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United States Department of Agriculture

Agricultural Research Service

Research Project: DIOXINS AND OTHER ENVIRONMENTAL CONTAMINANTS IN FOOD

Location: Animal Metabolism-Agricultural Chemicals Research

Title: The Effect of Dose on 2,3,7,8-TCDD Tissue Distribution, Metabolism and Elimination in CYP1A2 (-/-) Knockout and C57BL/6N Parental Strains of Mice

Authors
item Hakk, Heldur
item Diliberto, Janet -
item Birnbaum, Linda -

Submitted to: Journal of Toxicology and Applied Pharmacology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 10, 2009
Publication Date: August 18, 2009
Citation: Hakk, H., Diliberto, J.J., Birnbaum, L.S. 2009. The Effect of Dose on 2,3,7,8-TCDD Tissue Distribution, Metabolism and Elimination in CYP1A2 (-/-) Knockout and C57BL/6N Parental Strains of Mice. Journal of Toxicology and Applied Pharmacology. 241:119-126. DOI:10.1016/j.taap.2009.08.009.

Interpretive Summary: Numerous metabolism studies have demonstrated that the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is poorly metabolized. A key feature following TCDD exposure is the induction of liver CYP1A2 and the subsequent sequestration of TCDD, leading to high liver to fat concentration ratios. This study was conducted to determine whether TCDD was merely poorly metabolized, or rather, unavailable for metabolism due to its sequestration to CYP1A2. TCDD was administered as a single, oral dose at two low, environmentally-relevant levels to parental strain male mice (C57) or to mice that lacked the CYP1A2 gene (KO). At 96h, no great differences in urinary or fecal elimination were observed between the high and low dose treatments for either mouse strain. Less than 5% of the TCDD was eliminated in the urine of all groups, and was bound to mouse major urinary protein. Feces were the major elimination pathway, and fecal extracts and non-extractables, along with urine, were quantitated by HPLC for metabolites. Overall metabolism between C57 and KO mice showed no statistical differences at either the high or low doses. TCDD concentrations were the highest in adipose tissue for KO mice, but in liver for C57 mice, supporting the role of liver CYP1A2 in the sequestration of TCDD. The data suggested that metabolism of TCDD is inherently slow and due principally to another liver protein, i.e. CYP1A1. CYP1A2 was not an important contributor to the metabolism of TCDD in male mice at environmental levels, however, when highly induced may hinder CYP1A1 metabolism through sequestration.

Technical Abstract: Numerous metabolism studies have demonstrated that the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is poorly metabolized. A hallmark feature of TCDD exposure is induction of hepatic CYP1A2 and subsequent sequestration leading to high liver to fat concentration ratios. This study was initiated to determine whether TCDD was inherently poorly metabolized or unavailable for metabolism because of sequestration to CYP1A2. [3H]TCDD was administered as a single, oral dose (0.1 µg/kg and 10 µg/kg) to 12 male C57BL/6N mice or 12 CYP1A2 (-/-) mice. At 96h, less than 5% of the dose was eliminated in the urine of all groups, and TCDD detected in urine was bound to mouse major urinary protein (mMUP). Feces were the major elimination pathway (24-31% of dose). Urine, fecal extractables and non-extractables were quantitated by high performance liquid chromatography for metabolites. Overall metabolism (%dose) between parental and knockout strains showed no statistical differences at either the high or low dose treatments, i.e. 22.89 ± 4.12 vs. 19.20 ± 2.80 for high dose C57BL/6N and CYP1A2 (-/-) mice respectively, and 14.30 ± 3.17 vs. 15.45 ± 2.41 for low dose mice, respectively. TCDD concentrations were the highest in adipose tissue for CYP1A2 knockout mice but in liver for C57BL/6N mice, supporting the significant role of inducible, hepatic CYP1A2 in the sequestration of TCDD. The data suggested that metabolism of TCDD is inherently slow and due principally to CYP1A1 and that hepatic CYP1A2 is not an important contributor to the metabolism of TCDD in male mice.

Last Modified: 7/22/2014
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