Inhibition of pyruvate dehydrogenase multienzyme complex from Escherichia coli with mono- and bifunctional arsenoxides

Design and Synthesis of Thiourea-Conjugating Organic Arsenic D-Glucose with Anticancer Activities

Organic arsenic compounds such as p-aminophenylarsine oxide (p-APAO) are easier for structural optimization to improve drug-like properties such as pharmacokinetic properties, therapeutic efficacy, and target selectivity. In order to strengthen the selectivity of 4-(1,3,2-dithiarsinan-2-yl) aniline 7 to tumor cell, a thiourea moiety was used to strengthen the anticancer activity. To avoid forming a mixture of α/β anomers, the strategy of 2-acetyl's neighboring group participation was used to lock the configuration of 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate from 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl bromide. 1-(4-(1,3,2-dithiarsinan-2-yl) aniline)-2-N-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranos-1-yl)-thiourea 2 can increase the selectivity of human colon cancer cells HCT-116 (0.82 ± 0.06 μM vs. 1.82 ± 0.07 μM) to human embryonic kidney 293T cells (1.38 ± 0.01 μM vs. 1.22 ± 0.06 μM) from 0.67 to 1.68, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

Structural Modification of Aminophenylarsenoxides Generates Candidates for Leukemia Treatment via Thioredoxin Reductase Inhibition

Upregulation of the selenoprotein thioredoxin reductase (TrxR) is of pathological significance in maintaining tumor phenotypes. Thus, TrxR inhibitors are promising cancer therapeutic agents. We prepared different amino-substituted phenylarsine oxides and evaluated their cytotoxicity and inhibition of TrxR. Compared with our reported p-substituted molecule (8), the o-substituted molecule (10) shows improved efficacy (nearly a fourfold increase) to kill leukemia HL-60 cells. Although the compounds 8 and 10 display similar potency to inhibit the purified TrxR, the o-substitution 10 exhibits higher potency than the p-substitution 8 to inhibit the cellular TrxR activity. Molecular docking results demonstrate the favorable weak interactions of the o-amino group with the TrxR C-terminal active site. Efficient inhibition of TrxR consequently induces the oxidative stress-mediated apoptosis of cancer cells. Silence of the TrxR expression sensitizes the cells to the arsenic compound treatment, further supporting the critical involvement of TrxR in the cellular actions of compound 10.

Carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline as a cytotoxic agent against colorectal cancer

Arsenic trioxide (As₂O₃) has been approved for the treatment of acute promyelocytic leukemia (APL); however, its use in the treatment of solid tumors is limited due to its pharmacokinetic properties. Organic arsenic compounds provide better options for pharmaceutical optimization. p-Aminophenyl arsenoxide (p-APAO), an organic arsenic compound, was found to interact with the promyelocytic leukemia–retinoic acid receptor alpha (PML–RARα) fusion protein in a similar manner to arsenic trioxide. Analogs of p-APAO such as 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) were recently found to show improved cytotoxicity toward several solid tumor cell lines with lower toxicity to normal cells. Here, we synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) and showed that it exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

We synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline. It exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

Challenges in the evaluation of thiol-reactive inhibitors of human protein disulfide Isomerase

This paper addresses how to evaluate the efficacy of the growing inventory of thiol-reactive inhibitors of mammalian protein disulfide Isomerase (PDI) enzymes under realistic concentrations of potentially competing thiol-containing peptides and proteins. For this purpose, we introduce a variant of the widely-used reductase assay by using a commercially-available cysteine derivative (BODIPY FL L-Cystine; BD-SS) that yields a 55-fold increase in fluorescence (excitation/emission; 490/513nm) on scission of the disulfide bond. This plate reader-compatible method detects human PDI down to 5-10nM, can utilize a range of thiol substrates (including 5µM dithiothreitol, 10µM reduced RNase thiols, and 5mM glutathione; GSH), and can operate from pH 6-9.5 in a variety of buffers. PDI assays often employ low micromolar levels of substrates leading to ambiguities when thiol-directed inhibitors are evaluated. The present work utilizes 5mM GSH for both pre-incubation and assay phases to more realistically reflect the high concentration of thiols that an inhibitor would encounter intracellularly. Extracellular PDI faces a much lower concentration of potentially competing thiols; to assess reductase activity under these conditions, the pre-reduced PDI is treated with inhibitor and then fluorescence increase upon reduction of BD-SS is followed in the absence of additional competing thiols. Both assay modes were tested with four mechanistically diverse PDI inhibitors. Two reversible reagents, 3,4-methylenedioxy-β-nitrostyrene (MNS) and the arsenical APAO, were found to be strong inhibitors of PDI in the absence of competing thiols, but were ineffective in the presence of 5mM GSH. A further examination of the nitrostyrene showed that MNS not only forms facile Michael adducts with GSH, but also with the thiols of unfolded proteins (K_d values of 7 and <0.1µM, respectively) suggesting the existence of multiple potential intracellular targets for this membrane-permeant reagent. The inhibition of PDI by the irreversible alkylating agent, the chloroacetamide 16F16, was found to be only modestly attenuated by 5mM GSH. Finally, the thiol-independent flavonoid inhibitor quercetin-3-O-rutinoside was found to show equal efficacy in reoxidation and turnover assay types. This work provides a framework to evaluate inhibitors that may target the CxxC motifs of PDI and addresses some of the complexities in the interpretation of the behavior of thiol-directed reagents in vivo.

p-Azidophenylarsenoxide: An Arsenical "Bait" for the In Situ Capture and Identification of Cellular Arsenic-Binding Proteins

Identification of arsenic-binding proteins is important for understanding arsenic health effects and for developing arsenic-based therapeutics. We report here a strategy for the capture and identification of arsenic-binding proteins in living cells. We designed an azide-labeled arsenical, p-azidophenylarsenoxide (PAzPAO), to serve bio-orthogonal functions: the trivalent arsenical group binds to cellular proteins in situ, and the azide group facilitates click chemistry with dibenzylcyclooctyne. The selective and efficient capture of arsenic-binding proteins enables subsequent enrichment and identification by shotgun proteomics. Applications of the technique are demonstrated using the A549 human lung carcinoma cells and two in vitro model systems. The technique enables the capture and identification of 48 arsenic-binding proteins in A549 cells incubated with PAzPAO. Among the identified proteins are a series of antioxidant proteins (e.g., thioredoxin, peroxiredoxin, peroxide reductase, glutathione reductase, and protein disulfide isomerase) and glyceraldehyde-3-phosphate dehydrogenase. Identification of these functional proteins, along with studies of arsenic binding and enzymatic inhibition, points to these proteins as potential molecular targets that play important roles in arsenic-induced health effects and in cancer treatment.

Arsenic Trioxide Reduces Global Histone H4 Acetylation at Lysine 16 through Direct Binding to Histone Acetyltransferase hMOF in Human Cells

Histone post-translational modification heritably regulates gene expression involved in most cellular biological processes. Experimental studies suggest that alteration of histone modifications affects gene expression by changing chromatin structure, causing various cellular responses to environmental influences. Arsenic (As), a naturally occurring element and environmental pollutant, is an established human carcinogen. Recently, increasing evidence suggests that As-mediated epigenetic mechanisms may be involved in its toxicity and carcinogenicity, but how this occurs is still unclear. Here we present evidence that suggests As-induced global histone H4K16 acetylation (H4K16ac) partly due to the direct physical interaction between As and histone acetyltransferase (HAT) hMOF (human male absent on first) protein, leading to the loss of hMOF HAT activity. Our data show that decreased global H4K16ac and increased deacetyltransferase HDAC4 expression occurred in arsenic trioxide (As₂O₃)-exposed HeLa or HEK293T cells. However, depletion of HDAC4 did not affect global H4K16ac, and it could not raise H4K16ac in cells exposed to As₂O₃, suggesting that HDAC4 might not directly be involved in histone H4K16 de-acetylation. Using As-immobilized agarose, we confirmed that As binds directly to hMOF, and that this interaction was competitively inhibited by free As₂O₃. Also, the direct interaction of As and C2CH zinc finger peptide was verified by MAIDI-TOF mass and UV absorption. In an in vitro HAT assay, As₂O₃ directly inhibited hMOF activity. hMOF over-expression not only increased resistance to As and caused less toxicity, but also effectively reversed reduced H4K16ac caused by As exposure. These data suggest a theoretical basis for elucidating the mechanism of As toxicity.

Multivalency in the inhibition of oxidative protein folding by arsenic(III) species

The renewed use of arsenicals as chemotherapeutics has rekindled interest in the biochemistry of As(III) species. In this work, simple bis- and tris-arsenical derivatives were synthesized with the aim of exploiting the chelate effect in the inhibition of thiol-disulfide oxidoreductases (here, Quiescin sulfhydryl oxidase, QSOX, and protein disulfide isomerase, PDI) that utilize two or more CxxC motifs in the catalysis of oxidative protein folding. Coupling 4-aminophenylarsenoxide (APAO) to acid chloride or anhydride derivatives yielded two bis-arsenical prototypes, BA-1 and BA-2, and a tris-arsenical, TA-1. Unlike the monoarsenical, APAO, these new reagents proved to be strong inhibitors of oxidative protein folding in the presence of a realistic intracellular concentration of competing monothiol (here, 5 mM reduced glutathione, GSH). However, this inhibition does not reflect direct inactivation of QSOX or PDI, but avid binding of MVAs to the reduced unfolded protein substrates themselves. Titrations of reduced riboflavin-binding protein with MVAs show that all 18 protein −SH groups can be captured by these arsenicals. With reduced RNase, addition of substoichiometric levels of MVAs is accompanied by the formation of Congo Red- and Thioflavin T-positive fibrillar aggregates. Even with K_d values of ∼50 nM, MVAs are ineffective inhibitors of PDI in the presence of millimolar levels of competing GSH. These results underscore the difficulties of designing effective and specific arsenical inhibitors for folded enzymes and proteins. Some of the cellular effects of arsenicals likely reflect their propensity to associate very tightly and nonspecifically to conformationally mobile cysteine-rich regions of proteins, thereby interfering with folding and/or function.

Antibacterial evaluation of novel organoarsenic compounds by the microcalorimetric method

Antibacterial activities of novel organoarsenic compounds As(III)-containing Schiff bases on Escherichia coli (CCTCCAB91112) were investigated by microcalorimetry in this study. The experimental result showed that the arsenic(III)-containing Schiff bases at micromolar concentration exhibit strong inhibition on the E. coli. Specifically, the growth rate constant k decreased, and the generation time t G and the inhibitory ratio I (percentage) increased with the increased dose of the arsenicals as inhibitors. All of the arsenicals display the feature of considerable lag phase inhibition on the cell growth. The compound 4-(4-bromobenzaliminyl)phenylarsenoxide makes the lag phase of E. coli cell growth cycles to reach 650 min at 5 μmol/L. The compounds with donating electron groups at aromatic ring B have lower IC50 to present higher antibacterial activity. The compound 4-(4-hydroxyl-3-methoxylbenzaliminyl)phenylarsenoxide has the lowest IC50 (1.82 μmol/L) to show the strongest antibacterial activity among them.

α-(Substituted-phenoxyacetoxy)-α-heterocyclylmethylphosphonates: synthesis, herbicidal activity, inhibition on pyruvate dehydrogenase complex (PDHc), and application as postemergent herbicide against broadleaf weeds

Pyruvate dehydrogenase complex (PDHc) is the site of action of a new class of herbicides. On the basis of the previous work for O,O'-dimethyl α-(substituted-phenoxyacetoxy)alkylphosphonates (I), further synthetic modifications were made by introducing a fural and a thienyl group to structure I. A series of α-(substituted-phenoxyacetoxy)-α-heterocyclylmethylphosphonate derivatives (II) were synthesized as potential inhibitors of PDHc. The postemergent activity of the title compounds II was evaluated in greenhouse experiments. The in vitro efficacy of II against PDHc was also examined. Compounds II with fural as R(3) and 2,4-dichloro as X and Y showed significant herbicidal activity and effective inhibition against PDHc from plants. O,O'-Dimethyl α-(2,4-dichlorophenoxyacetoxy)-α-(furan-2-yl)methylphosphonate II-17 had higher inhibitory potency against PDHc from Pisum sativum than against PDHc from Oryza sativa in vitro and was most effective against broadleaf weeds at 50 and 300 ai g/ha. II-17 was safe for maize and rice even at the dose of 900-1200 ai g/ha. Field trials at different regions in China showed that II-17 (HWS) could control a broad spectrum of broad-leaved and sedge weeds at the rate of 225-375 ai g/ha for postemergent applications in maize fields. II-17 (HWS) displayed potential utility as a selective herbicide.

Effect of glucosamine and chitooligomer on the toxicity of arsenite against Escherichia coli

Escherichia coli was selected as the sample to study the toxicity of arsenite in the presence of saccharides. The effect of glucosamine, N-acetylglucosamine, glucose, lactose, sucrose, glucosamine and cyclodextrin on the toxicity of arsenite against E. coli was investigated by microcalorimetry. The glucosamine and the tested chitooligomer decreased the toxicity of arsenite on cells of E. coli, and the effect of glucosamine was stronger than that of the chitooligomer. These results suggest that the glucosamine and chitooligomer may be employed as the assistant antidote for arsenite.

Cysteine-3 and cysteine-4 are essential for the thioredoxin-like oxidoreductase and antioxidant activities of Plasmodium falciparum macrophage migration inhibitory factor

Plasmodium falciparum macrophage migration inhibitory factor (PfMIF) exhibits thioredoxin (Trx)-like oxidoreductase activity but the active site for this activity and its function have not been evaluated. A bioinformatics search revealed that the conserved CXXC motif, which is responsible for Trx-like oxidoreductase activity, is absent from PfMIF. In contrast, the adjacent N-terminal Cys-3 and Cys-4 are conserved in MIF across species of malarial parasites. Mutation of either vicinal Cys-3 or Cys-4 of PfMIF abolished the Trx-like activity, whereas the mutation of the remaining Cys-59 or Cys-103 did not affect it. PfMIF has an antioxidant function. It prevents reactive oxygen species-mediated lipid peroxidation and oxidative damage of DNA as evident from DNA nicking assay. Interestingly, chemical modification of the vicinal cysteines by phenylarsine oxide (PAO), a specific vicinal thiol modifier, significantly prevented this antioxidant activity. Modification of Cys-3 and Cys-4 was confirmed by MALDI-TOF mass spectroscopy of peptide fragments obtained after cyanogen bromide digestion of PAO-modified PfMIF. Furthermore, mutation of either Cys-3 or Cys-4 of PfMIF resulted in the loss of both Trx-like oxidoreductase and antioxidant activities of PfMIF. Altogether, our results suggest that the vicinal Cys-3 and Cys-4 play a critical role in the Trx-like oxidoreductase activity and antioxidant property of PfMIF.

Inhibition by methylated organo-arsenicals of the respiratory 2-oxo-acid dehydrogenases

Inorganic arsenic that is ingested through drinking water or inhalation is metabolized by biological methylation pathways into organoarsenical metabolites. It is now becoming understood that this metabolism that was formerly considered to be detoxification may contribute as much or more to increasing the toxicity of arsenic. One proposed mode of the toxic action of arsenic and its organoarsenic metabolites is through its binding to proteins and inactivating their enzymatic activity. The classic case has been considered the affinity of the proximal 1,3 sulfhydryl groups of the lipoic acid cofactor of the pyruvate dehydrogenase complex for arsenic. A 2:1 stoichiometry of sulfhydryl to arsenic groups has been measured in proteins and arsenical complexes can be synthesized using free D,L-lipoic acid. The relative importance of this site for arsenic binding has come in to question through the use of methylating bifunctional arsenic complexes that suggested the methylation of an active site histidine may also be important, and the suggestion that arsenic inhibits the pyruvate dehydrogenase complex indirectly by elevating mitochondrial hydrogen peroxide generation. In order to separate the effects of direct trivalent arsenite toxicity from that of hydrogen peroxide and activated oxygen, we studied the inhibition of the PDH complex under conditions that did not generate hydrogen peroxide but did expose the lipoic acid group in its reduced state to arsenicals. We also studied the effects of arsenicals in the inhibition of the α-ketoglutarate dehydrogenase complex. We found that only trivalent arsenical compounds inhibited the activity of both dehydrogenase complexes and only when the lipoic acid was in its reduced form. Arsenite inhibited both enzyme complexes approximately equivalently while monomethylarsenite inhibited the PDH complex to a greater extent than the KGDH complex - although both complexes were very sensitive to inhibition by this complex. Dimethylarsenite inhibition of both complexes was only observed with longer pre-incubation periods. Cumulative inhibition by the reduced arsenical was observed for all complexes indicating a binding mode of inhibition that is dependent upon lipoic acid being in its reduced state.

An improved synthesis of arsenic-biotin conjugates

An amide linked conjugate of p-aminophenylarsine oxide and biotin is conveniently prepared in a one-pot procedure by the reaction of biotinyl chloride, formed in situ, with p-aminophenyldichloroarsine. The reaction of the arsine oxide-biotin conjugate with 1,2-ethanedithiol produces the stabilized dithiarsolane. These reagents are now readily available for a variety of applications.

Identification of arsenic-binding proteins in human breast cancer cells

As a cancer chemotherapeutic drug, arsenic acts on numerous intracellular signal transduction pathways in cancer cells. However, its mechanism of actions is still not fully understood. Previous studies suggest that arsenic reacts with closely spaced cysteine (Cys) residues of proteins with high Cys content and accessible sulfhydryl (SH) groups. In this study, human breast cancer cell line MCF-7 was examined as a cellular model to explore arsenic-binding proteins and the mechanism of binding. An arsenic-biotin conjugate was synthesized by coupling the pentafluorophenol ester of biotin with p-aminophenylarsenoxide. Arsenic-binding proteins were eluted with streptavidin resin from arsenic-biotin treated MCF-7 cells, separated by polyacrylamide gel electrophoresis, and identified by matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). Arsenic-binding properties of two of these proteins, beta-tubulin and pyruvate kinase M2 (PKM2), were studied further in vitro and the biological consequences of this binding was evaluated. Binding assay with Western blotting confirmed binding of beta-tubulin and PKM2 by arsenic in a concentration-dependent manner. Arsenic binding inhibited tubulin polymerization, but surprisingly had no effect on PKM2 activity. Molecular modeling showed that binding of Cys(12) alone or vicinal Cys residues (Cys(12) and Cys(213)) of beta-tubulin by arsenic blocked the active site for access of GTP, which is necessary for tubulin polymerization. On the contrary, all Cys residues of PKM2 were far away from the active site of the enzyme. In summary, this study confirmed beta-tubulin and PKM2 as arsenic-binding proteins in MCF-7 cells. Functional consequence of such binding may depend on whether arsenic binding causes conformational changes or blocks active sites of target proteins.

Effects of As(III) Binding on β-Hairpin Structure

While arsenic(III) compounds can exert profound toxicological and pharmacological effects, their modes of action and, in particular, the structural consequences of their binding to cysteinyl side chains in proteins, remain poorly understood. To gain an understanding of how arsenic binding influences beta-structure, pairs of cysteines were introduced into a model monomeric beta-hairpin to yield a family of peptides such that coordination occurs either across the strands or within the same strand of the beta-hairpin. Circular dichroism, NMR, UV-vis spectroscopy, and rapid-reaction studies were used to characterize the binding of monomethylarsonous acid or p-succinylamidephenyl arsenoxide (PSAO) to these peptides. Placement of cysteines at non-hydrogen bond (NHB) positions across the beta-hairpin, such that they occupy the same face of the sheet, was found to enhance the structure as assessed by CD. Cross-strand cysteine residues that project on opposite faces close to the termini of the hairpin can still bind arsenic tightly and show modestly increased beta-sheet content. NMR and modeling studies suggest that arsenic can be accommodated at this locus without disrupting the core interactions stabilizing the turn. However, As(III) binding to nonopposed cysteines, or to cysteines at HB and NHB positions along one strand of the hairpin, caused loss of structure. UV-vis titrations show that all these hairpin peptides bind PSAO stoichiometrically with K(d) values from 13 to 106 nM. Further, binding is moderately rapid, with second-order rate constants for association of 10,000-22,000 M(-1) s(-)1 irrespective of the placement of the cysteines within the hairpin and the consequent extent of structural reorganization required as a result of binding. These studies complement recent work with alpha-helices and further demonstrate that capture of a pair of thiols by As(III) may result in significant changes in local secondary structure in the protein targets of these potent bioactive agents.

Optimized spectrophotometric assay for the completely activated pyruvate dehydrogenase complex in fibroblasts

BACKGROUND

Analysis of the pyruvate dehydrogenase complex (PDHc) activity in human skin fibroblasts is hampered by low enzyme activity in the cells. The most commonly used radiochemical method detects the formation of (14)CO(2), an endproduct of the E1 component of PDHc, from [1-(14)C]pyruvate.

METHODS

We report a spectrophotometric method for the analysis of PDHc activity in fibroblasts based on detection of NADH formation via a p-iodonitrotetrazolium violet (INT)-coupled system. We investigated in detail the specific requirements of this assay, such as cofactor requirements and the effects of suggested stimulatory compounds and different cell disruption procedures. The reliability of the optimized assay was studied by investigation of patients previously diagnosed with PDHc deficiency and by comparison with results from the radiochemical method.

RESULTS

Mean (SD) total PDHc activities were 136 (31) and 58 (21) mU/U of citrate synthase in fibroblast homogenates from 10 healthy volunteers and 7 PDHc-deficient patients, respectively, by the spectrophotometric assay. Similar results were obtained in a mitochondrial fraction. Dithiothreitol (DTT) increased the nonspecific inhibitor-insensitive rate with less pronounced effect on the specific rate of PDHc activity. Administration of DTT increased PDHc activity to 193 (3)% of control activity (without DTT), but decreased the inhibitor-sensitive rate from 99 (0.3)% (without DTT) to 69 (2)% (with 0.3 mmol/L DTT).

CONCLUSION

The simple, optimized spectrophotometric assay for PDHc analysis allows reliable investigation of the enzyme complex in human skin fibroblasts.

Effects of As(III) binding on alpha-helical structure

As(III) displays a wide range of effects in cellular chemistry. Surprisingly, the structural consequences of arsenic binding to peptides and proteins are poorly understood. This study utilizes model alpha-helical peptides containing two cysteine (Cys) residues in various sequential arrangements and spatial locations to study the structural effects of arsenic binding. With i, and i + 1, i + 2, or i + 3 arrangements, CD spectroscopy shows that As(III) coordination causes helical destabilization when Cys residues are located at central or C-terminal regions of the helix. Interestingly, arsenic binding to i, i + 3 positions results in the elimination of helical structure and the formation of a relatively stable alternate fold. In contrast, helical stabilization is observed for peptides containing i, i + 4 Cys residues, with corresponding pseudo pairwise interaction energies (Delta G(pw) degrees) of -1.0 and -0.7 kcal/mol for C-terminal and central placements, respectively. Binding affinities and association rate constants show that As(III) binding is comparatively insensitive to the location of the Cys residues within these moderately stable helices. These data demonstrate that As(III) binding can be a significant modulator of helical secondary structure.

Inhibitors of protein-disulfide isomerase prevent cleavage of disulfide bonds in receptor-bound glycoprotein 120 and prevent HIV-1 entry

We previously reported that monoclonal antibodies to protein-disulfide isomerase (PDI) and other membrane-impermeant PDI inhibitors prevented HIV-1 infection. PDI is present at the surface of HIV-1 target cells and reduces disulfide bonds in a model peptide attached to the cell membrane. Here we show that soluble PDI cleaves disulfide bonds in recombinant envelope glycoprotein gp120 and that gp120 bound to the surface receptor CD4 undergoes a disulfide reduction that is prevented by PDI inhibitors. Concentrations of inhibitors that prevent this reduction and inhibit the cleavage of surface-bound disulfide conjugate prevent infection at the level of HIV-1 entry. The entry of HIV-1 strains differing in their coreceptor specificities is similarly inhibited, and so is the reduction of gp120 bound to CD4 of coreceptor-negative cells. PDI inhibitors also prevent HIV envelope-mediated cell-cell fusion but have no effect on the entry of HIV-1 pseudo-typed with murine leukemia virus envelope. Importantly, PDI coprecipitates with both soluble and cellular CD4. We propose that a PDI.CD4 association at the cell surface enables PDI to reach CD4-bound virus and to reduce disulfide bonds present in the domain of gp120 that binds to CD4. Conformational changes resulting from the opening of gp120-disulfide loops may drive the processes of virus-cell and cell-cell fusion. The biochemical events described identify new potential targets for anti-HIV agents.

Effects of P-Aminophenyl Dichloroarsine on Reduced High-affinity [3H]Nicotine Binding Sites from Chick Brain: A Covalent, Yet Reversible, Agent for Neuronal Nicotinic Receptors

Neuronal nicotinic acetylcholine receptor (nAChR) alpha-subunits contain a conserved disulphide that is essential for function. Here, we have examined the effects of sulphydryl redox reagents on [3H]nicotine binding to chick brain nAChR immunoisolated with the monoclonal antibody mAb35. The disulphide reducing agent, dithiothreitol (DTT), inhibited [3H]nicotine binding [50% inhibitory concentration (IC50)=146 microM] but this effect was reversed (93 +/- 1.5%) by subsequent reoxidation with 1 mM dithio-bis(nitrobenzoic acid) (DTNB). The trivalent arsenical, p-aminophenyl dichloroarsine (APA), which reacts with pairs of spatially close sulphydryls, was a potent inhibitor of reoxidation by DTNB (IC50=35 nM). However, application of the 'anti-arsenical', 2,3-dimercaptopropane sulphonic acid (DMPS), restored agonist binding after APA treatment (50% effective concentration=120 microM). Paradoxically, DMPS was also found to be a potent oxidizing agent of these receptors. Affinity alkylation of reduced nAChRs with bromoacetylcholine (BAC; 100 microM) irreversibly blocked nicotine binding (>90%). We propose (but have not proven) that APA interacts with the cysteines homologous to Cys192 - 193 in Torpedo AChRs, since APA pretreatment of reduced neuronal receptors protected against irreversible BAC alkylation, as shown by subsequent reversal of DMPS (2 mM; 20 min). This study illustrates the potent and reversible nature of the arsenical's covalent interaction with an isolated nAChR and suggests that modified arsenicals could be useful nAChR probes.

Transplasma membrane electron transport in Leishmania donovani promastigotes

Leishmania donovani promastigotes are capable of reducing certain electron acceptors with redox potential at pH 7 down to -125 mV; outside the plasma membrane promastigotes can reduce ferricyanide. Ferricyanide has been used as an artificial electron acceptor probe for studying the mechanism of transplasma membrane electron transport. Transmembrane ferricyanide reduction by L. donovani promastigotes was not inhibited by such mitochondrial inhibitors as antimycin A or cyanide, but it responded to inhibitors of glycolysis. Transmembrane ferricyanide reduction by Leishmania appears to involve a plasma membrane electron transport chain dissimilar to that of hepatocyte cells. As with other cells, transmembrane electron transport is associated with proton release, which may be involved in internal pH regulation. The Leishmania transmembrane redox system differs from that of mammalian cells in being 4-fold less sensitive to chloroquine and 12-fold more sensitive to niclosamide. Sensitivities to these drugs suggest that transplasma membrane electron transport and associated proton pumping may be targets for the drugs used against leishmaniasis.

Evidence for the extracellular reduction of alpha-lipoic acid by Leishmania donovani promastigotes: a transplasma membrane redox system

Leishmania donovani cells, capable of reducing certain electron acceptors with redox potentials at pH 7.0 down to -290 mV, outside the plasma membrane, can reduce the oxidised form of alpha-lipoic acid. alpha-Lipoic acid has been used as natural electron acceptor probe for studying the mechanism of transplasma membrane electron transport. Transmembrane alpha-lipoic acid reduction by Leishmania was not inhibited by mitochondrial inhibitors as azide, cyanide, rotenone or antimycin A, but responded to hemin, modifiers of sulphhydryl groups and inhibitor of glycolysis. The protonophores carbonyl cyanide chlorophenylhydrazone and 2,4-dinitrophenol showed inhibition of alpha-lipoic acid reduction. This transmembrane redox system differs from that of mammalian cells in respect to its sensitivity of UV irradiation and stimulation by diphenylamine. Thus a naphthoquinone coenzyme appears to be involved in alpha-lipoic acid reduction by Leishmania cells.

Presence of closely spaced protein thiols on the surface of mammalian cells

It has been proposed that certain cell-surface proteins undergo redox reactions, that is, transfer of hydrogens and electrons between closely spaced cysteine thiols that can lead to reduction, formation, or interchange of disulfide bonds. This concept was tested using a membrane-impermeable trivalent arsenical to identify closely spaced thiols in cell-surface proteins. We attached the trivalent arsenical, phenylarsenoxide, to the thiol of reduced glutathione to produce 4-(N-(S-glutathionylacetyl)amino)phenylarsenoxide (GSAO). GSAO bound tightly to synthetic, peptide, and protein dithiols like thioredoxin, but not to monothiols. To identify cell-surface proteins that contain closely spaced thiols, we attached a biotin moiety through a spacer arm to the primary amino group of the gamma-glutamyl residue of GSAO (GSAO-B). Incorporation of GSAO-B into proteins was assessed by measuring the biotin using streptavidin-peroxidase. Up to 12 distinct proteins were labeled with GSAO-B on the surface of endothelial and fibrosarcoma cells. The pattern of labeled proteins differed between the different cell types. Protein disulfide isomerase was one of the proteins on the endothelial and fibrosarcoma cell surface that incorporated GSAO-B. These findings demonstrate that the cell-surface environment can support the existence of closely spaced protein thiols and suggest that at least some of these thiols are redox active.

Regulation of the reduced-folate transporter by nitric oxide in cultured human retinal pigment epithelial cells

The regulation of the reduced-folate transporter (RFT) by nitric oxide (NO) was analyzed in human retinal pigment epithelial (HRPE) cells. NO inhibited specifically and reversibly the uptake of N5-methyltetrahydrofolate by a cGMP-independent mechanism. The inhibition was associated with a decrease in substrate affinity. The NO-induced inhibition was prevented by antioxidants and NO scavengers. Agents capable of modifying thiol groups in proteins inhibited RFT, indicating that the likely mechanism of NO-induced inhibition is via modification of essential thiol groups in this protein. These studies suggest that NO produced during retinal disease may affect the function of RFT in adjacent RPE cells.

Selective bridging of bis-cysteinyl residues by arsonous acid derivatives as an approach to the characterization of protein tertiary structures and folding pathways by mass spectrometry

Bis-cysteine selective modifications were successfully applied with melarsen oxide (MEL), an arsonous acid derivative, for tertiary structural studies of peptides and a model protein. The arsonous acid modified peptides and proteins were amenable to direct characterizations by mass spectrometry, e.g., direct molecular weight determinations and mass spectrometric peptide mapping that identified stoichiometry and sites of modification, respectively. Proteolytic digestion and mass spectrometric fragmentation of modified oxytocin showed that MEL-bridged peptide derivatives are structural homologues to the disulfide-bonded macrocyclic peptides. Mass spectrometric analyses determined the MEL modification site in partially reduced and selectively modified bovine pancreatic trypsin inhibitor (BPTI) bridging Cys-14 and Cys-38. The BPTI.MEL derivative was resistant to proteolysis by both Lys-C and trypsin and thus represented a rigid structure like native BPTI. MEL exhibited several advantageous features such as (i) cross-linking two closely spaced thiol groups, providing detailed tertiary structure information; (ii) high solubility as monomeric ortho acid in aqueous and organic solutions; (iii) adding a relatively large mass increment to proteins upon single modification; (iv) enabling UV monitoring of the derivatization due to a strong chromophor; and (v) performing fast and specific modifications of bis-thiol groups in proteins to form stable structures without any side reactions even with a high molar excess of MEL. The investigated physical and chemical properties of MEL suggest general applicability for selective bis-thiol modifications, enabling protein structure-function studies in both soluble and membrane proteins and the study of protein-folding reactions.

Arsenic toxicity is enzyme specific and its affects on ligation are not caused by the direct inhibition of DNA repair enzymes

The molecular mechanism of arsenic toxicity is believed to be due to the ability of arsenite [As(III)] to bind protein thiols. Numerous studies have shown that arsenic is cytotoxic at micromolar concentrations. Micromolar As can also induce chromosomal damage and inhibit DNA repair. The mechanism of arsenic-induced genotoxicity is very important because arsenic is a human carcinogen, but not a mutagen, and there is a need to establish recommendations for safe levels of As in the environment. We have measured the dose-response for arsenic inhibition of several purified human DNA repair enzymes, including DNA polymerase beta, DNA ligase I and DNA ligase III and have found that most enzymes, even those with critical SH groups, are very insensitive to As. Many repair enzymes are activated by millimolar concentrations of As(III) and/or As(V). Only pyruvate dehydrogenase, one of eight purified enzymes examined so far, is inhibited by micromolar arsenic. In contrast to the purified enzymes, treatment of human cells in culture with micromolar arsenic produces a significant dose-dependent decrease in DNA ligase activity in nuclear extracts from the treated cells. However, the ligase activity in extracts from untreated cells is no more sensitive to arsenic than the purified enzymes. Our results show that direct enzyme inhibition is not a common toxic effect of As and that only a few sensitive enzymes are responsible for arsenic-induced cellular toxicity. Thus, arsenic-induced co-mutagenesis and inhibition of DNA repair is probably not the result of direct enzyme inhibition, but may be an indirect effect caused by As-induced changes in cellular redox levels or alterations in signal transduction pathways and consequent changes in gene expression.

Regulation of the thyroid NADPH-dependent H2O2 generator by Ca2+: studies with phenylarsine oxide in thyroid plasma membrane

Pig thyroid plasma membranes contain a Ca(2+)-dependent NADPH:O2 oxidoreductase, the thyroid NADPH-dependent H2O2 generator. This provided the H2O2 for the peroxidase-catalysed synthesis of thyroid hormones. The effect of the tervalent arsenical, phenylarsine oxide (PAO), on the NADPH oxidase was studied. PAO caused two directly related dose-dependent effects with similar half-effect concentrations of PAO (3 nmol of PAO/mg of protein): (i) partial inactivation of H2O2 formation by the Ca(2+)-stimulated enzyme, and (ii) desensitization of the enzyme activity to Ca2+. PAO had no effect on membranes that had been Ca(2+)-desensitized by alpha-chymotrypsin treatment. The NADPH oxidase in membranes treated with excess PAO had the same Vmax with and without Ca2+. This value was half the Vmax of the native enzyme. However, the K(m) for NADPH determined with Ca2+ (18 microM, identical with that of the native enzyme) was approx, one-third of the K(m) measured without Ca2+, showing the direct action of Ca2+ on the PAO-enzyme complex. PAO had the same effects, partial inactivation and Ca2+ desensitization, on the NADPH: ferricyanide oxidoreductase activity of the NADPH oxidase, suggesting that PAO acts on the flavodehydrogenase entity of the enzyme. Both partial inactivation and Ca2+ desensitization were completely and specifically reversed by 2.3-dimercaptopropanol, partly reversed by dithiothreitol and not reversed by 2-mercaptoethanol, indicating that PAO binds to vicinal thiol groups. These results suggest that thiol groups are involved in the control of thyroid NADPH oxidase by Ca2+; PAO bound to vicinal thiols might alter the structure of the enzyme so that electron transfer occurs without Ca2+ but more slowly.

Effect of substitutions in the thiamin diphosphate-magnesium fold on the activation of the pyruvate dehydrogenase complex from Escherichia coli by cofactors and substrate

The homotropic regulation of the Escherichia coli pyruvate dehydrogenase multienzyme complex (PDHc) by its coenzyme thiamin diphosphate and its substrate pyruvate was re-examined with complexes containing three and one lipoyl domains per E2 chain, and several variants of the latter, containing substitutions in the putative thiamin diphosphate fold of E1 (G231A, G231S, C259S, C259N, and N258Q). It was found that all of the E1 variants had significantly reduced specific activities, as reported elsewhere (Russell, G. C., Machado, R. S., and Guest, J. R. (1992) Biochem. J. 287, 611-619). In addition, extensive kinetic studies were performed in an attempt to determine the effects of the amino acid substitutions on the Hill coefficients with respect to thiamin diphosphate and pyruvate. All but one of the variants were incapable of being saturated with thiamin diphosphate, even at concentrations > 5 mM. Most importantly, the striking activation lag phase lasting for many seconds in the parental complexes containing three and one lipoyl domains per E2 chain was totally eliminated in the variants. Furthermore, activation by the coenzyme was localized to the E1 subunit, because resolved E1 exhibits virtually the same behavior during the activation lag phase as does the complex. In the parental complexes two distinct lag phases could be resolved, the duration of both decreases with increasing ThDP concentration. A mechanism that is consistent with all of the kinetic data on the parental complexes involves rapid equilibration of the first ThDP with the E1 dimer, followed by a slow conformational equilibration, that in turn is followed by slow addition of the second ThDP to form the fully activated dimer. When the diphosphate site is badly impaired, the binding affinity is very much reduced, this perhaps eliminates the slow step leading to the activated dimer form of the E1.

A library of monoclonal antibodies to Escherichia coli K-12 pyruvate dehydrogenase complex. A biochemical analysis and their ability to inhibit the enzyme complex

A library of monoclonal antibodies to K-12 Escherichia coli pyruvate dehydrogenase complex (PDHc) and its pyruvate decarboxylating (EC 1.2.4.1; E1) subunit is reported. 21 monoclonal antibodies were generated, and 20 were investigated, of which 9 were elicited to PDHc and 11 to pure E1 subunit; 19 were of the IgG1 isotype and one of the IgG3 isotype. According to an enzyme immunoassay, all 20 of the monoclonal antibodies bound the PDHc, and 17 bound the E1 subunit. According to Western blot analysis, 14 of the 19 monoclonal antibodies bound to the E1 subunit. The monoclonal antibodies inhibited PDHc from 0 to > 98%. The six monoclonal antibodies that displayed greater than 30% inhibition of E. coli PDHc were unable to inhibit porcine heart PDHc nor did they bind porcine heart PDHc according to dot blot analysis. Radiolabeling gave binding constants ranging from 5 to 10 x 10(8) M-1 on these six monoclonal antibodies, with greater than 80% of maximal inhibition achieved in less than 1 min. One of the six, 18A9, gave > 98% inhibition, required two antibodies/E1 subunit for maximum inhibition, and was shown to be a non-competitive inhibitor. Monoclonal antibody 15A9 was shown to counteract GTP-induced inhibition, while 1F2 influenced the conformation of E1, allowing two antibodies, which did not previously bind E1, to bind to it. A new mechanism-based kinetic assay is presented that is specific for the E1 component of 2-keto acid dehydrogenases. This assay confirmed that the three most strongly inhibitory monoclonal antibodies specifically inhibited the E1 function while antibody 1F2 led to enhanced activity, suggesting an induced conformational change in PDHc or in E1.

Interaction of p-aminophenyldichloroarsine, an arsenical with specificity for vicinal cysteines, with [3H]cytisine binding sites in rat brain membranes

The arsenical compound p-aminophenyldichloroarsine (APA) is selective for spatially close thiols with which it forms a stable complex. The alpha subunits of nicotinic acetylcholine receptors are defined by the presence of a pair of adjacent cysteines close to the agonist binding site. Here the interaction of APA with [3H]cytisine binding sites, which correspond to the major subtype of nicotinic receptors in rat brain has been examined. Incubation of brain membranes with 10 microM APA abolished [3H]cytisine binding. The action of APA was dependent on prior reduction of sulphydryls with dithiothreitol. APA effects could not be reversed by oxidizing agents but could be reversed by the antiarsenical reagent 2,3-dimercapto-1-propane sulphonic acid. Under the conditions used, the concentration of APA producing a half-maximal decrease in binding was 130 nM. The loss of [3H]cytisine binding was due to a decrease in the number of binding sites (Bmax) with no effect on affinity for the radioligand (Kd). Nicotinic ligands failed to protect against the reduction and arsenylation of neuronal receptor sites. These observations are consistent with the potent interaction of APA with this neuronal nicotinic receptor.

A reactive nucleophile proximal to vicinal thiols is an evolutionarily conserved feature in the mechanism of Arg aminoacyl-tRNA protein transferase

Aminoacyl-tRNA protein transferases post-translationally aminoacylate protein N-termini. At least in part, these enzymes function to allow a subset of cellular proteins to be targeted for protein degradation. A eukaryotic enzyme of this class, Arg aminoacyl-tRNA protein transferase, arginylates N-terminal Glu or Asp residues of proteins, allowing such proteins to be recognized by a specific ubiquitin-protein ligase. We showed previously that inorganic arsenite, a reagent expected to bind specifically to protein vicinal thiol groups, inhibited Arg aminoacyl-tRNA transferase activity in rabbit reticulocyte lysate (N. S. Klemperer and C. M. Pickart, 1989, J. Biol. Chem. 264, 19245-19252). We now report that a bifunctional arsenoxide reagent, p-[(bromoacetyl)-amino]phenylarsenoxide, is a potent and irreversible inactivator of the same enzyme (K0.5 = 11.5 microM). Bromoacetyl aniline, which lacks the arsenoxide moiety, has no effect. These results show that the transferase has a reactive nucleophile proximal to the site which binds arsenoxides. The related monofunctional arsenoxide reagent, p-aminophenylarsenoxide, is a reversible inhibitor whose potency (K0.5 = 7.7 microM) is 20-fold greater than that of inorganic arsenite. As expected for a mechanism in which p-aminophenylarsenoxide binds to vicinal thiol groups: (i) pretreatment of reticulocyte lysate with a thiol-blocking reagent prevents binding of the transferase to a phenylarsenoxide-Sepharose column; and (ii) inhibition by p-aminophenylarsenoxide is reversed by a competing chemical dithiol, but not by a monothiol reagent. Like the rabbit enzyme, Arg aminoacyl-tRNA protein transferase from the yeast Saccharomyces cerevisiae (expressed in Escherichia coli) is reversibly inhibited by the monofunctional phenylarsenoxide and irreversibly inactivated by the bifunctional phenylarsenoxide (but not by bromoacetylaniline). Thus, a reactive nucleophile proximal to vicinal thiol groups is a conserved feature of the activity of the transferase. We speculate that these groups are catalytic elements in the transferase active site.

Characterization and chemical modification of the Na(+)-dependent bile-acid transport system in brush-border membrane vesicles from rabbit ileum

The Na(+)-dependent uptake system for bile acids in the ileum from rabbit small intestine was characterized using brush-border membrane vesicles. The uptake of [3H]taurocholate into vesicles prepared from the terminal ileum showed an overshoot uptake in the presence of an inwardly-directed Na(+)-gradient ([Na+]out > [Na+]in), in contrast to vesicles prepared from the jejunum. The Na(+)-dependent [3H]taurocholate uptake was cis-inhibited by natural bile acid derivatives, whereas cholephilic organic compounds, such as phalloidin, bromosulphophthalein, bilirubin, indocyanine green or DIDS - all interfering with hepatic bile-acid uptake - did not show a significant inhibitory effect. Photoaffinity labeling of ileal membrane vesicles with 3,3-azo- and 7,7-azo-derivatives of taurocholate resulted in specific labeling of a membrane polypeptide with apparent molecular mass 90 kDa. Bile-acid derivatives inhibiting [3H]taurocholate uptake by ileal vesicles also inhibited labeling of the 90 kDa polypeptide, whereas compounds with no inhibitory effect on ileal bile-acid transport failed to show a significant effect on the labeling of the 90 kDa polypeptide. The involvement of functional amino-acid side-chains in Na(+)-dependent taurocholate uptake was investigated by chemical modification of ileal brush-border membrane vesicles with a variety of group-specific agents. It was found that (vicinal) thiol groups and amino groups are involved in active ileal bile-acid uptake, whereas carboxyl- and hydroxyl-containing amino acids, as well as tyrosine, histidine or arginine are not essential for Na(+)-dependent bile-acid transport activity. The irreversible inhibition of [3H]taurocholate transport by DTNB or NBD-chloride could be partially reversed by thiols like 2-mercaptoethanol or DTT. Furthermore, increasing concentrations of taurocholate during chemical modification with NBD-chloride were able to protect the ileal bile-acid transporter from inactivation. These findings suggest that a membrane polypeptide of apparent M(r) 90,000 is a component of the active Na(+)-dependent bile-acid reabsorption system in the terminal ileum from rabbit small intestine. Vicinal thiol groups and amino groups of the transport system are involved in Na(+)-dependent transport activity, whereas other functional amino acids are not essential for transport activity.

Aromatic trivalent arsenicals: covalent yet reversible reagents for the agonist binding site of nicotinic receptors

The agonist binding site of nicotinic acetylcholine receptors (AChRs) includes a disulfide bond that is easily reduced with dithiothreitol to a pair of thiols, and can be then either reoxidized with dithiobis(nitrobenzoic acid) (DTNB) or irreversibly alkylated with bromoacetylcholine (BAC). Aromatic trivalent arsenicals form stable complexes with pairs of appropriately-spaced thiols, but not single thiols. Furthermore, once complexed in proteins, trivalent arsenicals can be removed with dimercaptans, such as 2,3-dimercaptopropanesulfonic acid (DMPS). In an effort to develop reagents that will covalently, yet reversibly label AChRs, we investigated the effects of two model arsenicals, p-aminophenyldichloroarsine (APA) and 4-bromoacetyl-aminophenylarsenoxide (BAPA) on two types of nicotinic receptors: AChRs from Torpedo electroplax and neuronal receptors from chick retina. APA and BAPA significantly decrease the number of 125I-alpha-bungarotoxin binding sites in reduced Torpedo AChRs. Furthermore, arsenylation of neuronal and Torpedo receptors with APA or BAPA (1) prevents reoxidation with DTNB, (2) is reversible with DMPS, and (3) protects against irreversible alkylation by BAC. In Torpedo receptors, the EC50 of protection against BAC alkylation with APA or BAPA is approximately 30 nM. APA arsenylation of Torpedo receptors persists up to 20 h, but can be reversed at any time with DMPS. These results suggest that heterobifunctional arsenicals could anchor labeling groups in the agonist binding site in order to map the agonist binding site, quantitate receptors, or purify and reconstitute functional receptors.