A new approach on the purification of recombinant human soluble catechol-O-methyltransferase from an Escherichia coli extract using hydrophobic interaction chromatography

Catechol-O-methyltransferase (COMT) is a significant target in protein engineering due to its role not only in normal brain function but also to its possible involvement in some human disorders. In this work, a new approach was employed for the purification of recombinant human soluble COMT (hSCOMT) using hydrophobic interaction chromatography, as the main isolation method, from an Escherichia coli culture broth. A simplified overall process flow is proposed. Indeed, with an optimized heterologous expression system for recombinant hSCOMT production, such as E. coli, it was possible to produce and recover the active monomeric enzyme directly from the cell crude culture broth either by a freeze/thaw or ultrasonication lysis step. The recombinant enzyme present in the bacterial soluble fraction, exhibited similar affinity for epinephrine (K(m) 276 [215; 337] microM) and the methyl donor (S-adenosyl-L-methionine, SAMe) (K(m) 36 [30; 41]microM) as human SCOMT. After the precipitation step by 55% of ammonium sulphate, a HIC step on the butyl-sepharose resin was found to be highly effective in selectively eluting a range of contaminating key proteins present in the concentrate soluble extract. Consequently, the partially purified eluate from HIC could then be loaded and polished by gel filtration in order to increase the process efficiency. The final product appeared as a single band in sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The procedure resulted in a global 10.9-fold purification with a specific activity of 5500 nmol/h/mg of protein. The widespread applicability of the process, here described, to different COMT sources could make this protocol highly useful for all studies requiring purified and active COMT proteins.

Characterization of SafC, a catechol 4-O-methyltransferase involved in saframycin biosynthesis

Members of the saframycin/safracin/ecteinascidin family of peptide natural products are potent antitumor agents currently under clinical development. Saframycin MX1, from Myxococcus xanthus, is synthesized by a nonribosomal peptide synthetase, SafAB, and an O-methyltransferase, SafC, although other proteins are likely involved in the pathway. SafC was overexpressed in Escherichia coli, purified to homogeneity, and assayed for its ability to methylate a variety of substrates. SafC was able to catalyze the O-methylation of catechol derivatives but not phenols. Among the substrates tested, the best substrate for SafC was L-dihydroxyphenylalanine (L-dopa), which was methylated specifically in the 4'-O position (k(cat)/K(m) = 5.5 x 10(3) M(-1) s(-1)). SafC displayed less activity on other catechol derivatives, including catechol, dopamine, and caffeic acid. The more labile l-5'-methyldopa was an extremely poor substrate for SafC (k(cat)/K(m) = approximately 2.8 x 10(-5) M(-1) s(-1)). L-dopa thioester derivatives were also much less reactive than L-dopa. These results indicate that SafC-catalyzed 4'-O-methylation of L-dopa occurs prior to 5'-C-methylation, suggesting that 4'-O-methylation is likely the first committed step in the biosynthesis of saframycin MX1. SafC has biotechnological potential as a methyltransferase with unique regioselectivity.

Rapid assay for catechol-O-methyltransferase activity by high-performance liquid chromatography-fluorescence detection

A rapid assay for measuring the activities of catechol-O-methyltransferase (COMT) is described. The method is based on high-performance liquid chromatography (HPLC)-fluorescence detection, and includes on-line extraction of catecholamines with a precolumn, separation of norepinephrine (NE) and normetanephrine (NMN) on an ODS column, electrochemical oxidation, and post-column fluorogenic derivatization using ethylenediamine. The method took less than 25 min for one sample, which is half that of the previous method and the sensitivity was similar. The intra-day assay precisions were 0.52-1.6%, and the inter-day assay precisions were 3.6-5.8% for rat liver and cerebral cortex (n = 5). The method is suitable for the rapid measurement of COMT activities of many biological samples.

Separation methods for catechol O-methyltransferase activity assay: physiological and pathophysiological relevance

Catechol O-methyltransferase (COMT) transfers a methyl group from S-adenosyl-L-methionine to the catechol substrate in the presence of magnesium. After the characterisation of COMT more than four decades ago, a wide variety of COMT enzyme assays have been introduced. COMT activity analysis usually consists of the handling of the sample and incubation followed by separation and detection of the reaction products. Several of these assays are validated, reliable and sensitive. Besides the studies of the basic properties of COMT, the activity assay has also been applied to explore the relation of COMT to various disease states or disorders. In addition, COMT activity analysis has been applied clinically since COMT inhibitors have been introduced as adjuvant drugs in the treatment of Parkinson's disease.

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.

Purification and characterization of Streptomyces griseus catechol O-methyltransferase

A soluble (100,000 x g supernatant) methyltransferase catalyzing the transfer of the methyl group of S-adenosyl-L-methionine to catechols was present in cell extracts of Streptomyces griseus. A simple, general, and rapid catechol-based assay method was devised for enzyme purification and characterization. The enzyme was purified 141-fold by precipitation with ammonium sulfate and successive chromatography over columns of DEAE-cellulose, DEAE-Sepharose, and Sephacryl S-200. The purified cytoplasmic enzyme required 10 mM magnesium for maximal activity and was catalytically optimal at pH 7. 5 and 35 degrees C. The methyltransferase had an apparent molecular mass of 36 kDa for both the native and denatured protein, with a pI of 4.4. Novel N-terminal and internal amino acid sequences were determined as DFVLDNEGNPLENNGGYXYI and RPDFXLEPPYTGPXKARIIRYFY, respectively. For this enzyme, the K(m) for 6,7-dihydroxycoumarin was 500 +/- 21.5 microM, and that for S-adenosyl-L-methionine was 600 +/- 32.5 microM. Catechol, caffeic acid, and 4-nitrocatechol were methyltransferase substrates. Homocysteine was a competitive inhibitor of S-adenosyl-L-methionine, with a K(i) of 224 +/- 20.6 microM. Sinefungin and S-adenosylhomocysteine inhibited methylation, and the enzyme was inactivated by Hg(2+), p-chloromercuribenzoic acid, and N-ethylmaleimide.

Purification methods of mammalian catechol-O-methyltransferases

The protein purification strategies used for obtaining homogeneous rat and human soluble catechol-O-methyltransferase (S-COMT) polypeptides are reviewed. Expression and purification of recombinant rat and human S-COMT in Escherichia coli and for human S-COMT in baculevirus-infected insect cells made it possible to elucidate the S-COMT polypeptides in more detail. The application of these purification methods has allowed the crystallization of the rat S-COMT protein and the analysis of the kinetic properties of the enzyme in great detail. The availability of the pure S-COMT protein together with the structural data has also greatly enhanced the development of more potent COMT inhibitors.

Catechol-O-methyltransferase as a target for melanoma destruction?

Catechols may interfere in melanogenesis by causing increased levels of toxic quinones. Several catechols and known inhibitors of the enzyme catechol-O-methyltransferase (COMT) were therefore tested for their toxicity towards a pigmented melanoma cell line, UCLA-SO-(M14). The inhibition of thymidine incorporation as a result of exposure to the compounds was measured. All agents were compared to 4-hydroxyanisole (4HA), a depigmenting agent extensively studied as an antimelanoma drug. The compounds were also tested on the epithelial cell line, CNCM-I-(221) in the presence and absence of tyrosinase. All the compounds were more effective than 4HA towards the M14-cells at either 10(-4) M or 10(-5) M. The toxicity of 4HA towards the 221-cells was shown to be completely dependent on the presence of tyrosinase. Effects of the test agents on the 221-cells were also observed in the absence of tyrosinase. Although some of them were shown to be good substrates for tyrosinase only small changes in toxicity were observed as a result of the presence of the enzyme in comparison with 4HA. No direct correlation of the toxicity of the agents and COMT inhibition was observed. The possible mode of action of the compounds through inhibition of COMT and interference in melanogenesis is discussed together with other possibilities and factors involved.

Molecular cloning and expression of a new class of ortho-diphenol-O-methyltransferases induced in tobacco (Nicotiana tabacum L.) leaves by infection or elicitor treatment

In tobacco (Nicotiana tabacum L. cv Samsun NN), three distinct enzymes account for ortho-diphenol-O-methyltransferase (OMT) activity. OMT I is the major enzyme of healthy leaves, whereas enzymes OMT II and III are preferentially induced during the hypersensitive reaction to tobacco mosaic virus (TMV). Using an anti-OMT III antiserum, we isolated a partial OMT III cDNA clone by immunoscreening an expression library made from mRNA of TMV-infected tobacco leaves. Using this OMT III clone as a probe, we isolated a full-length clone with a deduced amino acid sequence encompassing all of the sequences obtained by Edman degradation of both purified proteins II and III. Thus, OMT II and III of tobacco are likely to be encoded by the same genes and to arise from different posttranslational modifications. Sequence analysis showed that this OMT clone represents a new class of OMT enzymes (class II) with a low level of similarity (53-58%) to OMTs cloned previously from other dicotyledonous plants. Southern analysis indicated that a small family of class II OMT genes inherited from ancestors related to Nicotiana sylvestris and Nicotiana tomentosiformis occurs in the tobacco genome. RNA blot analysis demonstrated that class II OMT genes, unlike class I OMT genes, are not expressed at a high constitutive level in lignified tissues of tobacco. Class II OMT transcripts were found to accumulate in tobacco leaves infected with TMV or treated with megaspermin, a proteinaceous elicitor from Phytophthora megasperma, but not in leaves treated with salicylic acid, a molecule known to trigger many defense genes. In TMV-infected or elicitor-treated tissues, a marked increase in catechol-methylating activity accompanied the accumulation of class II OMT gene products.

Catechol O-methyltransferase pharmacogenetics: photoaffinity labelling and western blot analysis of human liver samples

The level of catechol O-methyltransferase (COMT) activity and COMT thermal stability in human tissue are controlled by a common genetic polymorphism. We studied individual hepatic biopsy samples shown previously to have phenotypically high, low or intermediate COMT activities and thermal stabilities to test the hypothesis that the molecular mass (M(r)) and/or isoelectric point (pI) of the enzyme might differ in tissue from subjects with different presumed genotypes for the COMT genetic polymorphism. COMT was partially purified from each hepatic tissue sample by sequential ion exchange and gel filtration chromatography, and photoaffinity labelling was performed with [3H-methyl]-S-adenosyl-L-methionine ([3H-methyl]-Ado-Met), the methyl donor for the COMT enzymatic reaction. Two-dimensional sodium dodecylsulfate polyacrylamide gel electrophoresis (2-D SDS-PAGE) analysis of individual samples consistently showed the presence of three [3H-methyl]-Ado-Met photoaffinity labelled proteins with pI values of 5.4, 5.5 and 5.7, all three of which had M(r) values of approximately 27.1 kDa. The same pattern was observed in all samples irrespective of COMT phenotype. Western blot analysis of 2-D SDS-PAGE gels performed with rabbit polyclonal antibodies to partially purified human kidney COMT showed a pattern similar to that found during photoaffinity labelling. Once again, the same pattern was found in all samples irrespective of COMT phenotype. Therefore, neither photoaffinity labelling nor Western blot analysis revealed differences in either M(r) or pI of cytoplasmic COMT in hepatic tissue from subjects selected on the basis of different phenotypic expression of the COMT genetic polymorphism.

Interferon-induced Mx proteins form oligomers and contain a putative leucine zipper

Interferons induce a number of different proteins that mediate the antiproliferative, antiviral, and immunomodulatory functions of interferons. At least three different proteins mediate the antiviral response, and one of them, Mx protein, specifically inhibits the replication of influenza virus and (vesicular stomatitis virus). Mouse and rat Mx1 proteins are nuclear, whereas other presently known Mx proteins are cytoplasmic. The cellular functions of Mx proteins are unknown, but all of them contain a consensus GTP binding site. Very little information is available on the structure and characteristics of the mouse Mx1 protein itself. For biochemical characterization, we expressed mouse Mx1 protein in a baculovirus system and purified it to homogeneity. The purified protein as well as the authentic murine cellular Mx1 protein exists in dimers and trimers in the presence of dissociating solvents, whereas in physiological buffers they form aggregates. Cross-linking experiments done on Mx-expressing cells from various species revealed that mouse, rat, and human Mx proteins exist predominantly in trimers. Amino acid sequence analysis shows that all known Mx proteins have conserved leucine repeats typical for a leucine zipper at their COOH-terminal end. In vitro translation of chimeric catechol O-methyltransferase-Mx1 gene constructs revealed that the leucine zipper domain of Mx1 protein is responsible for the oligomerization. The COOH terminus also functions as a nuclear localization signal. Microinjection of purified oligomers into the cell cytoplasm resulted in a fast accumulation of the protein in the resulted in a fast accumulation of the protein in the nucleus. Immunoelectron microscopy revealed that nuclear murine Mx1 protein exists in distinct, electron-dense structures separate from nuclear membrane, and chromatin, or nucleolus. These observations reveal that a COOH-terminal leucine zipper domain is an important structural element of all Mx proteins. Its relevance to the biology and functions of Mx proteins is presently not known.

Kinetic and inhibition studies on catechol-O-methyltransferase affinity labelling by N-(3,4-dihydroxyphenyl)maleimide

Initial velocity and product inhibition studies have been performed on soluble catechol-O-methyltransferase which has been partially purified from pig liver. The results are consistent with an ordered reaction mechanism, in which S-adenosyl-L-methionine (AdoMet) is the leading substrate. The enzyme is irreversibly inhibited by maleimide derivatives in a biphasic manner, which suggests a differential reaction with two thiol groups. N-(3,4-Dihydroxyphenyl)maleimide, which has a reactive moiety (maleimide ring) and an affinity moiety (catechol ring), acts as an affinity labelling compound on the more reactive SH group; AdoMet and Mg2+ protect against this modification. Total protection of this SH group results in a pseudo-first-order inhibition of the enzyme, with the apparent rate constant being proportional to the inhibitor concentration. All the other maleimide derivatives studied inhibited the enzyme by reacting with one of the two SH groups in a non-specific manner. The reaction of the other, more reactive, SH group was either specific (active-site-directed) or non-specific, depending on the substituent present in the affinity moiety and also on the length of an intermediate chain of methylene groups present between this moiety and the reactive maleimide ring. In the presence of both AdoMet and Mg2+, 3,5-dinitrocatechol, a reversible inhibitor of the enzyme which is competitive with respect to the catechol substrate, protects the enzyme from inactivation by any of the maleimide derivatives. The adducts of these maleimide derivatives formed with dithiothreitol inhibit the enzyme reversibly, showing inhibition patterns that are consistent with the mechanism deduced from the initial velocity and product inhibition studies.

Expression of enzymatically active rat liver and human placental catechol-O-methyltransferase in Escherichia coli; purification and partial characterization of the enzyme

To produce sufficient amounts of recombinant catechol-O-methyltransferase (COMT) for structural and functional studies the coding regions of the rat liver and human placental COMT genes have been introduced into a bacterial expression vector pKEX14. Recombinant COMT was produced in Escherichia coli up to 10% of total bacterial protein after the induction of the T7 RNA polymerase gene with isopropyl-beta-D-thiogalactopyranoside. Both the rat and human enzymes were enzymatically active, soluble and reacted with anti-COMT antiserum in Western blotting. Both enzymes were purified from E. coli cells and partially characterized by determining their specific activity, apparent molecular weight and pI.

Immunoaffinity purification and partial amino acid sequence analysis of catechol-O-methyltransferase from pig liver

Monoclonal antibodies (mAbs) against the soluble form (S-COMT) of catechol-O-methyltransferase (COMT, EC 2.1.1.6) were produced using a purified preparation of the enzyme from pig liver as antigen. The selected monoclonal antibodies recognized the enzyme with different capacities. One of them (Co60-1B/7) showed a significant cross reaction with S-COMT from rat and human liver. A protein band of 23 kDa was recognized by the mAbs on Western blots of the soluble fraction of pig liver. The mAbs were also able to recognize the membrane-bound form of the enzyme, which was found to be mainly localized in the microsomal fraction of pig and rat liver as well as of the human hepatoma cell line Hep G2. The protein bands detected in microsomes had a molecular mass of 26 kDa in pig and rat liver and displayed a slightly higher molecular mass (29 kDa) in the Hep G2 cell line. A single step method for the immunoaffinity purification of pig liver S-COMT was developed by using a Sepharose 4B column to which the mAb Co54-5F/8 was covalently coupled. Acid elution conditions were optimized to obtain the enzyme in active form with a good yield. SDS-PAGE analysis of the purified preparation revealed a single protein band with a molecular mass of 23 kDa with 154-fold enrichment in enzyme activity over the starting material. Since the N-terminus was blocked, purified enzyme preparations were cleaved with trypsin. Two fragments of 22 and 33 amino acids in length could be sequenced by Edman degradation.

Purification and partial characterization of rat liver soluble catechol-O-methyltransferase

The rat liver soluble catechol-O-methyltransferase (EC 2.1.1.6.) has been purified utilizing a combination of conventional chromatography and HPLC. The purified enzyme has a molecular mass of 25 kDa, a pI of 5.1, and exists in two forms which differ in the nature of their intramolecular disulfide bonds. This difference causes these two protein forms to behave differently in reversed phase chromatography.

Inhibition of catechol-O-methyltransferase by N-(3,4-dihydroxyphenyl) maleimide

Catechol-O-methyltransferase (COMT) is inhibited rapidly and irreversibly by N-(3,4-dihydroxyphenyl) maleimide. S-adenosylmethionine (AdoMet) and magnesium ions protect the enzyme from inactivation by this compound, but no protection is observed by the catechol substrate. However, the corresponding succinimide analogue shows a reversible inhibition of COMT, which is competitive with pyrocatecholphthalein and non-competitive with AdoMet. Amino-group reagents also inhibit COMT and this inhibition is protected by AdoMet, suggesting that sulphydryl and amino groups essential for activity are located in an AdoMet-binding site on COMT. The maleimide derivative may be considered to be an active-site directed inhibitor.

Behaviour and properties of catechol-O-methyltransferase from human placenta

A procedure is reported for the purification of human placental catechol-O-methyltransferase. The preparation is apparently homogeneous and behaves as a monomer with an approximate Mr of 23,000. The sequence of the first 21 amino acid residues from the N-terminal end of the protein is reported. The activity of the enzyme is strongly influenced by the nature of the buffer in which it is assayed.

The specificity of interaction between S-adenosyl-L-methionine and a nucleolar 2'-O-methyltransferase

The structural features of S-adenosyl-L-methionine (SAM)3 required for optimal binding to a nucleolar 2'-O-methyltransferase were elucidated using various analogs of SAM with modifications of the amino acid, sugar, sulfonium center, and base portions of the molecule. Equilibrium binding constants for SAM and each analog were determined by a nitrocellulose filter binding assay. To ensure the chiral and chemical purity of the 3H-labeled SAM used in the binding experiments, a cation-exchange HPLC procedure was developed to separate degradation products of SAM such as adenine and 5'-deoxy-5'-methylthioadenosine, as well as to separate the (S,S)-SAM from the biologically inactive (R,S)-SAM stereoisomer. Results from these studies demonstrated that S-adenosyl-L-homocysteine, a product of the methyltransferase reaction, bound equally as well as (S,S)-SAM, indicating that neither the charge nor the methyl group at the sulfonium center of (S,S)-SAM is essential for maximal binding. Other modifications of the sulfonium center demonstrated that a sulfur to carbon atom replacement had little effect on binding affinity, whereas substituting an ethyl group for the methyl group greatly reduced the binding affinity. In addition, the chirality at the sulfonium center was important. The naturally occurring S-chiral form had a 10-fold higher binding affinity than the R-chiral form. No significant stereospecificity was observed relative to the chiral alpha-carbon of the methionine moiety in SAM. The alpha-amino group of methionine and the 6-amino group of adenine were both required for maximal binding, while the loss of the 2'-hydroxyl group on the ribose moiety was not. Taken together, these results defined some of the specific geometric and functional group requirements which affect the specificity of interaction between S-adenosyl-L-methionine and the nucleolar 2'-O-methyltransferase.

Purification of rat liver soluble catechol-O-methyltransferase by high performance liquid chromatography

The soluble form of catechol-O-methyltransferase (EC 2.1.1.6) from rat liver was purified to homogeneity by high-performance anion-exchange chromatography and high-performance gel-filtration chromatography. The specific activity of the final pool was 270 U/mg protein. The purification was 1180-fold and recovery of the enzyme activity was 15%. During this rapid and gentle purification there were no problems with loss of activity, and the estimated half of the final purified enzyme pool was 5.5 days at +4 degrees C. The only additive used was phenylmethylsulfonylfluoride in the homogenizing buffer.

Kinetic reaction mechanism for magnesium binding to membrane-bound and soluble catechol O-methyltransferase

Catechol O-methyltransferase (COMT, EC 2.1.1.6) from human brain occurs in both a membrane-bound (MB-COMT) and a soluble form (SOL-COMT). While these enzymes appear to be distinct molecular entities, both catalyze the O-methylation of catecholamines through an ordered reaction mechanism in which S-adenosylmethionine (SAM) is the leading substrate [Rivett, A. J., & Roth, J. A. (1982) Biochemistry 21, 1740-1742; Jeffery, D. R., & Roth, J. A. (1985) J. Neurochem. 44, 881-885]. Both MB-COMT and SOL-COMT require the presence of divalent cations for catalytic activity. This series of experiments provides evidence indicating that magnesium ions bind to both MB-COMT and SOL-COMT in a rapid equilibrium sequence prior to the addition of SAM. An equation is presented that predicts the qualitative results obtained in all kinetic experiments carried out with either MB-COMT or SOL-COMT.

Isolation of the low-molecular-mass form of catechol O-methyltransferase from rat liver and immunocytochemical localization of the enzyme in the glycogen compartment

The low-molecular-mass form of two distinct catechol O-methyltransferase activities (S-adenosyl-L-methionine: catechol O-methyltransferase, COMT, EC 2.1.1.6) has been purified to homogeneity from rat liver using 40-70% ammonium sulfate precipitation, gel filtration on Sephadex G-100, adsorption on hydroxyapatite C and ion-exchange chromatography on DEAE-Sepharose CL-6B. The relative molecular mass Mr, determined by sodium dodecyl sulfate/polyacrylamide gel electrophoresis is 22 400 +/- 500. Irradiation of the enzyme in the presence of 8-azido-[methyl-3H]AdoMet results in the specific labeling of the catalytic site of the enzyme. Photolabeling was successful with crude COMT preparations and with the isolated enzyme. Immunocytochemical studies present new information about the localization of the low-molecular-mass form in the liver parenchyma. Subcellularly COMT immunoreactivity could be attributed exclusively to the compartment with glycogen granules. Nucleus, mitochondria and endoplasmic reticulum showed no immunostaining.

External and internal standards in the single-isotope derivative (radioenzymatic) measurement of plasma norepinephrine and epinephrine

In plasma from normal humans (n = 9, 35 samples) and from patients with diabetes mellitus (n = 12, 24 samples) single-isotope derivative (radioenzymatic) plasma norepinephrine and epinephrine concentrations calculated from external standard curves constructed in a normal plasma pool were identical to those calculated from internal standards added to an aliquot of each plasma sample. In plasma from patients with end-stage renal failure receiving long-term dialysis (n = 34, 109 samples), competitive catechol-O-methyltransferase (COMT) inhibitory activity resulted in a systematic error when external standards in a normal plasma pool were used, as reported previously; values so calculated averaged 21% (+/- 12%, SD) lower than those calculated from internal standards. However, when external standard curves were constructed in plasma from a given patient with renal failure and used to calculate that patient's values, or in a renal failure plasma pool and used to calculate all renal failure values, norepinephrine and epinephrine concentrations were not significantly different from those calculated from internal standards. We conclude: (1) External standard curves constructed in plasma from a given patient with renal failure can be used to measure norepinephrine and epinephrine in plasma from that patient; further, external standards in a renal failure plasma pool can be used for assays in patients with end-stage renal failure receiving long-term dialysis. (2) Major COMT inhibitory activity is not present commonly if samples from patients with renal failure are excluded. Thus, it would appear that external standard curves constructed in normal plasma can be used to measure norepinephrine and epinephrine precisely in samples from persons who do not have renal failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and kinetic mechanism of human brain soluble catechol-O-methyltransferase

The soluble form of human brain catechol-O-methyltransferase (EC 2.1.1.6, COMT) has been purified approximately 4,000-fold from a 250,000 X g supernatant solution. The purified enzyme exhibits a molecular weight near 27,500 and a pI value equal to approximately pH 5.0. Initial velocity and product inhibition studies are consistent with an ordered reaction mechanism for soluble COMT. Tropolone, a dead-end inhibitor, exhibited a competitive pattern of inhibition when dopamine (DA) was the varied substrate and an uncompetitive pattern when S-adenosyl-L-methionine (SAM) was the varied substrate. These observations strongly suggest that the soluble form of COMT from human brain catalyzes the O-methylation of catecholamines via an ordered reaction mechanism in which SAM is the leading substrate. Since the membrane-bound form of COMT catalyzes the O-methylation of catecholamines through an identical reaction mechanism, these data provide further evidence that two forms of COMT, while being localized in distinct subcellular compartments, are quite similar in their molecular structure.

Catechol-O-methyltransferase: a method for autoradiographic visualization of isozymes in cellogel

An electrophoretic procedure for separating the molecular forms of catechol-O-methyltransferase in cellulose acetate gel is described; the zones of enzyme activity were revealed by autoradiography. The electrophoretic patterns of the enzyme in several tissues and cell lines derived from four different species are presented.

Distinct cellular localization of membrane-bound and soluble forms of catechol-O-methyltransferase in brain

The cellular localization of the two forms of catechol-O-methyltransferase (COMT) was investigated by measuring their activities in rat striatum following unilateral stereotaxic injection of kainic acid, which causes degeneration of striatal neurons followed by proliferation of astroglial cells. Membrane-bound COMT activity was decreased in the lesioned striatum, while soluble COMT activity was increased. There was a statistically significant correlation between the ratio of lesioned to control activity for membrane-bond COMT and the neuronal marker enzyme glutamate decarboxylase. Similarly the increase in soluble COMT activity paralleled that of the astroglial marker enzyme, glutamine synthetase. These results indicate that the low-Km membrane-bound catechol-O-methyltransferase may be localized predominantly in neurons, whereas the high-Km soluble enzyme is found in glial cells.

A highly sensitive radioenzymatic assay for simultaneous estimation of norepinephrine, dopamine, and epinephrine

We have developed a radioenzymatic assay for simultaneous estimation of norepinephrine (NE), dopamine (DA), and epinephrine (E) that was the result of the integration of several unique features of previously described assay procedures. Catecholamines in sera or tissue homogenates were enzymatically O-methylated in the presence of partially purified catechol-O-methyltransferase with S-[methyl-3H] adenosyl methionine serving as the methyl donor. The O-methylated products were then separated by thin-layer chromatography, eluted from the gel, and their tritium content determined. The assay allows measurement of catecholamines with a sensitivity in the ranges of 15-20 pg. In addition, the assay is highly specific, reproducible, relatively rapid and simple, and inexpensive.

Purification and properties of a catechol methyltransferase of the yeast Candida tropicalis

In an effort to investigate catechol methyltransferase activity in sources other than mammalian tissues and cells, a high level of enzyme activity was found in the yeast fungus Candida tropicalis CBS 94. Partial purification of the enzyme (approx. 550 fold with a recovery of 7%) could be achieved by using ion-exchange and gel filtration techniques. The molecular weight was estimated at 32,000 +/- 2,000 by gel filtration on Sephadex G-100. In isoelectric focusing experiments on Sephadex G-75 the enzyme exhibited a pI-value of 5.0 +/- 0.1. In contrast to catechol methyltransferase from various mammalian tissues the enzyme activity was prepared from the pH 5-sediment. The substrate specifity is comparable to other catechol methyltransferases.

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.

The purification and properties of pig-liver catechol-O-methyl transferase

A procedure utilising affinity chromatography is described for the large-scale purification of pig-liver catechol-O-methyl transferase. The enzyme prepared by this method appears to be homogeneous by polyacrylamide gel electrophoretic criteria and gel chromatography. It is stable for prolonged periods when stored at -5 degrees C in 20% (v/v) glycerol. The enzyme has a molecular weight of about 23000 and does not appear to be a compound of subunits, or to associate to any appreciable degree. The pH optimum of the enzyme's activity is approximately pH 7.1--7.4, it does not catalyse the methylation of benzimidazole and has a Km of 0.64 mM and 0.056 mM towards 3,4-dihydroxyphenylacetic acid and S-adenosyl-L-methionine, respectively. Amino acid analysis showed the presence of five cysteine residues.

Purification and characterization of rat heart and brain catechol methyltransferase

In an effort to detect the similarities and differences in the properties of rat heart, brain and liver catechol methyltransferase (S-adenosyl-L-methionine:catechol O-methyltransferase, EC 2.1.1.6), we have determined the cellular distribution of this enzyme activity and extensively purified the soluble and microsomal enzymes present in these tissues. Purification of soluble heart (688-fold) and brain enzymes (240-fold) were achieved using an affinity chromatographic system. The properties of these enzymes were compared with respect to their molecular weights, substrate specificities, inhibitor specificities and immunological properties. The characteristics of the enzyme active sites were investigated using various methyl acceptor substrates and various analogs of S-adenosylmethionine as methyl donors. A series of analogs of S-adenosylhomocysteine was also evaluated as inhibitors of these enzymes. The immunological properties of the purified soluble and microsomal enzymes from heart and brain were investigated using an antibody isolated from rabbits which had been immunized with the soluble rat liver enzyme. In general the properties of catechol methyltransferases isolated from heart and brain were similar to the properties of the enzyme isolated from liver. Some minor differences in substrate and inhibitor specificities were observed which might suggest slight differences in the active sites of these enzymes.

Solubilization and partial purification of particulate catechol-O-methyltransferase from rat liver

Particulate catechol-O-methyltransferase (COMT) from rat liver has been solubilized by acetone treatment and partially purified. Results from the present study demonstrate that the solubilized, partially purified enzyme is similar to the cytosol COMT with respect to molecular weight, pH profile, sensitivity toward inhibitors, Mg2+ requirement, and substrate affinities. However, a comparison of the crude particulate COMT and the solubilized enzyme shows that there is a significant difference in their affinity for catechol substrates. This finding suggests that membrane protein and (or) lipid components may play an important role in catecholamine metabolism. The relationship of particulate COMT to [3H]norepinephrine binding was investigated. No correlation between the COMT and [3H]norepinephrine binding activities was observed in vitro.

[Purification and properties of rat kidney catechol-O-methyltransferase]

The S-adenosyl-methionine: catechol-O-methyltransferase (EC 2.1.1.6) from rat kidney was purified about 650 fold as compared with the homogenate and the result of disc electrophoresis presented. The purification involved extraction, precipitation at pH 5, ammonium sulfate fractionation, Chromatographies on Biogel 0.5 m, Ultrogel AcA 44 and DE Sephadex A 50. Affinity chromatography was tried but unsuccessful. The enzyme exhibited two pH optima at 7.9 and 9.6 with a minimum at about 8.9. The COMT had a temperature optimum of 50 degrees C, with activation energy of 23.1 Kcal/Mole between 25-35 degrees C, 18.9 Kcal/mole between 35-45 degrees C and the Q10 within the range of 25-35 degrees amounted to 3.5. The molecular weight was estimated to be 21500+/-1000 daltons from its behavior on Ultrogel AcA 44 and the pH1 determined by electrofocalisation was near 5.50. The time of half life of the best purified enzymatic extract was found to be 2 h 10 min. at -20 degrees C. At basic pH the instability of the enzyme was increased. Since O-methylation required the presence of divalent cations, our results show that apparent Michaelis constants for Mg++ and Mn++ were respectively 0.50 X 10(-3) M and 0.33 X 10(-5) M. The study of their Hill's number indicated that there was only one point of fixation on the enzyme. The Km value determined by Florini and Vestling's method were 2.5 X 10(-4) M and 11.9 X 10(-5) M for epinephrine and S-adenosyl-methionine respectively. All results were discussed with respect to other investigations.

Affinity labeling of catechol O-methyltransferase by N-haloacetyl derivatives of 3,5-dimethoxy-4-hydroxyphenylethylamine and 3,4-dimethoxy-5-hydroxyphenylethylamine. Kinetics of inactivation

In an attempt to elucidate the relationship between the chemical structure and the catalytic function of catechol O-methyltransferase (COMT), several classes of affinity labeling reagents have been synthesized and their interaction with COMT has been studied. Earlier studies have shown that various N-haloacetyl derivatives of 3,5-dimethoxy-4-hydroxyphenylethylamine were effective affinity labeling reagents for this enzyme. In this report we have shown that N-haloacetyl derivatives of the isomeric 3,4-dimethoxy-5-hydroxyphenylethylamine also rapidly and irreversibly inactivate COMT ant they satisfy many of the criteria established for affinity labeling reagents. This latter group of agents appear to modify a nucleophilic residue at the active site of COMT different from that modified by the 3,5-dimethoxy-4-hydroxyphenylethylamine series. Evidence to support this conclusion has been obtained by comparing the kinetics of COMT inactivation and the substrate protection profiles for these two classes of affinity labeling reagents.