Structure of the rat catechol-O-methyltransferase gene: separate promoters are used to produce mRNAs for soluble and membrane-bound forms of the enzyme

Difference in Methylation and Expression of Brain-Derived Neurotrophic Factor in Alzheimer's Disease and Mild Cognitive Impairment

Due to the increasing number of progressive dementias in the population, numerous studies are being conducted that seek to determine risk factors, biomarkers and pathological mechanisms that could help to differentiate between normal symptoms of aging, mild cognitive impairment (MCI) and dementia. The aim of this study was to investigate the possible association of levels of BDNF and COMT gene expression and methylation in peripheral blood cells with the development of Alzheimer's disease (AD). Our results revealed higher expression levels of BDNF (p < 0.001) in MCI subjects compared to individuals diagnosed with AD. However, no difference in COMT gene expression (p = 0.366) was detected. DNA methylation of the CpG islands and other sequences with potential effects on gene expression regulation revealed just one region (BDNF_9) in the BDNF gene (p = 0.078) with marginally lower levels of methylation in the AD compared to MCI subjects. Here, we show that the level of BDNF expression in the periphery is decreased in subjects with AD compared to individuals with MCI. The combined results from the gene expression analysis and DNA methylation analysis point to the potential of BDNF as a marker that could help distinguish between MCI and AD patients.

Membrane bound catechol-O-methytransferase is the dominant isoform for dopamine metabolism in PC12 cells and rat brain

Impaired dopamine activity in the dorsolateral prefrontal cortex (DLPFC) is thought to contribute to cognitive deficits in diseases such as schizophrenia, attention deficit hyperactivity disorder (ADHD) and traumatic brain injury. Catechol-O-methyltransfease (COMT) metabolizes dopamine and is an important regulator of dopamine signaling in the DLPFC. In mammalian species, two isoforms of COMT protein, membrane-bound COMT (MB-COMT) and soluble COMT (S-COMT), are encoded by one COMT gene and expressed widely. While S-COMT is thought to play a dominant role in the peripheral tissues, MB-COMT is suggested to have a greater role in dopamine metabolism in the brain. However, whether a selective inhibitor for MB-COMT may effectively block dopamine metabolism remains unknown. We generated a knockout of MB-COMT in PC12 cells using CRISPR-cas9 technology to evaluate the effect of both MB and S-COMT on dopamine metabolism. Deletion of MB-COMT in PC12 cells significantly decreased homovanillic acid (HVA), completely depleted 3-methyoxytyramine (3-MT), and significantly increased 3,4-dihydroxyphenylacetic acid (DOPAC) levels. Comparison of the effect of a MB-COMT selective inhibitor LI-1141 on dopamine metabolism in wild type and MB-COMT knockout PC12 cells allowed us to confirm the selectivity of LI-1141 with respect to MB-COMT in cells. Under conditions in which LI-1141 was shown to inhibit only MB-COMT but not S-COMT, it effectively changed dopamine metabolites similar to the effect induced by tolcapone, a non-selective COMT inhibitor, suggesting that selective inhibition of MB-COMT will be effective in blocking dopamine metabolism, providing an attractive therapeutic approach in improving cognition for patients.

Tissue and interspecies comparison of catechol-O-methyltransferase mediated catalysis of 6-O-methylation of esculetin to scopoletin and its inhibition by entacapone and tolcapone

Catechol-O-methyltransferase (COMT) methylates both endogenous and exogenous catechol compounds to inactive and safe metabolites. We first optimised conditions for a convenient and sensitive continuous fluorescence-based 6-O-methylation assay of esculetin, which we used for investigating the COMT activity in human, mouse, rat, dog, rabbit, and sheep liver cytosols and microsomes and in ten different rat tissues. Furthermore, we compared the inhibition potencies and mechanisms of two clinically used COMT inhibitors, entacapone and tolcapone, in these species. In most tissues, the COMT activity was at least three times higher in cytosol than in microsomes. In the rat, the highest COMT activity was found in the liver, followed by kidney, ileum, thymus, spleen, lung, pancreas, heart, brain, and finally, skeletal muscle. Entacapone and tolcapone were characterised as highly potent mixed type tight-binding inhibitors. The competitive inhibition type dominated over the uncompetitive inhibition with entacapone, whereas uncompetitive inhibition dominated with tolcapone. Rats, dogs, pigs, and sheep are high COMT activity species, in contrast to humans, mice, and rabbits; COMT activity is highest in the liver. Both entacapone and tolcapone are potent COMT inhibitors, but their inhibition mechanisms differ.

Novel, non-nitrocatechol catechol-O-methyltransferase inhibitors modulate dopamine neurotransmission in the frontal cortex and improve cognitive flexibility

RATIONALE

Cognitive impairment is a primary feature of many neuropsychiatric disorders and there is a need for new therapeutic options. Catechol-O-methyltransferase (COMT) inhibitors modulate cortical dopaminergic function and have been proposed as potential cognitive enhancers. Unfortunately, currently available COMT inhibitors are not good candidates due to either poor blood-brain barrier penetration or severe toxicity.

OBJECTIVES

To address the need for safe, brain-penetrant COMT inhibitors, we tested multiple novel compounds in a set of preclinical in vivo efficacy assays in rats to determine their ability to inhibit COMT function and viability as potential clinical candidates.

METHODS

We measured the change in concentration of dopamine (DA) metabolites in cerebrospinal fluid (CSF) from the cisterna magna and extracellular fluid (ECF) from the frontal cortex produced by our novel compounds. Additionally, we tested the effects of our brain-penetrant COMT inhibitors in an attentional set-shifting assay (ASST). We benchmarked the performance of the novel COMT inhibitors to the effects produced by the known COMT inhibitor tolcapone.

RESULTS

We found that multiple COMT inhibitors, exemplified by LIBD-1 and LIBD-3, significantly modulated dopaminergic function measured as decreases in homovanillic acid (HVA) and increases in 3,4-Dihydroxyphenylacetic acid (DOPAC), two DA metabolites, in CSF and the frontal cortex. Additionally, we found that LIBD-1 significantly improved cognitive flexibility in the ASST, an effect previously reported following tolcapone administration.

CONCLUSIONS

These results demonstrate that LIBD-1 is a novel COMT inhibitor with promising in vivo activity and the potential to serve as a new therapy for cognitive impairment.

The Association Between C-Reactive Protein and Postoperative Delirium Differs by Catechol-O-Methyltransferase Genotype

OBJECTIVE

Catechol-O-methyltransferase (COMT), a key enzyme in degrading catecholamines associated with the stress response, may influence susceptibility to delirium. Individuals with the COMT (rs4680) Val/Val genotype (designated "warriors") withstand the onset of neuropsychiatric disorders and cognitive decline, whereas individuals with Met/Met and Val/Met genotypes ("nonwarriors") are more susceptible to these conditions. We evaluated whether COMT genotype modifies the established association between acute phase reactant (stress marker) C-reactive protein (CRP) and postoperative delirium.

METHODS

This was a prospective cohort study conducted at two academic medical centers. The study involved 547 patients aged 70 or older undergoing major noncardiac surgery. We collected blood, extracted DNA, and performed COMT genotyping using allele-specific polymerase chain reaction assays, considering warriors versus nonwarriors. High plasma CRP, measured on postoperative day 2 using enzyme-linked immunosorbent assay, was defined by the highest sample-based quartile (≥234.12 mg/L). Delirium was determined using the Confusion Assessment Method, augmented by a validated chart review. We used generalized linear models adjusted for age, sex, surgery type, and race/ethnicity, stratified by COMT genotype, to determine whether the association between CRP and delirium differed by COMT.

RESULTS

Prevalence of COMT warriors was 26%, and postoperative delirium occurred in 23%. Among COMT warriors, high CRP was not associated with delirium (relative risk [RR] 1.0, 95% confidence interval [CI] 0.4-2.6). In contrast, among nonwarriors, we found the expected relationship of high CRP and delirium (RR 1.5, 95% CI 1.1-2.2).

CONCLUSION

COMT warriors may be protected against the increased risk of delirium associated with high CRP on postoperative day 2. With further confirmation, COMT genotype may help target interventions for delirium prevention in the vulnerable nonwarrior group.

Delayed O-methylation of l-DOPA in MB-COMT-deficient mice after oral administration of l-DOPA and carbidopa

1. Catechol-O-methyltransferase (COMT) is involved in the O-methylation of l-DOPA, dopamine, and other catechols. The enzyme is expressed in two isoforms: soluble (S-COMT), which resides in the cytoplasm, and membrane-bound (MB-COMT), which is anchored to intracellular membranes. 2. To obtain specific information on the functions of COMT isoforms, we studied how a complete MB-COMT deficiency affects the total COMT activity in the body, peripheral l-DOPA levels, and metabolism after l-DOPA (10 mg kg^-1) plus carbidopa (30 mg kg^-1) administration by gastric tube in wild-type (WT) and MB-COMT-deficient mice. l-DOPA and 3-O-methyl-l-DOPA (3-OMD) levels were assayed in plasma, duodenum, and liver. 3. We showed that the selective lack of MB-COMT did not alter the total COMT activity, COMT enzyme kinetics, l-DOPA levels, or the total O-methylation of l-DOPA but delayed production of 3-OMD in plasma and peripheral tissues.

Lack of Cell Proliferative and Tumorigenic Effects of 4-Hydroxyestradiol in the Anterior Pituitary of Rats: Role of Ultrarapid O-Methylation Catalyzed by Pituitary Membrane-Bound Catechol-O-Methyltransferase

In animal models, estrogens are complete carcinogens in certain target sites. 4-Hydroxyestradiol (4-OH-E₂), an endogenous metabolite of 17β-estradiol (E₂), is known to have prominent estrogenic activity plus potential genotoxicity and mutagenicity. We report here our finding that 4-OH-E₂ does not induce pituitary tumors in ACI female rats, whereas E₂ produces 100% pituitary tumor incidence. To probe the mechanism, we conducted a short-term animal experiment to compare the proliferative effect of 4-OH-E₂ in several organs. We found that, whereas 4-OH-E₂ had little ability to stimulate pituitary cell proliferation in ovariectomized female rats, it strongly stimulates cell proliferation in certain brain regions of these animals. Further, when we used in vitro cultured rat pituitary tumor cells as models, we found that 4-OH-E₂ has similar efficacy as E₂ in stimulating cell proliferation, but its potency is approximately 3 orders of magnitude lower than that of E₂. Moreover, we found that the pituitary tumor cells have the ability to selectively metabolize 4-OH-E₂ (but not E₂) with ultrahigh efficiency. Additional analysis revealed that the rat pituitary expresses a membrane-bound catechol-O-methyltransferase that has an ultralow K_m value (in nM range) for catechol estrogens. On the basis of these observations, it is concluded that rapid metabolic disposition of 4-OH-E₂ through enzymatic O-methylation in rat anterior pituitary cells largely contributes to its apparent lack of cell proliferative and tumorigenic effects in this target site.

From Pregnancy to Preeclampsia: A Key Role for Estrogens

Preeclampsia (PE) results in placental dysfunction and is one of the primary causes of maternal and fetal mortality and morbidity. During pregnancy, estrogen is produced primarily in the placenta by conversion of androgen precursors originating from maternal and fetal adrenal glands. These processes lead to increased plasma estrogen concentrations compared with levels in nonpregnant women. Aberrant production of estrogens could play a key role in PE symptoms because they are exclusively produced by the placenta and they promote angiogenesis and vasodilation. Previous assessments of estrogen synthesis during PE yielded conflicting results, possibly because of the lack of specificity of the assays. However, with the introduction of reliable analytical protocols using liquid chromatography/mass spectrometry or gas chromatography/mass spectrometry, more recent studies suggest a marked decrease in estradiol levels in PE. The aim of this review is to summarize current knowledge of estrogen synthesis, regulation in the placenta, and biological effects during pregnancy and PE. Moreover, this review highlights the links among the occurrence of PE, estrogen biosynthesis, angiogenic factors, and cardiovascular risk factors. A close link between estrogen dysregulation and PE occurrence might validate estrogen levels as a biomarker but could also reveal a potential approach for prevention or cure of PE.

Meta-Analysis of the COMT Val158Met Polymorphism in Major Depressive Disorder: Effect of Ethnicity

The COMT (catechol-O-methyltransferase) Val158Met polymorphism (rs4680) is a potential susceptibility variant for major depressive disorder (MDD). Although many genetic studies have examined the association between MDD and this polymorphism, the results were inconclusive. In the present study, we conducted a series of meta-analyses of samples consisting of 2905 MDD cases and 2403 controls with the goal of determining whether this variant indeed has any effect on MDD. We revealed a significant association in the comparison of Val/Val + Val/Met vs. Met/Met (OR =1.180; 95 % CI = 1.019, 1.367; P = 0.027), Val/Met vs. Val/Val (OR =1.18; 95 % CI = 1.038, 1.361; P = 0.013), and Val/Met vs. Met/Met (OR =1.229; 95 % CI = 1.053, 1.435; P = 0.009). Further meta-analyses of samples with European ancestry demonstrated a significant association of this SNP with MDD susceptibility in Val/Val + Val/Met vs. Met/Met (OR =1.231, 95 % CI = 1.046, 1.449; P = 0.013) and Val/Met vs. Met/Met (OR =1.284, 95 % CI = 1.050, 1.484; P = 0.012). For the samples with East Asian ancestry, we found a significant association in both allelic (Val vs. Met: OR =0.835; 95 % CI = 0.714, 0.975; P = 0.023) and genotypic (Met/Met + Val/Met vs. Val/Val: OR =1.431, 95 % CI = 1.143, 1.791; P = 0.002; Val/Met vs. Val/Val: OR =1.482, 95 % CI = 1.171, 1.871; P = 0.001) analyses. No evidence of heterogeneity among studies or publication bias was observed. Together, our results indicate that the COMT Val158Met polymorphism is a vulnerability factor for MDD with distinct effects in different ethnic populations.

Common single nucleotide variants underlying drug addiction: more than a decade of research

Drug-related phenotypes are common complex and highly heritable traits. In the last few years, candidate gene (CGAS) and genome-wide association studies (GWAS) have identified a huge number of single nucleotide polymorphisms (SNPs) associated with drug use, abuse or dependence, mainly related to alcohol or nicotine. Nevertheless, few of these associations have been replicated in independent studies. The aim of this study was to provide a review of the SNPs that have been most significantly associated with alcohol-, nicotine-, cannabis- and cocaine-related phenotypes in humans between the years of 2000 and 2012. To this end, we selected CGAS, GWAS, family-based association and case-only studies published in peer-reviewed international scientific journals (using the PubMed/MEDLINE and Addiction GWAS Resource databases) in which a significant association was reported. A total of 371 studies fit the search criteria. We then filtered SNPs with at least one replication study and performed meta-analysis of the significance of the associations. SNPs in the alcohol metabolizing genes, in the cholinergic gene cluster CHRNA5-CHRNA3-CHRNB4, and in the DRD2 and ANNK1 genes, are, to date, the most replicated and significant gene variants associated with alcohol- and nicotine-related phenotypes. In the case of cannabis and cocaine, a far fewer number of studies and replications have been reported, indicating either a need for further investigation or that the genetics of cannabis/cocaine addiction are more elusive. This review brings a global state-of-the-art vision of the behavioral genetics of addiction and collaborates on formulation of new hypothesis to guide future work.

Association of COMT and TPH-2 genes with DSM-5 based PTSD symptoms

BACKGROUND

Dopaminergic and serotonergic systems have been implicated in PTSD. The present study evaluated the association of four catechol-O-methyltransferase (COMT) gene loci, and the joint effect of COMT and tryptophan hydroxylase 2 (TPH2) genes on PTSD symptoms.

METHODS

Subjects included 200 Caucasian Armenian adults exposed to the 1988 Spitak earthquake from 12 multigenerational (3-5 generations) families. Instruments used included the UCLA PTSD Reaction Index based on DSM-5 criteria, and the Beck Depression Inventory.

RESULTS

The adjusted heritabilitiy of vulnerability to DSM-5 based PTSD symptoms was 0.60 (p<10(-4)). There was a significant association of the COMT allele rs4633C with total PTSD (p<0.03), and D category (p<0.04) (negative alterations in cognitions and mood) severity scores, but not with C category (avoidance) scores. There was no genetic correlation between C and D category severity scores. COMT allele rs4633C and the TPH-2 allele rs11178997T together accounted for 7% of the variance in PTSD severity scores (p<0.001). None of the COMT alleles were associated with depression.

LIMITATIONS

The ratings of earthquake exposure and prior trauma may have been subject to recall bias. The findings may not be generalizable to other ethnic/racial populations.

CONCLUSION

COMT allele rs4633C may be causally related and/or is in linkage disequilibrium with gene(s) that are causally related to PTSD symptoms. Carriers of these COMT and the TPH-2 alleles may be at increased risk for PTSD. The findings provide biological support for dividing DSM-IV category C symptoms into DSM-5 categories C and D.

Epistasis between polymorphisms in COMT, ESR1, and GCH1 influences COMT enzyme activity and pain

Abnormalities in the enzymatic activity of catechol-O-methyltransferase (COMT) contribute to chronic pain conditions, such as temporomandibular disorders (TMD). Thus, we sought to determine the effects of polymorphisms in COMT and functionally related pain genes in the COMT pathway (estrogen receptor 1 [ESR1], guanosine-5-triphosphate cyclohydrolase 1 [GCH1], methylenetetrahydrofolate reductase [MTHFR]) on COMT enzymatic activity, musculoskeletal pain, and pain-related intermediate phenotypes among TMD cases and healthy control subjects. Results show that the COMT rs4680 (val(158)met) polymorphism is most strongly associated with outcome measures, such that individuals with the minor A allele (met) exhibit reduced COMT activity, increased TMD risk, and increased musculoskeletal pain. Epistatic interactions were observed between the COMT rs4680 polymorphism and polymorphisms in GCH1 and ESR1. Among individuals with the COMT met allele, those with 2 copies of the GCH1 rs10483639 minor G allele exhibit normalized COMT activity and increased mechanical pain thresholds. Among individuals with the COMT val allele, those with 2 copies of the ESR1 rs3020377 minor A allele exhibit reduced COMT activity, increased bodily pain, and poorer self-reported health. These data reveal that the GCH1 minor G allele confers a protective advantage among met carriers, whereas the ESR1 minor A allele is disadvantageous among val carriers. Furthermore, these data suggest that the ability to predict the downstream effects of genetic variation on COMT activity is critically important to understanding the molecular basis of chronic pain conditions.

Medicinal chemistry of catechol O-methyltransferase (COMT) inhibitors and their therapeutic utility

Catechol O-methyltransferase (COMT) is the enzyme responsible for the O-methylation of endogenous neurotransmitters and of xenobiotic substances and hormones incorporating catecholic structures. COMT is a druggable biological target for the treatment of various central and peripheral nervous system disorders, including Parkinson's disease, depression, schizophrenia, and other dopamine deficiency-related diseases. The purpose of this perspective is fourfold: (i) to summarize the physiological role of COMT inhibitors in central and peripheral nervous system disorders; (ii) to provide the history and perspective of the medicinal chemistry behind the discovery and development of COMT inhibitors; (iii) to discuss how the physicochemical properties of recognized COMT inhibitors are understood to exert influence over their pharmacological properties; and (iv) to evaluate the clinical benefits of the most relevant COMT inhibitors.

Gene-sex interactions in schizophrenia: focus on dopamine neurotransmission

Schizophrenia is a severe mental disorder, with a highly complex and heterogenous clinical presentation. Our current perspectives posit that the pathogenic mechanisms of this illness lie in complex arrays of gene × environment interactions. Furthermore, several findings indicate that males have a higher susceptibility for schizophrenia, with earlier age of onset and overall poorer clinical prognosis. Based on these premises, several authors have recently begun exploring the possibility that the greater schizophrenia vulnerability in males may reflect specific gene × sex (G×S) interactions. Our knowledge on such G×S interactions in schizophrenia is still rudimentary; nevertheless, the bulk of preclinical evidence suggests that the molecular mechanisms for such interactions are likely contributed by the neurobiological effects of sex steroids on dopamine (DA) neurotransmission. Accordingly, several recent studies suggest a gender-specific association of certain DAergic genes with schizophrenia. These G×S interactions have been particularly documented for catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO), the main enzymes catalyzing DA metabolism. In the present review, we will outline the current evidence on the interactions of DA-related genes and sex-related factors, and discuss the potential molecular substrates that may mediate their cooperative actions in schizophrenia pathogenesis.

Genetic polymorphisms of catechol-O-methyltransferase modify the neurobehavioral effects of mercury in children

Mercury (Hg) is neurotoxic and children may be particularly susceptible to this effect. A current major challenge is identification of children who may be uniquely susceptible to Hg toxicity because of genetic disposition. This study examined the hypothesis that genetic variants of catechol-O-methyltransferase (COMT) that are reported to alter neurobehavioral functions that are also affected by Hg in adults might modify the adverse neurobehavioral effects of Hg exposure in children. Five hundred and seven children, 8-12 yr of age at baseline, participated in a clinical trial to evaluate the neurobehavioral effects of Hg from dental amalgam tooth fillings. Subjects were evaluated at baseline and at seven subsequent annual intervals for neurobehavioral performance and urinary Hg levels. Following the clinical trial, genotyping assays were performed for single-nucleotide polymorphisms (SNPs) of COMT rs4680, rs4633, rs4818, and rs6269 on biological samples provided by 330 of the trial participants. Regression-modeling strategies were employed to evaluate associations between allelic status, Hg exposure, and neurobehavioral test outcomes. Similar analysis was performed using haplotypes of COMT SNPs. Among girls, few interactions for Hg exposure and COMT variants were found. In contrast, among boys, numerous gene-Hg interactions were observed between individual COMT SNPs, as well as with a common COMT haplotype affecting multiple domains of neurobehavioral function. These findings suggest increased susceptibility to the adverse neurobehavioral effects of Hg among children with common genetic variants of COMT, and may have important implications for strategies aimed at protecting children from the potential health risks associated with Hg exposure.

Children under stress - COMT genotype and stressful life events predict cortisol increase in an acute social stress paradigm

Dopamine and norepinephrine are key regulators of cognitive and affective processes. The enzyme catechol-O-methyltransferase (COMT) catabolizes catecholamines and the COMT Val158Met polymorphism has been linked to several neuropsychiatric variables. Additionally, stressful life events (SLEs) contribute substantially to affective processes. We used the stress-induced activation of the hypothalamic-pituitary-adrenal axis to investigate the effects of COMT and SLEs on the cortisol response in 119 healthy children (8-12 yr). Saliva cortisol was measured during and after the Trier Social Stress Test for Children. SLEs were assessed with a standardized interview with one of the children's parents. Linear regression analysis revealed a significant effect for COMT, with Met allele carriers showing a higher cortisol response (β=0.300, p=0.001). In turn, more SLEs lead to a less pronounced cortisol increase (β=-0.192, p=0.029) probably indicating increased resilience. Our results further underscore the essential and differential role of genetic variation and environmental factors on stress responsivity.

A single-nucleotide polymorphism in the fetal catechol-O-methyltransferase gene is associated with spontaneous preterm birth in African Americans

Catechol-O-methyltransferase (COMT) activity has been reported to be higher in African Americans (AA) than Caucasians (Cau). COMT converts 2- and 4-hydroxy (OH) estrogens to 2- and 4-methoxyestrogens, respectively, and can increase estrogenic milieu locally in tissues. To assess whether the increased incidence of preterm birth (PTB) among AA women is associated with single-nucleotide polymorphism (SNP) in the COMT gene, we examined variations in maternal and fetal COMT genes and their association with pregnancy outcomes (term vs preterm pregnancies) using 4 functional SNPs: rs4633, rs4680, rs4818, and rs6269 in both AA and Cau. We analyzed samples from 267 AA women (191 term and 76 preterm pregnancies) and 339 Cau (194 term and 145 preterm pregnancies) in this study. The results showed a significant difference (P < .05) in allele and genotype frequencies between term and preterm AA and Cau women in 3 SNPs in both maternal and fetal DNA. The analysis revealed that in AA fetal COMT genes, SNP rs4818 is associated with PTB at the allele (C; P < .001), genotype (C/C; P < .01), and 2- (P < .03) and 3 (P < .04)-window haplotype levels. Multidimensionality reduction analysis also showed a significant (P < .01) association between rs4818 and PTB. In conclusion, our study demonstrated that a synonymous polymorphism, rs4818 in the fetal COMT gene, is associated with PTB in AA.

Variation in genes involved in dopamine clearance influence the startle response in older adults

The dopamine transporter (DAT) and the enzyme catechol-O-methyltransferase (COMT) both terminate synaptic dopamine action. Here, we investigated the influence of two polymorphisms in the respective genes: DAT1 (SLC6A3) VNTR and COMT val(158)met (rs4680). Startle magnitudes to intense noise bursts as measured with the eye blink response were recorded during the presentation of pictures of three valence conditions (unpleasant, pleasant and neutral) and during baseline without additional pictorial stimulation in a sample of healthy older adults (N = 94). There was a significant Bonferroni corrected main effect of COMT genotype on the overall startle responses, with met/met homozygotes showing the highest and participants with the val/val genotype showing the lowest startle response, while participants with the val/met genotype displayed intermediate reactions. There was also a DAT1 VNTR main effect, which, after Bonferroni correction, still showed a tendency toward significance with carriers of at least one 9-repeat (R) allele showing smaller overall startle responses compared to 10R/10R homozygotes. Thus, older adult carriers of COMT variants, which result in lower enzyme activity and therefore probably enhanced dopamine signaling, showed stronger startle activity. Although the functional significance of DAT1 VNTR is less defined, our results point to a potential influence of SLC6A3 on startle magnitude.

Association study of a functional catechol-O-methyltransferase polymorphism and smoking in healthy Caucasian subjects

Tobacco smoking is a global health problem. The association of a functional common polymorphism in the catechol-o-methyltransferase gene (COMT Val158Met) with smoking behavior has been extensively studied, but with divergent findings. In the present study the frequency of COMT genotypes and alleles was evaluated in 578 male and a smaller group of 79 female unrelated, medication-free Caucasian healthy subjects of Croatian origin. Smokers were classified as subjects smoking <or=10 cigarettes per day, while subjects who never smoked in their life were regarded as nonsmokers. A chi(2)-test with standardized residuals and Bonferroni correction revealed significant (P=0.017) differences in Met/Met, Met/Val or Val/Val genotype frequency between male smokers and nonsmokers. This significant association between COMT Val158Met polymorphism and smoking was not detected in female subjects, due to the small number of women, which represents a limitation of the study. Our results confirmed the significant association between COMT variants and smoking, which was due to the higher frequency of Val/Val homozygotes in male smokers compared to male nonsmokers. These results suggest that carriers of the high activity COMT variant are more prone to develop a higher level of nicotine dependence, or that they release more dopamine than carriers of Met/Met or Met/Val genotypes.

Gonadectomy and hormone replacement exert region- and enzyme isoform-specific effects on monoamine oxidase and catechol-O-methyltransferase activity in prefrontal cortex and neostriatum of adult male rats

Sex differences and gonadal hormone influences are well known for diverse aspects of forebrain amine and indolamine neurotransmitter systems, the cognitive and affective functions they govern and their malfunction in mental illness. This study explored whether hormone regulation/dysregulation of these systems could be related to gonadal steroid effects on catechol-O-methyltransferase and monoamine oxidase which are principal enzymatic controllers of forebrain dopamine, serotonin and norepinephrine levels. Driven by male over female differences in cortical enzyme activities, by male-specific associations between monoamine oxidase and catechol-O-methyltransferase gene polymorphisms and cognitive and dysfunction in disease and by male-specific consequences of gene knockouts in mice, the question of hormone sensitivity was addressed here using a male rat model where prefrontal dopamine levels and related behaviors are also known to be affected. Specifically, quantitative O-methylation and oxidative deamination assays were used to compare the activities of catechol-O-methyltransferase's soluble and membrane-bound isoforms and of monoamine oxidase's A and B isoforms in the pregenual medial prefrontal cortex and dorsal striatum of male rats that were sham operated, gonadectomized or gonadectomized and supplemented with testosterone propionate or with estradiol for 28 days. These studies revealed significant effects of hormone replacement but not gonadectomy on the soluble but not the membrane-bound isorfom of catechol-O-methyltransferase in both striatum and cortex. A significant, cortex-specific testosterone-but not estradiol-attenuated effect (increase) of gonadectomy on monoamine oxidase's A but not B isoform was also observed. Although none of these actions suggest potential roles in the regulation/dysregulation of prefrontal dopamine, the suppressive effects of testosterone on cortical monoamine oxidase-A that were observed could have bearing on the increased incidence of cognitive deficits and symptoms of depression and anxiety that are repeatedly observed in males in conditions of hypogonadalism related to aging, other biological factors or in prostate cancer where androgen deprivation is used as a neoadjuvant treatment.

Variation in the COMT gene: implications for pain perception and pain treatment

Catechol-O-methyltransferase (COMT) is an enzyme that inactivates biologically-active catechols, including the important neurotransmitters dopamine, noradrenaline and adrenaline. These neurotransmitters are involved in numerous physiological processes, including modulation of pain. Genetic variation in the COMT gene has been implicated in variable response to various experimental painful stimuli, variable susceptibility to develop common pain conditions, as well as the variable need for opioids in the treatment of cancer pain. Increased insight into how genetic variants within the COMT locus affect pain perception will contribute to improved understanding of the mechanisms involved in the development of common human pain disorders and may lead to improved strategies for pain treatment. So far, a remarkable complex relationship between COMT genotypes or haplotypes and pain phenotypes has been revealed.

A hotspot of inactivation: The A22S and V108M polymorphisms individually destabilize the active site structure of catechol O-methyltransferase

Human catechol O-methyltransferase (COMT) contains three common polymorphisms (A22S, A52T, and V108M), two of which (A22S and V108M) render the protein susceptible to deactivation by temperature or oxidation. We have performed multiple molecular dynamics simulations of the wild-type, A22S, A52T, and V108M COMT proteins to explore the structural consequences of these mutations. In total, we have amassed more than 1.4 micros of simulation time, representing the largest set of simulations detailing the effects of polymorphisms on a protein system to date. The A52T mutation had no significant effect on COMT structure in accord with experiment, thereby serving as a good negative control for the simulation set. Residues 22 (alpha2) and 108 (alpha5) interact with each other throughout the simulations and are located in a polymorphic hotspot approximately 20 A from the active site. Introduction of either the larger Ser (22) or Met (108) tightens this interaction, pulling alpha2 and alpha5 toward each other and away from the protein core. The V108M polymorphism rearranges active-site residues in alpha5, beta3, and alpha6, increasing the S-adenosylmethionine site solvent exposure. The A22S mutation reorients alpha2, moving critical catechol-binding residues away from the substrate-binding pocket. The A22S and V108M polymorphisms evolved independently in Northern European and Asian populations. While the decreased activities of both A22S and V108M COMT are associated with an increased risk for schizophrenia, the V108M-induced destabilization is also linked with improved cognitive function. These results suggest that polymorphisms within this hotspot may have evolved to regulate COMT activity and that heterozygosity for either mutation may be advantageous.

Species differences in pharmacokinetic and pharmacodynamic properties of nebicapone

The present study was designed to characterize pharmacodynamic and pharmacokinetic properties of nebicapone in rats and mice. Upon oral administration of nebicapone the extent of mouse liver catechol-O-methyltransferase (COMT) inhibition is half that in the rat. Nebicapone was rapidly absorbed reaching plasma C(max) within 30min and being completely eliminated by 8h. Nebicapone was metabolized mainly by glucuronidation and methylation in both species, but rat had an extra major metabolite, resulting from sulphation. Administration of nebicapone by the intraperitoneal route significantly increased compound AUC in the rat while in the mouse a significant increase in AUC of metabolites was observed. These results show that nebicapone exhibited marked species differences in bioavailability and metabolic profile. Evaluation of COMT activity in rat and mice liver homogenates revealed that both had similar methylation efficiencies (K(cat) values, respectively 7.3 and 6.4min(-1)), but rat had twice active enzyme units as the mouse (molar equivalency respectively 150 and 83). Furthermore, nebicapone inhibited rat liver COMT with a lower K(i) than mouse liver COMT (respectively 0.2nM vs. 1.2nM). In conclusion, the results from the present study show that mice and rats respond differently to COMT inhibition by nebicapone. The more pronounced inhibitory effects of nebicapone in the rat may be related to an enhanced oral availability and less pronounced metabolism of nebicapone in this specie, but also concerned with the predominant expression of S-COMT over MB-COMT, the latter of which is less sensitive to inhibition by nebicapone than the former.

Low enzymatic activity haplotypes of the human catechol-O-methyltransferase gene: enrichment for marker SNPs

Catechol-O-methyltransferase (COMT) is an enzyme that plays a key role in the modulation of catechol-dependent functions such as cognition, cardiovascular function, and pain processing. Three common haplotypes of the human COMT gene, divergent in two synonymous and one nonsynonymous (val(158)met) position, designated as low (LPS), average (APS), and high pain sensitive (HPS), are associated with experimental pain sensitivity and risk of developing chronic musculoskeletal pain conditions. APS and HPS haplotypes produce significant functional effects, coding for 3- and 20-fold reductions in COMT enzymatic activity, respectively. In the present study, we investigated whether additional minor single nucleotide polymorphisms (SNPs), accruing in 1 to 5% of the population, situated in the COMT transcript region contribute to haplotype-dependent enzymatic activity. Computer analysis of COMT ESTs showed that one synonymous minor SNP (rs769224) is linked to the APS haplotype and three minor SNPs (two synonymous: rs6267, rs740602 and one nonsynonymous: rs8192488) are linked to the HPS haplotype. Results from in silico and in vitro experiments revealed that inclusion of allelic variants of these minor SNPs in APS or HPS haplotypes did not modify COMT function at the level of mRNA folding, RNA transcription, protein translation, or enzymatic activity. These data suggest that neutral variants are carried with APS and HPS haplotypes, while the high activity LPS haplotype displays less linked variation. Thus, both minor synonymous and nonsynonymous SNPs in the coding region are markers of functional APS and HPS haplotypes rather than independent contributors to COMT activity.

Characterization of NF-kB-mediated inhibition of catechol-O-methyltransferase

BACKGROUND

Catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines, has recently been implicated in the modulation of pain. Specifically, low COMT activity is associated with heightened pain perception and development of musculoskeletal pain in humans as well as increased experimental pain sensitivity in rodents.

RESULTS

We report that the proinflammatory cytokine tumor necrosis factor alpha (TNFalpha) downregulates COMT mRNA and protein in astrocytes. Examination of the distal COMT promoter (P2-COMT) reveals a putative binding site for nuclear factor kappaB (NF-kappaB), the pivotal regulator of inflammation and the target of TNFalpha. Cell culture assays and functional deletion analyses of the cloned P2-COMT promoter demonstrate that TNFalpha inhibits P2-COMT activity in astrocytes by inducing NF-kappaB complex recruitment to the specific kappaB binding site.

CONCLUSION

Collectively, our findings provide the first evidence for NF-kappaB-mediated inhibition of COMT expression in the central nervous system, suggesting that COMT contributes to the pathogenesis of inflammatory pain states.

Identification of a novel haplotype of the human catechol-O-methyltransferase gene

Human catechol-O-methyltransferase (COMT; EC 2.1.1.6) catalyzes the transfer of the methyl group to a variety of endogenous and exogenous catechol substrates using S-adenosyl-L-methionine as the methyl donor. This enzymatic O-methylation plays an important role in the inactivation of biologically active and toxic catechols. A number of studies in recent years have sought to characterize the polymorphism of human COMTs and also to determine the catalytic activity of polymorphic enzymes. We report here the identification of a new haplotype of the human COMT gene with triplet point mutations, which encodes the D51G/S60F/K162R mutant of the soluble COMT and the D101G/S110F/K212R mutant of the membrane-bound COMT. Kinetic analysis showed that these new COMT variants had essentially the same kinetic characteristics and catalytic activity as the wild-type COMTs for the O-methylation of 2-hydroxyestradiol and 4-hydroxyestradiol in vitro, but they have a significantly reduced thermostability at 37 degrees C.

Catechol-O-methyltransferase and its inhibitors in Parkinson's disease

Parkinson's disease (PD) is a neurological disorder characterized by the degeneration of dopaminergic neurons, with consequent reduction in striatal dopamine levels leading to characteristic motor symptoms. The most effective treatment for this disease continues to be the dopamine replacement therapy with levodopa together with an inhibitor of aromatic amino acid decarboxylase (AADC). The efficacy of this therapy, however, decreases with time and most patients develop fluctuating responses and dyskinesias. The last decade showed that the use of catechol-O-methyltransferase inhibitors as adjuvants to the levodopa/AADC inhibitor therapy, significantly improves the clinical benefits of this therapy. The purpose of this article is to review the current knowledge on the enzyme catechol-O-methyltransferase (COMT) and the role of COMT inhibitors in PD as a new therapeutic approach to PD involving conversion of levodopa to dopamine at the target region in the brain and facilitation of the continuous action of this amine at the receptor sites. A historical overview of the discovery and development of COMT inhibitors is presented with a special emphasis on nebicapone, presently under clinical development, as well as entacapone and tolcapone, which are already approved as adjuncts in the therapy of PD. This article reviews human pharmacokinetic and pharmacodynamic properties of these drugs as well as their clinical efficacy and safety.

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.

Biochemical and molecular modeling studies of the O-methylation of various endogenous and exogenous catechol substrates catalyzed by recombinant human soluble and membrane-bound catechol-O-methyltransferases

Catechol-O-methyltransferase (COMT, EC 2.1.1.6) catalyzes the O-methylation of a wide array of catechol-containing substrates using s-adenosyl-L-methionine as the methyl donor. In the present study, we have cloned and expressed the human soluble and membrane-bound COMTs (S-COMT and MB-COMT, respectively) in Escherichia coli and have studied their biochemical characteristics for the O-methylation of representative classes of endogenous catechol substrates (catecholamines and catechol estrogens) as well as exogenous catechol substrates (bioflavonoids and tea catechins). Enzyme kinetic analyses showed that these two recombinant human COMTs are functionally active, with catalytic and kinetic properties nearly identical to those of crude or purified enzymes prepared from human tissues or cells. Kinetic parameters for the O-methylation of various substrates were characterized. In addition, computational modeling studies were conducted to better understand the molecular mechanisms for the different catalytic behaviors of human S- and MB-COMTs with respect to s-adenosyl-L-methionine, various substrates, and also the regioselectivity for the formation of mono-methyl ether products. Our modeling data showed that the binding energy values (Delta G) calculated for most substrates agreed well with the measured kinetic parameters. Also, our modeling data precisely predicted the regioselectivity for the O-methylation of these substrates at different hydroxyl groups, the predicted values matched nearly perfectly with the experimental data.

Comparisons of brain, uterus, and liver mRNA expression for cytochrome p450s, DNA methyltransferase-1, and catechol-o-methyltransferase in prepubertal female Sprague-Dawley rats exposed to a mixture of aryl hydrocarbon receptor agonists

Non-ortho polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) are ubiquitous environmental contaminants that exert their toxicity mostly through activation of the aryl-hydrocarbon receptor (AhR), and are referred to as AhR agonists. The objective was to study, by real time reverse-transcriptase-polymerase chain reaction (RT-PCR), the effects of postnatal exposure to a reconstituted mixture of AhR agonists present in breast milk (3 non-ortho PCBs, 6 PCDDs, and 7 PCDFs, referred to here-in-after as AhRM) on mRNA expression of estrogen receptor (ERalpha), enzymes involved with the metabolism of estrogens [catechol-o-methyltransferase (Comt), cytochrome P450 (Cyp)1A1, 1B1 and 2B1], and DNA methyltransferase-1 (Dnmt1), in brain areas, liver and uterus of immature female rats. Neonates were exposed by gavage during postnatal day (PND) 1-20 with dosages equivalent to 1, 10, 100, and 1000 times the estimated average human exposure level, and were sacrificed at PND 21. None of the end points were affected in uterine cross-sections, or in samples of uterine tissue layers collected by laser capture microdissection. At 1000x, the AhRM reduced Dnmt1 mRNA abundance to 28% and 32% of control in the liver and hypothalamus, respectively. In the brain, Cyp1A1 was increased (409%) but ERalpha was reduced (66%). Similarly, mRNA abundance for Comt isoforms was reduced in the liver (45%) and brain areas (55-70%). AhRM at 100x, the lowest effective dose, exerted a 220% increase in brain cortex Comt [membrane bound (Mb)], a 219% increase in hepatic Cyp1B1, and a 63% decrease in hepatic Comt (soluble (S)+Mb). These results support the possibility that early exposure to environmental contaminants could lead to effects mediated by changes in DNA methylation and/or estrogen metabolism and signaling.

Functional and structural comparisons of cysteine residues in the Val108 wild type and Met108 variant of human soluble catechol O-methyltransferase

Catechol O-methyltransferase (COMT) plays an important role in the inactivation of biologically active and toxic catechols. This enzyme is genetically polymorphic with a wild type and a variant form. Numerous epidemiological studies have shown that the variant form is associated with an increased risk of developing estrogen-associated cancers and a wide spectrum of mental disorders. There are seven cysteine residues in human S-COMT, all of which exist as free thiols and are susceptible to electrophilic attack and/or oxidative damage leading to enzyme inactivation. Here, the seven cysteine residues were systematically replaced by alanine residues by means of site-directed mutagenesis. The native forms and cysteine/alanine mutants were assayed for enzymatic activity, thermal stability, methylation regioselectivity, and reactivity of cysteine residues to thiol reagent. Our data showed that although there is only one encoding base difference between these two COMT forms, this difference might induce structural changes in the local area surrounding some cysteine residues, which might further contribute to the different roles they might play in enzymatic activity, and to the different susceptibility to enzyme inactivation.

Crystallization and preliminary X-ray diffraction studies of a catechol-O-methyltransferase/inhibitor complex

Inhibitors of the enzyme catechol-O-methyltransferase (COMT) are used as co-adjuvants in the therapy of Parkinson's disease. A recombinant form of the soluble cytosolic COMT from rat has been co-crystallized with a new potent inhibitor, BIA 8-176 [(3,4-dihydroxy-2-nitrophenyl)phenylmethanone], by the vapour-diffusion method using PEG 6K as precipitant. Crystals diffract to 1.6 A resolution on a synchrotron-radiation source and belong to the monoclinic space group P2(1), with unit-cell parameters a = 52.77, b = 79.63, c = 61.54 A, beta = 91.14 degrees.

Functional analysis of genetic variation in catechol-O-methyltransferase (COMT): effects on mRNA, protein, and enzyme activity in postmortem human brain

Catechol-O-methyltransferase (COMT) is a key enzyme in the elimination of dopamine in the prefrontal cortex of the human brain. Genetic variation in the COMT gene (MIM 116790) has been associated with altered prefrontal cortex function and higher risk for schizophrenia, but the specific alleles and their functional implications have been controversial. We analyzed the effects of several single-nucleotide polymorphisms (SNPs) within COMT on mRNA expression levels (using reverse-transcriptase polymerase chain reaction analysis), protein levels (using Western blot analysis), and enzyme activity (using catechol methylation) in a large sample (n = 108) of postmortem human prefrontal cortex tissue, which predominantly expresses the -membrane-bound isoform. A common coding SNP, Val158Met (rs4680), significantly affected protein abundance and enzyme activity but not mRNA expression levels, suggesting that differences in protein integrity account for the difference in enzyme activity between alleles. A SNP in intron 1 (rs737865) and a SNP in the 3' flanking region (rs165599)--both of which have been reported to contribute to allelic expression differences and to be associated with schizophrenia as part of a haplotype with Val--had no effect on mRNA expression levels, protein immunoreactivity, or enzyme activity. In lymphocytes from 47 subjects, we confirmed a similar effect on enzyme activity in samples with the Val/Met genotype but no effect in samples with the intron 1 or 3' SNPs. Separate analyses revealed that the subject's sex, as well as the presence of a SNP in the P2 promoter region (rs2097603), had small effects on COMT enzyme activity. Using site-directed mutagenesis of mouse COMT cDNA, followed by in vitro translation, we found that the conversion of Leu at the homologous position into Met or Val progressively and significantly diminished enzyme activity. Thus, although we cannot exclude a more complex genetic basis for functional effects of COMT, Val is a predominant factor that determines higher COMT activity in the prefrontal cortex, which presumably leads to lower synaptic dopamine levels and relatively deleterious prefrontal function.

CoMFA modeling of human catechol O-methyltransferase enzyme kinetics

Three-dimensional QSAR models with different charge calculation methods (MOPAC-AM1-ESP, MOPAC-AM1-Coulson and Gasteiger-Hückel) were developed for predicting all three enzyme kinetic parameters Km, Vmax and Vmax/Km for catecholic substrates of human soluble catechol O-methyltransferase (S-COMT). The empirical parameters of 45 substrates were correlated to the steric and electronic molecular fields of the substrates utilizing Comparative Molecular Field Analysis (CoMFA). Alignment rules for CoMFA were developed based on the catalytic mechanism and crystal structure of S-COMT, and the analysis was optimized using an all-space search technique. Leave-one-out and leave-n-out cross-validation (with 5 and 10 cross-validation groups) was carried out, and all developed models proved to be statistically significant with q2 values up to 0.84. The models based on MOPAC charge calculations predicted the empirical values clearly better than the Gasteiger-Hückel method. The derived CoMFA coefficient contour maps of steric and electrostatic interactions correlated clearly with the S-COMT crystallographic structures.

Catechol O-methyltransferase mRNA expression in human and rat brain: evidence for a role in cortical neuronal function

Catechol O-methyltransferase (COMT) is involved in the inactivation of catecholamines, including the neurotransmitter dopamine. A Val(108/158) Met functional polymorphism of the COMT gene has been shown to affect working memory-associated frontal lobe function in humans. In the present study, in situ hybridization histochemistry was employed to determine the mRNA expression profile of COMT in the human prefrontal cortex, striatum and midbrain and in the rat forebrain. In both species, COMT mRNA signals were observed in large pyramidal and smaller neurons in all cortical layers of the prefrontal cortex as well as in medium and large neurons in the striatum. Levels of COMT mRNA were obviously higher in neurons than in glia. The striatum, which receives a dense dopaminergic input, expressed lower levels of COMT mRNA as compared with the prefrontal cortex. Consistent with previous protein expression data, COMT mRNA was abundant in ependymal cells lining the cerebral ventricles. In the midbrain, COMT mRNA was detected in dopaminergic neurons in both species, albeit at low levels. In the rat forebrain, dense labeling was also detected in choroid plexus and hippocampal dentate gyrus and Ammon's horn neurons. Contrary to expectations that COMT would be expressed predominantly in non-neuronal cells, the present study shows that neurons are the main cell populations expressing COMT mRNA in the prefrontal cortex and striatum. Combined with previous data about protein localization, the present results suggest that the membrane-bound isoform of COMT having a high affinity for dopamine is expressed at neuronal dendritic processes in human cortex, consistent with functional evidence that it plays an important role in dopaminergic neurotransmission.

Kinetics and crystal structure of catechol-o-methyltransferase complex with co-substrate and a novel inhibitor with potential therapeutic application

Catechol-O-methyltransferase (COMT; E.C. 2.1.1.6) is a ubiquitous enzyme in nature that plays an important role in the metabolism of catechol neurotransmitters and xenobiotics. In particular, inactivation of drugs such as L-3,4-dihydroxyphenylalanine (L-DOPA) via O-methylation is of relevant pharmacological importance, because L-DOPA is currently the most effective drug used in the treatment of Parkinson's disease. This justified the interest in developing COMT inhibitors as potential adjuncts to L-DOPA therapy. The kinetics of inhibition by BIA 3-335 (1-[3,4-dihydroxy-5-nitrophenyl]-3-(N-3'-trifluormethylphenyl)-piperazine-1-propanone dihydrochloride) were characterized using recombinant rat soluble COMT. BIA 3-335 was found to act as a potent, reversible, tight-binding inhibitor of COMT with a K(i) of 6.0 +/- 1.6 nM and displaying a competitive inhibition toward the substrate binding site and uncompetitive inhibition toward the S-adenosyl-L-methionine (SAM) binding site. The 2.0-A resolution crystal structure of COMT in complex with its cosubstrate SAM and a novel inhibitor BIA 3-335 shows the atomic interactions between the important residues at the active site and the inhibitor. This is the first report of a three-dimensional structure determination of COMT complexed with a potent, reversible, and tight-binding inhibitor that is expected to have therapeutic applications.

Kinetics of Rat Brain and Liver Solubilized Membrane-Bound Catechol-O-Methyltransferase

Catechol-O-methyltransferase (COMT), an enzyme involved in the metabolism of catecholamines, is present in mammals as soluble (S-COMT) and membrane-bound (MB-COMT) forms. The kinetic properties of rat liver and brain solubilized MB-COMT were evaluated and compared with the ones of the respective native enzymes. Treatment with Triton X-100 did not affect the affinity of S-COMT for the substrate (adrenaline) or the activity of the enzyme. Conversely, solubilized MB-COMT presented a lower affinity for the substrate than the native protein, as evidenced by a significant increase in the Km values: 9.3 (6.2, 12) vs 2.5 (0.8, 4.3) microM for the liver enzyme and 12 (11, 13) vs 1.4 (1.0, 1.9) microM for the brain enzyme. A 1.6- and 1.5-fold increase in Vmax was also observed for the liver and brain solubilized enzymes, respectively. The actual enzyme concentrations (molar equivalence, Meq) and their efficiency in the O-methylation reaction (catalytic number, Kcat) were determined from Ackermann-Potter plots. Both liver and brain solubilized MB-COMT were more efficient in methylating adrenaline than the respective native enzymes as revealed by higher Kcat values (P < 0.05): 16.4+/-0.9 vs 10.9+/-0.8 min(-1) (brain) and 5.9+/-0.3 vs 3.3+/-0.2 min(-1) (liver). Subjecting liver solubilized MB-COMT to further purification increased the Km of the enzyme to the levels of liver S-COMT, 252 (127; 377) vs 257 (103; 411) microM. The solubilization process significantly alters MB-COMT kinetic properties but only after partial purification does the enzyme present an affinity for the subtrate identical to S-COMT.

Soluble and membrane-bound catechol-O-methyltransferase in normal and malignant mammary gland

The levels of 26 kDa-soluble (S) and 30 kDa-membrane-bound (MB) catechol-O-methyltransferase (COMT) polypeptides were determined in paired samples from normal and neoplastic breast tissue of 32 patients with breast cancer. Immunohistochemical staining showed that the COMT reaction in normal mammary tissue was restricted to the epithelial cells in the ducti and lobuli, whereas in the tumors a strong reaction was also seen in the malignant cells. The amounts of COMT proteins in tumors could not be correlated with various clinical or pathological parameters. Quantitative immunoblotting analysis revealed that the total amount of COMT proteins in tumors was more than 50% higher than in respective normal samples in 26 out of 32 patients. Five cases showed less than a 50% difference and in one case less COMT was detected in the tumor. In most cases the amount of both S- and MB-COMT forms was increased. The average amount of total COMT was 178 +/- 57 pg/microg total protein in normal tissue and 566 +/- 94 pg/microg total protein in tumor. Respective values for S-COMT were 137 +/- 52 pg/microg total protein in normal tissue and 369 +/- 62 pg/microg total protein in tumor and for MB-COMT 41 +/- 10 and 197 +/- 41 pg/microg total protein, respectively. Analysis of COMT-specific transcripts suggested that the COMT enzyme level in tumors is determined in some cases by transcriptional and in some cases by post-transcriptional mechanisms.

Methylation pharmacogenetics: catechol O-methyltransferase, thiopurine methyltransferase, and histamine N-methyltransferase

Methyl conjugation is an important pathway in the biotransformation of many exogenous and endogenous compounds. Pharmacogenetic studies of methyltransferase enzymes have resulted in the identification and characterization of functionally important common genetic polymorphisms for catechol O-methyltransferase, thiopurine methyltransferase, and histamine N-methyltransferase. In recent years, characterization of these genetic polymorphisms has been extended to include the cloning of cDNAs and genes, as well as a determination of the molecular basis for the effects of inheritance on these methyltransferase enzymes. The thiopurine methyltransferase genetic polymorphism is responsible for clinically significant individual variations in the toxicity and therapeutic efficacy of thiopurine drugs such as 6-mercaptopurine. Phenotyping for the thiopurine methyltransferase genetic polymorphism represents one of the first examples in which testing for a pharmacogenetic variant has entered standard clinical practice. The full functional implications of pharmacogenetic variation in the activities of catechol O-methyltransferase and histamine N-methyltransferase remain to be determined. Finally, experimental strategies used to study methylation pharmacogenetics illustrate the rapid evolution of biochemical, pharmacologic, molecular, and genomic approaches that have been used to determine the role of inheritance in variation in drug metabolism, effect, and toxicity.

Gamma-glutamyl transpeptidase gene organization and expression: a comparative analysis in rat, mouse, pig and human species

Gamma-glutamyl transpeptidase (GGT) is an enzyme located at the external surface of epithelial cells. It initiates extracellular glutathione (GSH) breakdown, provides cells with a local cysteine supply and contributes to maintain intracellular GSH level. GGT expression, highly sensitive to oxidative stress, is a part of the cell antioxidant defense mechanisms. We describe recent advances in GGT gene structure and expression knowledge and put emphasis on the complex transcriptional organization of that gene and its conservation among different species. GGT gene structure has been elucidated in rat and mouse where a single gene is transcribed from multiple promoters into several transcripts which finally yield a unique polypeptidic chain. Analysis of rat, mouse, human and pig cDNA and gene sequences reveals a large conservation of the transcriptional organization of that gene. This complex structure provides flexibility in GGT expression controlled at the promoter level, through multiple regulatory sites, and at RNA level by alternate 5' untranslated sequences which may create a diversity in the stability and translational efficiency of the different transcripts. In conclusion, transcription of the GGT gene from several promoters offers multiple DNA and RNA targets for various oxidative stimuli and contributes to a broad antioxidant cell defense through GGT induction and subsequent cysteine supply from extracellular glutathione.

Effects of tolcapone upon soluble and membrane-bound brain and liver catechol-O-methyltransferase

The present study was aimed to evaluate the sensitivity of soluble (S) and membrane bound (MB) catechol-O-methyltransferase (COMT) from rat brain and liver to inhibitors which interact with the enzyme as competitive (tropolone), non-competitive (S-adenosyl-l-homocysteine; SAHC) and tight-binding (tolcapone and 3,5-dinitrocatechol) inhibitors. COMT activity was evaluated by the ability to methylate adrenaline (0.1 to 2000 microM) to metanephrine in the presence of a saturating concentration of the methyl donor (S-adenosyl-l-methionine). When using a fixed amount of total protein (2 micrograms/ml), but variable concentrations of COMT, the inhibitory potency of tolcapone upon S- and MB-COMT activity in the brain was in the low nM range (IC50's of 2 and 3 nM, respectively), whereas in liver the IC50 values for tolcapone against liver MB- and S-COMT (IC50's of 123 and 795 nM, respectively) were markedly higher than those observed in the brain. By contrast, when inhibition studies were performed with a fixed concentration of COMT (15 nM), as determined by the Ackermann-Potter equation, tolcapone was found to be endowed with the same potency (in the low nM range) in inhibiting S- and MB-COMT from both brain and liver. As for tolcapone, 3,5-dinitrocatechol was more potent against MB- than against S-COMT when a fixed amount of total protein was used, but showed the same potency when a fixed concentration of COMT was used. Tropolone, a competitive inhibitor, was much less potent than tolcapone and 3,5-dinitrocatechol in inhibiting S- and MB-COMT from both brain and liver and its potency was found not to depend on enzyme concentration. SAHC, a non-competitive inhibitor, behaved similarly to tight-binding inhibitors when a fixed amount of total protein was used. By contrast, when a fixed amount of enzyme was used, SAHC was found to be endowed with the same potency against S- and MB-COMT from brain and liver. In the final series of experiments the inhibitory effect of tolcapone was examined under in vitro ex vivo conditions, using the same concentration of COMT (15 nM). One hour after its oral administration, tolcapone (0.3 to 30 mg/kg) was found to be much more potent against MB-COMT than against S-COMT. In the liver, 0.3 mg/kg tolcapone resulted in 82% inhibition of MB-COMT and 31% inhibition of S-COMT. In the brain, 3.0 mg/kg tolcapone inhibited 78% MB-COMT, whereas S-COMT activity was reduced by 38% only. In conclusion, the results reported here show that tolcapone is particularly potent in inhibiting MB-COMT from liver and brain under in vivo experimental conditions, though it does not discriminate between MB- and S-COMT under in vitro experimental conditions when using the same amount of enzyme in the assay.

New, selective catechol-O-methyltransferase inhibitors as therapeutic agents in Parkinson's disease

Levodopa remains the most effective drug for Parkinson's disease (PD). However, its benefits are limited owing to extensive metabolism by catechol-O-methyltransferase (COMT), especially if levodopa is used in combination with peripheral dopa-decarboxylase inhibitors. A new generation of potent, orally active, selective, and reversible COMT inhibitors has become available recently. Among these, tolcapone and entacapone have been best characterised. Preclinical and clinical studies have shown that COMT inhibitors markedly enhance levodopa availability and prolong its plasma half-life. In recent large clinical trials they proved to be able to ameliorate motor fluctuations, reduce disability, and decrease levodopa requirements in PD patients. The tolerability profiles of entacapone and tolcapone are good. COMT inhibition promises to become an important means of extending the benefits of levodopa therapy in PD.

Induction of UDP-glycosyltransferase family 1 genes in rat liver: different patterns of mRNA expression with two inducers, 3-methylcholanthrene and beta-naphthoflavone

Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), presently called UDP-glycosyltransferases, catalyse the detoxification of many toxic and carcinogenic compounds. Glucuronidation is also a major metabolic pathway for numerous drugs. The UGT1A6 gene (formerly known as UGT1*06 and UGT1A1) has been suggested to belong to the aryl hydrocarbon (Ah) gene battery, which consists of several genes encoding for drug-metabolising enzymes regulated by dioxin and other ligands of the Ah receptor. In this study, we analysed the localisation of UGT1A6 expression in rat liver by in situ hybridisation to mRNA. Two different RNA probes were used, one which was specific to UGT1A6 and the other against the C terminal sequence shared by all UGT1 genes. In this study, no UGT1A6 mRNA was detected in the control animals. However, other gene(s) of the UGT1 family were expressed in the perivenous region surrounding the central veins as detected by hybridisation with the probe against the common region of the UGT1 genes. Treatment with the lower dose (5 mg/kg) of 3-methylcholanthrene (3MC) induced expression of UGT1A6 perivenously. Treatment with the higher dose (25 mg/kg) of 3-Methylcholanthrene resulted in a more panacinar expression pattern. In contrast to the perivenous induction observed with 3-methylcholanthrene, treatment with 15 mg/kg of beta-naphthoflavone (BNF) resulted in strong induction in the periportal region. The results reveal an inducer-specific pattern of UGT1A6 expression similar to that demonstrated earlier for other Ah battery genes, namely CYP1A1, CYP1A2, GSTYalpha and ALDH3. The finding further supports the notion that common factors regulate the regional hepatic expression of Ah battery genes.

Rat liver catechol-O-methyltransferase kinetics and assay methodology

In mammals, catechol-O-methyltransferase (COMT) is distributed throughout various organs, the highest activities being found in the liver and kidney. However, comparisons of the kinetic parameters are difficult to perform, since the experimental procedures in the enzyme assay vary quite considerably. The present work was aimed at studying the optimal liver COMT assay conditions for determining the kinetics of the enzyme. The COMT assay was performed with liver homogenates from 60 days old male Wistar rats with adrenaline (AD) as the substrate. Time course experiments using 100 microM S-adenosyl-L-methionine (SAMe) and 300 microM AD showed linearity of O-methylation reaction upto 10 min. Using 100 microM SAMe, Vmax (nmol mg protein-1 h-1) and Km (microM) values progressively decreased respectively from 22.1 and 104.8 at 5 min down to 5.8 and 24.62 at 60 min incubation periods. This decrease was not due to end-product inhibition. Using 2500 microM AD, Km values (microM) for the methyl donor SAMe increased progressively from 174 at 5 min upto 1192.5 at 60 min; upto 30 min of incubation Vmax values did not change. When a 5 min incubation period and 500 microM SAMe were used, Vmax and Km values for liver COMT were 63.4 nmol mg protein-1 h-1 and 261.1 microM, respectively. It is concluded that an incubation period of 5 min and a SAMe concentration of 500 microM provide optimal conditions for the liver homogenate COMT assay.

Expression and intracellular localization of catechol O-methyltransferase in transfected mammalian cells

The intracellular localization of soluble and membrane-bound isoforms of rat and human catechol O-methyltransferase (COMT) was studied by expressing the recombinant COMT proteins either separately or together in mammalian cell lines (HeLa and COS-7 cells) and in rat primary neurons. The distribution of soluble and membrane-bound COMT enzyme was visualized by immunocytochemistry. For comparison, the localization of native COMT was studied in rat C6 glioma cells by immunoelectron microscopy. Staining of cells expressing membrane-bound COMT with a COMT-specific antiserum revealed an immunofluorescence signal in intracellular reticular structures and in the nuclear membrane. Double-staining of the cells with antisera against proteins specific for the rough endoplasmic reticulum indicated that they colocalized with membrane-bound COMT, suggesting that it resided in the endoplasmic reticulum. Notably, no COMT-specific fluorescence of plasma membranes was detected. The signal in the endoplasmic reticulum was also evident in the cells expressing both recombinant COMT forms. Intracellular native COMT reaction was detected by immunoelectron microscopy in rat C6 glioma cells and an intense cytoplasmic signal was seen in the primary neurons infected with the recombinant Semliki Forest virus. The cells expressing recombinant soluble COMT revealed intense nuclear staining together with diffuse cytoplasmic immunoreactivity, suggesting that a part of soluble COMT is transported to nuclei. Western blotting from rat liver and brain revealed soluble COMT in the nuclei. Enzyme activity measurements from liver cytoplasmic and nuclear fractions suggested that about 5% of the soluble COMT resided in nuclei. The intracellular localization of both COMT forms implies that COMT acts in the cytoplasm and possibly also in the nuclear compartment, and that the physiological substrates of COMT enzymes may have to be internalized before their methylation by COMT.

Structure and the promoter region of the mouse gene encoding the 67-kD form of glutamic acid decarboxylase

We have cloned and determined the complete structure of the murine gene encoding the 67-kD form of glutamic acid decarboxylase (GAD67), the gamma-aminobutyric acid synthetic enzyme. Its coding region comprises 18 exons spanning 42 kb of genomic DNA. Exon 1 together with 64 bp of exon 2 defines the 5' untranslated region of GAD67 mRNA. Exon 18 specifies the protein's carboxyl terminal and the entire 3' untranslated region. Exons 7/A and 7/B are solely contained in the coding regions of two alternatively spliced bicistronic embryonic mRNAs, which code for the truncated embryonic GAD forms. The promoter region (P1) corresponding to the main group of transcription initiation sites is devoid of TATA and CAAT boxes but has putative binding sites for the transcription factor SP1 and is embedded in a large G + C-rich domain of a CpG island, features shared by the promoters of constitutively expressed housekeeping genes. Primer extension data suggests the existence of additional transcription start sites at 130 bp and 295 bp upstream from the major initiation site that are utilized less frequently in adult brain. The tentative distal promoters (P2 and P3) that correspond to the minor start sites resemble tissue-specific promoters with TATA and CAAT-like boxes. In 1.3 kb of the 5'-upstream region, we identified several putative transcription factor binding sites such as AP2, Hox, E-box, egr-1, and NF-kappaB and putative neuronal-specific regulatory elements, including the neuronal-restrictive silencer element, which may have functional significance in the developmental and tissue-specific expression of the GAD67 gene.