Equine catechol estrogen 4-hydroxyequilenin is a more potent inhibitor of the variant form of catechol-O-methyltransferase.

The study investigates the role of catechol estrogens, particularly 4-hydroxyequilenin (4-OHEN), in inhibiting the variant form of Catechol-O-methyltransferase (COMT), an enzyme involved in inactivating harmful substances in the body. The findings suggest that COMT’s variant form is particularly susceptible to 4-OHEN, which could affect the body’s ability to detoxify harmful estrogens and potentially contribute to higher breast cancer risks.

Background and Significance

• The study focused on the enzyme Catechol-O-methyltransferase (COMT), which is crucial in the inactivation process of harmful substances called catechols.
• The researchers pointed out that COMT’s genetic polymorphism leads to a wild-type form and a variant form in which a valine is substituted with a methionine.
• The variant form of COMT has been linked to an increased risk of breast cancer in several epidemiological studies.
• The research also focused on 4-hydroxyequilenin (4-OHEN), a cytotoxic/genotoxic equine catechol estrogen metabolite, which has been found to be both a substrate of COMT and an irreversible inhibitor of COMT’s methylation activity.

Methodology and Findings

• The researchers used Michaelis-Menten analysis and found no significant difference in the two COMT forms’ ability to methylate 4-OHEN. However, they discovered that the variant form of COMT was more susceptible to irreversible inactivation by 4-OHEN.
• Using electrospray ionization mass spectrometry and SDS-gel analysis, they found that 4-OHEN’s inhibition mechanisms include alkylation and/or oxidation of certain amino acids.
• Site-directed mutagenesis experiments revealed that Cys33 plays a more significant role in the variant form of COMT, as its mutation in the variant form resulted in a more dramatic decrease in catalytic capability compared to the wild-type form.
• Thermotropic studies indicated that the variant form was more thermolabile, suggesting the valine to methionine substitution might have altered the secondary/tertiary structure of the variant COMT form, making it more susceptible to 4-OHEN and heat inactivation.

Implications

• The study suggests that 4-OHEN-mediated inhibition of the variant form of COMT might affect the body’s ability to detoxify endogenous and/or exogenous catechol estrogens.
• This discovery might explain the association between high breast cancer risk and COMT polymorphism, providing useful insight into the ways in which genetic differences can contribute to cancer risk.