Showing papers on "Catechol-O-methyl transferase published in 1984"

Characterization of membrane-bound and soluble catechol-O-methyltransferase from human frontal cortex

Abstract: Catechol-O-methyltransferase (COMT; E.C. 2.1.1.6) from human frontal cortex occurs in both a soluble and membrane-bound form. Attempts to solubilize the membrane-bound transferase by repeated washing or by extraction into solutions of high ionic strength were unsuccessful. The finding that Triton X-100 was capable of solubilizing membrane-bound COMT suggested that the membrane-bound transferase is an integral membrane protein. The membrane-bound and soluble enzymes did not differ in their requirements for magnesium ions or in their pH-activity profiles; both enzymes showed an optimum near pH 8.0 when assayed in phosphate buffer. In addition, the two enzymes did not differ in the degree of inhibition caused by CaCl2, both enzymes displaying 65% inhibition at 2.5 mM CaCl2. The competitive inhibitors tropolone and nordihydroguaiaretic acid displayed Ki values for the membrane-bound transferase five- to 10-fold lower than those observed for the soluble transferase. Solubilization of membrane-bound COMT in Triton X-100 resulted in an increase in the apparent Km value of the membrane-bound transferase for dopamine. The increase in Km appeared to be due to apparent competitive inhibition by Triton X-100 and reached a limiting value of approximately 80 microM. These results confirm that membrane-bound COMT is an integral membrane protein that may be structurally distinct from soluble COMT.

Platelet phenol sulfotransferase and erythrocyte catechol-O-methyltransferase activities: correlation with methyldopa metabolism.

Abstract: Methyldopa is metabolized by sulfate conjugation catalyzed by phenol sulfotransferase (PST), O-methylation catalyzed by catechol-O-methyltransferase (COMT), and decarboxylation catalyzed by aromatic L-amino acid decarboxylase. These experiments were performed to determine whether individual variations in red blood cell (RBC) COMT and platelet PST activities might reflect variations in the metabolism of methyldopa in man. Methyldopa, 3.5 mg/kg, was taken orally by 28 subjects. Blood samples were obtained from these subjects for the assay of platelet PST and RBC COMT activities, and a 24-hr urine sample was collected for the measurement of methyldopa and its major metabolites. Human platelets contain two independently regulated forms of PST. One form is thermolabile (TL), and the other is thermostable (TS). Methyldopa and alpha-methyldopamine are substrates for the TL but not for the TS form of PST. The results of the experiment showed significant correlations between TL platelet PST activity and the proportion of alpha-methyldopamine excreted as a sulfate conjugate, and between RBC COMT activity and the proportion of methyldopa excreted as an O-methyl metabolite. There was no significant correlation, however, between TL platelet PST activity, and the proportion of methyldopa itself excreted as a sulfate conjugate. These results are compatible with the conclusion that differences among subjects in drug metabolizing enzyme activities are one factor responsible for wide individual variations in methyldopa metabolism in man.

Catechol-O-methyltransferase and Parkinson's disease.

Abstract: OMD inhibits L-DOPA utilization in rat corpus striatum. This effect is probably mediated through competition with L-DOPA for brain uptake mechanism. Such competition may explain the inhibition exerted by OMD on L-DOPA-induced rotation in rats with unilateral destruction of the nigrostriatal pathway. U-0521, a potent COMT inhibitor, was shown, after i.p. injection, to effectively block the accumulation of OMD in the plasma and to enhance L-DOPA metabolism in rat brain. This drug, however, was not active when given orally to rats and to a single patient with Parkinson's disease. It is suggested that the combined use of L-DOPA plus an orally active COMT inhibitor may be useful in future treatment of Parkinson's disease.

Human pharmacogenetics of methyl conjugation.

Abstract: Methyl conjugation is an important pathway in drug metabolism. Activities of three human drug-metabolizing methyltransferase enzymes, catechol-O-methyltransferase (COMT) (EC 2.1.1.6), thiopurine methyltransferase ( TPMT ) (EC 2.1.1.67), and thiol methyltransferase (TMT) (EC 2.1.1.9), are controlled by inheritance. COMT activity in the red blood cell (RBC) is regulated by a single genetic locus with two alleles, COMTL for low activity and COMTH for high activity. Gene frequencies of these two alleles were approximately equal in a white population sample of Northern European origin. The genetically controlled level of COMT activity in the RBC reflects the level of enzyme activity in other tissues and is significantly correlated with individual variations in the methyl conjugation of catechol drugs such as L-dopa and methyldopa. TPMT catalyzes the S-methylation of thiopurines and thiopyrimidines . RBC TPMT activity is also controlled by a single genetic locus with two alleles, TPMTL for low and TPMTH for high activity. The gene frequencies of these two alleles were 0.06 and 0.94, respectively, in a white population sample. RBC TPMT activity reflects the level of enzyme activity in other cells and tissues such as the lymphocyte and kidney. TMT catalyzes the S-methylation of aliphatic sulfhydryl compounds such as the drugs captopril and D-penicillamine. The heritability of the level of RBC membrane TMT activity has been estimated on the basis of family studies to be approximately 0.98. Regulation of these three methyl-conjugating enzymes by inheritance raises the possibility that genetically determined methylator status may be one factor responsible for variations in drug metabolism in humans.

Major gene model for the inheritance of catechol-O-methyltransferase activity in five large families.

Abstract: Five large families including 1,189 individuals were each ascertained through one proband with essential hypertension. Four of the probands were white and one was black. Erythrocyte catechol-o-methyltransferase (COMT) activity was measured in 551 family members. Standard statistical methods were used to investigate sex, age, and family differences in COMT activity. Maximum-likelihood methods were used to fit mixtures of normal distributions to COMT activity. COMT activity is distinctly bimodal. Pedigree segregation analyses were performed on the untransformed COMT values, their square roots, and natural logarithms in each family. In no family and under none of the three transformations was it possible to reject the hypothesis of Mendelian transmission of a major gene with two alleles in Hardy-Weinberg equilibrium. In most cases a genetic hypothesis with complete dominance or recessiveness, or a hypothesis of equal transmission probabilities was rejected. While the different transformations had a large effect on the skewness and kurtosis of the overall distribution of the data, they had little effect on the outcome of these segregation analyses. Therefore, this study strongly supports the concept that variation in COMT activity is due in large part to the effects of a major gene.

Erythrocyte catechol O-methyltransferase activity in schizophrenia: analysis of family data.

Abstract: The authors determined the erythrocyte catechol O-methyltransferase (COMT) activity of 38 chronic schizophrenic patients, 69 of their first-degree relatives, and 39 normal controls. COMT activity did not distinguish patients from controls. Within families, COMT activity was not associated with schizophrenia spectrum disorders. The data suggest that COMT activity is not an indicator of vulnerability to schizophrenia.

Linkage relationships between a major gene for catechol-o-methyltransferase activity and 25 polymorphic marker systems

Abstract: Segregation analysis has provided evidence suggesting the existence of a major gene for catechol-o-methyltransferase (COMT) activity in man. Five large families (4 Caucasian, 1 black), with a total of 1,189 individuals, were ascertained as part of a genetic study of blood pressure. Erythrocyte COMT activity and status at 25 polymorphic genetic marker loci were determined on more than 518 individuals in these pedigrees. Genetic linkage analysis of COMT with each of the 25 marker loci was performed in two ways: 1) using parameter estimates from segregation analysis of untransformed COMT activity, and 2) using parameter estimates from segregation analysis of the power transformation of the COMT activity that maximized the likelihood of the genetic hypothesis in each family. Tight and close linkage were excluded at 21 and 15 loci, respectively. A lod score of 1.27 at theta = 0.1 was found between the loci for COMT activity and phosphogluconate dehydrogenase (PGD). Transformation of the data had little effect on the outcome of the linkage analysis.