Chemical Basis of the Action of Glyoxalase I, an Anticancer Target Enzyme

Enantioselective Synthesis of a New Non-Natural Gabosine

The preparation of a new non-natural gabosine is reported, in which the chirality is transferred from the toluene’s biotransformed metabolite (1R,2S)-3-methylcyclohexa-3.5-diene-1,2-diol. Further chemical transformations to introduce additional functionality and chirality to the molecule were also accomplished.

Proteome analysis of multidrug resistance in vincristine-resistant human gastric cancer cell line SGC7901/VCR

In order to elucidate the mechanisms of multidrug resistance (MDR) of vincristine-resistant human gastric carcinoma cell line SGC7901/VCR, 2-DE was used to separate the total proteins of SGC7901/VCR and its parental cell line SGC7901. PDQuest software was applied to analyze 2-DE images, and the differential protein spots were identified by both MALDI-TOF-MS and ESI-Q-TOF-MS. Then the differential expressional levels of partially identified proteins were determined by Western blot analysis and real-time RT-PCR. Furthermore, the association of heat shock protein (HSP27), one of the highly expressed proteins in sgc7901/vcr, with MDR was analyzed using antisense inhibition of HSP27. In this study, the well-resolved, reproducible 2-DE patterns of SGC7901/VCR and SGC7901 were established, and yielded about 1100 protein-spots each. All the 24 differential proteins between the two cell lines were identified, and the differential expression levels of the partial proteins were confirmed. The suppression of HSP27 expression by HSP27 antisense oligonucleotides could enhance vincristine chemosensitivity in sgc7901/vcr and induce the cells to exhibit apoptotic morphological features after vincristine treatment. The differentially expressed proteins could be divided into six groups based on their functions: calcium-binding proteins, chaperones, proteins involved in drug detoxification or repair of DNA damage, metabolic enzymes, proteins related to cellular structure, and proteins relative to signal transduction, some of which may contribute to MDR of human gastric carcinoma cell line SGC7901/VCR. These data will be valuable for further study of the mechanisms of MDR in human gastric cancer.

Active site structure and mechanism of human glyoxalase I-an ab initio theoretical study

The structure of the active site of human glyoxalase I and the reaction mechanism of the enzyme-catalyzed conversion of the thiohemiacetal, formed from methylglyoxal and glutathione, to S-D-lactoylglutathione has been investigated by ab initio quantum chemical calculations. To realistically represent the environment of the reaction center, the effective fragment potential methodology has been employed, which allows systems of several hundred atoms to be described quantum mechanically. The methodology and the active site model have been validated by optimizing the structure of a known enzyme-inhibitor complex, which yielded structures in good agreement with the experiment. The same crystal structure has been used to obtain the quantum motif for the investigation of the glyoxalase I reaction. The results of our study confirm that the metal center of the active site zinc complex plays a direct catalytic role by binding the substrate and stabilizing the proposed enediolate reaction intermediate. In addition, our calculations yielded detailed information about the interactions of the substrate, the reaction intermediates, and the product with the active site of the enzyme and about the mechanism of the glyoxalase I reaction. The proton transfers of the reaction proceed via the two highly flexible residues Glu172 and Glu99. Information about the structural and energetic effect of the protein on the first-shell complex has been attained by comparison of the structures optimized in the local protein environment and in a vacuum. The environment of the zinc complex disturbs the Cs symmetry found for the complex in a vacuum, which suggests an explanation for the stereochemical behavior of glyoxalase I.

Small molecule probes of glyoxalase I and glyoxalase II

A number of synthetic tropolones and hydrophobic S-blocked glutathione analogues were investigated as potential inhibitors of glyoxalase I from Saccharomyces cerevisiae and glyoxalase II from bovine liver. Several tropolones containing a free C-2 hydroxy group were found to be potent inhibitors of glyoxalase I, whereas the glutathione conjugates were found to be modest to poor inhibitors of this enzyme. Most tropolones and glutathione conjugates, except 5-p-tolylazotropolone and S-carbobenzoxy-L-glutathione, were found to be poor inhibitors of glyoxalase II. A recent report on an extremely active glyoxalase system from Plasmodium falciparum suggested that several of the more potent inhibitors may have antimalarial properties. A number of these compounds in fact, exhibited antimalarial activity in the low micromolar range. Further studies are required to fully elucidate the mechanism(s) of the antimalarial properties of these compounds.

Inhibition of proliferation of human leukaemia 60 cells by methylglyoxal in vitro

Methylglyoxal (2-oxopropanal) is the physiological substrate of the glyoxalase system. When exogenous methylglyoxal (50 microM-1 mM) was added to human leukaemia 60 (HL60) cells in culture (5 x 10(4) cells/ml), inhibition of growth and toxicity was induced. The median growth inhibitory concentration IC50 value was 238 +/- 2 microM. There was little differentiation of HL60 cells induced by methylglyoxal (a maximum of 2% differentiation with 500 microM methylglyoxal). There was no similar toxicity induced by methylglyoxal in corresponding differentiated cells, neutrophils, under the same culture conditions. Cell growth and toxicity induced by methylglyoxal (250 microM) in HL60 cells occurred in the initial 24 h of culture, after which residual surviving cells exhibited normal growth kinetics. It could also be prevented by replacing the culture medium in the initial 6 h of culture; thereafter, irreversible toxicity developed, reaching the maximum value after 24 h of culture. Growth arrest and toxicity induced by methylglyoxal increased with increasing serum composition of the medium. The mechanism of toxicity is unknown.

Nutritional and physiological consequences of tumour glycolysis

A frequent characteristic of many malignant tumours is an increase in anaerobic glycolysis, that is the conversion of glucose to lactate, when compared to normal tissues. The causes of this intensification involve changes in enzyme and glucose transporter levels, shifts of the isoenzyme patterns in the cancer cells to those similar to foetal tissues and a breakdown in the normal control mechanisms, most notably the Pasteur effect. The host must adapt, with a corresponding increase in gluconeogenesis. This change, along with other adaptations made by the host, eventually results in the syndrome known as cancer cachexia, which is characterized by anorexia and depletion and redistribution of the host energy stores. In some ways many malignant tumours behave much like parasites, drawing upon the host for nutrients such as glucose and returning waste products such as lactate to the host for recycling or disposal. This cycling of glucose and lactate between host and tumour has been the target for a number of proposed and tested treatments, with regard to the possible inhibition of tumour growth and/or possible prevention of some or all of the cachectic effects. Some of these suggested treatments have reached the point of clinical testing and show promise for continued research.

Inhibition of proliferation of human leukaemia 60 cells by diethyl esters of glyoxalase inhibitors in vitro

Diethyl esters of the glutathione S-conjugate S-p-bromobenzylglutathione, an inhibitor of glyoxalase I, and S-p-nitrobenzoxycarbonylglutathione, an inhibitor of glyoxalase II, induced growth arrest and toxicity in human leukaemia 60 cells in culture. The median growth inhibitory concentration IC50 values were 8.3 microM (95% C.I. 7.0-9.9 microM) for S-p-bromobenzylglutathione diethyl ester and 56 microM (95% C.I. 36-86 microM) for p-nitrobenzoxycarbonylglutathione. Monoethyl ester and unesterified derivatives were inactive. The diethyl ester derivatives were also toxic to mature human neutrophils under the same culture conditions where the respective median toxic concentration IC50 values were 39.7 (95% C.I. 35.4-44.5 microM) and 127 (95% C.I. 123-132 microM) microM. Diester derivatives may be of future interest in studying the cytotoxicity of glutathione S-conjugates and for the development of cytotoxic anti-tumour agents.

Why do cancer cells have such a high glycolytic rate?

The high glycolytic rate of most tumor cells has puzzled biochemists for a long time. In the tumor, glucose is aerobically converted to lactate in the cytoplasm rather than being oxidized in the mitochondria. The molecular mechanisms underlying this phenomenon are still fragmentary. The present study supports the view that the origin of the glucose disposal by the tumor is multifactorial.

Investigation on glyoxalase I inhibitors

Several classes of compounds - flavones, coumarins, S- and N-substituted glutathione analogs, transition state analogs, porphyrins, nucleotides and nucleosides - have been reported to inhibit the enzyme gloxalase I. In the current study, examination of some of the aforementioned compounds has revealed that squaric acid does not function as an inhibitor of glyoxalase I and several other compounds are much less effective in this regard than previously reported. Several new potent inhibitors of yeast glyoxalase I have been identified. Compounds containing the tropolone structure were especially inhibitory. Glutathione adducts of benzoquinone and naphthoquinone were also inhibitory and may be of particular interest with regard to the toxicology of normal aromatic metabolites in vivo.

The oxidation of methylglyoxal by mammalian pyruvate dehydrogenase

Mammalian pyruvate dehydrogenase actively catalyzed the oxidation of methylglyoxal to acetyl-CoA. The reaction was fully enzymatic with an estimated Km of 1.89 mM. On the other hand, methylglyoxal was a competitive inhibitor of the enzyme for pyruvate, the Ki being in the 1 mM range. The reaction was inhibited in the presence of HgCl2. The reaction products were quantitatively identified as acetyl-CoA and formic acid. A mechanism for the reaction is proposed.

Chemical modification of tyrosine residues in glyoxalase I from yeast and human erythrocytes

1. Yeast glyoxalase I was inactivated by N-acetylimidazole and tetranitromethane, the latter process following pK app 7.31 and irreversibly producing a protein with a spectrum typical of 3-nitrotyrosine. 2. For yeast glyoxalase I, amino-acid analysis and protection studies with S-(p-bromobenzyl)glutathione, a competitive inhibitor, indicated two classes of tetranitromethane-reactive tyrosine residues, fast- and slow-reacting, with the latter class containing the crucial tyrosine(s). 3. Human erythrocyte glyoxalase I was inactivated by tetranitromethane in fast and slow processes, protection studies in this case indicating the important tyrosine(s) as fast-reacting.

Modification of the glyoxalase system in human HL60 promyelocytic leukaemia cells during differentiation to neutrophils in vitro

The glyoxalase system of human promyelocytic leukaemia HL60 cells was substantially modified during differentiation to neutrophils. The activity of glyoxalase I was decreased and the activity of glyoxalase II was markedly increased relative to the level in control HL60 promyelocytes. There was a decrease in the apparent maximum velocity, Vmax, of glyoxalase I, and an increase in the Vmax of glyoxalase II. The apparent Michaelis constants for both enzymes remained unchanged. The flux of intermediates metabolised via the glyoxalase system increased during differentiation, as judged by the formation of D-lactic acid, whereas the percentage of glucotriose metabolised via the glyoxalase system remained unchanged. The cellular concentrations of the glyoxalase substrates, methylglyoxal and S-D-lactoylglutathione, were markedly decreased during differentiation. The maturation of HL60 promyelocytes is associated with an increased ability to metabolise S-D-lactoylglutathione by glyoxalase II and a concomitant decrease in the mean intracellular concentrations of S-D-lactoylglutathione and methylglyoxal. The maintenance of a high concentration of S-D-lactoylglutathione in HL60 promyelocytes may be related to the status of the microtubular cytoskeleton, since S-D-lactoylglutathione potentiates the GTP-promoted assembly of microtubules.

Inhibition by glutathione derivatives of bovine liver glyoxalase II (hydroxyacylglutathione hydrolase) as a probe of the N- and S-sites for substrate binding

The nature of the binding determinants used in the interaction of glutathione-based derivatives and bovine liver glyoxalase II (S-(2-hydroxyacyl)glutathione hydrolase, EC 3.1.2.6) has been investigated. Linear competitive inhibition was observed for S-blocked and S,N-blocked glutathiones with bovine liver glyoxalase II (molecular weight 22 500 by sodium dodecyl sulphate polyacrylamide gel electrophoresis; pI = 7.48 by analytical isoelectric focussing). There is a significant hydrophobic region on the enzyme to bind substituents around the sulphydryl-derived moiety of the substrate--a hydrophobic S-site. However, there is no evidence for binding of the N-site of the substrate (or inhibitor) to glyoxalase II. In contrast to glyoxalase I, there is no linkage between binding forces used at the S- and N-sites. Binding of S,N-dicarbobenzoxyglutathione is pH-dependent, showing dependence on an ionisation with pKapp approximately equal to 7.2 (binding more tightly at higher pH), as is the kcat value (pKapp approximately equal to 7.8) for S-D-lactoylglutathione.

Inhibition of mammalian glyoxalase I (lactoylglutathione lyase) by N-acylated S-blocked glutathione derivatives as a probe for the role of the N-site of glutathione in glyoxalase I mechanism

A series of twelve S-blocked and N,S-blocked glutathione derivatives has been studied as inhibitors of glyoxalase I [R)-S-lactoylglutathione methylglyoxal-lyase (isomerising), EC 4.4.1.5) from human erythrocytes. A number of new N,S-blocked glutathiones have been synthesised. Inhibition at pH 7.0, 25 degrees C was linear-competitive in all cases and the Ki values were interpreted in terms of the absence of a specific binding interaction for the N-site of the inhibitor and the absence of coupling between binding processes at N- and S-sites (the regions around the NH2 and HS groups, respectively, of GSH analogues bound to enzyme). These observations are in strong contrast to previous results with the yeast enzyme. Some Ki values were measured for yeast glyoxalase I. A special binding interaction of the phenyl groups with enzyme from both species was found for glutathione derivatives with N-acyl groups of structure -NH X CO X X X Y X Ph but not for -NH X COPh, where X and Y were variously -CH2-, -NH- and -O-. Studies were made of the range of stability of human erythrocyte glyoxalase I to pH. The pH profiles for the Ki values of S-p-bromobenzyl)glutathione and N-acetyl-S-(p-bromobenzyl)glutathione indicated no pH dependence for the latter and little, if any, for the former inhibitor. The mean Ki over the pH range 5-8.5 for S-(p-bromobenzyl)glutathione was 1.21 +/- 0.37 microM and for N-acetyl-S-(p-bromobenzyl)glutathione in the same pH range, Ki decreased from 1.45 +/- 0.26 microM to 0.88 +/- 0.11 M.

Partial transition-state inhibitors of glyoxalase I from human erythrocytes, yeast and rat liver

Glyoxalase I (lactoylglutathione lyase, EC 4.4.1.5) converts the hemithiolacetal of glutathione and an alpha-ketoaldehyde to S-D-lactoylglutathione which is hydrolysed under the catalytic influence of glyoxalase II to produce D-lactate and regenerate glutathione. There is much evidence that glyoxalase I operates via an enediol intermediate, and in this study a number of inhibitors are described which were designed based on the enediol moiety of this reactive intermediate. These enediol and paene-enediol moieties were combined with groups designed to make use of an adjacent hydrophobic site and can be described as partial transition-state analogues. Derivatives of lapachol and kojic acid were good competitive inhibitors of glyoxalase I from various sources unless the free hydroxy group was blocked or replaced. Flavones with strong inhibitors of glyoxalase I and gallocyanine (a dye) showed spectral changes on binding to glyoxalase I indicative of binding to a metal-ion site (probably Zn2+ or Mg2+). The use of the enediol-binding determinant to produce glyoxalase I inhibitors is discussed as a route to potential antitumour derivatives.