Oxidative inhibition of human soluble catechol-O-methyltransferase

Unveiling the biopathway for the design of novel COMT inhibitors

Catechol-O-methyltransferase (COMT) is an enzyme responsible for the O-methylation of biologically active catechol-based molecules. It has been associated with several neurological disorders, especially Parkinson's disease (PD), because of its involvement in catecholamine metabolism, and has been considered an important therapeutic target for central nervous system disorders. In this review, we summarize the biophysical, structural, and therapeutical relevance of COMT; the medicinal chemistry behind the development of COMT inhibitors and the application of computer-aided design to support the design of novel molecules; current methodologies for the biosynthesis, isolation, and purification of COMT; and revise existing bioanalytical approaches for the assessment of enzymatic activity in several biological matrices.

Advances in Membrane-Bound Catechol-O-Methyltransferase Stability Achieved Using a New Ionic Liquid-Based Storage Formulation

Membrane-bound catechol-O-methyltransferase (MBCOMT), present in the brain and involved in the main pathway of the catechol neurotransmitter deactivation, is linked to several types of human dementia, which are relevant pharmacological targets for new potent and nontoxic inhibitors that have been developed, particularly for Parkinson’s disease treatment. However, the inexistence of an MBCOMT 3D-structure presents a blockage in new drugs’ design and clinical studies due to its instability. The enzyme has a clear tendency to lose its biological activity in a short period of time. To avoid the enzyme sequestering into a non-native state during the downstream processing, a multi-component buffer plays a major role, with the addition of additives such as cysteine, glycerol, and trehalose showing promising results towards minimizing hMBCOMT damage and enhancing its stability. In addition, ionic liquids, due to their virtually unlimited choices for cation/anion paring, are potential protein stabilizers for the process and storage buffers. Screening experiments were designed to evaluate the effect of distinct cation/anion ILs interaction in hMBCOMT enzymatic activity. The ionic liquids: choline glutamate [Ch][Glu], choline dihydrogen phosphate ([Ch][DHP]), choline chloride ([Ch]Cl), 1- dodecyl-3-methylimidazolium chloride ([C12mim]Cl), and 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) were supplemented to hMBCOMT lysates in a concentration from 5 to 500 mM. A major potential stabilizing effect was obtained using [Ch][DHP] (10 and 50 mM). From the DoE 146% of hMBCOMT activity recovery was obtained with [Ch][DHP] optimal conditions (7.5 mM) at −80 °C during 32.4 h. These results are of crucial importance for further drug development once the enzyme can be stabilized for longer periods of time.

Catechol-O-Methyltransferase Inhibitors Isolated From Thai Propolis

Propolis is an aggregate of functional components found in plant resins and has been reported to exhibit a variety of valuable biological activities. This study investigated the inhibitory properties of propolis from Thailand toward human catechol- O-methyltransferase (COMT), a key neurotransmitter involved in Parkinson's disease and depression. Samples collected from Chanthaburi and Chiang Mai exhibited relatively high inhibitory activity against COMT. γ-Mangostin (1) and 6-prenyleriodictyol (3) were identified as COMT inhibitors with IC50 values of 62 and 75 μM, respectively. In an enzyme inhibition assay, 1 exhibited mixed inhibition toward COMT. The results suggest that both 1 and propolis have potential applications in the prevention and treatment of psychological illness.

Interaction of silver nanoparticles with catechol O-methyltransferase: Spectroscopic and simulation analyses

Catechol O-methyltransferase, an enzyme involved in the metabolism of catechol containing compounds, catalyzes the transfer of a methyl group between S-adenosylmethionine and the hydroxyl groups of the catechol. Furthermore it is considered a potential drug target for Parkinson’s disease as it metabolizes the drug levodopa. Consequently inhibitors of the enzyme would increase levels of levodopa. In this study, absorption, fluorescence and infrared spectroscopy as well as computational simulation studies investigated human soluble catechol O-methyltransferase interaction with silver nanoparticles. The nanoparticles form a corona with the enzyme and quenches the fluorescence of Trp143. This amino acid maintains the correct structural orientation for the catechol ring during catalysis through a static mechanism supported by a non-fluorescent fluorophore–nanoparticle complex. The enzyme has one binding site for AgNPs in a thermodynamically spontaneous binding driven by electrostatic interactions as confirmed by negative ΔG and ΔH and positive ΔS values. Fourier transform infrared spectroscopy within the amide I region of the enzyme indicated that the interaction causes relaxation of its β−structures, while simulation studies indicated the involvement of six polar amino acids. These findings suggest AgNPs influence the catalytic activity of catechol O-methyltransferase, and therefore have potential in controlling the activity of the enzyme.

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A recombinant soluble human catechol O-methyltransferase was inhibited by silver nanoparticles.

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Inhibition by AgNPs was concentration and size dependent.

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The binding mechanism was through spontaneous static quenching, driven by positive ΔS, and negative ΔH and ΔG.

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Stern-Volmer analysis suggested binding of AgNPs with Trp143.

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In silico indicate relaxation of β-sheets and the interaction of AgNPs with 6 amino acids in the enzyme’s helical structures.

Targeting redox regulation to treat substance use disorder using N‐acetylcysteine

Substance use disorder (SUD) is a chronic relapsing disorder characterized by transitioning from acute drug reward to compulsive drug use. Despite the heavy personal and societal burden of SUDs, current treatments are limited and unsatisfactory. For this reason, a deeper understanding of the mechanisms underlying addiction is required. Altered redox status, primarily due to drug-induced increases in dopamine metabolism, is a unifying feature of abused substances. In recent years, knowledge of the effects of oxidative stress in the nervous system has evolved from strictly neurotoxic to include a more nuanced role in redox-sensitive signaling. More specifically, S-glutathionylation, a redox-sensitive post-translational modification, has been suggested to influence the response to drugs of abuse. In this review we will examine the evidence for redox-mediating drugs as therapeutic tools focusing on N-acetylcysteine as a treatment for cocaine addiction. We will conclude by suggesting future research directions that may further advance this field.

Effects of bovine lactoferrin on l-DOPA absorption and metabolism in mice

Bioactive natural products, habitually ingested with milk or its derivative nutrients, have been studied for their bioavailability. In this study, we investigated the effects of the co-administration of bovine milk-derived lactoferrin (bLF) and bioactive products, with a focus on catechol-O-methyltransferase (COMT), an enzyme in the catechol metabolism. bLF showed inhibitory activity on COMT in vitro, and acidic pretreatment of bLF enhanced its inhibitory activity. Moreover, partially digested products of bLF by pepsin retained inhibitory activity. Based on these results, bLF was co-administered with levodopa (l-DOPA), which is a catechol compound and a precursor of dopamine, and the effect of bLF on l-DOPA absorption and metabolism was investigated in a mouse model. The co-administration of l-DOPA and bLF alone showed no effect on the concentration of l-DOPA in plasma. However, with the additional administration of carbidopa, the concentration of l-DOPA was significantly enhanced. Furthermore, the ratio of l-DOPA/3-O-methyldopa significantly increased. On the other hand, casein, which is a major milk protein, was not effective. In addition, COMT activity in the intestines was lowered with bLF administration. We concluded that the co-administration of bLF and carbidopa enhances the concentration of l-DOPA.

Inhibitory Effect of Bovine Lactoferrin on Catechol-O-Methyltransferase

Lactoferrin (LF) is a well-known multifunctional protein. In this study, we report the inhibitory potency of bovine LF (bLF) on catechol-O-methyltransferase (COMT), which catalyzes methylation of catechol substrates. We found that bLF binds to and inhibits COMT using its N-terminal region. An N-terminal peptide fragment obtained from bLF by trypsin digestion showed a higher inhibitory activity than intact bLF. A synthetic fragment of the bLF N-terminal residues 6-50, with two pairs of disulfide bonds, also showed higher inhibitory activity than intact bLF. Enzyme kinetic studies proved that bLF did not compete with S-adenosylmethionine (the methyl donor substrate) as well as methyl acceptor substrates such as dihydroxybenzoic acid, (-)-epicatechin, norepinephrine, or l-3,4-dihydroxyphenylalanine. The inhibitory potency of bLF decreased against a COMT preparation pretreated with dithiothreitol, suggesting that the oxidation status of COMT is relevant to interaction with bLF. We further confirmed that COMT activity in the cell extracts form Caco-2 and HepG2 cells was inhibited by bLF and by the synthesized fragment. Enzyme kinetic study indicated that bLF functions as a non-competitive inhibitor by binding to an allosteric surface of COMT.

S-Glutathionylation and Redox Protein Signaling in Drug Addiction

Drug addiction is a chronic relapsing disorder that comes at a high cost to individuals and society. Therefore understanding the mechanisms by which drugs exert their effects is of prime importance. Drugs of abuse increase the production of reactive oxygen and nitrogen species resulting in oxidative stress. This change in redox homeostasis increases the conjugation of glutathione to protein cysteine residues; a process called S-glutathionylation. Although traditionally regarded as a protective mechanism against irreversible protein oxidation, accumulated evidence suggests a more nuanced role for S-glutathionylation, namely as a mediator in redox-sensitive protein signaling. The reversible modification of protein thiols leading to alteration in function under different physiologic/pathologic conditions provides a mechanism whereby change in redox status can be translated into a functional response. As such, S-glutathionylation represents an understudied means of post-translational protein modification that may be important in the mechanisms underlying drug addiction. This review will discuss the evidence for S-glutathionylation as a redox-sensing mechanism and how this may be involved in the response to drug-induced oxidative stress. The function of S-glutathionylated proteins involved in neurotransmission, dendritic spine structure, and drug-induced behavioral outputs will be reviewed with specific reference to alcohol, cocaine, and heroin.

The enzymatic activities of brain catechol-O-methyltransferase (COMT) and methionine sulphoxide reductase are correlated in a COMT Val/Met allele-dependent fashion

AIMS

The enzyme catechol-O-methyltransferase (COMT) plays a primary role in the metabolism of catecholamine neurotransmitters and is implicated in the modulation of cognitive and emotional responses. The best characterized single nucleotide polymorphism (SNP) of the COMT gene consists of a valine (Val)-to-methionine (Met) substitution at codon 108/158. The Met-containing variant confers a marked reduction in COMT catalytic activity. We recently showed that the activity of recombinant COMT is positively regulated by the enzyme Met sulphoxide reductase (MSR), which counters the oxidation of Met residues of proteins. The current study was designed to assess whether brain COMT activity may be correlated to MSR in an allele-dependent fashion.

METHODS

COMT and MSR activities were measured from post-mortem samples of prefrontal cortices, striata and cerebella of 32 subjects by using catechol and dabsyl-Met sulphoxide as substrates, respectively. Allelic discrimination of COMT Val(108/185) Met SNP was performed using the Taqman 5'nuclease assay.

RESULTS

Our studies revealed that, in homozygous carriers of Met, but not Val alleles, the activity of COMT and MSR was significantly correlated throughout all tested brain regions.

CONCLUSION

These results suggest that the reduced enzymatic activity of Met-containing COMT may be secondary to Met sulphoxidation and point to MSR as a key molecular determinant for the modulation of COMT activity.

Glutathionylspermidine in the modification of protein SH groups: the enzymology and its application to study protein glutathionylation

Cysteine is very susceptible to reactive oxygen species. In response; posttranslational thiol modifications such as reversible disulfide bond formation have arisen as protective mechanisms against undesired in vivo cysteine oxidation. In Gram-negative bacteria a major defense mechanism against cysteine overoxidation is the formation of mixed protein disulfides with low molecular weight thiols such as glutathione and glutathionylspermidine. In this review we discuss some of the mechanistic aspects of glutathionylspermidine in prokaryotes and extend its potential use to eukaryotes in proteomics and biochemical applications through an example with tissue transglutaminase and its S-glutathionylation.

Methionine sulfoxide reductase regulates brain catechol-O-methyl transferase activity

Catechol-O-methyl transferase (COMT) plays a key role in the degradation of brain dopamine (DA). Specifically, low COMT activity results in higher DA levels in the prefrontal cortex (PFC), thereby reducing the vulnerability for attentional and cognitive deficits in both psychotic and healthy individuals. COMT activity is markedly reduced by a non-synonymous single-nucleotide polymorphism (SNP) that generates a valine-to-methionine substitution on the residue 108/158, by means of as-yet incompletely understood post-translational mechanisms. One post-translational modification is methionine sulfoxide, which can be reduced by the methionine sulfoxide reductase (Msr) A and B enzymes. We used recombinant COMT proteins (Val/Met108) and mice (wild-type (WT) and MsrA knockout) to determine the effect of methionine oxidation on COMT activity and COMT interaction with Msr, through a combination of enzymatic activity and Western blot assays. Recombinant COMT activity is positively regulated by MsrA, especially under oxidative conditions, whereas brains of MsrA knockout mice exhibited lower COMT activity (as compared with their WT counterparts). These results suggest that COMT activity may be reduced by methionine oxidation, and point to Msr as a key molecular determinant for the modulation of COMT activity in the brain. The role of Msr in modulating cognitive functions in healthy individuals and schizophrenia patients is yet to be determined.

Catechol-O-methyltransferase: characteristics, polymorphisms and role in breast cancer

Catechol estrogens are carcinogenic, probably because of their estrogenicity and potential for further oxidative metabolism to reactive quinones. Estrogenic quinones cause oxidative DNA damage as well as form mutagenic depurinating adenine and guanine adducts. O-Methylation by catechol-O-methyltransferase (COMT) blocks their estrogenicity and prevents their oxidation to quinones. A single gene encodes both membrane bound (MB) and soluble (S) forms of COMT. The COMT gene contains 34 single nucleotide polymorphisms (SNPs). The valine108 (S-COMT)/158 (MB-COMT) SNP encodes a low activity form of COMT and has been widely studied as a putative risk factor for breast cancer, with inconsistent results. Investigations of two other SNPs in the promoter of MB-COMT that may affect its expression have also provided mixed results. Future studies on the role of COMT in breast cancer should incorporate measurement of biomarkers that reflect COMT activity and its protective effects.

The role of glutathione S-transferase P in signaling pathways and S-glutathionylation in cancer

Glutathione S-transferase P is abundantly expressed in some mammalian tissues, particularly those associated with malignancies. While the enzyme can catalyze thioether bond formation between some electrophilic chemicals and GSH, novel nondetoxification functions are now ascribed to it. This review summarizes recent material that implicates GSTP in mediating S-glutathionylation of specific clusters of target proteins and in reactions that define a negative regulatory role in some kinase pathways through ligand or protein:protein interactions. It is becoming apparent that GSTP participates in the maintenance of cellular redox homeostasis through a number of convergent and divergent mechanisms. Moreover, drug platforms that have GSTP as a target have produced some interesting preclinical and clinical candidates.

Study: the lack of significant association of the catechol-O-methyl transferase (COMT) gene polymorphism in violent offenders with mental retardation

Little is known about criminality of cognitively impaired people and also there have been no reports on the relationship between catechol-O-methyl transferase (COMT) and committed Mental Retardation (MR) subjects. In the present study, the association between committed (violent offences) MR subjects and genetic variants of COMT were investigated by using polymerase chain reaction and based restriction fragment length polymorphism methods. During 6 years of follow-up, 36 violent offenders with mild MR were investigated. Thirty-six control volunteers were included in the study as a control group. H/L polymorphism of the COMT gene was investigated in these two groups. In conclusion, the COMT gene genotype distribution and allele frequency is not significantly different between the two groups (p > 0.05). This result suggests that the H/L polymorphism of the COMT gene does not show an association with the potential of "commits-violent offense" of Turkish subjects with mental retardation, compared with control group.

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

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

Four human thiopurine s-methyltransferase alleles severely affect protein structure and dynamics

Thiopurine S-methyltransferase (TPMT) metabolizes cytotoxic thiopurine drugs used in the treatment of leukemia and inflammatory bowel disease. TPMT is a major pharmacogenomic target with 23 alleles identified to date. Several of these alleles cause rapid protein degradation and/or aggregation, making it extremely difficult to study the structural impact of the TPMT polymorphisms experimentally. We, therefore, have performed multiple molecular dynamics simulations of the four most common alleles [TPMT*2 (A80P), *3A (A154T/Y240C), *3B (A154T) and *3C (Y240C)] to investigate the molecular mechanism of TPMT inactivation at an atomic level. The A80P polymorphism in TPMT*2 disrupts helix alpha3 bordering the active site, which breaks several salt-bridge interactions and opens up a large cleft in the protein. The A154T polymorphism is located within the co-substrate binding site. The larger threonine alters the packing of substrate-binding residues (P68, L69, Y166), increasing the solvent exposure of the polymorphic site. This packing rearrangement may account for the complete lack of activity in the A154T mutant. The Y240C polymorphism is located in beta-strand 9, distant from the active site. Side-chain contacts between residue 240 and helix alpha8 are lost in TPMT*3C. Residues 154 and 240 in TPMT*3A are connected through a hydrogen-bonding network. The dual polymorphisms result in a flattened, slightly distorted protein structure and an increase in the thiopurine-binding site solvent accessibility. The two variants that undergo the most rapid degradation in vivo, TPMT*2 and *3A, are also the most deformed in the simulations.

S-glutathionylation: indicator of cell stress and regulator of the unfolded protein response

The specific posttranslational modification of protein cysteine residues by the addition of the tripeptide glutathione is termed S-glutathionylation. This process is promoted by oxidative and nitrosative stress but also occurs in unstressed cells. Altered levels of S-glutathionylation in some proteins have been associated with numerous pathologies, many of which have been linked to redox stress in the endoplasmic reticulum (ER). Proper protein folding is dependent upon controlled redox conditions within the ER, and it seems that ER conditions can in turn affect rates of S-glutathionylation. This article seeks to bring together the ways through which these processes are interrelated and considers the implications of these interrelationships upon therapeutic approaches to disease.

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.

Comparative study on the interaction of recombinant human soluble catechol-O-methyltransferase on some hydrophobic adsorbents

The main scope of this work is the evaluation and potential application of hydrophobic interaction chromatography in the isolation of recombinant human soluble catechol-O-methyltransferase (hSCOMT) from an Escherichia coli cell extract. Therefore, a comparative study on the interaction of recombinant hSCOMT with different hydrophobic adsorbents (butyl-, octyl-, phenyl- and epoxy-Sepharose), was developed. The four adsorbents were evaluated in terms of selectivity, recovery and fractionation of recombinant hSCOMT from its Escherichia coli-free culture broth. Our data shows that the adjustment of the ionic strength on the mobile phase and the type of hydrophobic ligand are the most useful factors for a complete binding of hSCOMT and a selective fractionation of contaminating proteins. The results of these studies demonstrate that, although epoxy-Sepharose is used as a last resort due to the high salt concentrations needed, hSCOMT bind to the other three resins at low concentrations of ammonium sulfate (< or = 0.6 M) and eluted just by decreasing the ionic strength on the eluent to 0 M, without loss of specific of activity. The stepwise gradient with 0.6, 0.2, 0.075 and 0 M of ammonium sulfate onto a butyl-Sepharose media was found to be the most effective in the isolation of hSCOMT, leading to an enzyme solution with a 3.9-fold increased in specific activity. Since biochemical and structural studies require significant quantities of the enzyme in an active form, the above described approach may give some insight into the optimization and development of new purification strategies of mammalian COMTs.

Direct molecular haplotyping of multiple polymorphisms within exon 4 of the human catechol-O-methyltransferase gene by liquid chromatography–electrospray ionization time-of-flight mass spectrometry

The applicability of ion-pair reversed-phase high-performance liquid chromatography hyphenated to electrospray ionization time-of-flight mass spectrometry (ICEMS) for the haplotyping of five SNPs (rs769223, rs4818, rs4986871, rs8192488, rs4680) located within exon 4 of the human catechol-O-methyltransferase (COMT, EC 2.1.1.6) gene is demonstrated. Two differently sized products of polymerase chain reaction--a 71-bp amplicon partially covering the sequence of a 124-bp amplicon--were used to determine unequivocally the allelic states of the single nucleotide polymorphisms linked on both chromosomes. The two amplicons were co-loaded onto the chromatographic column and simultaneously analyzed within a single gradient run. Using the described strategy, 101 individuals representing an Austrian population sample were typed. The obtained haplotype frequencies will serve as reference values in future association studies to examine the impact of the COMT gene on neuropsychiatric disorders. Additionally, two newly discovered polymorphic sites within the sequence of the COMT gene are described (a synonymous C>T mutation at the third position of the amino acid codon 99 in the soluble COMT protein or 149 in the membrane-bound COMT protein; a non-synonymous G>A substitution at the second position of the amino acid codon 95 in the soluble COMT protein or 145 in the membrane-bound-COMT protein).

Redox proteomics: identification and functional role of glutathionylated proteins

Although radical oxygen and nitrogen species are harmful molecules that destroy cell functions, many operate as mediators of important cell signaling pathways when not in excess. Oxidants can modify protein function through the covalent, reversible addition of glutathione to cysteine. This review addresses different proteomic methods of identifying glutathionylation targets and emphasizes ways of defining their pattern of modification in response to oxidative stimuli in cells. Finally, the literature on nonproteomic studies that investigate the functional changes induced by glutathionylation are reviewed and future studies are commented on.

The 108M polymorph of human catechol O-methyltransferase is prone to deformation at physiological temperatures

The human gene for catechol O-methyltransferase (COMT) contains a common polymorphism that results in substitution of methionine (M) for valine (V) at residue 108 of the soluble form of the protein. While the two proteins have similar kinetic properties, 108M COMT loses activity more rapidly than 108V COMT at 37 degrees C. The cosubstrate S-adenosylmethionine (SAM) stabilizes the activity of 108M COMT at 40 degrees C. The 108M allele has been associated with increased risk for breast cancer, obsessive-compulsive disorder, and aggressive and highly antisocial manifestations of schizophrenia. In the current work, we have constructed homology models for both human COMT polymorphs and performed molecular dynamics simulations of these models at 25, 37, and 50 degrees C to explore the structural consequences of the 108V/M polymorphism. The simulations indicated that replacing valine with the larger methionine residue led to greater solvent exposure of residue 108 and heightened packing interactions between M108 and helices alpha2, alpha4 (especially with R78), and alpha5. These altered packing interactions propagated subtle changes between the polymorphic site and the active site 16 A away, leading to a loosening of the active site. At physiological temperature, 108M COMT sampled a larger distribution of conformations than 108V. 108M COMT was more prone to active-site distortion and had greater overall, and SAM binding site, solvent accessibility than 108V COMT at 37 degrees C. Similar structural perturbations were observed in the 108V protein only at 50 degrees C. Addition of SAM tightened up the cosubstrate pocket in both proteins and prevented the altered packing at the polymorphic site in 108M COMT.

Oxidation of thiol-proteases in the hippocampus of Alzheimer's disease

The hippocampus of Alzheimer's disease brain has been shown to be highly oxidized compared to age-matched controls. One of the most sensitive targets of oxidation is anionic sulfur which can be found within the active site of members of the cysteine-protease family. Thus, while members of the cysteine-protease family such as the calpains and caspases have been found to be in an active conformation in vulnerable brain regions in AD it is possible that their proteolytic activity is hampered due to the robust oxidative stress found at these locations. To address this issue, the amount of caseinolytic activity present in the hippocampus from post-mortem brain samples of AD and age-matched controls was determined. No difference in caseinolytic activity in the absence of exogenous reducing agent was observed between AD and control. However, after addition of the thiol-specific reducing agent, dithiothreitol (DTT), the amount of caseinolytic activity was significantly increased in AD compared to the DTT-mediated increase in control. This suggests that the cysteine proteases are more oxidized in AD brain and that latent proteolytic activity in AD brain can be released by antioxidants. Further testing revealed that the calcium-dependent caseinolytic activity was significantly lower in AD brain compared to controls. This is despite the fact that the major calcium-dependent thiol protease, calpain, is threefold more activated in AD brain based on autolytic activation measured by Western blotting. This calcium-dependent protease difference between AD and control brains was negated by addition of DTT. These data suggest that cysteine protease activity in AD brain is inactivated by oxidants, which is evident by the ability of thiol-specific reducing agents such as DTT to rescue and recover activity.