Linking 7-Nitrobenzo-2-oxa-1,3-diazole (NBD) to Triphenylphosphonium Yields Mitochondria-Targeted Protonophore and Antibacterial Agent

Appending lipophilic cations to small molecules has been widely used to produce mitochondria-targeted compounds with specific activities. In this work, we obtained a series of derivatives of the well-known fluorescent dye 7-nitrobenzo-2-oxa-1,3-diazole (NBD). According to the previous data [Denisov et al. (2014) Bioelectrochemistry, 98, 30-38], alkyl derivatives of NBD can uncouple isolated mitochondria at concentration of tens of micromoles despite a high pK_a value (~11) of the dissociating group. Here, a number of triphenylphosphonium (TPP) derivatives linked to NBD via hydrocarbon spacers of varying length (C5, C8, C10, and C12) were synthesized (mitoNBD analogues), which accumulated in the mitochondria in an energy-dependent manner. NBD-C10-TPP (C10-mitoNBD) acted as a protonophore in artificial lipid membranes (liposomes) and uncoupled isolated mitochondria at micromolar concentrations, while the derivative with a shorter linker (NBD-C5-TPP, or C5-mitoNBD) exhibited no such activities. In accordance with this data, C10-mitoNBD was significantly more efficient than C5-mitoNBD in suppressing the growth of Bacillus subtilis. C10-mitoNBD and C12-mitoNBD demonstrated the highest antibacterial activity among the investigated analogues. C10-mitoNBD also exhibited the neuroprotective effect in the rat model of traumatic brain injury.

Effect of Cyanide on Mitochondrial Membrane Depolarization Induced by Uncouplers

In this work, it was found that the ability of common uncouplers - carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and 2,4-dinitrophenol (DNP) - to reduce membrane potential of isolated rat liver mitochondria was diminished in the presence of millimolar concentrations of the known cytochrome c oxidase inhibitor - cyanide. In the experiments, mitochondria were energized by addition of ATP in the presence of rotenone, inhibiting oxidation of endogenous substrates via respiratory complex I. Cyanide also reduced the uncoupling effect of FCCP and DNP on mitochondria energized by succinate in the presence of ferricyanide. Importantly, cyanide did not alter the protonophoric activity of FCCP and DNP in artificial bilayer lipid membranes. The causes of the effect of cyanide on the efficiency of protonophoric uncouplers in mitochondria are considered in the framework of the suggestion that conformational changes of membrane proteins could affect the state of lipids in their vicinity. In particular, changes in local microviscosity and vacuum permittivity could change the efficiency of protonophore-mediated translocation.

Uncoupling and Toxic Action of Alkyltriphenylphosphonium Cations on Mitochondria and the Bacterium Bacillus subtilis as a Function of Alkyl Chain Length

A series of permeating cations based on alkyl derivatives of triphenylphosphonium (C(n)-TPP(+)) containing linear hydrocarbon chains (butyl, octyl, decyl, and dodecyl) was investigated in systems of isolated mitochondria, bacteria, and liposomes. In contrast to some derivatives (esters) of rhodamine-19, wherein butyl rhodamine possessed the maximum activity, in the case of C(n)-TPP a stimulatory effect on mitochondrial respiration steadily increased with growing length of the alkyl radical. Tetraphenylphosphonium and butyl-TPP(+) at a dose of several hundred micromoles exhibited an uncoupling effect, which might be related to interaction between C(n)-TPP(+) and endogenous fatty acids and induction of their own cyclic transfer, resulting in transport of protons across the mitochondrial membrane. Such a mechanism was investigated by measuring efflux of carboxyfluorescein from liposomes influenced by C(n)-TPP(+). Experiments with bacteria demonstrated that dodecyl-TPP(+), decyl-TPP(+), and octyl-TPP(+) similarly to quinone-containing analog (SkQ1) inhibited growth of the Gram-positive bacterium Bacillus subtilis, wherein the inhibitory effect was upregulated with growing lipophilicity. These cations did not display toxic effect on growth of the Gram-negative bacterium Escherichia coli. It is assumed that the difference in toxic action on various bacterial species might be related to different permeability of bacterial coats for the examined triphenylphosphonium cations.

Mitochondria-targeted plastoquinone derivatives as tools to interrupt execution of the aging program. 1. Cationic plastoquinone derivatives: synthesis and in vitro studies

Synthesis of cationic plastoquinone derivatives (SkQs) containing positively charged phosphonium or rhodamine moieties connected to plastoquinone by decane or pentane linkers is described. It is shown that SkQs (i) easily penetrate through planar, mitochondrial, and outer cell membranes, (ii) at low (nanomolar) concentrations, posses strong antioxidant activity in aqueous solution, BLM, lipid micelles, liposomes, isolated mitochondria, and cells, (iii) at higher (micromolar) concentrations, show pronounced prooxidant activity, the "window" between anti- and prooxidant concentrations being very much larger than for MitoQ, a cationic ubiquinone derivative showing very much lower antioxidant activity and higher prooxidant activity, (iv) are reduced by the respiratory chain to SkQH2, the rate of oxidation of SkQH2 being lower than the rate of SkQ reduction, and (v) prevent oxidation of mitochondrial cardiolipin by OH*. In HeLa cells and human fibroblasts, SkQs operate as powerful inhibitors of the ROS-induced apoptosis and necrosis. For the two most active SkQs, namely SkQ1 and SkQR1, C(1/2) values for inhibition of the H2O2-induced apoptosis in fibroblasts appear to be as low as 1x10(-11) and 8x10(-13) M, respectively. SkQR1, a fluorescent representative of the SkQ family, specifically stains a single type of organelles in the living cell, i.e. energized mitochondria. Such specificity is explained by the fact that it is the mitochondrial matrix that is the only negatively-charged compartment inside the cell. Assuming that the Deltapsi values on the outer cell and inner mitochondrial membranes are about 60 and 180 mV, respectively, and taking into account distribution coefficient of SkQ1 between lipid and water (about 13,000 : 1), the SkQ1 concentration in the inner leaflet of the inner mitochondrial membrane should be 1.3x10(8) times higher than in the extracellular space. This explains the very high efficiency of such compounds in experiments on cell cultures. It is concluded that SkQs are rechargeable, mitochondria-targeted antioxidants of very high efficiency and specificity. Therefore, they might be used to effectively prevent ROS-induced oxidation of lipids and proteins in the inner mitochondrial membrane in vivo.

Cations SkQ1 and MitoQ accumulated in mitochondria delay opening of ascorbate/FeSO4-induced nonspecific pore in the inner mitochondrial membrane

It is known that an addition of FeSO4 in the presence of ascorbic acid to cells or mitochondria can injure energy coupling and some other functions in mitochondria. The present study demonstrates that decrease in ascorbate concentration from 4 to 0.2 mM in the presence of the same low concentrations of FeSO4 accelerates the nonspecific pore opening, while cyclosporin A prevents and under some conditions reverses the pore opening. Hydrophobic cations SkQ1 and MitoQ (structural analogs of plastoquinone and coenzyme Q(10), respectively) delay pore opening, SkQ1 being more efficient. It is known that an increase in matrix ADP concentration delays pore opening, while an addition of carboxyatractylate to mitochondria accelerates the beginning of pore opening. Preliminary addition of SkQ1 into a mitochondrial suspension increased the effect of ADP and decreased the effect of carboxyatractylate. These results suggest that under the conditions used SkQ1 protects mitochondria from oxidative damage as an antioxidant when added at extremely low concentrations.

Involvement of mitochondrial inner membrane anion carriers in the uncoupling effect of fatty acids

This paper considers stages of the search (initiated by V. P. Skulachev) for a receptor protein for fatty acids that is involved in their uncoupling effect. Based on these studies, mechanism of the ADP/ATP antiporter involvement in the uncoupling induced by fatty acids was proposed. New data (suppression by carboxyatractylate of the SDS-induced uncoupling, pH-dependence of the ADP/ATP and the glutamate/aspartate antiporter contributions to the uncoupling, etc.) led to modification of this hypothesis. During discussion of the uncoupling effect of fatty acids caused by opening of the Ca(2+)-dependent pore, special attention is given to the effects of carboxyatractylate added in the presence of ADP. The functioning of the uncoupling protein UCP2 in kidney mitochondria is considered, as well as the diversity observed by us in effects of 200 microM GDP on decrease in Deltapsi under the influence of oleic acid added after H(2)O(2) (in the presence of succinate, oligomycin, malonate). A speculative explanation of the findings is as follows: 1) products of lipid and/or fatty acid peroxidation (PPO) modify the ADP/ATP antiporter in such a way that its involvement in the fatty acid-induced uncoupling is suppressed by GDP; 2) GDP increases the PPO concentration in the matrix by suppression of efflux of fatty acid hydroperoxide anions through the UCP and/or of efflux of PPO anions with involvement of the GDP-sensitive ADP/ATP antiporter; 3) PPO can potentiate the oleate-induced decrease in Deltapsi due to inhibition of succinate oxidation.

Chemical modification of the essential arginine residues of pyruvate dehydrogenase prevents its phosphorylation by kinase

The mechanism of regulatory phosphorylation of the pyruvate dehydrogenase component (E1) of muscle pyruvate dehydrogenase complex was studied. Chemical modification of the arginine residues essential for substrate binding was shown to prevent incorporation of 32P from [gamma-32P]ATP into E1 catalyzed by kinase and to exclude completely the interaction of holo-E1 with pyruvate. It is proposed that negatively charged phosphoseryl residues may compete with pyruvate for the active site arginine and thereby block the substrate binding.

Application of anti-E1 monoclonal antibodies to the study of the pyruvate dehydrogenase complex

It has been shown that monoclonal antibody (mAb) F7F10 raised against pyruvate dehydrogenase component (E1) of pigeon breast muscle pyruvate dehydrogenase complex (PDC) has no influence on the E1 activity, measured in the system with artificial oxidants. However it inhibited the full NAD+ and coenzyme A dependent activity of PDC. The competition of the F7F10 antibody with the E2 component of PDC for the binding with E1 was revealed by immunoenzymatic and kinetic analysis. It is suggested that F7F10 mAb interacts with an antigenic determinant, located in the immediate vicinity of or overlapping with the E1 region, responsible for the interaction with the E2 component of PDC.

The effect of phosphorylation on pyruvate dehydrogenase

Phosphorylation of the pyruvate dehydrogenase component (E1) of the muscle pyruvate dehydrogenase complex (PDC) by E1-kinase inhibits substrate conversion both in oxidative and non-oxidative reactions. Circular dichroism spectra were used to monitor the effect of phosphorylation on the following stages of the process: holoform formation from apo-E1 and thiamine pyrophosphate (TPP), substrate binding and active site deacetylation. It has been shown that phosphorylation of E1 reduces its affinity for TPP and prevents holo-E1 interaction with pyruvate. Phosphorylated and dephosphorylated PDC convert 2-hydroxyethyl-TPP in similar ways involving half of their active sites; all active sites of E1 function in the presence of deacetylating agents. The data obtained suggest that the phosphorylation and substrate binding sites interact with each other.

Production and characterization of a monoclonal antibody specific for the E1 component of the pyruvate dehydrogenase complex

The monoclonal antibody F7F10 against the E1 component of the pigeon breast muscle pyruvate dehydrogenase complex has been produced. The dissociation constant of the E1-mAb F7F10 complex was determined to be 5.93 x 10(-8)M. The cross-reaction of the mAb with the E1 components of the pyruvate dehydrogenase complexes from various species (including human) was established. The competitive solid-phase immunoenzyme assay of the E1 component and PDC concentrations has been developed.

Half-of-the-site reactivity of the decarboxylating component of the pyruvate dehydrogenase complex from pigeon breast muscle with respect to 2-hydroxyethyl thiamine pyrophosphate

The holopyruvate dehydrogenase is characterized by the charge transfer complex formation between tryptophan residue and thiamine pyrophosphate in each of two active centres. Interaction of apoenzyme with one mole of 2-hydroxyethyl thiamine pyrophosphate results in appearance of the same spectral band which does not change in intensity with further increase in ligand concentration. 2-hydroxyethyl thiamine pyrophosphate: acceptor oxidoreductase activity abolishes after oxidation of only one tryptophan residue per mole of the protein or blocking of one of the active centres with inactive analogue of the coenzyme. In the latter case the charge transfer complex band induced by interaction of apoenzyme with 2-hydroxyethyl thiamine pyrophosphate was not shown at all. These facts testify to half-of-the-site reactivity of pyruvate dehydrogenase with respect to 2-hydroxyethyl thiamine pyrophosphate.

Substrate-dependent inactivation of muscle pyruvate dehydrogenase: identification of the acetyl-substituted enzyme form

The properties of the pyruvate dehydrogenase component isolated from the pigeon breast muscle pyruvate dehydrogenase complex were studied upon inactivation of the enzyme in an incomplete reaction mixture: in the presence of cofactors and pyruvate, and in the absence of electron acceptors. The substrate-dependent inactivation was shown to result in the modification of two sulfhydryl groups per mole of the enzyme, in the appearance of a maximum at 235 nm in the protein absorption spectrum, and in the involvement of 1.5 moles of the [2-14C]-pyruvate fragment per mole of the pyruvate dehydrogenase. The fragment-protein bond is acid-stable, labile in alkali, and breaks up in the presence of performic acid, neutral hydroxylamine and dithiothreitol. An acetyl-substituted form of pyruvate dehydrogenase appearing with the participation of sulfhydryl enzyme groups is suggested.

Localization of tryptophan residues in thiamine pyrophosphate-binding sites of pyruvate dehydrogenase from pigeon breast muscle

Interaction of N-bromosuccinimide with the pyruvate dehydrogenase component of the pigeon breast muscle pyruvate dehydrogenase complex results in a rapid and specific modification of two tryptophan residues per mole of the protein and in complete inactivation of the enzyme. Modification of the enzyme excludes the development of the negative Cotton effect with a maximum at 330 nm, characteristic of the charge transfer complex between the protein tryptophan residue and thiamine pyrophosphate. Modification of one tryptophan residue was shown to result in the absence of the band at 330 nm in one of the active centers of pyruvate dehydrogenase, while oxidation of two tryptophan residues excludes the formation of a charge transfer complex in both centers. The conclusion was drawn about the presence in the pyruvate dehydrogenase active centers of two tryptophan residues involved in the formation of the charge transfer complex with the thiazolium ring of thiamine pyrophosphate.

Arginine residues in the active centers of muscle pyruvate dehydrogenase

The pyruvate dehydrogenase component isolated from the pigeon breast muscle pyruvate dehydrogenase complex is inactivated by the arginine-specific reagents, 2,3-butanedione and phenylglyoxal. The kinetics of inactivation is biphasic. The reaction order with respect to the inhibitor concentration at the fast and slow steps of the inactivation reaction is close to unity. This suggests that complete inactivation of the enzyme results from the interaction of two molecules of 2,3-butanedione or phenylglyoxal with essential groups of pyruvate dehydrogenase. In the presence of thiamine pyrophosphate and Mg2+ pyruvate protects the enzyme against inactivation. The experimental results are suggestive of the presence of two arginine residues in the substrate-binding sites of two active centres of the holoenzyme.

The substrate-mediated inactivation of the pyruvate dehydrogenase component of the pigeon breast muscle pyruvate dehydrogenase complex

Incubation of the pyruvate dehydrogenase component isolated from the pigeon breast muscle pyruvate dehydrogenase complex with Mg2+, thiamine pyrophosphate and low concentrations of pyruvic acid in the absence of electron acceptors results in irreversible time-dependent inactivation of the enzyme. The rate of the enzyme inactivation is markedly decreased in the presence of high concentrations of pyruvate; in this case acetoin and acetolactate are detected in the reaction mixture. The enzyme activity is stabilized when the artificial electron acceptor, 2,6-dichlorophenolindophenol, is present in the reaction mixture. The substrate-mediated inactivation of the enzyme is accompanied by incorporation of the 2-[14C]-substrate fragment and labelled thiamine pyrophosphate into the protein fraction. The enzyme reactivation by neutral hydroxylamine and the protective effect of dithiothreitol suggest that the SH-group(s) may be involved in the substrate-mediated inactivation of pyruvate dehydrogenase.

Study on the role of SH-groups in the activity of muscle pyruvate dehydrogenase

The kinetics of inactivation of the pyruvate dehydrogenase component of the pigeon breast muscle pyruvate dehydrogenase complex in the presence of 5,5'-dithiobis (2-nitrobenzoate) is biphasic. The rate constants for the fast and slow phases of the inactivation reaction are close to those for modification of two classes of SH-groups differing in their reactivities towards the inhibitor. The reaction order with respect to the inhibitor concentration suggests that the two distinct SH-groups are essential for the enzyme activity. Modification of these SH-groups results in inhibition of the overall activity of the pyruvate dehydrogenase complex and of the 2-hydroxyethyl thiamine pyrophosphate - acceptor oxidoreductase activity of its decarboxylating component. Thiamine pyrophosphate exerts a protective effect on the enzyme only at the slow phase of the enzyme inactivation and SH-modification. As a result of interaction between the holoenzyme and pyruvate (or apoenzyme and 2-hydroxyethyl thiamine pyrophosphate) the rate of the enzyme inactivation is increased. This is associated with masking of non-essential SH-groups and with an increase of the accessibility of two essential SH-groups to the inhibitor. The data obtained suggest the interrelationship between the essential SH-groups and the 2-hydroxyethyl thiamine pyrophosphate-acceptor oxidoreductase activity of pyruvate dehydrogenase.

[Cooperative interaction of pyruvate dehydrogenase from pigeon breast muscle]

Cooperative interaction of pyruvate with the pyruvate dehydrogenase (PD) complex from pigeon breast muscle was shown. The sigmoidal dependence of the reaction rate on pyruvate concentration was observed for the PD complex. The Hill coefficient is equal to 1,5; no inhibition by the substrate (up to 2.2.10(-3) M) was found. The kinetic behaviour of the isolated pyruvate dehydrogenase component (PDH) analyzed under similar conditions, is more complex; this may be probably due to the presence of oligomeric forms with different molecular weights and specific activities. The competitive inhibitor of the PD complex--an amide of pyruvic acid (PA) (Ki=6.3-10(-6) M) activates the enzyme at low concentrations (less than 2,10(-6) M). When PA is present, the dependence of the reaction rate on pyruvate concentration gives a usual hyperbolic curve, v of [S]o. It is concluded that pyruvate may have a regulatory effect on the activity of muscle PD complex.