Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “COMT”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27Linked to original sources
PubMed2004

Phytochemicals inhibit catechol-O-methyltransferase activity in cytosolic fractions from healthy human mammary tissues: implications for catechol estrogen-induced DNA damage.

Phytochemicals are natural dietary constituents of fruits and vegetables. Some of these phytochemicals are known to affect estrogen-metabolizing enzymes. In breast tissue, estradiol can be metabolized to the catechol estrogens 2- and 4-hydroxyestradiol (2-OHE(2) and 4-OHE(2)). Catechol estrogens are suspected carcinogens potentially involved in the etiology of breast cancer. Catechol-O-methyltransferase (COMT) converts the catechol estrogens to their inactive methoxy derivatives (2-MeOE(2) and 4-MeOE(2)). In this study we investigated the effects of several phytochemicals on COMT activity in cytosolic fractions of seven healthy human mammary tissues from reduction mammoplasty. Large interindividual variations were observed in the constitutive levels of COMT activity. However, in all cytosol samples the catalytic efficiency of COMT was greater for 2-MeOE(2) formation than for 4-MeOE(2) formation. The known COMT inhibitor Ro 41-0960 and several phytochemicals with a catechol structure (quercetin, catechin, and (-)-epicatechin) concentration-dependently inhibited COMT activity, while phytochemicals without a catechol structure (genistein, chrysin, and flavone) showed no effect up to 30 microM. Distinct interindividual variations were observed in sensitivity toward COMT inhibition among the various tissue samples, as was shown by the range in IC(50) values for Ro 41-0960 (5-42 nM). The toxicological relevance of COMT inhibition and the effect of reduced inactivation of catechol estrogens was studied by determining the amount of catechol estrogen-induced DNA damage in MCF-7 cells using the comet assay. Catechol estrogens alone caused no increase of DNA damage compared with control treated cells. However, both Ro 41-0960 and quercetin caused a decrease of methoxy estradiol formation and an increase of catechol estrogen-induced DNA damage in MCF-7 cells. This suggests that phytochemicals with a catechol structure have the potential to reduce COMT activity in mammary tissues and may consequently reduce the inactivation of potentially mutagenic estradiol metabolites and increase the chance of DNA damage.

Benzophenones↗
PubMed1997

Exercise-induced changes in enzymatic O-methylation of catecholestrogens by erythrocytes of eumenorrheic women.

The present study was designed to assess the effects of acute exercise and short-term intensive training on catechol-O-methyltransferase (COMT) activity. COMT inactivates catecholamines and converts primary catecholestrogens (CE) into their O-methylated form yielding the 2- (2-MeOE) and 4-methoxyestrogens (4-MeOE). Blood samples were obtained from 15 previously untrained eumenorrheic women (mean +/- SE, VO2max: 43.8 mL x kg-1 x min-1 +/- 0.6) before and after a 5-d intensive training period, at rest and during incremental exercise. COMT activity was determined in the erythrocytes (RBC-COMT) after incubation of blood lysate with primary CE. The formation of both 2- and 4-MeOE was significantly higher (P < 0.05) during the luteal (LPh) than during the follicular phase (FPh). The amount of 2-MeOE formed (FPh: 4.2 +/- 0.2%; LPh: 4.9 +/- 0.2%) was significantly greater than the produced amount of 4-MeOE (FPh: 1.4 +/- 0.1%; LPh: 1.5 +/- 0.1%) (P < 0.05). Both before and after training, incremental exercise did not significantly alter RBC-COMT activity although we observed a trend for RBC-COMT activity increasing proportionally with the exercise intensity. After a brief period of exhaustive training, during rest the formation of 2-MeOE (FPh: +16.7%, LPh: +15.7%) and 4-MeOE (FPh: +28.6%; LPh: +40%) was significantly (P < 0.05) increased. The results of the present study are consistent with earlier findings reporting increased plasma concentrations of O-methylated CE following training. It is concluded that RBC-COMT activity is increased by brief intensive training, but not by acute exercise. We speculate that an increase in COMT-catalyzed O-methylation of CE may indicate that less COMT is available to deactivate norepinephrine.

Adolescent↗
PubMed2001

Catechol-O-methyltransferase gene polymorphism in schizophrenia: evidence for association between symptomatology and prognosis.

Catechol-O-methyltransferase (COMT) gene has long been implicated to play a role in the pathogenesis of schizophrenia. The aim of this study is to assess the relationship of schizophrenia and its subgroups with COMT gene polymorphism. We have attempted to evaluate a possible correlation between the severity and prognosis of the illness (the psychopathology of symptoms) and COMT gene polymorphisms. The study comprised 129 unrelated subjects who strictly met DSM-IV criteria for schizophrenia, and 65 healthy unrelated controls. All subjects were of Turkish origin. A clinical evaluation of all patients was accomplished by applying the Brief Psychiatric Rating Scale (BPRS) test. The analysis of COMT polymorphism was performed using the polymerase chain reaction technique. Regarding COMT gene polymorphisms, no statistically significant difference was found between schizophrenic patients and control subjects. However, within the schizophrenic group, the average of BPRS points of patients with the L/L genotype was significantly higher than those of the L/H and H/H genotypes (F = 6.25, degrees of freedom = 2, P = 0.003). Although no statistically significant difference was found between the duration of illness and COMT variations, a higher frequency of hospitalization was found in patients with the L/L genotype compared with other groups (t = 3.048, P = 0.003). In conclusion, the findings indicate that COMT gene polymorphisms were not statistically significant between patient and control groups. However, the patients with the L/L genotype may have much more severe clinical signs in Turkish schizophrenics. COMT variations, however, do not help to evaluate the susceptibility of the patients, but can help in the estimation of severity of clinical manifestations. Further studies are required to better understand the association of symptomatology of schizophrenia and other psychiatric disorders with COMT gene polymorphism.

Adult↗
PubMed1995

The brown midrib3 (bm3) mutation in maize occurs in the gene encoding caffeic acid O-methyltransferase.

The brown midrib mutations are among the earliest described in maize. Plants containing a brown midrib mutation exhibit a reddish brown pigmentation of the leaf midrib starting when there are four to six leaves. These mutations are known to alter lignin composition and digestibility of plants and therefore constitute prime candidates in the breeding of silage maize. Here, we show that two independent brown midrib3 (bm3) mutations have resulted from structural changes in the COMT gene, which encodes the enzyme O-methyltransferase (COMT; EC 2.1.1.6), involved in lignin biosynthesis. Our results indicate that the bm3-1 allele (the reference mutant allele) has arisen from an insertional event producing a COMT mRNA altered in both size and amount. By sequencing a COMT cDNA clone obtained from bm3-1 maize, a retrotransposon with homology to the B5 element has been found to be inserted near the junction of the 3' coding region of the COMT gene intron. The second bm3 allele, bm3-2, has resulted from a deletion of part of the COMT gene. These alterations of the COMT gene were confirmed by DNA gel blot and polymerase chain reaction amplification analyses. These results clearly demonstrate that mutations at the COMT gene give a brown midrib3 phenotype. Thus, the gene genetically recognized as bm3 is the same as the one coding for COMT.

Alleles↗
PubMed1992

Expression of functional membrane-bound and soluble catechol-O-methyltransferase in Escherichia coli and a mammalian cell line.

Human catechol-O-methyltransferase (hCOMT) cDNA was used to express the recombinant hCOMT enzyme in sufficient quantities in prokaryotic as well as in eukaryotic cells to allow kinetic studies. When human membrane-bound catechol-O-methyltransferase (MB-COMT; amino acids 1-271) and the soluble catechol-O-methyltransferase COMT (S-COMT; delta membrane anchor hCOMT; amino acids 27-271), with the latter lacking the first 26 hydrophobic amino acids, were expressed in Escherichia coli, a relatively high-level synthesis of catalytically active enzymes was obtained. Insertion of the human MB-COMT-coding sequence into an eukaryotic expression vector under transcriptional control of the cytomegalovirus (CMV) promoter and enhancer yielded large quantities of hCOMT in human kidney 293 cells. Subcellular fractionation of 293 cells transfected with pBC12/CMV-hCOMT showed hCOMT to be located predominantly in the membrane fraction. The catechol-O-methyltransferase (COMT) activity was measured in cytosolic and membrane fractions at 37 degrees C, giving values of 33 and 114 units/mg of protein, respectively (1 unit produces 1 nmol of guaiacol/h). Km values were 10 microM for MB-COMT and 108 microM for S-COMT, indicating that recombinant MB-COMT exhibits a higher affinity for catechol as the substrate than the soluble form. RNA blot analysis of human hepatome cells (Hep G2), kidney, liver, and fetal brain revealed only one species of hCOMT mRNA of approximately 1.4 kb. Its level in these various tissues was similar to those of COMT protein in each tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed2000

Catechol-O-methyltransferase inhibition attenuates levodopa toxicity in mesencephalic dopamine neurons.

Inhibition of catechol-O-methyltransferase (COMT; EC 2.1.1.6) is a new therapeutic strategy in the treatment of Parkinson's disease. However, nothing is known about the effects of COMT inhibition on levodopa (L-dopa)-induced toxicity in dopamine (DA) neurons. Therefore we evaluated the effects of the selective COMT inhibitors Ro 41-0960, OR-486, and tolcapone alone and in combination with L-dopa in primary mesencephalic cultures from rat. Neither COMT inhibitor affected the growth of tyrosine hydroxylase immunoreactive (THir) cells with concentrations up to 10 microM when studied alone. However, Ro 41-0960 reduced the L-dopa-induced THir cell loss after 24 h in a dose-dependent manner, shifting the TD(50) value from 21 microM in the absence to 71 microM in the presence of 1 microM Ro 41-0960 (P <.01) without affecting survival of non-DA neurons. OR-486 and the clinically used COMT inhibitor tolcapone showed similar effects. In contrast, toxicity induced by D-dopa was not altered by COMT inhibitors. Furthermore, the primary metabolite of L-dopa formed by COMT, 3-O-methyldopa, and the methyl group donor S-adenosyl-L-methionine used by COMT did not alter THir neuron survival and L-dopa-induced toxicity, respectively, with concentrations up to 100 microM. These data demonstrate that COMT inhibition attenuates L-dopa toxicity toward DA neurons in vitro, but probably not by preventing 3-O-methyldopa production or cellular S-adenosyl-L-methionine depletion.

Animals↗
PubMed1991

Induction of catechol-O-methyltransferase in the luminal epithelium of rat uterus by progesterone.

We performed light microscopic immunocytochemical observations of the localization of catechol-O-methyltransferase (COMT) in rat uterus, using a rabbit anti-rat serum specific for the soluble form of rat liver COMT, biotinylated goat anti-rabbit immunoglobulin, and peroxidase conjugated with streptavidin. In the non-pregnant rat, COMT was minimal but detectable in the uterine luminal and glandular epithelium, with greater amounts present in uteri from rats in estrus than those in diestrus. In early pregnancy a robust accumulation of COMT was observed in the luminal epithelium. To more precisely define both the timing and the factors contributing to the appearance of COMT, uteri were examined on Days 1-5 in pregnant and pseudopregnant rats. Accumulation of COMT in the luminal epithelium was observed by Day 3 in uteri from pregnant and pseudopregnant rats and by Day 4 in lactating post-partum rats. No immunostaining of COMT was observed in uteri from non-lactating post-partum rats. Ovariectomy on Day 0 or 1 but not on Day 2 of pregnancy prevented the appearance of COMT on Day 4. Progesterone treatment immediately after ovariectomy on Day 0 or 1 of pregnancy restored the COMT.

Animals↗
PubMed2001

The effects of catechol-O-methyltransferase inhibition on estrogen metabolite and oxidative DNA damage levels in estradiol-treated MCF-7 cells.

Many of the major identified risk factors for breast cancer are associated with exposure to endogenous estrogen. In addition to the effects of estrogen as a growth factor, experimental and epidemiological evidence suggest that catechol metabolites of estrogen also contribute to estrogen carcinogenesis by both direct and indirect genotoxic mechanisms. O-Methylation catalyzed by catechol-O-methyltransferase (COMT) is a Phase II metabolic inactivation pathway for catechol estrogens. We and others have found that a polymorphism in the COMT gene, which codes for a low activity variant of the COMT enzyme, is associated with an increased risk of developing breast cancer; therefore, the goal of the current study was to investigate the role of decreased COMT activity on estrogen catechol levels and on oxidative DNA damage, as measured by 8-hydroxy-2'-deoxyguanosine (8-oxo-dG) levels. MCF-7 cells were pretreated with dioxin as a means to increase estrogen metabolism to catechol estrogens, then treated with estradiol (E2) +/- Ro 41-0960, a COMT-specific inhibitor. After extraction from culture medium, estrogen metabolites were separated using an high-performance liquid chromatography-electrochemical detection method. As expected, dioxin dramatically increased E2 oxidative metabolism, primarily to its 2-OH and 2-methoxy metabolites. The COMT inhibitor blocked 2-methoxy E2 formation. This was associated with increased 2-hydroxy E2 (2-OH E2) and 8-oxo-dG levels. In the presence of COMT inhibition, increased oxidative DNA damage was detected in MCF-7 cells exposed to as low as 0.1 microM E2, whereas in the absence of COMT inhibition, no increase in 8-oxo-dG was detected at E2 concentrations < or =10 microM. This study is the first to show that O-methylation of 2-OH E2 by COMT is protective against oxidative DNA damage caused by 2-OH E2, a major oxidative metabolite of E2.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed2005

Genetic susceptibility of catechol-O-methyltransferase polymorphism in Japanese patients with breast cancer.

Polymorphic catechol-O-methyltransferase (COMT) catalyzes the O-methylation of catechol estrogens. It has been reported that COMT polymorphism is a representative genetic trait related to the susceptibility of an individual to breast cancer. However, there is no consensus concerning the association between breast cancer in Japanese patients and COMT polymorphism. To analyze the polymorphism distribution in Japanese patients with breast cancer, a molecular genotyping method using a polymerase chain reaction-based restriction fragment length polymorphism (PCR-RFLP) was used. Based on an analysis of 201 Japanese patients with breast cancer and 352 healthy control subjects, a significant difference was observed in either the distribution of genotypes (p=0.03) or allele frequencies between the two groups (p=0.01). The relative risk of breast cancer for genotypes (COMT(Met/Met) and COMT(Val/Met)) including the variant allele (COMT(Met)) was 1.47 compared to the wild allele (COMT(Val)) and homozygote (COMT(Val/Val)). Furthermore, the distribution of genotypes in post-menopausal patients with breast cancer showed a significant difference with that of healthy subjects of the same menopausal status (p=0.01). No significant difference was found between the distribution of genotypes and clinicopathological features of the cancer. These results suggest that COMT polymorphism may thus be implicated as a genetic trait affecting the susceptibility of an individual to breast cancer in a Japanese population and be an important genetic risk factor in the development of breast cancer in post-menopausal women.

Adult↗
PubMed1979

Multiple molecular forms of catechol-O-methyltransferase. Evidence for two distinct forms, and their purification and physical characterization.

Catechol-O-methyltransferase (COMT: EC 2.1.1.6) has been shown to exist in the soluble fraction of rat liver as two distinct molecular forms, designated COMT I and COMT II, which are separable by gel filtration, ion exchange chromatography, and sedimentation. The predominant form, COMT I, has a smaller Mr of about 24,000, as determined by gel filtration and sedimentation, and less negative charge, whereas the minor form, COMT II, has a larger Mr of about 47,500 and more negative charge. The COMT I and COMT II have been purified 450- and 205-fold, respectively, from rat liver by a newly developed procedure which gives homogeneous enzyme preparations with respect to catechol-methylating activities. The molecular properties of the predominant form, COMT I, were: s20,w, 2.7; D20,W, 10.5; Stokes radius, 20.1 A; f/fo, 1.08; and pI, 4.9. For the minor form, COMT II, the values were s20,w, 3.8; D20,w, 7.3; Stokes radius, 28.7 A; f/fo, 1.23; and pI, 4.8. Catechol-O-methyltransferase was found to exhibit tissue-specific isozymic patterns in the distribution of its two variant forms. In the rat tissues, the liver and kidney exhibited the presence of the two physically separable forms. Catechol-O-methyltransferase was also found as two distinct molecular forms in human tissues, including liver, brain, and placenta. The two forms of human catechol-O-methyltransferase were not distinguishable by the criteria of gel filtration from their counterparts in rat liver, indicating that the two molecular forms of human and rat liver catechol-O-methyltransferase are homologous. No interconversion of one molecular form of catechol-O-methyltransferase into the other was observed under experimental conditions employed. Available evidence indicates that the two molecular forms of catechol-O-methyltransferase are genetically dissimilar proteins.

Animals↗
PubMed1998

Ethnic differences in catechol O-methyltransferase pharmacogenetics: frequency of the codon 108/158 low activity allele is lower in Kenyan than Caucasian or South-west Asian individuals.

Catechol O-methyltransferase (COMT) inactivates neurotransmitters, hormones and drugs such as levodopa. COMT activity is inherited in an autosomal recessive manner and individuals with low activity have thermolabile COMT protein. A low activity allele has been demonstrated at codon 108/158 of the soluble and membrane bound COMT protein, respectively, whereby a G to A transition results in a valine to methionine substitution, rendering the protein more thermolabile. As ethnic differences in erythrocyte COMT activity have been previously demonstrated, the frequency of low activity alleles were investigated in 265 British Caucasian, 99 British South-west Asian and 102 Kenyan individuals. Genotyping of COMT codon 108/158 was performed using a minisequencing method. Erythrocyte COMT activity was measured in 60 British Caucasian individuals by radiochemical assay. The frequency of low activity alleles was 0.54 in Caucasians, 0.49 in South-west Asians, and 0.32 in Kenyans. There was a much lower frequency of individuals with homozygous low activity allele in the Kenyan population (9%) than in Caucasians (31%) or South-west Asians (27%). Erythrocyte COMT activity was lower and less thermostable in individuals with homozygous low activity alleles. The data provide molecular evidence that low COMT is less common in African individuals than the Caucasian population.

Adolescent↗
PubMedDate not supplied

Assessment of catechol-O-methyltransferase activity and its inhibition in erythrocytes of animals and humans.

A non-radiometric method to measure the catechol-O-methyltransferase (COMT) activity in erythrocytes was modified to increase its sensitivity four-fold as well as its reproducibility and applicability. The method is based on the COMT-mediated O-methylation of 4-(naphtho [1,2-d] thiazol-2-yl) pyrocatechol, the product of which was determined fluorometrically. COMT activities down to less than 1% of those present at baseline could be measured precisely and accurately. The intra- and inter-assay coefficients were below 3 and 5.3%, respectively. Basal COMT activity and the distribution between soluble and membrane-bound COMT was shown to be variable among different species (ten species tested). The applicability of the method was demonstrated by the characterization of COMT activity-time courses in human erythrocytes after oral administration of the COMT inhibitor tolcapone. The assay developed will be useful in the rapid screening and clinical development of new COMT inhibitors.

Animals↗
PubMed2003

Family-based and case-control study of catechol-O-methyltransferase in schizophrenia among Palestinian Arabs.

COMT is a ubiquitous enzyme crucial to catechol metabolism. The molecular basis of COMT thermolability, that leads to three to fourfold differences in enzyme activity, is due to a substitution of valine with methionine in the Val158/108Met polymorphism. Of special interest is the role of this gene in major psychoses especially since a microdeletion (22q11) containing the COMT gene (velo-cardio-facial syndrome) also carries with it several types of behavioral disorders, including an increased prevalence of schizophrenia. Almost 20 genetic studies have examined the role of COMT in schizophrenia with ambiguous results. Towards clarifying the role of this polymorphism in conferring risk for psychosis, we examined a large group of culturally and ethnically akin Palestinian Arab schizophrenic triads (N = 276) using both a case-control and family-based study. In 194 informative triads with at least one heterozygote parent, no preferential transmission of either COMT allele was observed in this sample (TDT statistic chi-square = 0.14 NS; 131 COMT valine alleles were transmitted and 125 alleles not transmitted). However, using a case-control design a significant increase (Likelihood ratio = 3.935, P = 0.047) in the valine allele was observed in the group of schizophrenic patients (N = 276) compared to an ethnically matched control group (N = 77). The association was stronger in female patients (P = 0.012) similar to other studies showing that some COMT behavioral effects are gender sensitive. In summary, by case-control design but not by a family-based study, there is a weak effect in female patients of the high activity COMT allele in conferring risk for schizophrenia.

Amino Acid Substitution↗
PubMed2003

Lack of association of catechol-O-methyltransferase gene polymorphism in obsessive-compulsive disorder.

The COMT gene has been implicated to be involved in the pathogenesis of obsessive-compulsive disorder (OCD) and various other psychiatric disorders. COMT enzyme activity is governed by a common genetic polymorphism at codon 158 that results in substantial 3- to 4-fold variation in enzymatic activity [a high-activity COMT variant (H) and a low activity variant (L)]. This study evaluates the association between OCD and the COMT gene polymorphism. Fifty-nine OCD patients that were diagnosed according to DSM-IV criteria and 114 healthy control subjects were included in the study. PCR technique was used for molecular analysis. The genotypic pattern of distribution of the COMT gene (H/H, H/L, and L/L genotypes) was not different between the OCD patients and controls. There were no significant differences among the patients with positive family history for OCD, those with negative family history for OCD, and the controls with respect to allele frequencies of the COMT gene polymorphisms. Patients that were homozygous or heterozygous for the L allele had significantly higher insight scores (i.e., poorer insight) on Y-BOCS compared to those homozygous for the H allele. We did not find an association between OCD, family history for OCD, and the COMT gene polymorphism. This study suggests that the COMT gene polymorphism is not directly associated with OCD in our patient group.

Adolescent↗
PubMed1996

The contribution by monoamine oxidase and catechol-O-methyltransferase to the total-body and pulmonary plasma clearance of catecholamines.

To study the effects of inhibition of catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) on the removal of circulating catecholamines, anaesthetized rabbits were infused for 120 min with 3H-labelled noradrenaline, adrenaline and dopamine. Total-body plasma clearances (Cltot) and pulmonary fractional extractions (ERp) of the infused amines and the cardiac output of plasma (CO(p)) were determined under steady-state conditions at the end of each of two consecutive 60-min treatment periods. MAO and COMT were inhibited by treatment with pargyline (40 mg/kg) and tolcapone (3 mg/kg followed by 1.5 mg/kg given every 30 min), respectively. Two groups of animals were studied. Group I involved animals treated with tolcapone throughout and given pargyline at the beginning of the second treatment period. In group II, pargyline was given at the beginning of the first, and the treatment with tolcapone was started at the beginning of the second treatment period. As previous experiments had shown that COMT inhibition alone is without any effect on Cltot of the three catecholamines considered here, the results obtained in the first treatment period of group I can be taken to reflect control results. At the end of the first treatment period, Cltot of noradrenaline, adrenaline and dopamine (expressed as a percentage of CO(p)) was 88%, 85% and 142%, respectively, in group I (COMT inhibition) and 67%, 77% and 115%, respectively, in group II (MAO inhibition; P < 0.05 for the group difference regarding Cltot of noradrenaline and dopamine). MAO inhibition on top of COMT inhibition (group I) lowered Cltot of noradrenaline, adrenaline and dopamine by 23%, 12% and 26%, respectively, and COMT inhibition on top of MAO inhibition (group II) reduced Cltot of these catecholamines by 13%, 20% and 17%, respectively. At the end of the first treatment period, the pulmonary plasma clearance (Clp = ERp x CO(p)) of noradrenaline and dopamine was 13 and 25 ml kg-1 min-1, respectively, in group I and 12 and 28 ml kg-1 min-1, respectively, in group II. Clp of adrenaline did not differ from zero in either group. Clp of noradrenaline and dopamine was reduced by 74% and 70%, respectively, when both enzymes were inhibited in group I and by 70% and 67%, respectively, when both enzymes were inhibited in group II. Hence, inhibition of either MAO or COMT alone had little, if any, effect on the removal of noradrenaline, adrenaline and dopamine on passage through the systemic and pulmonary circulation. Combined inhibition of both MAO and COMT was highly effective in reducing the pulmonary clearance of noradrenaline and dopamine, but produced only minor decreases in the total-body clearance of all three catecholamines.

Animals↗
PubMed1994

Catechol-O-methyltransferase in vitiligo.

Catechol-O-methyltransferase (COMT) is involved in the metabolism of neurotransmitters such as epinephrine, norepinephrine and dopamine. For melanocytes, the enzyme is of particular importance in preventing the formation of toxic o-quinones during melanin synthesis. It has been suggested that COMT plays a regulatory role in melanin synthesis. Indeed, when the melanin precursor molecule DHI(2C) is methylated by COMT it is no longer available for incorporation into melanin. Auto-destruction by intermediates of melanin metabolism has been implicated in the aetiology of vitiligo. Therefore enzyme activities in vitiligo patients and in healthy controls were compared. Systemic COMT activities were measured using red blood cells (RBC) as starting material. However, as local alterations in COMT activity may be specifically involved in vitiligo, the enzyme activity was also measured in epidermal homogenates. Finally, to ascribe epidermal COMT activity to the responsible cell type(s), enzyme activity was measured in cultured vitiligo non-lesional melanocytes and melanocytes from healthy controls as well as in cultured keratinocytes from lesional skin and in purified keratinocytes from control skin. It was found that epidermal homogenates from vitiligo patients expressed higher levels of COMT activity than homogenates from healthy controls. Such differences were not found at the systemic level (i.e. in RBC) nor could they be explained by measurements on separately cultured epidermal cell types, indicating that the COMT activity was induced at the tissue level by extracellular factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1976

The effect of inhibitors of extraneuronal uptake on the distribution of 3H-(+/-)noradrenaline in nerve-free rabbit aortic strips.

1. Nerve-free rabbit aortic strips were exposed to 1.18muM 3H-(+/-)noradrenaline for 30 min. When either MAO or COMT was inhibited, far more O-methylated (MAO inhibited) than deaminated metabolites (COMT inhibited) were formed during the incubation. The accumulation of unchanged amine in the extraneuronal stores was inversely related to the rate of metabolism. 2. After inhibition of both metabolizing enzymes, nerve-free strips were first incubated with the amine and then washed out with amine free solution. Compartmental analysis of the efflux curves showed that two extraneuronal compartments were involved in accumulation (with half times of efflux of 3 and 11 min, respectively). 3. 86muM corticosterone or 30muM phenoxybenzamine greatly decreased the accumulation of noradrenaline in these two compartments. 4. When corticosterone or phenoxybenzamine was added to the wash out solution only, the half time of the efflux from both compartments was greatly increased. However, this effect was seen only after inhibition of COMT and not after inhibition of MAO only (the filling of the extraneuronal stores with unchanged noradrenaline being better after inhibition of COMT than when this enzyme was intact). The effect of corticosterone appeared to be reversible, that of phenoxybenzamine irreversible. 5. Analysis of the efflux of metabolites (in experiments in which only one enzyme was inhibited) indicated that corticosterone affected the efflux of noradrenaline but not that of the metabolites. 6. When either COMT or MAO was inhibited throughout the experiment, very little or no metabolism of noradrenaline occured during prolonged wash out. On the other hand, dis-inhibition of COMT during wash out (by the omission of U-0521 from the wash out solution after it had been present during the initial incubation) revealed that noradrenaline, stored extraneuronally during the initial incubation, is quickly O-methylated during wash out, especially when the efflux of the parent amine is inhibited by corticosterone. 7. The results show that COMT is the major extraneuronal noradrenaline-metabolizing enzyme of rabbit aorta, that inhibition of COMT is a pre-requisite for any corticosterone-sensitive accumulation of noradrenaline, that there are two important extraneuronal compartments (compartments III and IV; Henseling et al., 1976a), and that inhibitors of extraneuronal uptake inhibit both, influx and efflux of noradrenaline.

Animals↗
PubMed2001

The role of dopamine-metabolizing enzymes in the regulation of renal sodium excretion in the rat.

The intrarenal natriuretic hormone dopamine (DA) is metabolized by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO). We have previously shown that inhibition of COMT by entacapone results in a potent D1-like receptor-mediated natriuretic response. The present study was performed using anaesthetized rats to compare the importance of MAO and COMT in DA-mediated natriuresis by use of the MAO inhibitor phenelzine. Urinary sodium and DA excretion remained unchanged after MAO inhibition, while excretion of the main metabolite dihydroxyphenylacetic acid (DOPAC) decreased by 55%. The response was unaltered if 5-hydroxytryptamine receptors (5-HT1A) were blocked during MAO inhibition. We also investigated the specific renal activities of MAO and COMT in rat renal cortex during DA-influenced natriuresis. Specific COMT activity in the renal cortex was reduced by 13% after isotonic sodium loading (5% of body mass) whereas renal MAO-A and MAO-B activities remained unaltered. Furthermore, preliminary data obtained from spontaneously hypertensive rats, whose basal urinary DA excretion is higher than that of normotensive Wistar-Kyoto rats, show a tendency for renal COMT activity to be lower. It is concluded that MAOinhibition by phenelzine does not alter sodium excretion. Furthermore, specific renal cortical COMT activity is reduced during partly D1-like receptor-mediated natriuresis, whereas MAO activity remains unchanged. The results suggest that MAO is less important than COMT in regulating DA-mediated natriuresis in the rat kidney.

Animals↗