Received for publication December 21, 2005.
Revised February 22, 2006.
Accepted for publication February 23, 2006.

Demethylation of the Pesticide Methoxychlor in Liver and Intestine from Untreated, Methoxychlor-Treated and 3-Methylcholanthrene-Treated Channel Catfish (Ictalurus punctatus): Evidence for roles of CYP1 and 3A family isozymes

Abstract

Exposure to the organochlorine pesticide methoxychlor is associated with endocrine disruption in several species, through biotransformation to mono-desmethyl-MXC (OH-MXC) and bis-desmethyl-MXC (HPTE), which interact with estrogen receptors. The biotransformation of [¹⁴C]-methoxychlor was examined in channel catfish (Ictalurus punctatus), a freshwater species found in the southern United States. Hepatic microsomes formed OH-MXC and HPTE, assessed by co-migration with authentic standards. The K_m for OH-MXC formation by control liver microsomes was 3.8 ± 1.3 mM (mean ± S.D., n = 4) and V_max was 131 ± 53 pmol/min/mg protein. These values were similar to those of catfish pretreated with 2 mg methoxychlor /kg ip, for 6 days (K_m 3.3 ± 0.8 mM and V_max 99 ± 17 pmol/min/mg), but lower (p<0.05) than the kinetic parameters for catfish treated with 3-methylcholanthrene (3-MC), which had K_m of 6.0 ± 1.1 µM and V_max 246 ± 6 pmol/min/mg protein. Liver microsomes from 3-MC-treated fish produced significantly more of the secondary metabolite and more potent estrogen, HPTE. Intestinal microsomes formed OH-MXC at lower rates than liver. Methoxychlor pretreatment significantly reduced intestinal metabolite formation from 32 ± 4 to 15 ± 6 pmol/min/mg (mean ± S.D., n=4) while 3-MC treatment significantly raised OH-MXC production to 72 ± 22 pmol/min/mg. Ketoconazole, clotrimazole and -naphthoflavone all decreased the production of OH-MXC in liver microsomes, while -naphthoflavone stimulated HPTE formation, suggesting CYP1 and CYP3 family isozymes demethylated methoxychlor. The results suggest that the formation of estrogenic metabolites from methoxychlor would be more rapid in catfish co-exposed to CYP1 inducers.

Key words: cytochrome P450 catalyzed oxidations, environmental toxicology, fish cytochrome P450, insecticides, monooxygenases

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