from the surface following an initial rapid absorption of 93% within 12 hr. 3,3′,4,4′-TCB was absorbed very slowly, especially at the higher doses where only about 50% was absorbed within 96 hr. Both of the PCBs entered the fly body by simultaneous rapid (predominated for 2,2′,4,4′-TCB) and slow (predominated for the poorly soluble 3,3′,4,4′-TCB) absorption processes. A small fraction of the absorbed dose was irreversibly bound to fly tissues. A greater amount of 2,2′,4,4′-TCB was found in the fly body, but the amount of irreversibly bound radioactivity was higher for 3,3′,4,4′-TCB. Since almost all of the 2,2′,4,4′-TCB in the fly body was parent PCB, it is hypothesized that the greater burden but lower binding of 2,2′,4,4′-TCB in fly tissues is a result of a lack of reactive metabolites. Higher irreversible tissue binding of 3,3′,4,4′-TCB may reflect its greater metabolism. About 73% of the absorbed 3,3′,4,4′-TCB was excreted by 96 hr, whereas 76% of the absorbed 2,2′,4,4′-TCB was excreted at the lowest dose, decreasing to 56% at the highest dose. Selective absorption, excretion, tissue retention, and greater metabolism of the lower doses more representative of environmental exposure suggest that the large mobile biomass of insects may play a crucial role in the conversion of environmental PCBs to more polar metabolites as well as the dispersion of parent PCBs and their metabolites in food chains due to the dominant role of insects in many food webs.