Diastereomer- and enantiomer-specific accumulation, depuration, bioisomerization, and metabolism of hexabromocyclododecanes (HBCDs) in two ecologically different species of earthworms

Abstract

In this study, two ecological types of earthworms were exposed to soil samples that were artificially contaminated with individual hexabromocyclododecane (HBCD) diastereomers (α-, β-, and γ-HBCDs) to investigate the bioaccumulation, depuration, enantiomer selectivity and isomerization of HBCDs in earthworms. The uptake rate constant (ku), bioaccumulation factor (BAF), biota soil accumulation factor (BSAF), and half-life (t1/2) for the α-HBCD were the highest among the three diastereomers. The bioaccumulation parameters of the three diastereoisomers differed between the two ecologically different species of earthworms. The BSAF values of α- and γ-HBCDs were substantially higher in Eisenia fetida than those in Metaphire guillelmi, with the higher lipid and protein contents in E. fetida as the primary reason for this difference. The other processes, such as uptake, depuration, metabolism and isomerization, also differed between the two species and led to a difference in the bioaccumulation of β-HBCD. The β- and γ-HBCDs were bioisomerized to α-HBCD in the earthworms, but to a greater extent in E. fetida. The highest BSAF, t1/2 of α-HBCD and the bioisomerization of β- and γ-HBCDs to α-HBCD might explain in part why α-HBCD was the dominant isomer in biota samples. Most of the enantiomer fractions (EFs) for the three HBCD diastereoisomers in the earthworms were different from those in standard samples (p<0.05), indicating that enantiomer selectivity occurred. Moreover, the trends and extent of the enantioselectivity were different between the two species. Additionally, the EFs of α-HBCD that was bioisomerized from β- or γ-isomers were also different from those in the standards (p<0.05), which likely reflect the integration of several processes, such as enantioselective isomerization and the subsequent selective metabolism of the produced α-HBCD or selective excretion of the enantiomers.