Phenolphthalein metabolite inhibits catechol-O-methyltransferase-mediated metabolism of catechol estrogens: a possible mechanism for carcinogenicity.
Phenolphthalein (PT), used in over-the-counter laxatives, has recently been identified as a multisite carcinogen in rodents, but the molecular species responsible for the carcinogenicity is not known. A catechol metabolite of PT, hydroxyphenolphthalein (PT-CAT), was recently identified and may be the molecular species responsible for at least part of the toxicity/carcinogenicity of PT. We hypothesize that PT-CAT inhibits the enzyme catechol-O-methyltransferase (COMT) and therefore potentiates genotoxicity by either PT-CAT itself or the endogenous catechol estrogens (CEs) in susceptible tissues. The present studies were conducted to determine the effects of PT treatment and PT-CAT itself on the COMT-mediated metabolism of 4- and 2-hydroxyestradiol both in vitro and in vivo. Female mice were treated with PT (50 mg/kg/d) for 21 days and then euthanized. PT-CAT concentration in urine reached plateau levels by 7 days of exposure. An O-methylated metabolite of PT-CAT was detected in feces. In vitro experiments demonstrated that PT treatment resulted in an increase in free CEs, which are normally cleared by COMT and a concurrent decrease in the capacity of hepatic catechol clearance by COMT. In vitro, PT-CAT was a substrate of COMT, with kinetic properties within the range measured with endogenous substrates. PT-CAT was an extremely potent mixed-type inhibitor of the O-methylation of the catechol estrogens, with 90-300 nM IC50s. The above data, when taken together, suggest that chronic administration of PT may enhance metabolic redox cycling of both PT-CAT and the catechol estrogens and this, in turn, may contribute to PT-induced tumorigenesis.
Garner
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CE
Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA. C.Edwin.Garner@Dupontpharma.com
Matthews
H B
HB
Burka
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LT
D000818:Animals; D002273:Carcinogens / Q000378:metabolism / Q000633:toxicity*; D002394:Catechol O-Methyltransferase / Q000378:metabolism; D065098:Catechol O-Methyltransferase Inhibitors; D002400:Cathartics / Q000378:metabolism / Q000633:toxicity*; D004791:Enzyme Inhibitors / Q000097:blood / Q000378:metabolism* / Q000633:toxicity* / Q000652:urine; D004958:Estradiol / Q000031:analogs & derivatives / Q000378:metabolism; D002393:Estrogens, Catechol / Q000378:metabolism*; D005260:Female; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology; D008745:Methylation / Q000187:drug effects; D051379:Mice; D008815:Mice, Inbred Strains; D020113:Phenolphthalein / Q000097:blood / Q000378:metabolism* / Q000633:toxicity* / Q000652:urine; D010635:Phenolphthaleins / Q000097:blood / Q000378:metabolism / Q000633:toxicity* / Q000652:urine; D013552:Swine
D002273:Carcinogens; D065098:Catechol O-Methyltransferase Inhibitors; D002400:Cathartics; D004791:Enzyme Inhibitors; D002393:Estrogens, Catechol; D010635:Phenolphthaleins; C404584:hydroxyphenolphthalein; D004958:Estradiol; D020113:Phenolphthalein; C001390:2-hydroxyestradiol; C014036:4-hydroxyestradiol; D002394:Catechol O-Methyltransferase