Kinetic properties of a soluble catechol O-methyltransferase of human liver

Sex, hormones and neuroeffector mechanisms

Incidence and rate of cardiovascular disease differ between men and women across the life span. Although hypertension is more prominent in men than women, there is a group of vasomotor disorders [i.e. Raynaud's disease, postural orthostatic tachycardia syndrome and vasomotor symptoms (hot flashes) of menopause and migraine] with a female predominance. Both sex and hormones interact to modulate neuroeffector mechanisms including integrated regulation of the Sry gene and direct effect of sex steroid hormones on synthesis, release and disposition of monoamine neurotransmitters, and distribution and sensitivity of their receptors in brain areas associated with autonomic control. The interaction of the sex chromosomes and steroids also modulates these effector tissues, that is, the heart, vascular smooth muscle and endothelium. Although involvement of central serotonergic centres has been studied in regard to mood disorders such as depression, their contribution to cardiovascular risk is gaining attention. Studies are needed to further evaluate how hormonal treatments and drugs used to modulate adrenergic and serotonergic activity affect progression and risk for cardiovascular disease in men and women.

A universal competitive fluorescence polarization activity assay for S-adenosylmethionine utilizing methyltransferases

A high-throughput, competitive fluorescence polarization immunoassay has been developed for the detection of methyltransferase activity. The assay was designed to detect S-adenosylhomocysteine (AdoHcy), a product of all S-adenosylmethionine (AdoMet)-utilizing methyltransferase reactions. We employed commercially available anti-AdoHcy antibody and fluorescein-AdoHcy conjugate tracer to measure AdoHcy generated as a result of methyltransferase activity. AdoHcy competes with tracer in the antibody/tracer complex. The release of tracer results in a decrease in fluorescence polarization. Under optimized conditions, AdoHcy and AdoMet titrations demonstrated that the antibody had more than a 150-fold preference for binding AdoHcy relative to AdoMet. Mock methyltransferase reactions using both AdoHcy and AdoMet indicated that the assay tolerated 1 to 3 microM AdoMet. The limit of detection was approximately 5 nM (0.15 pmol) AdoHcy in the presence of 3 muM AdoMet. To validate the assay's ability to quantitate methyltransferase activity, the methyltransferase catechol-O-methyltransferase (COMT) and a known selective inhibitor of COMT activity were used in proof-of-principle experiments. A time- and enzyme concentration-dependent decrease in fluorescence polarization was observed in the COMT assay that was developed. The IC(50) value obtained using a selective COMT inhibitor was consistent with previously published data. Thus, this sensitive and homogeneous assay is amenable for screening compounds for inhibitors of methyltransferase activity.

Catechol estrogen 4-hydroxyequilenin is a substrate and an inhibitor of catechol-O-methyltransferase

Redox and/or electrophilic metabolites formed during estrogen metabolism may play a role in estrogen carcinogenesis. 4-Hydroxyequilenin (4-OHEN) is the major phase I catechol metabolite of the equine estrogens equilenin and equilin, which are components of the most widely prescribed estrogen replacement formulation, Premarin. Previously, we have found that 4-OHEN rapidly autoxidized to an o-quinone in vitro and caused toxic effects such as the inactivation of human detoxification enzymes. 4-OHEN has also been shown to be a substrate for catechol-O-methyltransferase (COMT) in human breast cancer cells. In the present study, we demonstrated that 4-OHEN was not only a substrate of recombinant human soluble COMT in vitro with a K(m) of 2.4 microM and k(cat) of 6.0 min(-)(1) but it also inhibited its own methylation by COMT at higher concentrations in the presence of the reducing agent dithiothreitol. In addition, 4-OHEN was found to be an irreversible inhibitor of COMT-catalyzed methylation of the endogenous catechol estrogen 4-hydroxyestradiol with a K(i) of 26.0 microM and a k(2) of 1.62 x 10(-)(2) s(-)(1). 4-OHEN in vitro not only caused the formation of intermolecular disulfide bonds as demonstrated by gel electrophoresis, but electrospray ionization mass spectrometry and matrix-assisted laser desorption ionization time-of-flight mass spectrometry also showed that 4-OHEN alkylated multiple residues of COMT. Peptide mapping experiments further indicated that Cys33 in recombinant human soluble COMT was the residue most likely modified by 4-OHEN in vitro. These data suggest that inhibition of COMT methylation by 4-OHEN might reduce endogenous catechol estrogen clearance in vivo and further enhance toxicity.

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.

Characterization of human soluble high and low activity catechol-O-methyltransferase catalyzed catechol estrogen methylation

The major detoxification pathway of the carcinogenic catechol estrogens is methylation by catechol- -methyltransferase (COMT). It has been hypothesized that the enzyme encoded by the low-activity allele (COMT(L) ) has a lower catalytic activity for catechol estrogen methylation than that encoded by the high activity allele (COMT(H) ). We expressed and purified human soluble (S)-COMT(H) and S-COMT(L) in and characterized the methylation of 2- and 4-hydroxyestradiol (2- and 4-OH-E2). There were no differences between the kinetic parameters for COMT(H) and COMT(L). The kinetic parameters for S-adenosylmethionine (SAM), the methyl donor in these reactions, also did not differ for COMT(H) and COMT(L). S-adenosylhomocysteine, the demethylated SAM metabolite, inhibited methylation of the catechol estrogens in a non-competitive manner similarly for COMT(H) and COMT(L). Each COMT substrate tested inhibited the methylation of other substrates in a mixed competitive and non-competitive fashion similarly for COMT(H) and COMT(L). Furthermore, in cytosolic fractions of COMT(HH)(MCF-10A and ZR-75-1) and COMT(LL)(MCF-7 and T47D) human breast epithelial cell lines, no differences were detected between the kinetic parameters of COMT with respect to 2- and 4-OH-E2 methylation; nor were COMT protein levels associated with the COMT genotype. These data suggest that the decreased COMT enzymatic activity that has been detected in human tissue in association with the COMT(L) allele is not reflected by differences in the affinity or capacity of COMT(H) and COMT(L) for catechol estrogen methylation. These results raise the question of what accounts for the difference in COMT activity associated with the COMT(HH) and COMT(LL) genotypes in human tissue.

Epicatechin and its in vivo metabolite, 3'-O-methyl epicatechin, protect human fibroblasts from oxidative-stress-induced cell death involving caspase-3 activation

There is considerable current interest in the cytoprotective effects of natural antioxidants against oxidative stress. In particular, epicatechin, a major member of the flavanol family of polyphenols with powerful antioxidant properties in vitro, has been investigated to determine its ability to attenuate oxidative-stress-induced cell damage and to understand the mechanism of its protective action. We have induced oxidative stress in cultured human fibroblasts using hydrogen peroxide and examined the cellular responses in the form of mitochondrial function, cell-membrane damage, annexin-V binding and caspase-3 activation. Since one of the major metabolites of epicatechin in vivo is 3'-O-methyl epicatechin, we have compared its protective effects with that of epicatechin. The results provide the first evidence that 3'-O-methyl epicatechin inhibits cell death induced by hydrogen peroxide and that the mechanism involves suppression of caspase-3 activity as a marker for apoptosis. Furthermore, the protection elicited by 3'-O-methyl epicatechin is not significantly different from that of epicatechin, suggesting that hydrogen-donating antioxidant activity is not the primary mechanism of protection.

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 in rat sensory ganglia and spinal cord

The localization of catechol-O-methyltransferase immunoreactivity in rat dorsal root ganglia and in the spinal cord and its co-existence with substance P, calcitonin gene-related peptide and fluoride-resistant acid phosphatase in dorsal root ganglion cells was examined with immunohistochemical and histochemical double-staining methods. Analysis of dorsal of dorsal root ganglia at both cervical and lumbar levels revealed catechol-O-methyltransferase immunoreactivity in numerous dorsal root ganglion cells. Double-staining studies showed that catechol-O-methyltransferase and substance P immunoreactivities were located in different cells with a few exceptions, whereas both catechol-O-methyltransferase and calcitonin gene-related peptide immunoreactivities were detected in about 10% of all labeled cells positive for one of the two markers at both levels studied. The great majority of fluoride-resistant alkaline phosphatase-positive cells were also immunoreactive for catechol-O-methyltransferase. Again, no difference was found between cervical and lumbar levels. Catechol-O-methyltransferase immunoreactivity was also found in the neuropil of the dorsal horn of the spinal cord. The staining was most intense in the superficial laminae (I-III) and overlapped partly with substance P and calcitonin gene-related peptide immunoreactivity. Western blotting analysis revealed that soluble catechol-O-methyltransferase was the clearly dominating form of the enzyme in dorsal root ganglia. The distribution pattern of catechol-O-methyltransferase in dorsal horn and sensory neurons suggests that the enzyme may modulate sensory neurotransmission.

Characterization of a nucleolar 2'-O-methyltransferase and its involvement in the methylation of mouse precursor ribosomal RNA

A nucleolar 2'-O-methyltransferase, partially purified from isolated mouse nucleoli, catalyzes the methylation of each of the four nucleosides, although to different levels depending on the RNA substrate. Similar to most methyltransferases which use S-adenosyl-L-methionine (SAM) as the methyl donor, the nucleolar 2'-O-methyltransferase was shown to bind S-adenosyl-L-homocysteine (SAH) (Kd = 0.17 microM), a product of the transfer reaction, as tightly as SAM (Kd = 0.24 microM). Binding assays also demonstrated stereospecificity about the sulfonium center of SAM. The naturally occurring S-chiral form of SAM had a 10-fold higher binding affinity than the R-chiral form. In addition, the alpha-amino group of the methionine moiety and the 6-amino group of the adenine moiety were shown to be required for maximal binding. The relative high affinity for both SAM and SAH may reflect a mechanism by which ribosome biogenesis is, in part, coordinated with cell growth, since a decrease in SAM:SAH ratio correlates with decreasing levels of 2'-O-methylation. The availability of unmethylated, in vitro-derived rRNA transcripts has made it possible to explore questions relating to the specificity for the RNA substrate. Using an in vitro-derived 28S rRNA transcript, the enzyme selectively methylated the sequence AmGmCm that occurs in a single-stranded bridge spanning two highly conserved structural domains of 28S rRNA. These results demonstrated that the purified nucleolar 2'-O-methyltransferase was sufficient to accurately methylate this region of 28S rRNA, and were taken to support the involvement of this nucleolar enzyme in the posttranscriptional methylation of the 47S precursor ribosomal RNA transcript.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of the COMT inhibitor entacapone on haemodynamics and peripheral catecholamine metabolism during exercise

1. Catechol-O-methyltransferase (COMT) inhibition might be assumed to potentiate the effects of circulating catecholamines, particularly under conditions of enhanced catecholamine release. 2. The purpose of the present study was to establish whether the novel COMT inhibitor, entacapone, changes haemodynamic responses and catecholamine metabolism during exercise. 3. Entacapone was given orally to 12 healthy male subjects (age 23-30 years) in increasing single doses from 0 mg (control day) to 200 mg. A submaximal exercise test was performed on a bicycle ergometer, and blood pressure, heart rate and ECG were recorded. The concentrations of noradrenaline, adrenaline, DHPG (3,4-dihydroxyphenylglycol), MHPG (3-methoxy-4-hydroxyphenyl-glycol) and, DOPAC (3,4-dihydroxyphenylacetic acid) in plasma were determined. 4. Entacapone did not influence haemodynamics or ECG at rest or during exercise. 5. Entacapone did not influence plasma catecholamine levels, either at rest or during exercise. However, it altered the metabolic profile of catecholamines, which was shown by increases in the plasma concentrations of the monoamine oxidase-dependent metabolites DHPG (by up to 100%) and DOPAC (by up to 53%), and by a decrease of the COMT-dependent metabolite MHPG (by up to 29%).

Purification and partial sequence analysis of the soluble catechol-O-methyltransferase from human placenta: comparison to the rat liver enzyme

Catechol-o-methyltransferase from human placenta was purified 1400-fold by hydroxyapatite adsorption, ammonium sulfate precipitation, gel filtration, high performance anion- exchange and reversed-phase chromatography. The purified enzyme has an apparent molecular weight of 26.000, an isoelectric point of 5,3 and is activated ten-fold in the presence of 20mM cysteine. The enzyme shows primary structure homology to the corresponding rat liver soluble enzyme, based on the sequenced tryptic peptides.

Formation, metabolism, and physiologic importance of catecholestrogens

The metabolism of natural and synthetic estrogens is governed primarily by hydroxylations, leading to polyhydroxylated derivatives of the steroid molecule. In mammals aromatic hydroxylation is most prominent quantitatively. The 2- and 4-hydroxyestrogens (catecholestrogens) formed are secreted not only in high amounts in urine but are also present in significant quantities in different organs, such as the liver, pituitary gland, and hypothalamus. This A ring hydroxylation of primary estrogens is affected by peroxidases, tyrosinases, and unspecific monooxygenases by mechanisms still not completely understood. The activity of the aromatic hydroxylases is regulated not only with respect to the overall extent but also to the relative rate of hydroxylation at C-atoms 2 and 4. The metabolism of catecholestrogens may be divided into reversible and irreversible reactions, of which the reaction with the catechol-O-methyltransferase, and thereby the interaction with catecholamines, the conjugation, and the thioether formation are the most prominent. Low- and high-affinity binding is operative in binding to plasma proteins and receptors. Finally, irreversible binding to cellular macromolecules, such as proteins and deoxyribonucleic acid, and the oncogenic potential of natural and synthetic catecholestrogens are discussed.

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.

Comparison of assays for catechol estrogen synthase activity: product isolation vs radioenzymatic catechol-O-methyltransferase-coupled procedures

Reported values for the activity of enzymes mediating catechol estrogen formation by hamster kidney and liver, measured by catechol-O-methyltransferase-coupled radioenzymatic assay, have been uniformly low and there have been marked discrepancies in values reported from different laboratories. Therefore, we examined the validity of the radioenzymatic assay used in these studies. NADPH-dependent estrogen 2- and 4-hydroxylase activity of hamster liver microsomes measured by radioenzymatic assay was comparable to that reported in the literature but at least one order of magnitude lower than that obtained with a direct product isolation assay. Several features of the radioenzymatic assay were identified which, together, contribute to the underestimation of enzyme activity. They include, incomplete protection from oxidative degradation of both the catechol estrogens generated and of the catechol-O-methyltransferase and assay conditions which are suboptimal for O-methylation of the catechol estrogens. We conclude that results obtained using the catechol-O-methyltransferase-based radioenzymatic assay can only be considered valid if a consistent stoichiometric relationship can be demonstrated between the amounts of catechol estrogens and their O-methylated products.

Kinetics and inhibition studies of catechol O-methyltransferase from the yeast Candida tropicalis

The Kms for esculetin and S-adenosyl-L-methionine for catechol O-methyltransferase from the yeast Candida tropicalis were 6.2 and 40 microM, respectively. S-Adenosyl-L-homocysteine was a very potent competitive inhibitor with respect to S-adenosyl-L-methionine, with a Ki of 6.9 microM. Of the catechol-related inhibitors, purpurogallin, with a Ki of 0.07 microM, showed the greatest inhibitory effect. Sulfhydryl group-blocking reagents, such as thiol-oxidizing 2-iodosobenzoic acid and mercaptide-forming p-chloromercuribenzoic acid, provided evidence for sulfhydryl groups in the active site of the enzyme. Yeast catechol O-methyltransferase is a metal-dependent enzyme and requires Mg2+ for full activity. Zn2+ and Mn2+ but not Ca2+ were able to substitute for Mg2+. Mn2+ showed optimal enzyme activation at concentrations 50- to 100-fold lower than those of Mg2+.

Importance of A-ring substitution of estrogens for the physiology and pharmacology of reproduction

Estrogens substituted in the ortho-position of the phenolic hydroxy-group with an additional hydroxy- or methoxy-group are quantitatively important estrogen metabolites; first isolated and identified from the urine of man and rodents have been demonstrated in blood and different organs, e.g. the pituitary and hypothalamus. The physiological importance of the preeminent representatives of this group, the 2- and 4-hydroxyestrogens, the so-called catecholestrogens, is still equivocal. For example, numerous in vivo investigations in rodents have demonstrated that gonadotrophin secretion is influenced by these catecholestrogens. However, depending on the position of the A-ring substituent, major potency differences have been observed. The significant discrepancies between the quantitative and qualitative effects of catecholestrogens in in vitro and in vivo experiments have been presented and explained on the basis of different receptor affinities and the pharmacokinetics of catecholestrogens. An array of A-ring-substituted steroid model substances has been tested with respect to the effects of 2- and/or 4-substitution on stimulation or blockade of the estrogenic potency.

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.

Protein methylation in animal cells. II. Inhibition of S-adenosyl-L-methionine:protein(arginine) N-methyltransferase by analogs of S-adenosyl-L-homocysteine

1. Protein methylase I (S-adenosyl-L-methionine: protein (arginine) N-methyltransferase, EC 2.1.1.23) has recently been purified in our laboratory from Krebs II ascites cells (Casellas, P. and Jeanteur, P. (1978) Biochim. Biophys. Acta 519, 243--254). In order to probe its binding site for S-adenosyl-L-methionine, three series of compounds deriving from the most potent competitive inhibitor, S-adenosyl-L-homocysteine, by specific alterations in each of the three regions of the molecule (amino acid side chain, ribose and adenine) have been tested for inhibitor activity. A competitive type of inhibition was assumed for all of them and demonstrated for five representative ones. The contribution of each of these regions to the binding could therefore be established as follows: (i) Any modification of the side chain results in a drop in affinity of about two orders of magnitude. Adenosine itself remained significantly inhibitory thereby demonstrating that the presence of a side chain was not critical, although important. (ii) The ribose moiety appears to be an essential part of the molecule as the loss of either 2'- or 3'-hydroxyls or their change to arabino configuration resulted in a nearly complete loss of activity. (iii) The amino group at position 6 and the nitrogen atom at position 7 of the adenine ring also play a crucial role although some substitutions can be tolerated. 2. S-Isobutyladenosine was shown to specifically inhibit the methylation of arginine residues as compared to lysine.

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.

Catechol estrogens: presence in brain and endocrine tissues

Catechol estrogens have been identified and measured in rat brain and various endocrine tissues with the use of a sensitive radioenzymatic assay. The specificity of this assay was confirmed by thin-layer chromatography and mass spectral analysis of the reaction products. The concentration of catechol estrogens in the hypothalamus and pituitary are at least ten times higher than reported previously for the parent estrogens. Catechol estrogens have potent endocrine effects and, because of their normal occurrence in the hypothalamic-pituitary axis, they have an important role in neuroendocrine regulation.

[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.

Studies on the kinetic mechanism of S-adenosylmethionine: protein O-methyltransferase of calf thymus

Initial velocity studies have been carried out on protein methylase II (S-adenosyl-L-methionine:protein O-methyltransferase, EC 2.1.1.24) purified from calf thymus, using bovine pancreatic ribonuclease as the protein substrate. Initial velocity patterns converging at a point on or near the extended abcissa were obtained with either ribonuclease or S-adenosyl-L-methionine as the variable substrate. Inhibition by the product S-adenosyl-L-homocysteine was linear competitive against both S-adenysyl-L-methionine and ribonuclease, the apparent inhibition constants being dependent on the concentration of the nonvaried substrate. Adenosine was an inhibitor of the reaction, the inhibition being linear competitive against both S-adenosyl-L-methionine (Ki/1.2 times 10-3 mol/1.) and ribonuclease (Ki/4.6 times 10-3 mol/1.). These results are consistent with a random mechanism for the protein methylase II reaction in which the rate-limiting step may be the interconversion of the ternary complexes and all other steps may be in equilibrium. The limiting Michaelis constants for S-adenosyl-L-methionine and ribonuclease are 0.87 times 10-6 and 2.86 times 10-4 mol/1., respectively. The dissociation constants of S-adenosyl-L-homocysteine for its reaction with the free enzyme was 1.03 times 10-6 mol/1. Thus it has about equal affinity for calf thymus protein methylase II as S-adenosyl-L-methionine.

Properties of catechol O-methyltransferases from brain and liver of rat and human

Kinetic and electrophoretic properties of catechol O-methyltransferases (EC 2.1.1.6) from brain and liver were studied. The enzyme of either rat or human tissues exhibited a single molecular form when subjected to electrophoresis at pH7.9. At pH9 a second, apparently oxidized, form was detected. Isoelectric-focusing experiments also indicated only one enzyme form, which was identical from extracts of brain and liver of each species (pI = 5.2 for rat, 5.5 for human). Similarities between brain and liver catechol O-methyltransferase of a given species were also demonstrated by kinetic parameters, meta/para ratios of products, and inhibitor potencies. Human catechol O-methyltransferase exhibited lower Km values than did the rat enzyme for S-adenosyl-L-methionine, dopamine and dihydroxybenzoic acid. Adrenochrome inhibited both rat and human enzyme. It was concluded (1) that only a single enzyme form could be demonstrated in the physiological pH region; (2) that catechol O-methyltransferase of brain could not be distinguished from the liver enzyme of the same species; and (3) that species differences exist between the enzymes of rat and human tissues.