Vitamin K1 2,3-epoxide and quinone reduction: mechanism and inhibition

The chemical and enzymatic pathways of vitamin K1 epoxide and quinone reduction have been investigated. The reduction of the epoxide by thiols is known to involve a thiol-adduct and a hydroxy vitamin K enolate intermediate which eliminates water to yield the quinone. Sodium borohydride treatment resulted in carbonyl reduction generating relatively stable compounds that did not proceed to quinone in the presence of base. NAD(P)H:quinone oxidoreductase (DT-diaphorase, E.C. 1.6.99.2) reduction of vitamin K to the hydroquinone was a significant process in intact microsomes, but 1/5th the rate of the dithiothreitol (DTT)-dependent reduction. No evidence was found for DT-diaphorase catalyzed reduction of vitamin K1 epoxide, nor was it capable of mediating transfer of electrons from NADH to the microsomal epoxide reducing enzyme. Purified diaphorase reduced detergent- solubilized vitamin K1 10(-5) as rapidly as it reduced dichlorophenylindophenol (DCPIP). Reduction of 10 microM vitamin K1 by 200 microM NADH was not inhibited by 10 microM dicoumarol, whereas DCPIP reduction was fully inhibited. In contrast to vitamin K3 (menadione), vitamin K1 (phylloquinone) did not stimulate microsomal NADPH consumption in the presence or absence of dicoumarol. DTT-dependent vitamin K epoxide reduction and vitamin K reduction were shown to be mutually inhibitory reactions, suggesting that both occur at the same enzymatic site. On this basis, a mechanism for reduction of the quinone by thiols is proposed. Both the DTT-dependent reduction of vitamin K1 epoxide and quinone, and the reduction of DCPIP by purified DT-diaphorase were inhibited by dicoumarol, warfarin, lapachol, and sulphaquinoxaline.

Caffeine, aminoimidazolecarboxamide and dicoumarol, inhibitors of NAD(P)H dehydrogenase (quinone) (DT diaphorase), prevent both the cytotoxicity and DNA interstrand crosslinking produced by 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) in Walker cells

A form of NAD(P)H dehydrogenase (quinone) (DT diaphorase, menadione reductase (NMOR), phylloquinone reductase, quinone reductase, EC 1.6.99.2) has been isolated from Walker 256 rat carcinoma cells. This enzyme can convert 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) to a cytotoxic DNA interstrand crosslinking agent by reduction of its 4-nitro group to the corresponding hydroxylamino species (Knox et al. Biochem Pharmacol, 37: 4661-4669 and 4671-4677, 1988). 2-Phenyl-5(4)-aminoimidazole-4(5)-carboxamide and AICA [5(4)-aminoimidazole-4(5)-carboxamide] have previously been reported to be antagonists of the anti-tumour effects of CB 1954. We have shown that both these compounds are inhibitors of the above enzyme and that AICA protects against both the cytotoxicity and the formation of DNA interstrand crosslinks, produced by CB 1954 in Walker cells. Similarly, known inhibitors of NAD(P)H dehydrogenase (quinone) such as dicoumarol, also reduced the cytotoxicity and DNA-interstrand crosslinking of CB 1954 in Walker cells. Caffeine was shown to be a novel inhibitor of NAD(P)H dehydrogenase (quinone) and also elicited the above protective effects. All of the above inhibitors were also shown to potentiate the toxic effects of menadione against the Walker cell. This quinone is known to be detoxified by NAD(P)H dehydrogenase (quinone) and thus emphasises the ability of these compounds to inhibit this enzyme within the cell.

Inhibition of bovine heart mitochondrial and Paracoccus denitrificans NADH----ubiquinone reductase by dequalinium chloride and three structurally related quinolinium compounds

1. Dequalinium chloride (DECA) and three related quinolinium compounds inhibit bovine heart mitochondrial and Paracoccus denitrificans electron transport activity, with inhibition localized between NADH and ubiquinone in both electron transport chains. 2. Structure-activity studies reveal that two quinolinium rings and a long bridging group are necessary for significant inhibition of reduction of artificial electron acceptors and coenzyme Q, whereas only one quinolinium ring and a long hydrocarbon side chain are required for significant inhibition of NADH oxidase activity. 3. Inhibition of coenzyme Q reduction by DECA is not reversed by dialysis. 4. Studies comparing DECA inhibition of rotenone-sensitive with rotenone-insensitive preparations indicate that DECA acts by a different inhibitory mechanism than rotenone on mammalian mitochondrial and P. denitrificans NADH----ubiquinone reductase.

New 4-hydroxypyridine and 4-hydroxyquinoline derivatives as inhibitors of NADH-ubiquinone reductase in the respiratory chain

Many derivatives of 2,3-dimethoxy-4-hydroxypyridine, which were designed from examination of the structure-activity relationship of piericidins, were tested for inhibition of NADH-UQ reductase. The lipophilic side chain of those compounds was indicated to be a key part for activity and its optimal length was conjectured. By the use of two different phases of assay material, intact mitochondria and submitochondria, the size of a membrane effect was shown to depend on the structure of the side chain. 4-Hydroxyquinoline derivatives were also tested for an analogous role in relation to the electron transport function of menaquinone, and they were proven to be inhibitors of NADH-UQ reductase as good as the pyridine derivatives.

Reaction of rat liver DT-diaphorase (NAD(P)H:quinone acceptor reductase) with 5'-[p-(fluorosulfonyl)benzoyl]-adenosine

Rat liver DT-diaphorase is inactivated by 5'-[p-(fluorosulfonyl)benzoyl]adenosine (5'-FSBA), following pseudo-first-order kinetics. A double-reciprocal plot of 1/kobs versus 1/[5'-FSBA] yields a straight line with a positive y intercept, indicative of reversible binding of the inhibitor before an irreversible incorporation. The dissociation constant (Kd) for the initial reversible enzyme-inhibitor complex is estimated at 2.86 mM with k2 = 0.22 min-1 (at pH 7.5 and 25 degrees). A stoichiometry of 2 mol of 5'-FSBA bound/mol of enzyme (i.e., 1 mol of the inhibitor bound/mol of subunit), at 100% inactivation, was determined from inactivation kinetics and from incorporation studies using 5'-[p-(fluorosulfonyl)benzoyl]-[14C]-adenosine. The irreversible inactivation as well as the covalent incorporation could be completely prevented by the presence of NAD(P)H during the incubation. These results indicate that 5'-FSBA inactivated DT-diaphorase by occupying its NAD(P)H binding site. Reverse phase high pressure liquid chromatography of tryptic digests of [14C]5'-FSBA-labeled DT-diaphorase revealed one radioactive peak containing two comigrating peptides. They are 146I-T-T-G-G-S-G-S-M-Y155 and 262S-I-P-A-D-N-Q-I-K270. By comparison of these sequences to those of the nucleotide binding sites of several kinases and dehydrogenases, it is suggested that the peptide I-T-T-G-G-S-G-S-M-Y is the one modified by 5'-FSBA and would be predicted to be the region where the pyrophosphate group of NAD(P)H binds.

Photoaffinity labelling of mitochondrial NADH: ubiquinone reductase with pethidine analogues

1. Chemically reactive derivatives of pethidine analogues--novel potent inhibitors of the mitochondrial NADH: ubiquinone reductase (complex I)--were synthesized. 2. Dose-response curves of these components revealed that the photoactivatable aryl azido derivative has retained most of the inhibitory activity displayed by the parent substance. After introduction of a radioactive iodine isotope into the molecule, it was used as a probe for the localization of the inhibitor binding polypeptides within complex I. 3. Photolysis of the radiolabelled derivative bound to isolated complex I both from Neurospora crassa and beef heart resulted in a covalent incorporation of the inhibitor into 6-7 individual subunits of the enzyme. Essentially the same labelling patterns were obtained, when whole mitochondrial membranes were incubated with the reactive derivative. 4. Applying a double isotope labelling technique, the inhibitor-binding polypeptides in N. crassa were identified as mitochondrially synthesized constituents of complex I (ND gene products). In the beef heart enzyme the ND-1 product was detected to be among the polypeptides reacting with the inhibitor. 5. Competition experiments employing either NADH or decylbenzoquinone (DB), together with the pethidine analogue, showed that both enzyme substrates interfere specifically with the inhibitor binding to complex I.

Effect of N-methyltetrazolethiol on liver microsomal vitamin K reductase

NADH-dependent vitamin K reductase activity in rat liver microsomes was measured by detecting the amount of the reduced form of vitamin K from the oxidized form of the vitamin. The enzyme activity was not detected when intact microsomes were employed as the enzyme source, but the solubilization of the microsomal enzyme with 1.5% Triton X-100 caused a development of the activity. Although the enzyme activity decreased gradually with time after the solubilization, the enzyme was stabilized by the addition of 20% glycerol and 2 mM vitamin C. Some optimal assay conditions for the vitamin K reductase were determined using the solubilized enzyme, and the standard assay method is described. Vitamin K reductase activity was not affected by warfarin and N-ethylmaleimide (NEM), but pyridoxal-5-phosphate (PAL-P) inhibited the activity, especially when microsomes were preincubated with PAL-P. The enzyme activity was not inhibited by N-methyltetrazolethiol (NMTT) and NMTT-containing antibiotics, suggesting that the hypoprothrombinemia caused by beta-lactam antibiotics was not due to the inhibition of NADH-dependent vitamin K reductase.

Increased sensitivity of quinone resistant cells to mitomycin C

L5178Y cells resistant to the model quinone antitumor agent, hydrolyzed benzoquinone mustard, were four-fold more sensitive to mitomycin C compared to parental cells. Mitomycin C also produced increased DNA-DNA crosslinking in these cells compared to parental L5178Y cells, but did not induce DNA double strand breaks in either cell line. The resistant cells have a 24-fold increased level of DT-diaphorase activity, an enzyme that produces two electron reduction of quinone groups. Dicoumarol, an inhibitor of DT-diaphorase, significantly inhibited crosslinking and cytotoxicity by mitomycin C in the quinone resistant cells. These findings suggest that DNA-DNA cross-linking may be a major contributor to mitomycin C cytotoxic activity in L5178Y cells, and that the hydroquinone of mitomycin C may play a major role in the crosslinking activity of this agent.

The pathway of electron transfer in NADH:Q oxidoreductase

The pre-steady-state reduction by NADPH of NADH:Q oxidoreductase, as present in submitochondrial particles, has been further investigated with the rapid-mixing, rapid-freezing technique. It was found that trypsin treatment, that had previously been used to inactivate the transhydrogenase activity (Bakker, P.T.A. and Albracht, S.P.J. (1986) Biochim. Biophys. Acta 850, 413-422), considerably affected the stability at pH 6.2 of the NAD(P)H oxidation activity of submitochondrial particles. Use of the inhibitor butadione circumvented this problem, thus allowing a more careful investigation of the kinetics at pH 6.2. In the presence of the inhibitor rotenone it was found that 50% of the Fe-S clusters 3 and all of the Fe-S clusters 2 and 4 could be reduced by NADPH within 30 ms at pH 6.2. The remainder of the Fe-S clusters 3 and all of the Fe-S clusters 1 were reduced slowly (complete reduction only after more than 60 s). It was concluded that these latter Fe-S clusters play no role in the NADPH oxidation activity. In the absence of rotenone at pH 6.2 only 50% of the Fe-S clusters 2-4 could be reduced within 30 ms, while Fe-S cluster 1 was again not reduced. This difference was attributed to the fast reoxidation of part of the Fe-S clusters 2 and 4 by ubiquinone. At pH 8.0, where the NADPH oxidation activity is almost zero, 50% of the Fe-S clusters 2-4 could still be reduced by NADPH within 30 ms, while Fe-S cluster 1 was not reduced. The presence of rotenone had no effect on this reduction. From these observations it is concluded that the Fe-S clusters 2 and 4, which were rapidly reduced by NADPH and reoxidised by ubiquinone at pH 6.2, could not be reduced by NADPH at 8.0. This provides an explanation why NADH:Q oxidoreductase was not able to oxidise NADPH at pH 8.0, while part of the Fe-S clusters were still rapidly reduced. As a working hypothesis a dimeric structure for NADH:Q oxidoreductase is proposed. One protomer (B) contains FMN and Fe-S clusters 1-4 in equal amounts; the other protomer (A) is identical except for the absence of Fe-S cluster 1. NADH is able to react with both protomers, while NADPH only reacts with protomer A. A pH-dependent electron transfer from protomer A to protomer B is proposed, which would allow the reduction of Fe-S clusters 2 and 4 of protomer B by NADPH at pH 6.2, which is required for NADPH:Q oxidoreductase activity.

X-ray diffraction analyses of crystals of rat liver NAD(P)H:(quinone-acceptor) oxidoreductase containing cibacron blue

NAD(P)H:(quinone-acceptor) oxidoreductase (EC 1.6.99.2) is a widely distributed dicoumarol-inhibitable FAD-containing protein that catalyzes the obligatory two-electron reduction of quinones. The enzyme plays an important role in protecting animal cells against quinone toxicity and may be involved in the vitamin K-dependent blood coagulation cascade. Cocrystallization of rat liver quinone reductase with Cibacron blue, a potent inhibitor with respect to NAD(P)H, was achieved by the method of vapor diffusion in the presence of ammonium sulfate and low concentrations of polyethylene glycol. X-ray diffraction analysis showed these blue crystalline platelets to be monoclinic and to belong to the space group P2(1) (a = 71.6 A, b = 107.1 A, c = 87.8 A and beta = 92.60 degrees) with two dimers in the asymmetric unit. The crystals diffract to a resolution of at least 2.8 A.

DCCD sensitivity of electron and proton transfer by NADH: ubiquinone oxidoreductase in bovine heart submitochondrial particles--a thermodynamic approach

The sensitivity of the H+/2e- ratio of the redox-driven proton pumping by the NADH: ubiquinone reductase (complex I) of the submitochondrial particles to dicyclohexylcarbodiimide (DCCD) was studied by a thermodynamic approach, measuring the membrane potential and delta pH across the membrane and the redox potential difference across the complex I span of the respiratory chain. The delta Gr/delta muH+ ratio did not decrease upon additions of 50 or 100 nmol of DCCD per mg protein in the presence of oligomycin although the H+/2e- ratio has been demonstrated to decrease upon DCCD addition in kinetic experiments with mitochondria. Complex I then becomes reminiscent of the cytochrome bc1 complex, which shows DCCD sensitivity of the kinetically but not thermodynamically determined H+/2e- ratio.

Capsaicin and its analogs inhibit the activity of NADH-coenzyme Q oxidoreductase of the mitochondrial respiratory chain

Capsaicin and its analogs with different acyl moieties were found to inhibit the electron-transfer activity of NADH-coenzyme Q oxidoreductase isolated from beef heart mitochondria. The inhibitory potency of capsaicin was lower than those of dihydrocapsaicin and analogs with heptanoyl, capryl, undecanoyl, and lauroyl moieties, but was higher than those of analogs with palmitoyl and stearoyl moieties. The analog with the lauroyl moiety showed the strongest inhibition. These results suggest that hydrophobicity and the appropriate carbon chain length of the acyl moiety are important for the binding of compounds to the enzyme. On the other hand, capsaicin and its analogs did not interrupt rotenone-insensitive electron transfer from NADH to menadione. Furthermore, these compounds had almost no effect on the spectral properties and EPR signals arising from iron-sulfur clusters of the NADH-treated enzyme. Kinetic analyses with double-reciprocal plots showed that these compounds were competitive inhibitors with respect to coenzyme Q1, an electron acceptor. These results strongly suggest that capsaicin and its analogs bind to the coenzyme Q1 binding site of the enzyme.

Sulfaquinoxaline inhibition of vitamin K epoxide and quinone reductase

Sulfaquinoxaline (N1-(2-quinoxalinyl)sulfanilamide) has been shown to be a potent (Ki = 1 microM) freely reversible inhibitor of the dithiothreitol-dependent reduction of both vitamin K epoxide and vitamin K quinone by rat liver microsomes in vitro. This observation provides an explanation for the hemorrhagic syndrome occasionally seen in poultry on medicated feed and the efficacy of sulfaquinoxaline in anticoagulant based rodenticides. Sulfaquinoxaline inhibition resembled inhibition by coumarin anticoagulants (e.g., warfarin) and hydroxynaphthoquinones (e.g., lapachol). Inhibition was observed in assays using microsomes from control strain rats, but the enzyme was resistant to sulfaquinoxaline in microsomes from warfarin-resistant rats. Steady-state kinetics inhibition patterns were nearly competitive versus dithiothreitol and nearly uncompetitive versus vitamin K epoxide as is observed for warfarin and lapachol. These results suggest that this inhibitor binds to the oxidized form of vitamin K epoxide reductase in the same way as suggested for the coumarins and hydroxyquinones. Of 10 other sulfa drugs tested, none were inhibitors, and of fragments and related compounds tested, only 2-aminoquinoxaline benzenesulfonamide was active. These results provide a probably orientation in the binding site in relation to that for warfarin and lapachol.

Modulation of the antineoplastic efficacy of mitomycin C by dicoumarol in vivo

Dicoumarol (DIC) modulates the intracellular metabolism of mitomycin C (MC) in vitro, increasing the toxicity of MC to hypoxic EMT6 cells and decreasing its toxicity to aerobic cells. The present experiments assessed whether DIC could be used to increase the therapeutic ratio attainable in vivo when MC was used as an adjunct to radiotherapy. Experiments with transplanted EMT6 tumors in mice showed that DIC increased the toxicity of MC to hypoxic tumor cells and increased the antineoplastic efficacy of regimens combining MC with radiation. DIC did not increase the hematologic toxicity of MC, and pretreatment with DIC plus MC did not augment radiation-induced skin reactions. The increase in antineoplastic effect was therefore obtained without a concomitant increase in normal tissue toxicities, and therapeutic gain was obtained.

[Hysteresis behavior of complex I in delta mu H+-dependent reduction of NAD+ succinate]

It was shown that the membrane-bound complex I is fully inactive in the absence of NADH during the reverse electron transfer from succinate to NAD+. The enzyme activation is attained by preincubation of submitochondrial particles with low concentrations of NADH; the activating effect persists after a complete oxidation of the latter during long-term (several hours) aerobic incubation. The experimental results suggest that complex I contains a redox component, whose reduction by NADH and aerobic oxidation are not involved in the overall catalytic reaction. An experimental scheme is proposed, according to which the key role of such a component is ascribed to the tightly bound ubiquinone; the activation and inactivation of the enzyme are due to a slow reversible redox conversion (ubiquinone in equilibrium ubisemiquinone), whereas the catalytic act involves a rapid reversible conversion (ubisemiquinone in equilibrium ubiquinol). It was demonstrated that the "redox" mechanism of the inactivation-activation reaction determines the strong dependence of activity of the reverse electron transfer on the mode of preparation of submitochondrial particles. The coupling properties of the submitochondrial particulate membrane and the activities of enzymes involved in the reverse electron transfer are stable at room temperature for over 14 hours.

The nitroreductase enzyme in Walker cells that activates 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) to 5-(aziridin-1-yl)-4-hydroxylamino-2-nitrobenzamide is a form of NAD(P)H dehydrogenase (quinone) (EC 1.6.99.2)

A nitroreductase enzyme has been isolated from Walker 256 rat carcinoma cells which can convert 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) to a cytotoxic DNA interstrand crosslinking agent by reduction of its 4-nitro group to the corresponding hydroxylamino species (Roberts JJ et al., Biochem Biophys Res Commun 140: 1073-1078, 1986; Knox RJ et al., Biochem Pharmacol 37: 4661-4669, 1988). The enzyme has now been identified as a form of NAD(P)H dehydrogenase (quinone) (DT diaphorase, menadione reductase (NMOR), phylloquinone reductase, quinone reductase, EC 1.6.99.2) by comparison of partial protein sequences, coenzymes, substrate and inhibitor specificities, and spectroscopic data. 2-Phenyl-5(4)-aminoimidazole-4(5)-carboxamide and 5(4)-aminoimidazole-4(5)-carboxamide were shown to be inhibitors of the isolated Walker cell enzyme. This observation could explain the reported antagonistic action of the aminoimidazole carboxamides to the antitumour effects of CB 1954.

Purification and crystallization of rat liver NAD(P)H:(quinone-acceptor) oxidoreductase by cibacron blue affinity chromatography: identification of a new and potent inhibitor

Cytosolic NAD(P)H:(quinone-acceptor) oxidoreductase (EC 1.6.99.2) is a widely distributed, FAD-containing enzyme that catalyzes the obligatory two-electron reduction of quinones. Cibacron Blue is an inhibitor of this enzyme comparable in potency to dicoumarol. Pure quinone reductase was obtained from the livers of Sudan II (1-[2,4-dimethylphenylazo]-2-naphthol)-treated rats in a single step by Cibacron Blue-agarose chromatography. Cibacron Blue is a competitive inhibitor with respect to NADH (Ki = 170 nM) and is a noncompetitive inhibitor with respect to menadione (Ki = 540 nM). Addition of Cibacron Blue to quinone reductase resulted in a decrease and red shift of the enzyme-bound FAD peak at 450 nm. The titration of the absorbance changes for both FAD and Cibacron Blue could be fitted to curves describing an equilibrium binding equation with a KD of 300 nM and one binding site per enzyme subunit. Furthermore, the Cibacron Blue difference spectrum that resulted from binding to quinone reductase was abolished by dicoumarol. Significant amino acid homology between quinone reductase and the nucleotide binding regions of enzymes that bind to Cibacron Blue was found. These data indicate that Cibacron Blue is a useful ligand for the purification of quinone reductase and a new probe for its NAD(P)H binding site. Conditions for crystallizing rat liver quinone reductase are also described.

Structure-function relationship of NAD(P)H:quinone reductase: characterization of NH2-terminal blocking group and essential tyrosine and lysine residues

The amino terminal blocked peptide of rat liver NAD(P)H:quinone reductase (DT-diaphorase) was determined by amino acid sequence analysis and by mass spectrometry. The mature protein is composed of 273 amino acids and contains an acetylated amino terminus, which was not identified by previous cDNA analysis. The enzyme was inactivated by p-nitrobenzenesulfonyl fluoride (NBSF) or 2,4,6-trinitrobenzenesulfonate (TNBS) with pseudo-first-order kinetics. These studies suggest that essential tyrosine and lysine may be present in the active site of this enzyme. The NBSF inhibition was protected by 1-naphthol and 1-naphthylamine, but not by NAD+. However, TNBS inhibition was not prevented by the naphthalene derivatives or NAD+. Specific peptides labeled with NBSF or TNBS were isolated by high-performance liquid chromatography and were sequenced. These analyses revealed that the NBSF-labeled tyrosine resides in a predominantly hydrophobic region and TNBS-labeled lysine in a predominantly hydrophilic region.

Human DT-diaphorase, a potential cancer protecting enzyme. Its purification from abdominal adipose tissue

The flavoprotein DT-diaphorase (EC 1.6.99.2) is believed to play an important role in the body's defense system. This enzyme has been purified 13,000-fold with a recovery of 58% from a cytosolic fraction of abdominal fat obtained from an obese patient undergoing elective surgery. Purification of the enzyme to electrophoretic homogeneity was achieved after two chromatographic steps: (1) affinity chromatography on azodicumarol Sepharose 6B; (2) anion exchange chromatography on DEAE Sephacel. The enzyme exhibits a monomer molecular mass of 32 kDa in SDS-PAGE and has 1 FAD prosthetic group per 32 kDa monomer. The FAD prosthetic group appears to be firmly attached to the apoproprotein. The enzyme reduces azodyes and quinones and demonstrates a broad substrate specificity. The enzyme has characteristics that are similar to DT-diaphorase purified from rodent liver, especially the rat liver enzyme. Estimated Km values for NADH, NADPH and menadione are 200, 140 and 3.3 microM, respectively. Vmax values for these substrates in the same order are 762, 667 and 294 mumol/mg.min. Dicumarol and warfarin exhibited competitive inhibition with pyridine nucleotides. The inhibition constants (Ki) for the drugs were estimated to be 10 nM and 2.2 microM, respectively. When compared to several other tissues, abdominal fat has one of the highest DT-diaphorase activities (Martin, L.F., Patrick, S.D. and Wallin, R. (1987) DT-diaphorase in morbidly obese patients. Cancer Lett., 36, 341-347), but the specific role of the enzyme in human fat is unknown.

Mitomycin C is not metabolized by but is an inhibitor of human kidney NAD(P)H: (quinone-acceptor)oxidoreductase

It has been suggested that quinone reductase [NAD(P)H: (quinone-acceptor)oxidoreductase], also known as DT-diaphorase, protects hypoxic cells against mitomycin C cytotoxicity by metabolizing mitomycin C to less toxic metabolites. This hypothesis is based on an increase in mitomycin C's cytotoxicity in the presence of the potent quinone reductase inhibitor dicumarol. It has been suggested that under aerobic conditions the metabolism of mitomycin C by quinone reductase leads to the formation of cytotoxic metabolites. In the present study, mitomycin C was found not to be a substrate for partially purified quinone reductase from human kidney. Mitomycin C did not cause the oxidation of NADPH by quinone reductase and there was no utilization of mitomycin C and no appearance of its metabolites. Quinone reductase did not catalyze the formation of alkylating metabolites from mitomycin C, determined by the lack of formation of 4-(p-nitrobenzyl)pyridine conjugates. However, mitomycin C was a weak competitive inhibitor of quinone reductase with dichloroindophenol as the substrate, with Ki = 0.32 mM. Therefore, the alteration of mitomycin C's cytotoxicity by dicumarol in tumor cell lines appears to involve a mechanism other than the direct inhibition of mitomycin C reduction by quinone reductase.

Pethidine analogues, a novel class of potent inhibitors of mitochondrial NADH: ubiquinone reductase

Analogues of the analgetic drug pethidine were synthesized. Two N-aralkylen derivatives displayed a superior inhibitory effect on the activity of NADH:ubiquinone reductase in beef heart mitochondrial membranes. Dose-response curves revealed that the potency of these compounds is very comparable to that of the standard probe rotenone. The inhibitors were characterized by (a) their action on the reductase activity in various (eukaryotic and prokaryotic) organisms, (b) their influence on the enzyme kinetics, (c) their effects on the NADH dependent reduction of different electron acceptors, (d) their interference with the activities of other mitochondrial oxido-reductases. With regard to many of these aspects a close analogy between pethidine derivatives and rotenone was observed. A computer simulation of the steric structures of these molecules indicates that both classes of the chemically rather unrelated inhibitors may imitate very similar conformations. The potential advantages of the pethidine derivatives for the investigation of structure - function relationships within complex I of the respiratory chain is discussed.

Trypsin proteolysis of the cytochrome d complex of Escherichia coli selectively inhibits ubiquinol oxidase activity while not affecting N,N,N',N'-tetramethyl-p-phenylenediamine oxidase activity

The cytochrome d complex is one of two membrane-bound terminal oxidases of the Escherichia coli aerobic respiratory chain. Previous studies have shown that this enzyme reconstituted into proteoliposomes rapidly oxidizes ubiquinol-8 as well as the soluble homologue, ubiquinol-1, and that quinol oxidase activity is accompanied by the formation of a transmembrane H+ electrochemical gradient. The enzyme also oxidizes the artificial reductant, N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) with the generation of a H+ electrochemical gradient. In this work, it is established that trypsin digestion of the purified cytochrome d complex cleaves subunit I while subunit II is unaffected. Proteolysis of subunit I is correlated with loss of ubiquinol-8 and ubiquinol-1 oxidase activities. Trypsin digestion has no effect on TMPD oxidase activity. The cytochrome d complex is concluded to possess three distinct active sites for 1) ubiquinol oxidation, 2) TMPD oxidation, and 3) oxygen binding and reduction. Data also suggest that both sites of ubiquinol and TMPD oxidations are located on the periplasmic side of the E. coli membrane while the site of oxygen reduction is on the opposite side.

NN'-dicyclohexylcarbodi-imide-sensitivity of bovine heart mitochondrial NADH: ubiquinone oxidoreductase. Inhibition of activity and binding to subunits

Dicyclohexylcarbodi-imide (DCCD) inhibition of NADH: ubiquinone oxidoreductase was studied in submitochondrial particles and in the isolated form, together with the binding of the reagent to the enzyme. DCCD inhibited the isolated enzyme in a time- and concentration-dependent manner. Over the concentration range studied, a maximum inhibition of 85% was attained within 60 min. The time course for the binding of DCCD to the enzyme was similar to that of activity inhibition. The NADH:ubiquinone oxidoreductase activity of the submitochondrial particles was also sensitive to DCCD, and the locus of binding of the inhibitor was studied by subsequent resolution of the enzyme into subunit polypeptides. Only two subunits (molecular masses 13.7 and 21.5 kDa) were labelled by [14C]DCCD, whereas, when the enzyme in its isolated form was treated with [14C]DCCD, six subunits (13.7, 16.1, 21.5, 39, 43 and 53 kDa) were labelled. Comparison with the subunit labelling of F1F0-ATPase and ubiquinol:cytochrome c oxidoreductase indicated that the labelling pattern of NADH:ubiquinone oxidoreductase, and enzyme complex with a multitude of subunits, is unique and not due to contamination by other inner-membrane proteins. The correlation between the electron- and proton-transport functions and the DCCD-binding components remains to be established.

Mechanisms of mitochondrial photosensitization by the cationic dye, N,N-bis(2-ethyl-1,3-dioxylene)kryptocyanine (EDKC): preferential inactivation of complex I in the electron transport chain

We investigated mechanisms of mitochondrial phototoxicity caused by the cationic cyanine dye N,N'-bis(2-ethyl-1,3-dioxylene)kryptocyanine (EDKC), examining the role of the mitochondrial membrane potential on the dye uptake by carcinoma cells in vitro, and both the dark and photosensitizing effects of the dye on the function of isolated mouse liver mitochondria. When human bladder carcinoma cells (EJ) were pretreated with 2,4-dinitrophenol or nigericin, cellular uptake of EDKC decreased or increased, respectively, consistent with dye uptake that is dependent on membrane potentials. In isolated liver mitochondria, during NADH linked substrate oxidation (using glutamate plus malate or beta-hydroxybutyrate as substrates), low concentrations of the dye (0.25-0.5 microM) sensitized mitochondria to illumination with long wavelength light and inhibited both basal and ADP-stimulated respiration. Similar effects were observed during succinate oxidation, but only at higher concentrations of EDKC (greater than 5 microM) and at 10-fold greater light doses. NADH coenzyme Q reductase (Complex I) activity was inhibited by dye with or without light to an extent comparable to the inhibition of glutamate plus malate oxidation. Activity of cytochrome c oxidase, the terminal enzyme in the electron transport chain, was photosensitized with high dye doses (greater than 5 microM) and light, but the extent of inhibition was much less than the inhibition of respiration with succinate as substrate. ATP synthetase (F0F1 ATPase) activity was minimally affected by 4.0 microM EDKC with or without 24 J/cm2 light. We conclude that at low concentrations of dye, respiratory Complex I is a primary target for EDKC dark and light-induced toxicities. If Complex I is bypassed by using succinate as a respiratory substrate, the mitochondria can tolerate much higher dye concentrations and light doses.

Effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 1-methyl-4-phenylpyridinium ion on activities of the enzymes in the electron transport system in mouse brain

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ion (MPP+) on activities of enzyme complexes in the electron transport system were studied using isolated mitochondrial preparations from C57BL/6J mouse brains. Both MPTP and MPP+ dose-dependently inhibited activity of NADH-ubiquinone oxidoreductase (EC 1.6.5.3). The inhibition was reversible. Preincubation of freeze-thawed mitochondria with MPTP or MPP+ had no effect on the inhibition; however, when nonfrozen mitochondria were used, NADH-ubiquinone oxidoreductase activity was reduced to 46% of that in the nonincubated sample after a 5-min preincubation with MPTP and to 77% of that in the nonincubated sample after a 5-min preincubation with MPP+. Kinetic analyses revealed that inhibition of MPTP was noncompetitive and that of MPP+ uncompetitive with respect to NADH. On the other hand, inhibition of MPTP was uncompetitive and that of MPP+ noncompetitive with respect to ubiquinone. Succinate-ubiquinone oxidoreductase (complex II), dihydroubiquinone-cytochrome c oxidoreductase (complex III), and ferrocytochrome c-oxygen oxidoreductase (EC 1.9.3.1) activities were either slightly inhibited or not inhibited by MPTP or MPP+. The significance of these findings is discussed in relation to the mechanism of MPTP-induced neuronal degeneration.