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USE OF CYP1A2(-/-) KNOCKOUT AND CYP1A2(+/+) C57BL/6N PARENTAL STRAINS OF MICE TO COMPARE METABOLISM OF 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN (TCDD)
Citation:
Diliberto, J J. AND H. Hakk. USE OF CYP1A2(-/-) KNOCKOUT AND CYP1A2(+/+) C57BL/6N PARENTAL STRAINS OF MICE TO COMPARE METABOLISM OF 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN (TCDD). Presented at Society of Toxicology 42nd Annual Meeting, Salt Lake City, Utah, March 9-13, 2003.
Description:
USE OF CYP1A2 (-/-) KNOCKOUT AND CYP1A2 (+/+) C57BL/6N PARENTAL STRAINS OF MICE TO COMPARE METABOLISM OF 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN (TCDD). J J Diliberto1 and H Hakk2. 1USEPA ORD, NHEERL, ETD, PKB, Research Triangle Park, NC, USA; 2USDA-ARS, BRL, Fargo, ND, USA. Sponsor: L S Birnbaum.
The most toxic dioxin congener, TCDD, induces hepatic cytochrome CYP1A2 to which it subsequently binds, resulting in whole body half lives of 5-11 years in humans and 30 days in rats. In addition, metabolism of TCDD is very limited in both species. Whether TCDD is a poor substrate for metabolizing enzymes, or whether TCDD is unavailable for metabolism due to its strong affinity to CYP1A2 has not been firmly established. Thus, we tested the hypothesis that sequestration of TCDD by CYP1A2 makes TCDD unavailable for metabolism that would readily occur in the absence of CYP1A2. Male C57BL/6N mice which possess or lack the CYP1A2 gene were given a single oral dose of 156 ?g [14C]TCDD/kg. After 4 days, the mice (housed in metabolism cages with separate collection of urine and feces every 24h) were killed and tissues collected. Tissue deposition and overall metabolism in urine and feces were quantitated. Similar to previously reported studies, liver:fat ratio of the two groups was vastly different, i.e. 4.09 (C57BL/6N) vs. 0.57 (knockout, KO). Slightly higher levels of 14C-derived TCDD were excreted in urine and feces of the parental strain at each time point when compared to KO mice. The overall level of metabolism of TCDD was determined as sum of 14C in 0-96h urine, non-extractable feces, and metabolites in extractable feces. The parental strain of mice had greater overall metabolism than the KO mice, i.e. 11.1% vs. 6.5% of the dose, respectively. The lower overall metabolism in the KO than the parental strain of mice is probably due to low hepatic retention and rapid redistribution of TCDD into lipophilic tissues for storage, which made the TCDD unavailable to hepatic metabolizing enzymes. In conclusion, the data presented in this study contradicts the original hypothesis and confirms that TCDD has an inherently slow metabolism in mammals, perhaps via the inducible CYP1A1, 1A2, and 1B1 isozymes and/or non-P450 dependant mechanisms. (This abstract does not reflect USEPA and USDA policies.)