Metabolism of the Endocrine Disruptor Pesticide-Methoxychlor by Human P450s: Pathways Involving a Novel Catechol Metabolite

Abstract

ABSTRACT:The metabolism of methoxychlor, a proestrogenic pesticide (endocrine disruptor), was investigated with cDNA expressed human cytochrome P450s and liver microsomes (HLM). In addition to 1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane (mono-OH-M), 1,1,1-trichloro-2, 2-bis(4-hydroxyphenyl)ethane (bis-OH-M), and 1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(3, 4-dihydroxyphenyl)ethane (tris-OH-M), a new metabolite was identified as 1,1,1- Methoxychlor, 1 a biodegradable pesticide and a substitute for the banned DDT, has a relatively low toxicity and short half-life (Metcalf et al., 1971;Gardner and Bailey, 1975;Metcalf, 1976). Despite these favorable features of methoxychlor, there is considerable concern for exposure to methoxychlor because of its estrogenic activity (Bulger et al., 1978b;Ousterhout et al., 1981). Whereas in vitro methoxychlor has little or no affinity for the estrogen receptor (ER), in vivo methoxychlor exhibits pronounced estrogenic activity, indicating that methoxychlor per se is a proestrogen (Bulger et al., 1978a,b;Kupfer and Bulger, 1979;Bulger et al., 1985). Indeed, methoxychlor undergoes oxidative metabolism by hepatic cytochrome P450 forming metabolites with pronounced estrogenic activity (Bulger et al., 1978a,b;Bulger et al., 1985). Additionally, methoxychlor was found to elicit considerable endocrine and reproductive toxicity (Gray et al., 1988;Cummings and Gray, 1989;Cummings and Laskey, 1993). However, hitherto it has not been established whether the endocrine and reproductive toxicities are associated with methoxychlor per se or are primarily due to its metabolites. Based on such and other findings, methoxychlor was categorized as a prototype endocrine disruptor, and the nature of its toxicity has been under protracted intensive investigation (Chapin et al., 1997;Cummings, 1997;You et al., 2002). Interestingly, the mono-and bis-demethylated methoxychlor metabolites (mono-OH-M and bis-OH-M) exhibit opposing activities toward the ER␣ and ER␤ isoforms [i.e., both metabolites are agonists of ER␣, but antagonists of ER␤, as well as antagonists of the androgen receptor (Gaido et al., 2000)]. Additionally, it was reported that bis-OH-M activity differs considerably from that of estradiol-17␤ in affecting certain gene expression in mice ovaries (Waters et al., 2001), possibly explaining the distinctive but overlapping pathologies in trichloro-2-(4-methoxyphenyl)-2-(3, 4-dihydroxyphenyl)ethane (catechol-M; previously assumed to be ring-OH-M) and as a key metabolic intermediate. A novel metabolic route was proposed involving methoxychlor O-demethylation to mono-OH-M, followed by bifurcation of the pathway, both leading to the same final product tris-OH-M: pathway a, mono-OH-M is demethylated to bis-OH-M, followed by ortho-hydroxylation forming tris-OH-M and pathway b, mono-OH-M is ortho-hydroxylated forming catechol-M that is O-demethylatedThe project described was supported by an National Institutes of Health Grant ES00834 from the National Institute of Environmental Health Sci...

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 CYP3A FAMILY ISOZYMES

“…In human liver microsomes, several P450 isoforms, but predominantly CYP2C9 and CYP1A2, catalyze demethylation of methoxychlor to OH-MXC (Hu and Kupfer, 2002;Hu et al, 2004;Hazai and Kupfer, 2005). Demethylation of OH-MXC to form HPTE was catalyzed by human CYP1A2, CYP2C8, CYP2C19, CYP2D6, and to a lesser extent CYP3A4 (Hu and Kupfer, 2002). Although there have been several studies of the adverse effects of methoxychlor in various fish species, little is known of the biotransformation of methoxychlor in fish.…”

“…Because Hu and Kupfer (2002) showed that human CYP1A2 readily metabolized methoxychlor, it was somewhat surprising that the CYP1 family inducer ␤-naphthoflavone did not increase methoxychlor metabolism in catfish (Schlenk et al, 1997). However, it has been shown that residues of ␤-naphthoflavone can inhibit CYP1A (James et al, 1997); therefore, it was of interest to examine the effects of another polycyclic aromatic inducing agent, 3-methylcholanthrene (3-MC).…”

Glucuronidation of the Oxidative Cytochrome P450-Mediated Phenolic Metabolites of the Endocrine Disruptor Pesticide: Methoxychlor by Human Hepatic Udp-Glucuronosyl Transferases

“…Mono-OH-M was synthesized in our laboratory (Hu and Kupfer, 2002a). Bis-OH-M was kindly provided by Dr. J. Sanborn (Illinois State Natural History Survey, Urbana, IL).…”

“…In vitro and in vivo studies of methoxychlor metabolism have characterized several metabolites, some of which are estrogenic (Bulger et al, 1978;Kupfer and Bulger, 1979;Osterhout et al, 1981;Bulger et al, 1985;Hu and Kupfer, 2002a). The oxidative metabolism of methoxychlor catalyzed by the cytochrome P450 enzymes primarily yields a mono-[1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane (mono-OH-M)] and bis-demethylated product [1,1,1-trichloro-2,2-bis(4-hydroxy phenyl)ethane (bis-OH-M)].…”

Calbindin-D9k mRNA Expression in the Rat Uterus Following Exposure to Methoxychlor: A Comparison of Oral and Subcutaneous Exposure

“…MC is categorized as a prototype endocrine disruptor or proestrogen that shows estrogenic activity due to its metabolites [39][40][41][42]. The estrogenic effects of MC are believed to be caused by the action of its demethylated phenolic metabolites [16,43], which are major phase I metabolites catalysed by the cytochrome P450 monooxygenase system [44,45].…”