Mechanism of antibacterial action: electron transfer and oxy radicals

Bismuth(III) complexes of Schiff bases derived from aliphatic amines: interaction with biomolecules and antimicrobial activity

Schiff bases bearing a sterically hindered phenolic moiety and their Bi(III) complexes were synthesized and characterized by physicochemical, quantum chemical, and biological methods. The compounds were screened in vitro against bacterial and yeast strains. It was found that Bi(III) complexes demonstrate higher antimicrobial activity compared to the parent ligands as well as to the commonly used drug (De-Nol®). Moreover, the antibacterial activity of investigated compounds did not directly correlate with their hemolytic activity, indicating that the antimicrobial effect of Bi(III) complexes cannot be explained solely by their membranolytic properties. Spectrofluorometric studies of the interaction of the Bi(III) complexes with plasma proteins indicate their moderate to high affinity toward BSA and hemoglobin, which is crucial for the determination of their pharmacological profile as well as toxicity assessment. Additionally, molecular docking was performed to predict the possible interaction modes and binding energies of the tested compounds at the molecular level. The results obtained may provide the basis for the design and development of novel Bi(III)-based antimicrobial agents.

How to measure diffusion coefficients in biofilms: A critical analysis

Biofilm and granular sludge processes depend on diffusion of substrates. Despite their importance for the kinetic description of biofilm reactors, biofilm diffusion coefficients reported in literature vary greatly. The aim of this simulation study was to determine to what extent the methods that are used to measure diffusion coefficients contribute to the reported variability. Granular sludge was used as a case study. Six common methods were selected, based on mass balances and microelectrodes. A Monte Carlo simulation was carried out to determine the theoretical precision of each method, considering the uncertainty of various experimental parameters. A model-based simulation of a diffusion experiment was used to determine the theoretical accuracy as a result of six sources of error: solute sorption, biomass deactivation, mass transfer boundary layer, granule roughness, granule shape, and granule size distribution. Based on the Monte Carlo analysis, the relative standard deviation of the different methods ranged from 5% to 61%. In a theoretical experiment, the six error sources led to an 37% underestimation of the diffusion coefficient. This highlights that diffusion coefficients cannot be determined accurately with existing experimental methods. At the same time, the need for measuring precise diffusion coefficients as input value for biofilm modeling can be questioned, since the output of biofilm models has a limited sensitivity toward the diffusion coefficient.

Nitroaromatic compounds: Environmental toxicity, carcinogenicity, mutagenicity, therapy and mechanism

Vehicle pollution is an increasing problem in the industrial world. Aromatic nitro compounds comprise a significant portion of the threat. In this review, the class includes nitro derivatives of benzene, biphenyls, naphthalenes, benzanthrone and polycyclic aromatic hydrocarbons, plus nitroheteroaromatic compounds. The numerous toxic manifestations are discussed. An appreciable number of drugs incorporate the nitroaromatic structure. The mechanistic aspects of both toxicity and therapy are addressed in the context of a unifying mechanism involving electron transfer, reactive oxygen species, oxidative stress and antioxidants.

Stimulatory effect of xenobiotics on oxidative electron transport of chemolithotrophic nitrifying bacteria used as biosensing element

Electron transport chain (ETCh) of ammonium (AOB) and nitrite oxidizing bacteria (NOB) participates in oxidation of ammonium to nitrate (nitrification). Operation of ETCh may be perturbed by a range of water-soluble xenobiotics. Therefore, consortia of nitrifying bacteria may be used as a biosensor to detect water contamination. A surprising feature of this system is an increase of oxygen consumption, detected in the presence of certain inhibitors of ETCh. Thus, to shed light on the mechanism of this effect (and other differences between inhibitors) we monitored separately respiration of the bacteria of the first (AOB - Nitrosomonas) and second (NOB -Nitrobacter) stages of nitrification. Furthermore, we measured plasma membrane potential and the level of reduction of NAD(P)H. We propose a novel model of ETCh in NOB to explain the role of reverse electron transport in the stimulation of oxygen consumption (previously attributed to hormesis).

Phagomimetic action of antimicrobial agents

A wide variety of extracted and synthesised drug molecules have electron transfer capabilities which allow them to generate reactive oxygen species (ROS). In particular, many antibiotics that kill or inhibit bacteria, yeasts and cancer cells readily transfer electrons to oxygen making superoxide and hydrogen peroxide in the process. When suitable redox active forms of iron are available, Fenton chemistry occurs generating the highly damaging hydroxyl radical. This type of chemistry is very similar to that which evolved within phagocytic cells as part of their microbial killing armoury. Many antibiotics, when used in model systems, have well defined pharmacological actions against key cellular functions, but their clinical usefulness is also often demonstrable at concentrations in vivo well below their in vitro minimum inhibitory concentrations. These observations have led us to propose that a common mechanism exists whereby phagocytic cells and antibiotics exploit the use of ROS for microbial killing.

Antibacterial effect of 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1, 4-naphthoquinone-4-imine on Staphylococcus aureus

The mechanism by which a new naphthoquinone derivative, the 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1, 4-naphthoquinone-4-imine (INQI-E) has antibacterial effect against Staphylococcus aureus was studied. The interaction of INQI-E with the bacteria was followed by absorption spectroscopy at 323 and 490 nm. The absorption band of INQI-E at 490 nm undergoes a hypochromic shift with a decrease of intensity. This effect was found to be reversible by oxygenation during the first hours of incubation. The participation of an oxidation-reduction process related to the respiratory chain was demonstrated by oxygen consumption. An increase in O2 uptake and inhibition of S. aureus growth was observed. Experiments with three inhibitors of the respiratory chain demonstrated that the pathway induced by INQI-E was antimycin-resistant and KCN- and salicylhydroxamic acid (SHAM)-sensitive, which suggests that INQI-E is capable of diverting the normal electron flow to an alternate superoxide-producing route. On the other hand, experiments with Tiron, a specific scavenger of superoxide, hindered the effect of INQI-E against S. aureus, indicating that the inhibitory growth effect of this quinone-imine is mainly due to the production of the cytotoxic superoxide radical.

Effect of 5-nitroindole on adenylate energy charge, oxidative phosphorylation, and lipid peroxidation in rat hepatocytes

5-Nitroindole (NI), a mutagenic nitroarene, was assayed for cytotoxic effects on rat hepatocytes. After incubation with 25-100 microM NI, the adenylate energy charge of the hepatocytes decreased significantly as a result of the decrease in ATP and the increase in AMP. ATP depletion correlated well with the effects of NI on mitochondrial electron transfer and energy transduction in hepatocytes. Thus, NI (a) inhibited the antimycin-sensitive hepatocyte respiration; (b) inhibited NADH oxidation by disrupted hepatocyte mitochondria; (c) inhibited L-malate-L-glutamate oxidation by ADP-supplemented mitochondria; (d) in the absence of ADP, stimulated the same substrates and also succinate oxidation by mitochondria; (e) released the latent ATPase activity of mitochondrial F1F0-ATP synthase; (f) shifted the redox level of reduced cytochromes (c + c1) and b towards the oxidized state; (g) inhibited NADH oxidation by disrupted mitochondria in the vicinity of the NADH-dehydrogenase flavoprotein; (h) inhibited Ca2+ uptake by mitochondria using L-malate-L-glutamate as an energy source; (i) inhibited valinomycin-induced, endogenously energized K+ uptake, with little effect on the ATP-induced uptake; and (j) inhibited the MgATP-dependent contraction of Ca(2+)-swollen mitochondria. NI inhibited lipid peroxidation in hepatocytes and also in substrate-supplemented liver microsomes and mitochondria, thus ruling out hydroperoxides as a cause of NI cytotoxicity. Long-term incubation with NI produced loss of hepatocyte viability, as indicated by lactate dehydrogenase leakage.

DNA base damage by beta-lactam, tetracycline, bacitracin and rifamycin antibacterial antibiotics

Several antibacterial antibiotics have been shown to participate with transition metal ions in chemical reactions leading to the formation of reactive oxygen species. An important host defence mechanism for dealing with invading bacteria involves the production of reactive oxygen species, such as superoxide, hydrogen peroxide and hypochlorous acid, by phagocytic cells. The production of reactive oxygens by redox cycling antibacterial antibiotics has led us to suggest that a 'phagomimetic' contribution may also be made in vivo. Here we show that four structurally different antibacterial antibiotics, in the presence of added copper salt, bring about oxidative modification to bases in DNA detected using gas chromatography-mass spectrometry. The drug most damaging to DNA was rifamycin SV which was more active than a reference mixture of hydrogen peroxide and ascorbic acid.

Electrochemistry of Cu(I) bipyridyl complexes with alkene, alkyne, and nitrile ligands. Implications for plant hormone action of ethylene

The redox behavior was evaluated for several (BIPY)Cu(I) complexes (BIPY = 2,2'-bipyridyl) with unsaturated ligands by means of cyclic voltammetry in CH2Cl2 at reduced temperatures (-78 degrees, -23 degrees, 0 degree C). The complexes studied are [Cu(I)(BIPY)(C2H4)]PF6, [Cu(I)(BIPY)(3-hexyne)] PF6, [Cu(I)(BIPY)(DEAD)]PF6, ([Cu(I)(BIPY)]2 DEAD)[PF6]2 (DEAD = diethyl acetylene dicarboxylate) and [Cu(I)(BIPY)(CH3CN)]PF6. The oxidations are quasi-reversible at -78 degrees C for scan rates of 20 to 200 mV/sec. The reductions were irreversible on the CV time scale. Evidence is presented in support of a role for an electron transfer mechanism in the case of the plant hormone ethylene. Related literature data are also discussed.

Catalysis of nitrofuran redox-cycling and superoxide anion production by heart lipoamide dehydrogenase

Heart lipoamide dehydrogenase (LADH) catalyzed redox-cycling and O2-. production by (5-nitro-2-furfurylidene)amino derivatives using NADH as electron donor. NADH was a much more effective electron donor than NADPH for the nitroreductase activity. O2-. production was demonstrated by cytochrome c reduction, adrenochrome formation and the effect of superoxide dismutase. Under optimum conditions, nitroreductase activity was about 1% of LADH activity. One electron oxygen reduction and NADH oxidation correlated in 2:1 stoichiometry. The nitroreductase kinetics was in accordance with an ordered bi-bi mechanism. Nitrofuran derivatives bearing unsaturated five- or six-membered nitrogen heterocycles were more effective substrates than those bearing other groups, namely nifurtimox, nitrofurazone, nitrofurantoin and 5-nitro-2-furoic acid. Other nitro compounds (chloramphenicol, benznidazole, 2-nitroimidazole and 5-nitroindole) were ineffective. With the triazole, traizine and imidazole nitrofuran derivatives, the nitroreductase pH curve showed a maximum at pH 8.8, different from the pH optimum for the lipoamide reductase and diaphorase activities. Spectroscopic observations demonstrated pH-dependent structural changes in the triazole(I) and triazine derivatives which would affect their behavior as nitroreductase substrates. The nitroreductase activity was inhibited by p-chloromercuribenzoate and enhanced by cadmium and arsenite, whereas the NADH-induced LADH inactivation failed to affect the nitroreductase activity. In the absence of oxygen. LADH catalyzed nitrofuran reduction to products more reduced than the nitroanion, which were not reoxidized by oxygen. The anaerobic nitrofuran reduction was inhibited by cadmium and arsenite. The assayed nitrofuran compounds did not inhibit LADH lipoamide reductase activity, at variance with their action on glutathione reductase (Grinblat et al., Biochem Pharmacol 38: 767-772, 1989).

Generation of radical anions of nifurtimox and related nitrofuran compounds by ascorbate

Nifurtimox analogues bearing triazol-4-yl, benzimidazol-1-yl, triazin-4-yl or related groups as counterpart of the (5-nitro-2-furfurylidene) amino group were reduced to their nitro anion radicals by ascorbate in anaerobic solutions at high pH. The ESR spectra of the radical anions showed hyperfine spin couplings restricted to the nitrofuran moiety. With these compounds, the spin density at the nitro group was greater than with nifurtimox, nitrofurazone and nitrofurantoin. At neutral pH, solutions containing ascorbate and nitrofuran derivatives consumed oxygen, the compounds bearing unsaturated nitrogen heterocycles being the most effective. Superoxide dismutase and catalase decreased the rate of oxygen consumption, thus demonstrating the production of superoxide and hydrogen peroxide, respectively. NMR spectra of the triazol-4-yl and triazin-4-yl nitrofuran derivatives showed a deshielding effect for the azomethine proton, which was undetectable with nifurtimox and nitrofurazone.

Theoretical calculations on calcium channel drugs: is electron transfer involved mechanistically?

Theoretical studies were done on calcium channel drugs in order to gain insight into the mode of action. Empirical force field calculations with nifedipine, a calcium channel antagonist, indicate that the E-conformation at the ring juncture is lower in energy than the Z-conformation. This energy difference is only 0.2 kcal/mol when the esters in the 3- and 5-positions of the dihydropyridine (DHP) ring are both synperiplanar (sp, sp). Molecular orbital calculations on the ground and excited states in the Z-conformation with the esters in the (ap, sp) conformation show a low lying excited state with substantial intramolecular electron transfer (ET) character. This excited state is only 1.8 eV higher in energy than the ground state and corresponds to a transfer of approximately 0.3 electron from the DHP ring to the nitrobenzene moiety. We suggest that ET may play an important role in the mechanism of action, either intramolecular or, as previously proposed, intermolecular, along with lipophilicity and steric effects.

Minimum essential structural requirements for lactam antibiotic action

A mechanism of action encompassing mono- and bicyclic beta-lactams has been proposed previously, which stresses the importance of formation of an electron transfer (ET) entity (conjugated iminium) as a requirement for antibiotic activity, in association with enzyme inactivation. Additional evidence in support of this contention is now provided. Reduction potentials for several cephalosporins and pyrazolidinones, all of which contain an oximino functionality in the side chain, were observed in the range of -0.6 to -0.7 V. Comparison is made with related compounds lacking imine. Agents containing side chain hydrazone, oxamazins (mono beta-lactams), and lactivicin are discussed based on the ET approach.

Reduction potentials of anthelmintic drugs: possible relationship to activity

Electrochemical data were acquired for several categories of anthelmintic agents, namely, iminium-type ions, metal derivatives and chelators, quinones and iminoquinones, and nitroheterocycles. Reductions usually were in the favorable range of +0.2 to -0.7 V versus normal hydrogen electrode. The drug effect is believed to result in part from either the catalytic production of oxidative stress or disruption of helminth electron transport systems. Relevant literature results are discussed.

Electrochemistry of cyclic alpha-imino carboxylates and their metal complexes: correlation with physiological activity

Cyclic voltammetry data were obtained for delta 1-pyrroline-2-carboxylate, delta 3-thiazoline-4-carboxylate, delta 2-thiazoline-2-carboxylate and their complexes with Cu(II), Fe(III), and Fe(II). The free ligands were reduced at about -0.35 V and were oxidized in the range of 0.42-0.52 V. Complexing the imine carboxylates with metal ions produces reduction and oxidation in the ranges of 0.05-0.37 V and 0.52-0.74 V, respectively. Prior reports show that these ligands take part in various biological functions. We propose that electron transfer may be involved in some aspects of the physiological activity. The captodative effect can be applied.

Dioxathiadiaza-heteropentalenes mediate superoxide and hydrogen peroxide production in Escherichia coli

The dioxathiadiaza-heteropentalenes, HEP-I (4,4-dimethyl-1,7-dioxa-2,6-diaza- 7 alpha lambda 4-thia-3H,5H-benzo[cd]pentalene), HEP-II (1,7-dioxa-2, 6-diaza-4, 7 alpha lambda 4-dithia-3H, 5H-benzo[cd]pentalene), HEP-III (1,7-dioxa-2,6-diaza-4, 7 alpha lambda 4-dithia-3H, 5H-benzo[cd]pentalene-4-oxide), and HEP-IV (1,7-dioxa-2,6-diaza-4,7 alpha lambda 4-dithia-3H, 5H-benzo[cd]pentalene-4,4-dioxide), inhibited growth of Escherichia coli in a simple glucose-salt medium, with their toxicities following the order of HEP-IV greater than HEP-III greater than HEP-II greater than HEP-I. These toxicities could be suppressed by yeast extract added to the glucose-salt medium. Yeast extract also facilitated maximal induction of superoxide dismutase (SOD) and catalase. The redox potentials of HEP-I-HEP-IV and the rates of oxygen uptake dependent on heteropentalenes in cyanide-resistant respiration of E. coli were correlated with the induction of SOD and catalase. Thus, the higher the redox potential of the compounds, the more potent they were for induction of enzyme production. Under anaerobic conditions, HEP-IV did not inhibit E. coli growth. These results indicate that HEP-I-HEP-IV can be reduced within the cell of E. coli and then reoxidized by molecular oxygen, generating O2- and H2O2. The toxicities of the heteropentalenes depend largely upon superoxide and/or hydrogen peroxide toxicity, and SOD and catalase provide a defense against the potential cytotoxicity of these species.

Hydroxyl radical generation by the tetracycline antibiotics with free radical damage to DNA, lipids and carbohydrate in the presence of iron and copper salts

Tetracycline antibiotics caused the degradation of carbohydrate in the presence of a ferric salt at pH 7.4. This degradation appeared to involve hydroxyl radicals since the damage was substantially reduced by the presence of catalase, superoxide dismutase, scavengers of the hydroxyl radical and metal chelators. Similarly, the tetracycline antibiotics in the presence of a ferric salt greatly stimulated the peroxidation of liposomal membranes. This damage, which did not implicate the hydroxyl radical, was significantly reduced by the addition of chain-breaking antioxidants and metal chelators. Only copper salts in the presence of tetracycline antibiotics, however, caused substantial damage to linear duplex DNA. Studies with inhibitors suggested that damage to DNA did involve hydroxyl radicals.

Mode of action of antiprotozoan agents. Electron transfer and oxy radicals

Cyclic voltammetry data were obtained for most of the main classes of antiprotozoan agents, specifically, nitroheterocycles, quinones, metal complexes and derivatives, iminium-type ions, and azo compounds. The reductions were generally reversible in the range of -0.3 to -0.9 V. Catalytic production of oxidative pressure from redox cycling involving oxygen is believed to be an important mode of action by the medicinal agents. Literature data contribute support.

An integrated concept of amebicidal action: electron transfer and oxy radicals

Cyclic voltammetry data were obtained for most of the main categories of antiamebic agents, specifically, quinones, heterocyclic nitro compounds, metal derivatives and chelators, and iminium-type ions. The reductions (our data and literature values) were for the most part reversible, with potentials usually in the favorable range of +0.10 to -0.56 V. The drug effect is believed to result generally from the catalytic production of oxidative stress usually arising from the formation of superoxide via electron transfer. In addition, relevant literature data are provided.