A Glutathione Peroxidase Mimic 6,6′-Ditellurobis (6-Deoxy-β-Cyclodextrin) with High Substrate Specificity

Glutathione peroxidase (GPx) is one of the most important antioxidative selenoenzymes in living organisms. The novel GPx mimic 6,6′-ditellurobis(6-deoxy-β-cyclodextrin) (6-TeCD) was prepared and evaluated for its capacity to catalyze the reduction of H₂O₂, tert-butyl hydroperoxide (t-BuOOH), and cumene hydroperoxide (CuOOH) by glutathione (GSH) or 3-carboxy-4-nitrobenzenethiol (ArSH). Compared the ArSH assay with the coupled reductase assay, we found that 6-TeCD exhibited strong substrate specificity for aromatic thiol substrate. The specificity led to efficient peroxidase activity almost 100,000-fold than that for a well-known GPx mimic diphenyl diselenide (PhSeSePh). Furthermore, reduction of lipophilic CuOOH was proceeded ca. 30 times faster than the more hydrophilic H₂O₂, which cannot bind into the hydrophobic cavity of β-cyclodextrin. Thus, it seemed that catalytic activity of cyclodextrin-derived GPx models strongly depends on the structurally different both substrates hydroperoxides (ROOH) and thiols.     

Aryl thiol substrate 3-carboxy-4-nitrobenzenethiol strongly stimulating thiol peroxidase activity of glutathione peroxidase mimic 2, 2'-ditellurobis(2-deoxy-beta-cyclodextrin)

Artificial glutathione peroxidase (GPx) model 2, 2'-ditellurobis(2-deoxy-beta-cyclodextrin) (2-TeCD) which has the desirable properties exhibited high substrate specificity and remarkably catalytic efficiency when 3-carboxy-4-nitrobenzenethiol (ArSH) was used as a preferential thiol substrate. The complexation of ArSH with beta-cyclodextrin was investigated through UV spectral titrations, fluorescence spectroscopy, 1H NMR and molecular simulation, and these results indicated that ArSH fits well to the size of the cavity of beta-cyclodextrin. Furthermore, 2-TeCD was found to catalyze the reduction of cumene peroxide (CuOOH) by ArSH 200,000-fold more efficiently than diphenyl diselenide (PhSeSePh). Its steady-state kinetics was studied and the second rate constant kmax/KArSH was found to be 1.05 x 10(7) M(-1) min(-1) and similar to that of natural GPx. Moreover, the kinetic data revealed that the catalytic efficiency of 2-TeCD depended strongly upon the competitive recognition of both substrates for 2-TeCD. The catalytic mechanism of 2-TeCD catalysis agreed well with a ping-pong mechanism, in analogy with natural GPx, and might exert its thiol peroxidase activity via tellurol, tellurenic acid, and tellurosulfide.     

Cyclodextrin-derived mimic of glutathione peroxidase exhibiting enzymatic specificity and high catalytic efficiency

To elucidate the relationships between molecular recognition and catalytic ability, we chose three assay systems using three different thiol substrates, glutathione (GSH), 3-carboxyl-4-nitrobenzenethiol (CNBSH), and 4-nitrobenzenethiol (NBSH), to investigate the glutathione peroxidase (GPx) activities of 2,2'-ditellurobis(2-deoxy-beta-cyclodextrin) (2-TeCD) in the presence of a variety of structurally distinct hydroperoxides (ROOH), H2O2, tert-butyl peroxide (tBuOOH), and cumene peroxide (CuOOH), as the oxidative reagent. A comparative study of the three assay systems revealed that the cyclodextrin moiety of the GPx mimic 2-TeCD endows the molecule with selectivity for ROOH and thiol substrates, and hydrophobic interactions are the most important driving forces in 2-TeCD complexation. Furthermore, in the novel NBSH assay system, 2-TeCD can catalyze the reduction of ROOH about 3.4 x 10(5) times more efficiently than diphenyl diselenide (PhSeSePh), and its second-order rate constants for thiol are similar to some of those of native GPx. This comparative study confirms that efficient binding of the substrate is essential for the catalytic ability of the GPx mimic, and that NBSH is the preferred thiol substrate of 2-TeCD among the chosen thiol substrates. Importantly, the proposed mode of action of 2-TeCD imitates the role played by several possible noncovalent interactions between enzymes and substrates in influencing catalysis and binding.     

Tellurium‐Based Polymeric Surfactants as a Novel Seleno‐Enzyme Model with High Activity

Summary: A tellurium‐based polymeric sufactant as a seleno‐enzyme model has been constructed by employing 11‐acryloyloxyundecyltriethylammonium bromide (AUTEAB, 4 ) and a tellurium‐containing compound ( 1 ). It demonstrates strong substrate binding ability for thiols and high glutathione peroxidase (GPx) activity about 6 orders of magnitude more efficient than the well‐known GPx mimic PhSeSePh in an ArSH assay system. More importantly, a series of tellurium‐based polymeric micelle catalysts with the catalytic tellurium center located at various positions in the micelle have been constructed, and the dramatic difference in activity indicates that the exact match of the catalytic center and binding site plays a key role in enzyme catalytic efficiency.

Schematic representation of the proposed mode of the telluro‐micelle catalysts.     

Three-phase microemulsion/sol-gel system for aqueous catalysis with hydrophobic chemicals

A facile three-phase transport process is described that allows to carry out catalytic reactions in water, whereby all components are hydrophobic. According to this process a hydrophobic substrate is microemulsified in water and subjected to an organometallic catalyst, which is entrapped within a partially hydrophobized sol-gel matrix. The surfactant molecules, which carry the hydrophobic substrate, adsorb/desorb reversibly on the surface of the sol-gel matrix breaking the micellar structure, spilling their substrate load into the porous medium that contains the catalyst. A catalytic reaction then takes place within the ceramic material to form the desired products that are extracted by the desorbing surfactant, carrying the emulsified product back into the solution. The method is general and versatile and has been demonstrated with the catalytic hydrogenations of alkenes, alkynes, aromatic C=C bonds, and nitro and cyano groups.     

An Amphiphilic Selenide Catalyst Behaves Like a Hybrid Mimic of Protein Disulfide Isomerase and Glutathione Peroxidase 7

Protein disulfide isomerase (PDI) and glutathione peroxidase 7 (GPx7) cooperatively promote the oxidative folding of disulfide (SS)‐containing proteins in endoplasmic reticulum by recognizing the nascent proteins to convert them into the native folds by means of SS formation and SS isomerization and by catalyzing reoxidation of reduced PDI with H₂O₂, respectively. In this study, new amphiphilic selenides with a long‐chain alkyl group were designed as hybrid mimics of PDI and GPx7 and were applied to the refolding of reduced hen egg‐white lysozyme (HEL‐R). Competitive SS formation at pH 4 using HEL‐R and glutathione (GSH) in the presence of the selenide catalyst and H₂O₂ showed that the amphiphilic selenides can preferentially catalyze SS formation of HEL‐R, probably on account of hydrophobic interactions between the protein and the catalyst. In contrast, simple water‐soluble selenides did not exhibit such behavior. In addition, when the pH of the solution was adjusted to 8.5 after the SS formation, surviving GSH promoted the SS isomerization of misfolded HEL to recover the native SS linkages. Thus, the amphiphilic selenides designed here could mimic the function of the PDI‐GPx7 system. The combination of a water‐soluble selenide and a long‐chain alkyl group would be a useful motif in designing medicines for both protein misfolding diseases and antioxidant therapy.     

A semisynthetic glutathione peroxidase with high catalytic efficiency. Selenoglutathione transferase

Glutathione peroxidase (GPX) protects cells against oxidative damage by catalyzing the reduction of hydroperoxides by glutathione (GSH). GPX therefore has potential therapeutic value as an antioxidant, but its pharmacological development has been limited because GPX uses a selenocysteine as its catalytic group and it is difficult to generate selenium-containing proteins with traditional recombinant DNA technology. Here, we show that naturally occurring proteins can be modified to generate GPX activity. The rat theta-class glutathione transferase T2-2 (rGST T2-2) presents an ideal scaffold for the design of a novel GPX catalyst because it already binds GSH and contains a serine close to the substrate binding site, which can be chemically modified to bind selenium. The modified Se-rGST T2-2 efficiently catalyzes the reduction of hydrogen peroxide, and the GPX activity surpasses the activities of some natural GPXs.     

Synthesis of N-benzoyl-N''-aryl selenoureas under PTC

Recently many syntheses of selenium-containing compounds have been reported and studied, in which compounds selenoureas are used as the precursors for the syntheses of selenium-nitrogen heterocyclic compounds and their activities have received increasing attentions.Herein, we report the facile preparation of N-benzoyl-N'-aryl selenourea derivatives using potassium selenocyanate.In this typical procedure, Benzoyl chloride 1 was treated with potassium selenocyanate in CH2C12 under the condition of solid-liquid phase transfer catalysis using polyethylene glycal-400 as the catalyst to give the corresponding benzoyl isoselenocyanate 2. This compound did not need to be isolated and reacted with aromatic amine affording the N-benzoyl-N'-aryl selenourea derivatives 3.The reaction is described as:All the experiments were carried out under the condition of solid-liquid phase transfer catalysis using polyethylene glycal-400 as the catalyst and room temperature. And the structure was determined by IR, 1H NMR and 13C NMR. Selected data for N-benzoyl-N'-(4-fluoro)-selenourea:IR(KBr) 3426, 3274, 1672,1234,1155(C=Se); 1HMR(500MHz, DMSO) δ 12.85 (1H,S),11.86(1H,S), 7.27(2H,d,J=2.15), 7.98(2H,s,J=l.15), 7.30(2H,d,J=2.05), 7.56(2H.t,J=6.50),7.67(1H,t,J=6.20); 13C NMR(500MHz, DMSO)δ 181,168(C=Se),135,133, 132,115, 128.3, 128.8,161, 129.     

2-位碲桥联环糊精的制备、表征及其谷胱甘肽过氧化物酶活性的研究

该文以三种母体环糊精(CD),即α-、β-和γ-CD为修饰模板,将功能性基团有机碲引入到环糊精次面的2位羟基上,制备得到了三种具有谷胱甘肽过氧化物酶(GPX)活性的GPX模拟物。采用元素分析、红外光谱、核磁共振等手段对三种环糊精衍生物的结构进行了表征。运用GPX经典双酶体系法测定了三种环糊精衍生物的GPX活性,实验结果表明三者均具有很高的催化活性,其中2-位碲桥联γ-环糊精(2-Te-γ-CD)具有最高的GPX活性,其催化谷胱甘肽(GSH)还原过氧化氢(H2O2),叔丁基过氧化氢(t-BuOOH)和枯烯过氧化氢(CuOOH)的活力分别是传统"小分子硒酶"Ebselen的80.5,333.3和118.3倍。     

Cyclodextrin-derived chalcogenides as glutathione peroxidase mimics and their protection of mitochondria against oxidative damage

A series of novel glutathione peroxidase (GPx) mimics based on organochalcogen cyclodextrin (CD) dimer were synthesized. Their GPx-like antioxidant activities were studied using hydrogen peroxide H₂O₂, tert-butylhydroperoxide (BHP), and cumene hydroperoxide (CHP) as substrates and glutathione as thiol co-substrate. The results showed that 6A,6B-ditelluronic acid-A,6B′-tellurium bridged γ-cyclodextrin (6-diTe-γ-CD) had the highest peroxidase activity, which was ~670-fold higher than ebselen, a well-known GPx mimic. Reduction of lipophilic CHP often proceeded much faster than reduction of the more hydrophilic H₂O₂ or BHP, which cannot bind into the hydrophobic interior of the CD. The biological activities were also evaluated for their capacity to protect mitochondria against ferrous sulfate/ascorbate-induced oxidative damage. 6-diTe-γ-CD was the best inhibitor which significantly suppressed ferrous sulfate/ascorbate-induced cytotoxicity as determined by swelling of mitochondria, lipid peroxidation and cytochrome c oxidase activity. Our data suggests that 6-diTe-γ-CD has potential pharmaceutical application in the treatment of ROS-mediated diseases.     

Gene Manipulation Based Selenium-containing Peptide Exhibiting Synergism of SOD and GPx

In this paper, we constructed a novel bifunctional superoxide dismutase（SOD）/glutathione peroxi- dase（GPx） mimic, a selenium-, copper-containing 35-mer peptide conjugate（Se-Cu-35P） in which a three-amino acid linker（（31y-Asn-Gly） connects the C-terminus of 17-mer polypeptide SOD mimic with the N-terminus of 15-mer po- lypeptide GPx mimic. The SOD and GPx activities of Se-Cu-35P are two orders of magnitude lower than those of natural SOD and GPx, respectively. It provides a GPx activity 56-fold higher than Ebselen（a well-known GPx mimic）. The glutathione（GSH） binding constant is 5.6× 10^2 L.mol 1. Se-Cu-35P synergistically resists against the inactivation by H202 and protects the mitochondria from oxidative damage in a dose dependent manner. These results highlight the challenge of generating an efficient SOD/GPx synergism mimic. It could facilitate the studies of the cooperation of GPx and SOD and could be a potential therapeutic agent for the treatment of ROS-mediated diseases,     

Bicarbonate surfoxidants: micellar oxidations of aryl sulfides with bicarbonate-activated hydrogen peroxide

The mechanism and kinetics of bicarbonate-catalyzed oxidations of sulfides by H(2)O(2) at the aqueous /cationic micellar interface have been investigated. The general term surfoxidant is introduced to describe the combination of an ionic surfactant with a reactive counterion that is itself an oxidant or activates an oxidant from the bulk solution to form an oxidant counterion. It is shown that the new catalytic cationic surfoxidant CTAHCO(3) (cetyltrimethylammonium bicarbonate) significantly enhances the overall oxidation rates as compared to the addition of bicarbonate salts to CTACl and CTABr, for which the halide counterions must undergo equilibrium displacement by the oxidant anion (peroxymonocarbonate, HCO(4)(-)). General equations based on the classic pseudophase model have been derived to account for the preequilibrium reaction in the aqueous and micellar phases, and the resulting model can be used to describe any micellar reaction with associated preequilibria. Rate constants and relevant equilibrium constants for HCO(4)(-) oxidations of aryl sulfides at micellar surfaces have been estimated for CTAHCO(3), CTACl, and CTABr. The second-order rate constants in the Stern layer (k(2)(m)) for sulfide oxidations by HCO(4)(-) are estimated to be approximately 50-fold (PhSEtOH) and approximately 180-fold (PhSEt) greater than the background rate constant k(m)(0) for oxidation by H(2)O(2) at the micellar surface. The estimated values of k(2)(m) are lower than the corresponding values in water by a factor of 20-70 depending on the substrate, but the high local concentration of the bicarbonate activator in the surfoxidant and the local accumulation of substrate as a result of strong binding to the micelle lead to a net increase in the observed reaction rates. Comparisons of CTAHCO(3)-activated peroxide to other highly reactive oxidants such as peroxymonosulfate (HSO(5)(-)) in aqueous surfactant media suggest a wide variety of potential applications for this green oxidant.     

Mimetics of the Selenoenzyme Glutathione Peroxidase: Novel Structures and Unusual Catalytic Mechanisms

Glutathione peroxidase (GPx) mimetics comprise an important class of selenium-containing antioxidants that catalyze the destruction of biologically harmful peroxides in the presence of stoichiometric thiol reductants. The synthesis of two novel cyclic selenium compounds and their evaluation as GPx mimetics was achieved. The first is a cyclic seleninate ester that is formed in situ from the oxidation of allyl 3-hydroxypropyl selenide. The second is a spirodioxyselenurane that is similarly formed from di(3-hydroxypropyl) selenide. Both compounds were shown to be remarkably active catalysts in an assay based on the reduction of t-butyl hydroperoxide with benzyl thiol. The mechanisms of the catalytic cycles of the two novel selenium compounds were elucidated and were found to be distinct from each other and from that of GPx.     

Ephedrine-based diselenide: a promiscuous catalyst suitable to mimic the enzyme glutathione peroxidase (GPx) and to promote enantioselective C-C coupling reactions

The ephedrine-based diselenide appears as a new promiscuous catalyst, able to generate optically active alcohols by addition of organozinc to aldehydes (up to 97% ee), and shows powerful GPx like activity, reducing H(2)O(2) to water in only 16.33 min (eleven times faster than PhSeSePh).     

UV released factors induce antioxidant defense in A375 cells

UV light leads to release of different secretory factors from irradiated cells of which some of them have been characterized. We have reported earlier that cells exposed to the supernatant medium from irradiated cells were resistant to killing by some genotoxic agents. In this study, we present our finding that demonstrates DNA damage induced by UV or H(2)O(2) is lowered on prior exposure to the UV released factors (UVRF). Production of ROS in cells and lipid peroxidation was also lowered. It was found that treatment of unexposed cells with UVRF present in the supernatant medium altered the antioxidant defense activity in cells. Significant was the increase in catalase (CAT) and Cu-Zn superoxide dismutase (SOD) activity, whereas glutathione peroxidase (GPx) and reduced glutathione (GSH) levels remained unaffected. Cells exposed to UVRF prior to UV or H(2)O(2) treatment also experienced such upregulation; however, the remarkable increase in the GPx activity exhibited by these cells was not observed in cells exposed to H(2)O(2) or UV alone. It appears that exposure to UVRF tinkered with antioxidant defense in cells to facilitate its proliferation upon assault by an agent that can produce oxidative damage.     

CO2-Responsive wormlike micelles based on sodium oleate and hydrophobic tertiary amines

We reported two simple and novel CO₂-responsive surfactant wormlike micellar systems consisting of commercial anionic surfactant sodium oleate (NaOA) and common hydrophobic tertiary amine N,N-dimethylcyclohexylamine (DMCHA), N,N-dimethylbenzylamine (DMBA). The conductivity, pH, and rheological measurements demonstrated the CO₂-sensitive flowing behavior and property, which were attributed to the spherical-wormlike micelles transition, verified by cryogenic transmission electron microscopy (Cryo-TEM) and dynamic laser light scattering (DLS) measurements. Moreover, the transition can be easily cycled more than three times without deterioration of viscosity. Combined with the species distribution curve and ¹H NMR spectra, a mechanism of the intermolecular electrostatic interaction and hydrophobic effects was proposed.     

Reversible Ca2+ Switch of An Engineered Allosteric Antioxidant Selenoenzyme

A Ca²⁺‐responsive artificial selenoenzyme was constructed by computational design and engineering of recoverin with the active center of glutathione peroxidase (GPx). By combining the recognition capacity for the glutathione (GSH) substrate and the steric orientation of the catalytic selenium moiety, the engineered selenium‐containing recoverin exhibits high GPx activity for the catalyzed reduction of H₂O₂ by glutathione (GSH). Moreover, the engineered selenoenzyme can be switched on/off by Ca²⁺‐induced allosterism of the protein recoverin. This artificial selenoenzyme also displays excellent antioxidant ability when it was evaluated using a mitochondrial oxidative damage model, showing great potential for controlled catalysis in biomedical applications.     

NMR研究溶液中正十四烷基硫酸钠/正十四烷基聚氧乙烯醚(3)表面活性剂之间的相互作用

采用¹HNMR弛豫、自扩散系数和二维相敏(2DNOESY)实验研究了正十四烷基硫酸钠[n-CH₃(CH₂)₁₃OSO₃Na(STS)]和正十四烷基聚氧乙烯醚(3)[n-CH₃(CH₂)₁₃O(C₂H₄O)₃H(C₁₄E₃)]在溶液中的自聚集以及二者混合后的相互作用.结果表明,STS与C₁₄E₃混合后存在相互作用,并形成混合胶束;弛豫实验表明,混合胶束中STS疏水链质子运动更加受阻,C₁₄E₃的α-(4″)和β-CH₂(3″)处链堆积紧密.C₁₄E₃的亲水端(CH₂CH₂0)₃链卷曲紧贴在疏水壳表面外链堆积较紧密处.自扩散系数测量表明,混合胶束比单一阴离子表面活性剂形成的胶束大.单一非离子型胶束和混合胶束的亲水端(CH₂CH₂0)₃(5″)链构成相应较软和松散的外壳.单一C₁₄E₃在极性溶剂氯仿溶液中,质子运动比在水中自由度大,但2DNOESY谱中出现了少量分子间的交叉峰,也可能形成了一些小的聚集体.     

Dependence of enantioselectivity on the distribution of a chiral hydrogenation catalyst between an aqueous and a micellar phase: investigations using pulsed field gradient spin echo NMR spectroscopy

The enantioselectivity obtained from rhodium complex catalyzed hydrogenations conducted in water can often be increased considerably by the addition of amphiphiles. At present the reasons for this increase in selectivity are not fully understood. The application of pulsed field gradient spin echo NMR (PGSE-NMR) spectroscopy to determine the average diffusion coefficients of the catalysts in both known and novel examples of asymmetric hydrogenation shows definitively that the increase in enantioselectivity is coupled with an aggregation of the catalyst to the micelles. This aggregation or solubilization of the catalyst in the micelles leads to the formation of a new colloidal phase in the aqueous solution. This phase has stronger hydrophobic properties, and thus the hydrogenation is more comparable to those conducted in a hydrophobic or less polar organic solvent. In the case of anionic amphiphiles, which form amphiphilic salts with the cationic catalyst, the embedment of the catalyst complex into the micelle is generally complete. The whole hydrogenation then takes place exclusively inside the micelles, leading to high enantioselectivity. If the catalyst is not completely embedded into the micelle, for example in the cases of nonionic or cationic surfactant solutions, the solubility of the substrate plays an important role. For soluble substrates the hydrogenation of the substrate occurs predominately in the aqueous phase itself, leading to very poor enantioselectivities. In these cases, only the use of a large excess of amphiphile, far above the critical micelle concentration (cmc), will lead to higher enantioselectivities due to a shift of the equilibrium towards the micellar bonded forms of catalyst and substrate. In contrast, poorly soluble substrates exhibit a high tendency to be incorporated into micelles, which leads to much higher enantioselectivities if the cmc of the surfactant is small enough. Changes in the cmc of amphiphiles caused by their aggregation with catalysts could also be estimated. The variation in selectivity observed for the catalysts containing seven-membered, flexible chelate rings is apparently due to changes in their conformation in the less polar micellar medium, and this effect is also seen in organic solvents. As expected, catalysts containing smaller chelate rings show this effect to a considerably lower extent since they are conformationally more rigid.     

Antioxidant activity of peptide-based angiotensin converting enzyme inhibitors

Angiotensin converting enzyme (ACE) inhibitors are important for the treatment of hypertension as they can decrease the formation of vasopressor hormone angiotensin II (Ang II) and elevate the levels of vasodilating hormone bradykinin. It is observed that bradykinin contains a Ser-Pro-Phe motif near the site of hydrolysis. The selenium analogues of captopril represent a novel class of ACE inhibitors as they also exhibit significant antioxidant activity. In this study, several di- and tripeptides containing selenocysteine and cysteine residues at the N-terminal were synthesized. Hydrolysis of angiotensin I (Ang I) to Ang II by ACE was studied in the presence of these peptides. It is observed that the introduction of L-Phe to Sec-Pro and Cys-Pro peptides significantly increases the ACE inhibitory activity. On the other hand, the introduction of L-Val or L-Ala decreases the inhibitory potency of the parent compounds. The presence of an L-Pro moiety in captopril analogues appears to be important for ACE inhibition as the replacement of L-Pro by L-piperidine 2-carboxylic acid decreases the ACE inhibition. The synthetic peptides were also tested for their ability to scavenge peroxynitrite (PN) and to exhibit glutathione peroxidase (GPx)-like activity. All the selenium-containing peptides exhibited good PN-scavenging and GPx activities.