Formation of strand breaks and interstrand cross-links in DNA by methylglyoxal

Endogenous Cellular Metabolite Methylglyoxal Induces DNA-Protein Cross-Links in Living Cells

Methylglyoxal (MGO) is an electrophilic α-oxoaldehyde generated endogenously through metabolism of carbohydrates and exogenously due to autoxidation of sugars, degradation of lipids, and fermentation during food and drink processing. MGO can react with nucleophilic sites within proteins and DNA to form covalent adducts. MGO-induced advanced glycation end-products such as protein and DNA adducts are thought to be involved in oxidative stress, inflammation, diabetes, cancer, renal failure, and neurodegenerative diseases. Additionally, MGO has been hypothesized to form toxic DNA-protein cross-links (DPC), but the identities of proteins participating in such cross-linking in cells have not been determined. In the present work, we quantified DPC formation in human cells exposed to MGO and identified proteins trapped on DNA upon MGO exposure using mass spectrometry-based proteomics. A total of 265 proteins were found to participate in MGO-derived DPC formation including gene products engaged in telomere organization, nucleosome assembly, and gene expression. In vitro experiments confirmed DPC formation between DNA and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), as well as histone proteins H3.1 and H4. Collectively, our study provides the first evidence for MGO-mediated DNA-protein cross-linking in living cells, prompting future studies regarding the relevance of these toxic lesions in cancer, diabetes, and other diseases linked to elevated MGO levels.

The Role of Glyoxalase in Glycation and Carbonyl Stress Induced Metabolic Disorders

Glycation refers to the covalent binding of sugar molecules to macromolecules, such as DNA, proteins, and lipids in a non-enzymatic reaction, resulting in the formation of irreversibly bound products known as advanced glycation end products (AGEs). AGEs are synthesized in high amounts both in pathological conditions, such as diabetes and under physiological conditions resulting in aging. The body's anti-glycation defense mechanisms play a critical role in removing glycated products. However, if this defense system fails, AGEs start accumulating, which results in pathological conditions. Studies have been shown that increased accumulation of AGEs acts as key mediators in multiple diseases, such as diabetes, obesity, arthritis, cancer, atherosclerosis, decreased skin elasticity, male erectile dysfunction, pulmonary fibrosis, aging, and Alzheimer's disease. Furthermore, glycation of nucleotides, proteins, and phospholipids by α-oxoaldehyde metabolites, such as glyoxal (GO) and methylglyoxal (MGO), causes potential damage to the genome, proteome, and lipidome. Glyoxalase-1 (GLO-1) acts as a part of the anti-glycation defense system by carrying out detoxification of GO and MGO. It has been demonstrated that GLO-1 protects dicarbonyl modifications of the proteome and lipidome, thereby impeding the cell signaling and affecting age-related diseases. Its relationship with detoxification and anti-glycation defense is well established. Glycation of proteins by MGO and GO results in protein misfolding, thereby affecting their structure and function. These findings provide evidence for the rationale that the functional modulation of the GLO pathway could be used as a potential therapeutic target. In the present review, we summarized the newly emerged literature on the GLO pathway, including enzymes regulating the process. In addition, we described small bioactive molecules with the potential to modulate the GLO pathway, thereby providing a basis for the development of new treatment strategies against age-related complications.

Formation and repair of unavoidable, endogenous interstrand cross-links in cellular DNA

Genome integrity is essential for life and, as a result, DNA repair systems evolved to remove unavoidable DNA lesions from cellular DNA. Many forms of life possess the capacity to remove interstrand DNA cross-links (ICLs) from their genome but the identity of the naturally-occurring, endogenous substrates that drove the evolution and retention of these DNA repair systems across a wide range of life forms remains uncertain. In this review, we describe more than a dozen chemical processes by which endogenous ICLs plausibly can be introduced into cellular DNA. The majority involve DNA degradation processes that introduce aldehyde residues into the double helix or reactions of DNA with endogenous low molecular weight aldehyde metabolites. A smaller number of the cross-linking processes involve reactions of DNA radicals generated by oxidation.

Antimicrobial Prodrug Activation by the Staphylococcal Glyoxalase GloB

With the rising prevalence of multidrug resistance, there is an urgent need to develop novel antibiotics. Many putative antibiotics demonstrate promising in vitro potency but fail in vivo due to poor drug-like qualities (e.g., serum half-life, oral absorption, solubility, and toxicity). These drug-like properties can be modified through the addition of chemical protecting groups, creating "prodrugs" that are activated prior to target inhibition. Lipophilic prodrugging techniques, including the attachment of a pivaloyloxymethyl group, have garnered attention for their ability to increase cellular permeability by masking charged residues and the relative ease of the chemical prodrugging process. Unfortunately, pivaloyloxymethyl prodrugs are rapidly activated by human sera, rendering any membrane permeability qualities absent during clinical treatment. Identification of the bacterial prodrug activation pathway(s) will allow for the development of host-stable and microbe-targeted prodrug therapies. Here, we use two zoonotic staphylococcal species, Staphylococcus schleiferi and S. pseudintermedius, to establish the mechanism of carboxy ester prodrug activation. Using a forward genetic screen, we identify a conserved locus in both species encoding the enzyme hydroxyacylglutathione hydrolase (GloB), whose loss-of-function confers resistance to carboxy ester prodrugs. We enzymatically characterize GloB and demonstrate that it is a functional glyoxalase II enzyme, which has the capacity to activate carboxy ester prodrugs. As GloB homologues are both widespread and diverse in sequence, our findings suggest that GloB may be a useful mechanism for developing species- or genus-level prodrug targeting strategies.

Interplay between Cellular Metabolism and the DNA Damage Response in Cancer

Metabolism is a fundamental cellular process that can become harmful for cells by leading to DNA damage, for instance by an increase in oxidative stress or through the generation of toxic byproducts. To deal with such insults, cells have evolved sophisticated DNA damage response (DDR) pathways that allow for the maintenance of genome integrity. Recent years have seen remarkable progress in our understanding of the diverse DDR mechanisms, and, through such work, it has emerged that cellular metabolic regulation not only generates DNA damage but also impacts on DNA repair. Cancer cells show an alteration of the DDR coupled with modifications in cellular metabolism, further emphasizing links between these two fundamental processes. Taken together, these compelling findings indicate that metabolic enzymes and metabolites represent a key group of factors within the DDR. Here, we will compile the current knowledge on the dynamic interplay between metabolic factors and the DDR, with a specific focus on cancer. We will also discuss how recently developed high-throughput technologies allow for the identification of novel crosstalk between the DDR and metabolism, which is of crucial importance to better design efficient cancer treatments.

Synthesis and metabolism of methylglyoxal, S-D-lactoylglutathione and D-lactate in cancer and Alzheimer's disease. Exploring the crossroad of eternal youth and premature aging

Both cancer and Alzheimer's disease (AD) are emerging as metabolic diseases in which aberrant/dysregulated glucose metabolism and bioenergetics occur, and play a key role in disease progression. Interestingly, an enhancement of glucose uptake, glycolysis and pentose phosphate pathway occurs in both cancer cells and amyloid-β-resistant neurons in the early phase of AD. However, this metabolic shift has its adverse effects. One of them is the increase in methylglyoxal production, a physiological cytotoxic by-product of glucose catabolism. Methylglyoxal is mainly detoxified via cytosolic glyoxalase route comprising glyoxalase 1 and glyoxalase 2 with the production of S-D-lactoylglutathione and D-lactate as intermediate and end-product, respectively. Due to the existence of mitochondrial carriers and intramitochondrial glyoxalase 2 and D-lactate dehydrogenase, the transport and metabolism of both S-D-lactoylglutathione and D-lactate in mitochondria can contribute to methylglyoxal elimination, cellular antioxidant power and energy production. In this review, it is supposed that the different ability of cancer cells and AD neurons to metabolize methylglyoxal, S-D-lactoylglutathione and D-lactate scores cell fate, therefore being at the very crossroad of the "eternal youth" of cancer and the "premature death" of AD neurons. Understanding of these processes would help to elaborate novel metabolism-based therapies for cancer and AD treatment.

The glyoxalase pathway: the first hundred years... and beyond

The discovery of the enzymatic formation of lactic acid from methylglyoxal dates back to 1913 and was believed to be associated with one enzyme termed ketonaldehydemutase or glyoxalase, the latter designation prevailed. However, in 1951 it was shown that two enzymes were needed and that glutathione was the required catalytic co-factor. The concept of a metabolic pathway defined by two enzymes emerged at this time. Its association to detoxification and anti-glycation defence are its presently accepted roles, since methylglyoxal exerts irreversible effects on protein structure and function, associated with misfolding. This functional defence role has been the rationale behind the possible use of the glyoxalase pathway as a therapeutic target, since its inhibition might lead to an increased methylglyoxal concentration and cellular damage. However, metabolic pathway analysis showed that glyoxalase effects on methylglyoxal concentration are likely to be negligible and several organisms, from mammals to yeast and protozoan parasites, show no phenotype in the absence of one or both glyoxalase enzymes. The aim of the present review is to show the evolution of thought regarding the glyoxalase pathway since its discovery 100 years ago, the current knowledge on the glyoxalase enzymes and their recognized role in the control of glycation processes.

Fanconi-like crosslink repair in yeast

Interstrand crosslinks covalently link complementary DNA strands, block replication and transcription, and can trigger cell death. In eukaryotic systems several pathways, including the Fanconi Anemia pathway, are involved in repairing interstrand crosslinks, but their precise mechanisms remain enigmatic. The lack of functional homologs in simpler model organisms has significantly hampered progress in this field. Two recent studies have finally identified a Fanconi-like interstrand crosslink repair pathway in yeast. Future studies in this simplistic model organism promise to greatly improve our basic understanding of complex interstrand crosslink repair pathways like the Fanconi pathway.

Thioester hydrolysis reactivity of zinc hydroxide complexes: investigating reactivity relevant to glyoxalase II enzymes

A recently reported binuclear zinc hydroxide complex [(L(1)Zn(2))(mu-OH)](ClO(4))(2) (, L(1) = 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenolate monoanion) containing a single bridging hydroxide was examined for thioester hydrolysis reactivity. Treatment of it with hydroxyphenylthioacetic acid S-methyl ester in dry CD(3)CN results in no reaction after approximately 65 h at 45(1) degrees C. Binuclear zinc hydroxide complexes of the N-methyl-N-((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine (L(2)) and N-methyl-N-((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)ethyl)amine (L(3)) chelate ligands were prepared by treatment of each ligand with molar equivalent amounts of Zn(ClO(4))(2).6H(2)O and KOH in methanol. These complexes, [(L(2)Zn)(2)(mu-OH)(2)](ClO(4))(2) and [(L(3)Zn)(2)(mu-OH)(2)](ClO(4))(2) (), which have been structurally characterized by X-ray crystallography, behave as 1 : 1 electrolytes in acetonitrile, indicating that the binuclear cations dissociate into monomeric zinc hydroxide species in solution. Treatment of them with one equivalent of hydroxyphenylthioacetic acid S-methyl ester per zinc center in acetonitrile results in the formation of a zinc alpha-hydroxycarboxylate complex, [(L(2))Zn(O(2)CCH(OH)Ph)]ClO(4).1.5H(2)O or [(L(3))Zn(O(2)CCH(OH)Ph)]ClO(4).1.5H(2)O, and CH(3)SH. These reactions, to our knowledge, are the first reported examples of thioester hydrolysis mediated by zinc hydroxide complexes. The results of this study suggest that a terminal Zn-OH moiety may be required for hydrolysis reactivity with a thioester substrate.

Isolation of Ralstonia solanacearum hrpB constitutive mutants and secretion analysis of hrpB-regulated gene products that share homology with known type III effectors and enzymes

The Hrp type III secretion system (TTSS) is essential for the pathogenicity of the Gram-negative plant pathogen Ralstonia solanacearum. To examine the secretion of type III effector proteins via the Hrp TTSS, a screen was done of mutants constitutively expressing the hrpB gene, which encodes an AraC-type transcriptional activator for the hrp regulon. A mutant was isolated that in an hrp-inducing medium expresses several hrpB-regulated genes 4.9-83-fold higher than the wild-type. R. solanacearum Hrp-secreted outer proteins PopA and PopC were secreted at high levels into the culture supernatants of the hrpB constitutive (hrpB(c)) mutant. Using hrpB(c) mutants, the extracellular secretion of several hrpB-regulated (hpx) gene products that share homology with known type III effectors and enzymes was examined. Hpx23, Hpx24 and Hpx25, which are similar in sequence to Pseudomonas syringae pv. tomato effector proteins HopPtoA1, HolPtoR and HopPtoD1, are also secreted via the Hrp TTSS in R. solanacearum. The secretion of two hpx gene products that share homology with known enzymes, glyoxalase I (Hpx19) and Nudix hydrolase (Hpx26), was also examined. Hpx19 is accumulated inside the cell, but interestingly, Hpx26 is secreted outside the cell as an Hrp-secreted outer protein, suggesting that Hpx19 functions intracellularly but Hpx26 is a novel effector protein of R. solanacearum.

Structural studies on a mitochondrial glyoxalase II

Glyoxalase 2 is a beta-lactamase fold-containing enzyme that appears to be involved with cellular chemical detoxification. Although the cytoplasmic isozyme has been characterized from several organisms, essentially nothing is known about the mitochondrial proteins. As a first step in understanding the structure and function of mitochondrial glyoxalase 2 enzymes, a mitochondrial isozyme (GLX2-5) from Arabidopsis thaliana was cloned, overexpressed, purified, and characterized using metal analyses, EPR and (1)H NMR spectroscopies, and x-ray crystallography. The recombinant enzyme was shown to bind 1.04 +/- 0.15 eq of iron and 1.31 +/- 0.05 eq of Zn(II) and to exhibit k(cat) and K(m) values of 129 +/- 10 s(-1) and 391 +/- 48 microm, respectively, when using S-d-lactoylglutathione as the substrate. EPR spectra revealed that recombinant GLX2-5 contains multiple metal centers, including a predominant Fe(III)Z-n(II) center and an anti-ferromagnetically coupled Fe(III)Fe(II) center. Unlike cytosolic glyoxalase 2 from A. thaliana, GLX2-5 does not appear to specifically bind manganese. (1)H NMR spectra revealed the presence of at least eight paramagnetically shifted resonances that arise from protons in close proximity to a Fe(III)Fe(II) center. Five of these resonances arose from solvent-exchangeable protons, and four of these have been assigned to NH protons on metal-bound histidines. A 1.74-A resolution crystal structure of the enzyme revealed that although GLX2-5 shares a number of structural features with human GLX2, several important differences exist. These data demonstrate that mitochondrial glyoxalase 2 can accommodate a number of different metal centers and that the predominant metal center is Fe(III)Zn(II).

The binding of iron and zinc to glyoxalase II occurs exclusively as di-metal centers and is unique within the metallo-beta-lactamase family

Cytosolic glyoxalase 2 (GLX2-2) from Arabidopsis thaliana is a metalloenzyme that has been shown to bind a mixture of Zn, Fe, or Mn when produced in cells grown in rich media. In an effort to prepare metal-enriched samples, GLX2-2 was over-expressed in minimal media containing either Zn, Fe, or Mn. The resulting enzymes bound an average of 1 equivalent of metal ion and were partially enriched with a specific metal ion. The enzymes produced in minimal media were active towards the substrate S-D-lactoylglutathione, yielding kcat/ Km values similar to those of rich media GLX2-2. EPR studies on minimal media GLX2-2 samples revealed spectra which were identical to those over-expressed in rich media that contained nearly 2 equivalents of metal. The EPR spectra showed the presence of antiferromagnetically and ferromagnetically coupled, dinuclear metal centers. EXAFS spectra on the minimal media GLX2-2 samples over-expressed in the presence of Fe or Zn were also very similar to those of the rich media GLX2-2 samples, indicating the presence of dinuclear metal centers. The EXAFS studies also demonstrate that Zn(II) and Fe (in the Fe-enriched sample) are distributed in the dinuclear site. These data indicate that the minimal media GLX2-2 samples are a mixture of fully loaded, dinuclear metal-containing enzyme and metal-free enzyme. This characteristic of A. thaliana GLX2-2 makes it unique among the other members of the metallo-beta-lactamase family in that it does not ever appear to exist as a mononuclear metal ion containing enzyme and that it exhibits positive cooperativity in metal binding.

Glutathione analogues in cancer treatment

Glutathione is an important intracellular antioxidant and redox potential regulator that plays a vital role in drug detoxification and elimination and in cellular protection from damage by free radicals, peroxides, and toxins. Molecular alterations reported in many of the components of the glutathione system in various tumors and cancer cell lines can lead to increased survival and enhanced chemotherapy drug resistance. Several glutathione analogues that target this system have been developed and used experimentally and clinically in an attempt to improve cancer chemotherapy. These compounds include glutathione-S-transferase inhibitors and prodrugs, glyoxalase I inhibitors, and S-nitrosoglutathione. Work with these analogues has shown promising results, although lack of tumor specificity is a potential problem. Continuing development and optimization of glutathione analogues will lead to more specific targeting of this system in cancer treatment, allowing an increased therapeutic response.

Flexible Metal Binding of the Metallo-β-lactamase Domain: Glyoxalase II Incorporates Iron, Manganese, and Zinc in Vivo†

Glyoxalase II belongs to the metallo-beta-lactamase superfamily of proteins, possessing the characteristic dinuclear active site. Within this protein family, glyoxalase II from Arabidopsis thaliana is the first member to be isolated with significant amounts of iron, manganese, and zinc when being recombinantly produced in Escherichia coli. Enzyme preparations with different ratios of these three metals all yield k(cat)/K(M) values in the range of 1.5-1.9 s(-1) microM(-1) with the substrate S-d-lactoylglutathione. X-ray absorption spectroscopy reveals binding of all three metals to the dinuclear active site with 5-6-fold coordination consisting of 2.5 +/- 0.5 histidine and 2.5 +/- 0.5 oxygen ligands. This model does not distinguish site-specific or distributed binding. The metal-metal distance is determined to be 3.18 +/- 0.06 A. Electron paramagnetic resonance spectroscopy gives evidence for several different types of dimetal sites, including spin-coupled Fe(III)Fe(II), Fe(III)Zn(II), and Mn(II)Mn(II) centers. The metal-ligand distances measured by X-ray absorption spectroscopy vary depending on the metal type and comply with their element-specific, characteristic values. This reflects a high degree of structural flexibility within the glyoxalase II dinuclear active site, which is considered as the structural basis for its broad metal selectivity.

DNA damage by carbonyl stress in human skin cells

Reactive carbonyl species (RCS) are potent mediators of cellular carbonyl stress originating from endogenous chemical processes such as lipid peroxidation and glycation. Skin deterioration as observed in photoaging and diabetes has been linked to accumulative protein damage from glycation, but the effects of carbonyl stress on skin cell genomic integrity are ill defined. In this study, the genotoxic effects of acute carbonyl stress on HaCaT keratinocytes and CF3 fibroblasts were assessed. Administration of the alpha-dicarbonyl compounds glyoxal and methylglyoxal as physiologically relevant RCS inhibited skin cell proliferation, led to intra-cellular protein glycation as evidenced by the accumulation of N(epsilon)-(carboxymethyl)-L-lysine (CML) in histones, and caused extensive DNA strand cleavage as assessed by the comet assay. These effects were prevented by treatment with the carbonyl scavenger D-penicillamine. Both glyoxal and methylglyoxal damaged DNA in intact cells. Glyoxal caused DNA strand breaks while methylglyoxal produced extensive DNA-protein cross-linking as evidenced by pronounced nuclear condensation and total suppression of comet formation. Glycation by glyoxal and methylglyoxal resulted in histone cross-linking in vitro and induced oxygen-dependent cleavage of plasmid DNA, which was partly suppressed by the hydroxyl scavenger mannitol. We suggest that a chemical mechanism of cellular DNA damage by carbonyl stress occurs in which histone glycoxidation is followed by reactive oxygen induced DNA stand breaks. The genotoxic potential of RCS in cultured skin cells and its suppression by a carbonyl scavenger as described in this study have implications for skin damage and carcinogenesis and its prevention by agents selective for carbonyl stress.

Mechanistic implications of methylglyoxal synthase complexed with phosphoglycolohydroxamic acid as observed by X-ray crystallography and NMR spectroscopy

Methylglyoxal synthase (MGS) and triosephosphate isomerase (TIM) share neither sequence nor structural similarities, yet the reactions catalyzed by both enzymes are similar, in that both initially convert dihydroxyacetone phosphate to a cis-enediolic intermediate. This enediolic intermediate is formed from the abstraction of the pro-S C3 proton of DHAP by Asp-71 of MGS or the pro-R C3 proton of DHAP by Glu-165 of TIM. MGS then catalyzes the elimination of phosphate from this enediolic intermediate to form the enol of methylglyoxal, while TIM catalyzes proton donation to C2 to form D-glyceraldehyde phosphate. A competitive inhibitor of TIM, phosphoglycolohydroxamic acid (PGH) is found to be a tight binding competitive inhibitor of MGS with a K(i) of 39 nM. PGH's high affinity for MGS may be due in part to a short, strong hydrogen bond (SSHB) from the NOH of PGH to the carboxylate of Asp-71. Evidence for this SSHB is found in X-ray, 1H NMR, and fractionation factor data. The X-ray structure of the MGS homohexamer complexed with PGH at 2.0 A resolution shows this distance to be 2.30-2.37 +/- 0.24 A. 1H NMR shows a PGH-dependent 18.1 ppm signal that is consistent with a hydrogen bond length of 2.49 +/- 0.02 A. The D/H fractionation factor (phi = 0.43 +/- 0.02) is consistent with a hydrogen bond length of 2.53 +/- 0.01 A. Further, 15N NMR suggests a significant partial positive charge on the nitrogen atom of bound PGH, which could strengthen hydrogen bond donation to Asp-71. Both His-98 and His-19 are uncharged in the MGS-PGH complex on the basis of the chemical shifts of their Cdelta and C(epsilon) protons. The crystal structure reveals that Asp-71, on the re face of PGH, and His-19, on the si face of PGH, both approach the NO group of the analogue, while His-98, in the plane of PGH, approaches the carbonyl oxygen of the analogue. The phosphate group of PGH accepts nine hydrogen bonds from seven residues and is tilted out of the imidate plane of PGH toward the re face. Asp-71 and phosphate are thus positioned to function as the base and leaving group, respectively, in a concerted suprafacial 1,4-elimination of phosphate from the enediolic intermediate in the second step of the MGS reaction. Combined, these data suggest that Asp-71 is the one base that initially abstracts the C3 pro-S proton from DHAP and subsequently the 3-OH proton from the enediolic intermediate. This mechanism is compared to an alternative TIM-like mechanism for MGS, and the relative merits of both mechanisms are discussed.

Arabidopsis glyoxalase II contains a zinc/iron binuclear metal center that is essential for substrate binding and catalysis

Glyoxalase II participates in the cellular detoxification of cytotoxic and mutagenic 2-oxoaldehydes. Because of its role in chemical detoxification, glyoxalase II has been studied as a potential anti-cancer and/or anti-protozoal target; however, very little is known about the active site and reaction mechanism of this important enzyme. To characterize the active site and kinetic mechanism of the enzyme, a detailed mutational study of Arabidopsis glyoxalase II was conducted. Data presented here demonstrate for the first time that the cytoplasmic form of Arabidopsis glyoxalase II contains an iron-zinc binuclear metal center that is essential for activity. Both metals participate in substrate binding, transition state stabilization, and the hydrolysis reaction. Subtle alterations in the geometry and/or electrostatics of the binuclear center have profound effects on the activity of the enzyme. Additional residues important in substrate binding have also been identified. An overall reaction mechanism for glyoxalase II is proposed based on the mutational and kinetic data from this study and crystallographic data on human glyoxalase II. Information presented here provides new insights into the active site and reaction mechanism of glyoxalase II that can be used for the rational design of glyoxalase II inhibitors.

Methylglyoxal induces G:C to C:G and G:C to T:A transversions in the supF gene on a shuttle vector plasmid replicated in mammalian cells

We previously reported that the majority of base-pair substitutions induced by an endogenous mutagen, methylglyoxal, were G:C-->T:A transversions and G:C-->A:T transitions in wild-type and nucleotide excision repair (NER)-deficient (uvrA or uvrC) Escherichia coli strains. To investigate the mutation spectrum of methylglyoxal in mammalian cells and to compare the spectrum with those detected in other experimental systems, we analyzed mutations in a bacterial suppressor tRNA (supF) gene in the shuttle vector plasmid pMY189. We treated pMY189 with methylglyoxal and immediately transfected it into simian COS-7 cells. The cytotoxicity and the mutation frequency (MF) increased according to the dose of methylglyoxal. In the mutants induced by methylglyoxal, multi-base deletions were predominant (50%), followed by base-pair substitutions (35%), in which 89% of the substitutions occurred at G:C sites. Among them, G:C-->C:G and G:C-->T:A transversions were predominant. The overall distribution of methylglyoxal-induced mutations detected in the supF gene was different from that for the spontaneous mutations. These results suggest that methylglyoxal may take part in causing G:C-->C:G and G:C-->T:A transversions in vivo.

Mirroring perfection: the structure of methylglyoxal synthase complexed with the competitive inhibitor 2-phosphoglycolate

The crystal structure of the transition-state analogue 2-phosphoglycolate (2PG) bound to methylglyoxal synthase (MGS) is presented at a resolution of 2.0 A. This structure is very similar to the previously determined structure of MGS complexed to formate and phosphate. Since 2PG is a competitive inhibitor of both MGS and triosephosphate isomerase (TIM), the carboxylate groups of each bound 2PG from this structure and the structure of 2PG bound to TIM were used to align and compare the active sites despite differences in their protein folds. The distances between the functional groups of Asp 71, His 98, His 19, and the carboxylate oxygens of the 2PG molecule in MGS are similar to the corresponding distances between the functional groups of Glu 165, His 95, Lys 13, and the carboxylate oxygens of the 2PG molecule in TIM. However, these spatial relationships are enantiomorphic to each other. Consistent with the known stereochemical data, the catalytic base Asp 71 is positioned on the opposite face of the 2PG-carboxylate plane as Glu 165 of TIM. Both His 98 of MGS and His 95 of TIM are in the plane of the carboxylate of 2PG, suggesting that these two residues are homologous in function. While His 19 of MGS and Lys 13 of TIM appear on the opposite face of the 2PG carboxylate plane, their relative location to the 2PG molecule is quite different, suggesting that they probably have different functions. Most remarkably, unlike the coplanar structure found in the 2PG molecule bound to TIM, the torsion angle around the C1-C2 bond of 2PG bound to MGS brings the phosphoryl moiety out of the molecule's carboxylate plane, facilitating elimination. Further, the superimposition of this structure with the structure of MGS bound to formate and phosphate suggests a model for the enzyme bound to the first transition state.

Cloning and characterization of glyoxalase I from soybean

Glyoxalase I and glutathione transferase (GST) are two glutathione-dependent enzymes which are enhanced in plants during cell division and in response to diverse stress treatments. In soybean, a further connection between these two enzymes has been suggested by a clone (Accession No. X68819) resembling a GST being described as a glyoxalase I. To characterize glyoxalase I in soybean, GmGlyox I resembling the dimeric enzyme from animals has been cloned from a cDNA library prepared from soybean suspension cultures. When expressed in Escherichia coli, GmGlyox I was found to be a 38-kDa dimer composed of 21-kDa subunits and unlike the enzyme from mammals showed activity in the absence of metal ions. GmGlyox I was active toward the hemithioacetal adducts formed by reacting methylglyoxal, or phenylglyoxal, with glutathione, homoglutathione, or gamma-glutamylcysteine, showing no preference for homoglutathione adducts over glutathione adducts, even though homoglutathione is the dominant thiol in soybean. When the clone X68819 was expressed in E. coli, the respective recombinant enzyme was active as a GST rather than a glyoxalase and was termed GmGST 3. GmGST 3 was active as a homodimer (45 kDa) composed of 26-kDa subunits and showed a preference for glutathione over homoglutathione when conjugating 1-chloro-2,4-dinitrobenzene. Both enzymes are associated with cell division in soybean cultures, but GmGST 3 (0.4% total protein) was 40 times more abundant than GmGlyox I (0.01%).

The crystal structure of methylglyoxal synthase from Escherichia coli

BACKGROUND

The reaction mechanism of methylglyoxal synthase (MGS) is believed to be similar to that of triosephosphate isomerase (TIM). Both enzymes utilise dihydroxyacetone phosphate (DHAP) to form an enediol(ate) phosphate intermediate as the first step of their reaction pathways. However, the second catalytic step in the MGS reaction pathway is characterized by the elimination of phosphate and collapse of the enediol(ate) to form methylglyoxal instead of reprotonation to form the isomer glyceraldehyde 3-phosphate.

RESULTS

The crystal structure of MGS bound to formate and substoichiometric amounts of phosphate in the space group P6522 has been determined at 1.9 A resolution. This structure shows that the enzyme is a homohexamer composed of interacting five-stranded beta/alpha proteins, rather than the hallmark alpha/beta barrel structure of TIM. The conserved residues His19, Asp71, and His98 in each of the three monomers in the asymmetric unit bind to a formate ion that is present in the crystallization conditions. Differences in the three monomers in the asymmetric unit are localized at the mouth of the active site and can be ascribed to the presence or absence of a bound phosphate ion.

CONCLUSIONS

In agreement with site-directed mutagenesis and mechanistic enzymology, the structure suggests that Asp71 acts as the catalytic base. Further, Asp20 and Asp101 are involved in intersubunit salt bridges. These salt bridges may provide a pathway for transmitting allosteric information.

Genetic damage induced by methylglyoxal and methylglyoxal plus X-rays in Drosophila melanogaster germinal cells

The effect of methylglyoxal (MG) and MG administered prior to X-irradiation was investigated in Drosophila melanogaster germinal cells using the sex-linked recessive lethal (s.l.r.l.), II-III autosomal translocation (AT) and X-chromosome nondisjunction (ND) tests. For the s.l.r.l. test the males were either injected with MG (0.5 M, 0.75 M or 1.7 M) or fed for 24 h (1 M) and two 24 h broods (A and B) were obtained. For the AT test the males were injected with MG 1.7 M and the same brooding scheme was followed. ND was tested in females fed on MG 1 M. The only effect observed after MG treatment was a significant increase on the yield of s.l.r.l. with MG 1.7 M. In the combined treatments MG was administered prior to irradiation with 20 Gy of X-rays and the induction of s.l.r.l. and AT was assessed. Pre-treatment with MG 0.75 M and 1.7 M enhanced the frequency of s.l.r.l. in cells sampled in brood B, consisting mainly of the rather hypoxic late spermatids. It is suggested that this radiosensitizing effect could be ascribed to a decrease in the level of glutathione due to the metabolization of MG.

Identification of catalytic bases in the active site of Escherichia coli methylglyoxal synthase: cloning, expression, and functional characterization of conserved aspartic acid residues

Methylglyoxal synthase provides bacteria with an alternative to triosephosphate isomerase for metabolizing dihydroxyacetone phosphate (DHAP). In the present studies, the methylglyoxal synthase gene in Escherichia coli has been cloned and sequenced. The identified open reading frame (ORF) codes for a polypeptide of 152 amino acids, consistent with the 17 kDa purified protein. The sequence of this protein is not similar to any other protein of known function, including the functionally similar protein triosephosphate isomerase. The methylglyoxal synthase gene was amplified by PCR, subcloned into the pET16B expression vector, and expressed in the host E. coli BL21(DE3). Sequence comparison of the methylglyoxal protein and related ORFs from four different bacterial species revealed that four aspartic acid and no glutamic acid residues are absolutely conserved. The function of the four aspartic acid residues was tested by mutating them to either asparagine or glutamic acid. Thermal denaturation, CD spectroscopy, and gel filtration experiments showed that the mutant enzymes had the same secondary and quaternary structure as the wild-type enzyme. Kinetic characterization of both Asp 71 and Asp 101 mutant proteins shows reduced kcat/Km by 10(3)- and 10(4)-fold respectively, suggesting that they are both intimately involved in catalysis. A time-dependent inhibition of both Asp 20 and Asp 91 asparagine mutants by DHAP suggests that these two residues are involved with protecting the enzyme from DHAP or reactive intermediates along the catalytic pathway. In combination with the results of 2-phosphoglycolate binding studies, a catalytic mechanism is proposed.

Pharmacology of methylglyoxal: formation, modification of proteins and nucleic acids, and enzymatic detoxification--a role in pathogenesis and antiproliferative chemotherapy

1. Methylglyoxal is a reactive alpha-oxoaldehyde and physiological metabolite formed by the fragmentation of triose-phosphates, and by the metabolism of acetone and aminoacetone. 2. Methylglyoxal modifies guanylate residues to form 6,7-dihydro-6,7-dihydroxy-6-methyl-imidazo[2,3-b]purine-9(8)one and N2-(1-carboxyethyl)guanylate residues and induces apoptosis. 3. Methylglyoxal modifies arginine residues in proteins to form N(delta)-(4,5-dihydroxy-4-methylimidazolidin-2-yl) ornithine, N(delta)-(5-hydro-5-methylimidazol-4-on-2-yl)ornithine and N(delta)-(5)methylimidazol-4-on-2-yl)ornithine residues. 4. Methylglyoxal-modified proteins undergo receptor-mediated endocytosis and lysosomal degradation in monocytes and macrophages, and induce cytokine synthesis and secretion. 5. Methylglyoxal is detoxified by the glyoxalase system. Decreased detoxification of methylglyoxal may be induced pharmacologically by glyoxalase I inhibitors which have anti-tumor and anti-malarial activities. 6. The modification of nucleic acids and protein by methylglyoxal is a signal for their degradation and may have a role in the development of diabetic complications, atherosclerosis, the immune response in starvation, aging and oxidative stress.

Antimalarial activity in vitro of the glyoxalase I inhibitor diester, S-p-bromobenzylglutathione diethyl ester

S-p-Bromobenzylglutathione diethyl ester induced toxicity in the malarial parasite Plasmodium falciparum in infected human red blood cells in culture. The median inhibitory concentration, IC50, was 4.77 +/- 0.12 microM (N = 10) for incorporation of [3H]hypoxanthine in nucleotide synthesis and 5.20 +/- 0.1 microM (N = 10) for incorporation of [14C]isoleucine into protein. The prospective mechanism of action is inhibition of glyoxalase I by the de-esterified metabolite, S-p-bromobenzylglutathione, and accumulation of the cytotoxic substrate methylglyoxal.