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In common carp, environmental exposure to polybrominated diphenyl ethers (PBDEs) result in the bioconcentration of BDE-47 (2,2,4,4tetrabromodiphenyl ether) at higher concentrations than BDE-99 (2,2,4,4,5 pentabromodiphenyl ether). BDE-99 isMoreIn common carp, environmental exposure to polybrominated diphenyl ethers (PBDEs) result in the bioconcentration of BDE-47 (2,2,4,4tetrabromodiphenyl ether) at higher concentrations than BDE-99 (2,2,4,4,5 pentabromodiphenyl ether). BDE-99 is debrominated to BDE-47 by the removal of the meta positioned bromine. This research investigates carp PBDE debromination and the potential for effects to the thyroid and DNA by BDE-99 exposure. After extraction, PBDE congeners were quantified by gas chromatography, mass spectrometry operated in electron capture, negative chemical ionization mode.-Juvenile carp were fed BDE-99 amended food for 62 days followed by a 5 day depuration. BDE-47 bioaccumulated in liver and intestines of BDE-99 exposed fish with minimal or no accumulation of BDE-99. Radioactive assays indicated that BDE-99 exposure resulted in depressed plasma total T3:T4 ratios but exposure had little effect on T4 outer-ring deiodination (T4ORD) and T3 inner-ring deiodination (T3IRD). The comet assay revealed that BDE-99 exposure significantly increased hepatocyte DNA damage (day 62) and that this damage became insignificant after 5 days depuration.-BDE-99 in vitro incubations with carp intestinal microflora showed no debromination. However, liver and intestinal microsomes, without the addition of an energy cofactor (NADPH), converted BDE-99 to BDE-47 to a similar extent (p>0.05). Iodoacetate (IaC) and reverse thyronine (rT3) inhibited the production of BDE-47 and T4ORD in liver and intestine microsomes. BDE-99 did not modify liver nor intestinal T4ORD.-BDE-75 (2,4,4,6-tetrabromodiphenyl ether) did not debrominate but significantly inhibited T4ORD. BDE-209 (decabromodiphenyl ether) debromination was variable by organ and did not affect DI activity. T3IRD was not apparent in liver microsomes and did not occur in every intestine sample.-This research concludes that debromination of BDE-99 is enzymatic but that the DI enzyme(s) responsible for intestinal and hepatic T4ORD as well as intestinal T3IRD are not catalytically mediating the debromination of BDE-99 based on in vitro competitive substrate exposures. BDE-99 (670 +/- 40 ng day-1) exposure can lead to toxic homeostatic changes including thyroid hormone (TH) concentration alterations and DNA damage, in carp. The implications of these homeostatic changes include the potential for DNA mutations and additional adverse effects during critical TH developmental periods.