Catechol Estrogen 4-Hydroxyequilenin Is a Substrate and an Inhibitor of Catechol-O-Methyltransferase
- Authors
- Yao, JQ; Li, Y; Chang, Min Sun; Wu, HP; Yang, XF; Goodman, JE; Liu, XM; Liu, H; Mesecar, AD; van Breemen, RB; Yager, JD; Bolton, JL
- Issue Date
- May-2003
- Publisher
- American Chemical Society
- Citation
- Chemical Research in Toxicology, v.16, no.5, pp 668 - 675
- Pages
- 8
- Journal Title
- Chemical Research in Toxicology
- Volume
- 16
- Number
- 5
- Start Page
- 668
- End Page
- 675
- URI
- https://scholarworks.sookmyung.ac.kr/handle/2020.sw.sookmyung/149165
- DOI
- 10.1021/tx0340549
- ISSN
- 0893-228X
- Abstract
- Redox and/or electrophilic metabolites formed during estrogen metabolism may play a role in estrogen carcinogenesis. 4-Hydroxyequilenin (4-OHEN) is the major phase I catechol metabolite of the equine estrogens equilenin and equilin, which are components of the most widely prescribed estrogen replacement formulation, Premarin. Previously, we have found that 4-OHEN rapidly autoxidized to an o-quinone in vitro and caused toxic effects such as the inactivation of human detoxification enzymes. 4-OHEN has also been shown to be a substrate for catechol-O-methyltransferase (COMT) in human breast cancer cells. In the present study, we demonstrated that 4-OHEN was not only a substrate of recombinant human soluble COMT in vitro with a K-m of 2.4 muM and k(cat) of 6.0 min(-1) but it also inhibited its own methylation by COMT at higher concentrations in the presence of the reducing agent dithiothreitol. In addition, 4-OHEN was found to be an irreversible inhibitor of COMT-catalyzed methylation of the endogenous catechol estrogen 4-hydroxyestradiol with a K-i of 26.0 muM and a k(2) of 1.62 x 10(-2) s(-1). 4-OHEN in vitro not only caused the formation of intermolecular disulfide bonds as demonstrated by gel electrophoresis, but electrospray ionization mass spectrometry and matrix-assisted laser desorption ionization time-of-flight mass spectrometry also showed that 4-OHEN alkylated multiple residues of COMT. Peptide mapping experiments further indicated that Cys33 in recombinant human soluble COMT was the residue most likely modified by 4-OHEN in vitro. These data suggest that inhibition of COMT methylation by 4-OHEN might reduce endogenous catechol estrogen clearance in vivo and further enhance toxicity.
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