catechol-O-methyltransferase Expression Research Articles

Progesterone-Mediated Regulation of Catechol-O-Methyl Transferase Expression in Endometrial Cancer Cells

The effects of estrogen and progesterone on the expression of estrogen-metabolizing enzymes such as catechol-O-methyl transferase (COMT) are not known. COMT converts genotoxic catecholestrogens to anticarcinogenic methoxyestrogens in the endometrium. The aim of this study is to investigate the effect of progesterone on COMT expression in well-differentiated endometrial cancer cells. The wild-type Ishikawa cell line as well as progesterone receptor A- or progesterone receptor B-transfected Ishikawa cells were used for in vitro studies. The regulation of COMT expression by progesterone was studied using Western blots, Hoechst dye DNA proliferation studies, and wild-type and/or site-directed mutagenesis of COMT promoter 1-luciferase reporter gene. Progesterone upregulated COMT protein expression in Ishikawa cells through progesterone receptor A isoform. COMT promoter activity was differentially regulated by the 3 half-site progesterone response elements in the COMT promoter. High doses of 2-ME2 inhibited Ishikawa cell proliferation. These data suggest that COMT expression is hormonally regulated in well-differentiated human endometrial cancer cells. COMT regulation and 2-ME2 production in the endometrium may affect endometrial carcinogenesis.

Soy isoflavones decrease the catechol-O-methyltransferase-mediated inactivation of 4-hydroxyestradiol in cultured MCF-7 cells

The tissue concentrations of the female sex hormone 17beta-estradiol (E2) and its reactive catechol metabolites such as 4-hydroxyestradiol (4-HO-E2) play important roles in hormonal carcinogenesis. They are influenced by the activity of local enzymes involved in the metabolic activation and inactivation of E2. In the mammary gland, catechol estrogens are predominately inactivated by catechol-O-methyltransferase (COMT). Food supplements containing the soy isoflavones genistein and daidzein are consumed because they are believed to protect from breast cancer; however, this proposed benefit is controversial. The aim of the present study was to investigate the influence of soy isoflavones on the gene expression and activity of COMT in cultured human mammary adenocarcinoma MCF-7 cells. Levels of COMT messenger RNA (mRNA) were determined by reverse transcription/competitive polymerase chain reaction and COMT activity was determined by high-performance liquid chromatography analysis of the methylation products of both the model substrate quercetin and the physiological relevant substrate 4-HO-E2. Our study demonstrates for the first time that soy isoflavones at hormonally active concentrations cause a significant reduction of both COMT mRNA levels and COMT activity as well as of the methylation of 4-HO-E2. Experiments using the estrogen receptor (ER) antagonist ICI 182,780 support a role of the ER in the isoflavone-induced down-regulation of COMT expression. Thus, this study not only demonstrates that hormonally active concentrations of soy isoflavones inhibit the detoxification of catechols in this human breast cancer cell line but also implies that diet might influence COMT activity to a greater extent than heretofore recognized.

Analyses of variants located in estrogen metabolism genes (ESR1, ESR2, COMT and APOE) and schizophrenia

Relationships between gender, age-of-onset of schizophrenia and reproductive age strongly suggest a key role for gonadal hormones, and more specifically for estrogens, in the etiology of the illness. Also, estrogens act as neural growth and trophic factors influencing neuron and glial cells in many areas of the central nervous system. Therefore, we investigated the association between schizophrenia and 4 genes related to estrogen metabolism. These genes are ESR1 (estrogen receptor 1), ESR2 (estrogen receptor 2), APOE (apolipoprotein E) and COMT (catechol-O-methyltransferase). The expression of APOE and COMT, which contain estrogen response elements, have been demonstrated to be regulated by the estrogen receptors. In this current association study, we examined 59 single nucleotide polymorphisms (SNPs) located in the ESR1 (26), ESR2 (14), APOE (7) and COMT (12) loci. Allele frequencies were evaluated in the schizophrenia ( n = 585)-control ( n = 615) sample and no association was found with any of the four genes. In conclusion, our data suggest that the four analyzed genes do not play an important role in susceptibility to schizophrenia.

Linkage studies of catechol-O-methyltransferase (COMT) and dopamine-beta-hydroxylase (DBH) cDNA expression levels.

The COMT and DBH genes are physically located at chromosomes 22q11 and 9q34, respectively, and both COMT and DBH are involved in catecholamine metabolism and are strong candidates for certain psychiatric and neurological disorders. Although the genetic determinants for both enzymes' activities have been widely studied, their genetic involvement on gene mRNA expression levels remains unclear. In this study we performed quantitative linkage analysis of COMT and DBH cDNA expression levels, identifying transcriptional regulatory regions for both genes. Multiple Haseman-Elston regression was used to detect both additive and interactive effects between two loci. We found that the master transcriptional regulatory region 20q13 had an additive effect on the COMT expression level. We also found that chromosome 19p13 showed both additive and interactive effects with 9q34 on DBH expression level. Furthermore, a potential interaction between COMT and DBH was indicated.

Progesterone regulates catechol- O-methyl transferase gene expression in breast cancer cells: Distinct effect of progesterone receptor isoforms

There is strong evidence that catechol- O-methyl transferase (COMT) protects breast cells against estrogen-induced cancer by detoxifying catecholestrogens, the carcinogenic estrogen metabolites. COMT gene expression is controlled by two promoters – a proximal promoter (COMTP1) and a distal promoter (COMTP2) – that regulate the expression of soluble (S-COMT) and membrane-bound (MB-COMT) isoforms, respectively. We investigated the transcriptional regulation of the COMT gene by progesterone/progesterone receptors in breast cancer cells. Our results indicated that progesterone (P4) downregulates COMT gene expression in breast cancer cell lines. In addition, the COMTP1 and COMTP2 harbor several progesterone response elements (PREs). Electrophoretic mobility shift assay (EMSA) indicated that nuclear extracts of T47D cells bind to the identified PREs in COMTP1. Site-directed mutagenesis of PREs in COMTP1 not only reversed the P4-induced inhibition of COMTP1, but also increased its basal activity. The two progesterone receptor isoforms, PR-A and PR-B, were found to have opposite effects on the regulation of P4 in COMT expression; PR-A is associated with P4-induced upregulation of COMT, while PR-B is associated with P4-induced downregulation of COMT. In summary, our data demonstrated that P4 downregulates the COMT gene expression through multiple PREs in the COMT promoters and that different progesterone receptor isoforms have distinctive effects on COMT gene expression.

CLINICAL STUDY: Endogenous morphine signaling via nitric oxide regulates the expression of CYP2D6 and COMT: autocrine/paracrine feedback inhibition

We determined changes in mRNA expression in specific enzymes involved in the biosynthesis of morphine in human white blood cells via microarray. Leukocyte exposure to morphine down-regulated catechol-O-methyl transferase (COMT) and CYP2D6 by approximately 50% compared with control values. The treatment did not alter DOPA decarboxylase and dopamine beta-hydroxylase expression, demonstrating the specificity of morphine actions. The verification of the microarray data was accomplished via real-time Taqman reverse transcriptase polymerase chain reaction (RT-PCR) focused on CYP2D6 and COMT expression in different blood samples treated with morphine. The analysis showed similar changes in the expression of CYP2D6 and COMT mRNA. The expression was reduced by 47 +/- 7% for CYP2D6, substantiating the microarray finding of a 54% reduction. Furthermore, exposure of white blood cells to 10(-6) M S-nitroso-N-acetyl-DL-penicillamine (SNAP), a nitric oxide (NO) donor, reduced the expression of CYP2D6 and COMT. Prior naloxone (10(-6) M) or N-nitro-L-arginine methyl ester (L-NAME) (10(-4) M) addition abrogated morphine's down-regulating activity, demonstrating morphine was initiating its actions via stimulating constitutive NO synthase derived NO release via the mu3 opiate receptor splice variant. In the past we demonstrated that UDP-glucurosyltransferase is involved in metabolizing morphine to morphine 6-glucuronide in adrenal chromaffin cells. In the present study its expression was not found in controls and morphine-treated cells, suggesting that morphine 6-glucuronide may not be synthesized in white blood cells. Taken together, it appears that morphine has the ability to modulate its own synthesis via autocrine and paracrine signaling.

Lower Levels of Urinary 2-Hydroxyestrogens in Polycystic Ovary Syndrome

Women with polycystic ovary syndrome (PCOS) have anovulation due to arrested follicular maturation. The substrate (2-hydroxyestrogen) and product (2-methoxyestrogen) of catechol-O-methyl transferase (COMT) have been shown to modulate proliferation and angiogenesis of granulosa cells. The objective of the study was to evaluate COMT ovarian expression as well as the production of estrogen metabolites (2-hydroxyestrogen and 2-methoxyestrogen) in subjects with PCOS. Immunohistochemistry was used to assess COMT expression in ovarian tissues. Urinary levels of 10 different estrogens and estrogen metabolites were measured using enzyme-labeled immunoassays and/or liquid chromatography with tandem mass spectrometry. The study was conducted at a tertiary university referral center. Ovarian tissues were obtained from six control subjects and six subjects with PCOS. Fasting first-void urinary samples were collected from 49 subjects with PCOS and 36 healthy control subjects. COMT protein expression in ovarian tissues was measured. Urinary levels of 2-hydroxyestrogen and 2-methoxyestrogen levels in PCOS patients were also measured. Whereas immunohistochemistry showed that COMT was expressed in ovaries from control and PCOS subjects, its expression was significantly higher in ovaries from subjects with PCOS, in both the follicular structures and ovarian stroma. The urinary 2-hydroxyestrogen level was significantly lower in subjects with PCOS, compared with normal controls (P = 0.009). Additionally, urinary 2-hydroxyestrogen levels negatively correlated with serum insulin levels in subjects with PCOS (r = -0.333, P =0 .031). Urinary 2-hydroxyestrogen is decreased in subjects with PCOS, which could be due in part to increased ovarian expression of COMT. Further studies are needed to ascertain the role of estrogen metabolism in PCOS before this information can be used in clinical settings.

Expression of Cytochrome P450 1B1 and Catechol-O-Methyltransferase in Breast Tissue and Their Associations with Breast Cancer Risk

Cytochrome P450 1B1 (CYP1B1) and catechol-O-methyltransferase (COMT) are important estrogen-metabolizing enzymes that may affect breast cancer risk. Few studies have directly measured the expression of CYP1B1 and COMT genes in breast tissue samples. The subjects in this study were a subgroup of participants of the Shanghai Breast Cancer Study including 64 patients diagnosed with breast cancer and 68 patients diagnosed with benign breast diseases (BBD) who provided samples of tumor tissue and adjacent nontumor tissue to the study. We compared CYP1B1 and COMT mRNA expression in tumor tissue and adjacent nontumor tissue in both breast cancer patients and BBD patients. High levels of CYP1B1 expression and low levels of COMT expression in adjacent nontumor tissue were associated with a significantly increased breast cancer risk in a nonlinear manner. Odds ratios and 95% confidence intervals (in parentheses) for the midpoints of the first, second, fourth, and fifth quintiles of gene expression levels compared with the overall median levels in BBD subjects were 0.21 (0.07-0.67), 0.81 (0.69-0.95), 1.20 (1.05-1.38), and 1.55 (1.12-2.15) for CYP1B1 and 1.72 (1.17-2.55), 1.19 (1.05-1.35), 0.83 (0.73-0.95), and 0.78 (0.65-0.93) for COMT, respectively. These results support the hypothesis that the formation and accumulation of catechol estrogens in breast tissue through increased CYP1B1 expression and reduced COMT expression may play a significant role in breast cancer risk.

Traumatic brain injury stimulates hippocampal catechol- O-methyl transferase expression in microglia

Outcome following traumatic brain injury (TBI) is in large part determined by the combined action of multiple processes. In order to better understand the response of the central nervous system to injury, we utilized an antibody array to simultaneously screen 507 proteins for altered expression in the injured hippocampus, a structure critical for memory formation. Array analysis indicated 41 candidate proteins have altered expression levels 24 h after TBI. Of particular interest was catechol- O-methyl transferase (COMT), an enzyme involved in metabolizing catecholamines released following neuronal activity. Altered catecholamine signaling has been observed after brain injury, and may contribute to the cognitive dysfunctions and behavioral deficits often experienced after TBI. Our data shows that COMT expression in the injured ipsilateral hippocampus was elevated for at least 14 d after controlled cortical impact injury. We found strong co-localization of COMT immunoreactivity with the microglia marker Iba1 near the injury site. Since dopamine transporter expression has been reported to be down-regulated after brain injury, COMT-mediated catecholamine metabolism may play a more prominent role in terminating catecholamine signaling in injured areas.

Hormonal regulation of catechol- O-methyl transferase activity in women with uterine leiomyomas

Catechol-O-methyl transferase (COMT) expression is higher in leiomyomas compared with paired normal myometrium. The expression of COMT in leiomyoma cells is hormonally regulated-estrogen down-regulates, whereas P and dexamethasone up-regulate, COMT expression.

Catechol-O-methyltransferase haplotypes and breast cancer among women on Long Island, New York

The gene encoding catechol-O-methyltransferase (COMT), critical to the inactivation of reactive catechol estrogens, has several single nucleotide polymorphisms (SNPs) that influence enzyme activity. A 3-SNP haplotype (IVS1+255 C>T; Ex4-12 G>A; 3'UTR-521 A>G), which has been shown to reduce COMT expression in the human brain, has been identified. To evaluate the influence of genetic variation of COMT on breast cancer risk, these 3-SNPs were genotyped in 1052 cases and 1098 controls. We estimated the associations between breast cancer and individual SNPs, as well as, multilocus haplotypes. We also examined surrogates of hormone exposure as potential modifiers of the putatively functional Ex4-12 SNP-breast cancer association. Odds ratios (OR) and 95% confidence intervals (CI) were based on age-adjusted unconditional logistic regression models. We found no association between the individual SNPs alone and breast cancer. When examining the association between breast cancer and the 3-SNP haplotypes, we observed a 19% increase in risk associated with each copy of the TGG haplotype (OR=1.19, 95% CI 0.96-1.49), relative to the common TAA haplotype, which was statistically significant when assuming a dominant model (OR=1.32, 95% CI 1.05-1.67, p-value=0.02). In this report of COMT haplotypes and breast cancer, we found some evidence that additional genetic variability beyond the Ex4-12 G>A SNP contributes to risk of breast cancer among a small subgroup of women; however, these results need to be replicated in additional studies.

Prostaglandin J2 reduces catechol-O-methyltransferase activity and enhances dopamine toxicity in neuronal cells

There is clear evidence that an inflammatory reaction is mounted within the CNS following trauma, stroke, infection and seizures, thus augmenting brain damage. Furthermore, chronic inflammation of the CNS is implicated in many neurodegenerative disorders. However, the effects of products of inflammation on neuronal cells are poorly understood. Herein, we characterize the effects of a neurotoxic product of inflammation, prostaglandin J2 (PGJ2), on catechol-O-methyltransferase (COMT) in human dopaminergic-like neuroblastoma SK-N-SH cells and rat (P2) cortical neurons. COMT metabolizes catechols and catecholamines, a pathway relevant to neurodegeneration. PGJ2 treatment reduced the expression and activity of COMT, induced its sequestration into perinuclear aggregates and potentiated dopamine toxicity. The large COMT aggregates were co-localized with the centrosome, suggesting an aggresome-like structure. Our results indicate that COMT impairment induced by PGJ2 treatment may increase the concentration of dopamine (or its metabolites) to neurotoxic levels. Thus, COMT impairment following pro-inflammatory events may be a potential risk factor in neurodegeneration.

Relative imbalances in the expression of estrogen-metabolizing enzymes in the breast tissue of women with breast carcinoma

Estrogens are a known risk factor for breast cancer. Studies indicate that initiation of breast cancer may occur by metabolism of estrogens to form abnormally high levels of catechol estrogen-3,4-quinones, which can then react with DNA to form depurinating adducts and, subsequently, induce mutations that lead to cancer. Among the key enzymes metabolizing estrogens are two activating enzymes: cytochrome P450 (CYP)19 (aromatase), which converts androgens to estrogens, and CYP1B1, which converts estrogens predominantly to the 4-catechol estrogens that are further oxidized to catechol estrogen-3,4-quinones. Formation of the quinones is prevented by methylation of the 4-catechol estrogens by the enzyme, catechol-O-methyltransferase (COMT). In addition, catechol estrogen quinones can be reduced back to catechol estrogens by NADPH quinone oxidoreductase 1 (NQO1) and/or are coupled with glutathione, preventing reaction with DNA. Thus, COMT and NQO1 are key deactivating enzymes. In this initial study, we examined whether the expression of these four critical estrogen activating/deactivating enzymes is altered in breast cancer. Control breast tissue was obtained from four women who underwent reduction mammoplasty. Breast tissues from five women with breast carcinoma, who underwent mastectomy, were used as cases. The level of expression of CYP19, CYP1B1, COMT and NQO1 mRNAs was quantified from total RNA using a real time RT-PCR method in an ABI PRISM 7700 sequence detection system. The control breast tissues showed lower expression of the activating enzymes, CYP19 and CYP1B1, and higher expression of the deactivating enzymes, COMT and NQO1, compared to the cases. In the cases, the reverse pattern was observed: greater expression of activating enzymes and lower expression of deactivating enzymes. Thus, in women with breast cancer, estrogen metabolism may be related to altered expression of multiple genes. These unbalances appear to be instrumental in causing excessive formation of catechol estrogen quinones that, by reacting with DNA, initiate the series of events leading to breast cancer.

COMT Polymorphisms and Anxiety-Related Personality Traits

High neuroticism and low extraversion are characteristic of anxiety-prone individuals. A functional variant in the catechol-O-methyltransferase (COMT) gene, the Val158Met ('val/met') polymorphism, has been associated in some prior studies with several phenotypes, including neuroticism. We tested the hypothesis that the val158met polymorphism would be associated with both high neuroticism and low extraversion, making it a plausible candidate locus for anxiety susceptibility. To determine whether val158met is responsible for these effects, we also evaluated the association with haplotypes that included two other SNPs within the COMT gene. We collected a sample of 497 undergraduate college students who were phenotyped on a personality inventory (the NEO-Personality Inventory-Raised (NEO-PI-R)). Subjects were genotyped for three COMT polymorphisms: the well-studied nonsynonymous SNP rs4680 that generates a valine-to-methionine substitution (val158met), rs737865 (near exon #1), and rs165599 (also functional, near the 3'-UTR). Together, these three SNPs define a haplotype that is associated with reduced COMT expression in human brain. Logistic regression analyses were used to examine the effects of individual SNPs on extraversion and neuroticism scores. Score tests for association between these traits (quantitatively and dichotomously considered) and haplotypes were also conducted. We evaluated potential for population stratification artifact by genotyping a set of 36 unlinked highly polymorphic markers previously demonstrated to distinguish sufficiently ancestry of major American populations. Two of the SNPs (rs4680 ('val/met') and rs737865) were significantly associated with (low) extraversion and, less consistently, with (high) neuroticism, with effects confined to women. A significant association between COMT haplotype and (low) extraversion and (high) neuroticism was also observed. Formal testing showed that population structure did not explain the findings. These data suggest that involvement of the COMT locus in susceptibility to anxiety-related traits (ie low extraversion and high neuroticism) is unlikely to be wholly accounted for by the well-studied rs4680 ('val/met') polymorphism. Other functional variants may exist that contribute to this relationship. Possible sex-specific effects remain to be further studied and explained.

2,3,7,8-Tetrachlorodibenzo- p-dioxin induces CYP1B1 expression in human luteinized granulosa cells

The environmental contaminant 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) is a reproductive toxicant in multiple species; however, mechanisms and direct ovarian effects are poorly understood. DNA microarrays were used to characterize gene expression profiles of human luteinized granulosa cells (HLGCs) exposed to TCDD in primary cultures. Exposure to 10 nM TCDD for 24 h induced a significant increase in CYP1B1, while few other genes responded. TaqMan PCR and Western immunoblotting demonstrated that induction was dose-dependent. Additionally, the microsomal form of catechol- O-methyltransferase (COMT) was highly expressed in HLGCs, along with only fractional amounts of the soluble form. This is the first report of CYP1B1 and COMT expression, and CYP1B1 induction, in cells from the human ovary. The role of CYP1B1 in the oxidative metabolism of estrogens and potential generation of DNA adducts in the ovary may have significant consequences for oocyte quality, corpus luteum function, and ovarian carcinogenesis.

Association of Ala72Ser polymorphism with COMT enzyme activity and the risk of schizophrenia in Koreans

Catechol-O-methyltransferase (COMT) inactivates circulating catechol hormones, catechol neurotransmitters, and xenobiotic catecholamines by methylating their catechol moieties. The COMT gene has been suggested as a candidate gene for schizophrenia through linkage analyses and molecular studies of velo-cardio-facial syndrome. A coding polymorphism of the COMT gene at codon 108/158 (soluble/membrane-bound form) causing a valine to methionine substitution has been shown to influence enzyme activity, but its association with schizophrenia is inconclusive. We have screened 17 known polymorphisms of the COMT gene in 320 Korean schizophrenic patients and 379 controls to determine whether there is a positive association with a nonsynonymous single-nucleotide polymorphism (rs6267) at codon 22/72 (soluble/membrane-bound form) causing an alanine-to-serine (Ala/Ser) substitution. With the Ala/Ala genotype as a reference group, the combined genotype (Ala/Ser and Ser/Ser)-specific adjusted odds ratio was 1.82 (95% CI = 1.19-2.76; P = 0.005), suggesting the Ser allele as a risk allele for schizophrenia. However, the Val/Met polymorphism was not associated with an increased risk of schizophrenia in Koreans (OR = 0.88, 95% CI = 0.64-1.21; P = 0.43). The Ala72Ser substitution was correlated with reduced COMT enzyme activity. Our results support previous reports that the COMT haplotype implicated in schizophrenia is associated with low COMT expression.

A Haplotype Implicated in Schizophrenia Susceptibility Is Associated with Reduced COMT Expression in Human Brain

The gene encoding catechol-O-methyltransferase (COMT) is a strong candidate for schizophrenia susceptibility, owing to the role of COMT in dopamine metabolism, and the location of the gene within the deleted region in velocardiofacial syndrome, a disorder associated with high rates of schizophrenia. Recently, a highly significant association was reported between schizophrenia and a COMT haplotype in a large case-control sample (Shifman et al. 2002). In addition to a functional valine-->methionine (Val/Met) polymorphism, this haplotype included two noncoding single-nucleotide polymorphisms (SNPs) at either end of the COMT gene. Given the role of COMT in dopamine catabolism and that deletion of 22q11 (containing COMT) is associated with schizophrenia, we postulated that the susceptibility COMT haplotype is associated with low COMT expression. To test this hypothesis, we have applied quantitative measures of allele-specific expression using mRNA from human brain. We demonstrate that COMT is subject to allelic differences in expression in human brain and that the COMT haplotype implicated in schizophrenia (Shifman et al. 2002) is associated with lower expression of COMT mRNA. We also show that the 3' flanking region SNP that gave greatest evidence for association with schizophrenia in that study is transcribed in human brain and exhibits significant differences in allelic expression, with lower relative expression of the associated allele. Our results indicate that COMT variants other than the Val/Met change are of functional importance in human brain and that the haplotype implicated in schizophrenia susceptibility is likely to exert its effect, directly or indirectly, by down-regulating COMT expression.

X-ray Crystal Structure of a Bisubstrate Inhibitor Bound to the Enzyme Catechol-O-methyltransferase: A Dramatic Effect of Inhibitor Preorganization on Binding Affinity.

With an IC50 value of 9 nM, 1 is the most potent known disubstrate inhibitor for catechol-O-methyltransferase (COMT). Inhibition of COMT is of significant interest in the therapy of Parkinsonapos;s disease since it ensures that a larger percentage of orally administered L-dopa reaches-in the form of dopamine-its target in the brain. The X-ray crystal structure of a complex formed by COMT and 1 has been solved at 2.6-Å resolution.

Biogenesis and Inactivation of Catecholestrogens

Publisher Summary Catecholestrogens (CEs) can be generated by several discrete enzymatic pathways that can result in the formation of both 2- and 4- hydroxylated CEs (4-OH-CEs). These two classes of CEs differ in their properties both as catechols and as estrogens (Es). This chapter summarizes the value of these insights, availability of specific and sensitive assays for measuring CE synthase activities, and molecular biological tools now available for probing the role of CEs as local mediators of the physiological actions of Es. Hydroxylation of Es at C-2 and C-4 can be catalyzed by two distinct biochemical mechanisms. The two differ in their cofactor requirement and in the profile of CEs they generate. It generates predominantly either 2- Or 4-OH-CEs, depending on the form of P450 involved. The second generates 2- and 4-OH-CEs in similar amounts. As peroxidative activity generates 2- and 4-OH-CEs in similar amounts, and 2-OH-CEs can inhibit the inactivation of 4-OH-CEs by COMT, the peroxidative pathway of CE formation could be especially relevant to pathophysiology. The 2- and 4-OH-CEs differ also in their properties as catechols. Thus, although 2- and 4-OH-CEs have similar apparent affinity for catechol-O-methyltransferase (COMT) and similar maximal velocities of O-methylation, 2-OH-CEs can inhibit the O-methylation of 4-OH-CEs. COMT in liver and red blood cells plays a major role in bulk inactivation of catechols, including CEs generated in liver. In extrahepatic organs, such as brain, uterus, kidney, and mammary gland, COMT has been shown to be expressed only in a few anatomically defined loci and cell populations. COMT participates in regulation of local concentrations of catechols, including CEs. Studies of expression of COMT in uterus in relationship to implantation also support this notion.