The Na+-translocating NADH:ubiquinone oxidoreductase from Vibrio alginolyticus--redox states of the FAD prosthetic group and mechanism of Ag+ inhibition

The FAD prosthetic group of the Na+-motive NADH:ubiquinone oxidoreductase (Na+-NQR) from Vibrio alginolyticus was investigated by ultraviolet-visible and fluorescence spectroscopy. The reduction of Na+-NQR by excess NADH in the presence of 6-13 microM O2 resulted in the formation of the blue flavosemiquinone radical. If the concentration of dioxygen was further reduced to 0.1 microM O2, neither the reduction of Na+-NQR by NADH nor its reoxidation with ubiquinone-1 (Q-1) yielded a stable flavosemiquinone in equilibrium with reductant or oxidant, respectively, but the fully reduced (Fl(red)H2) or oxidized flavin (Fl(ox)) prevailed. During reoxidation of Fl(red)H2 with Q-1, the intermediate formation of an absorbance band around 800 nm was observed, which was tentatively assigned as the Fl(red)H(-)-NAD+ charge-transfer complex. Complete reoxidation of Fl(red)H2 in Na+-NQR was achieved by a fivefold excess of Q-1 over NADH. These results indicated that only a small fraction of FAD was in the flavosemiquinone redox state during turnover to mediate the electron transfer between the hydride donor, NADH, and the one-electron acceptor [2Fe-2S]. The titration of Na+-NQR with Ag+, a specific inhibitor, was followed by the fluorescence emission spectra of FAD (Fl(ox)). The addition of Ag+ resulted in a marked increase of the flavin fluorescence (16% at 200 nM Ag+), with half-maximal saturation at approximately 50 nM Ag+, indicating dissociation of FAD from the enzyme. The increase in fluorescence intensity correlated with the loss of enzyme activity. Gel filtration of the Ag+-treated Na+-NQR confirmed that FAD had been displaced from the holo-enzyme.

Purification of NADPH-linked and NADH-linked quinone reductases from liver cytosol of sea bream, Pagrus major

Quinone reductases in sea bream, Pagrus major, were investigated using menadione as a model quinone. Both NADPH-linked and NADH-linked quinone reductase activities were detected, in varying degrees, in all tissues examined. In the liver, these activities resided in its microsomal and cytosolic fractions. The cytosolic activity was markedly inhibited by cupric sulfate and p-chloromercuribenzoate. However, little effect was observed with dicoumarol, a potent inhibitor of DT-diaphorase. The NADH-linked activity was more resistant to heat inactivation than the NADPH-linked activity. The NADPH-linked quinone reductase was purified from the liver cytosol by chromatography with DEAE-cellulose, hydroxyapatite and AF-Blue Toyopearl. The molecular weight of the enzyme was estimated to be 68,000 by gel filtration and 32,000 by SDS-PAGE. The NADH-linked quinone reductase was purified from the liver cytosol by heat treatment, fractionation with ammonium sulfate and chromatography with phenyl-Toyopearl, hydroxyapatite, DEAE-cellulose and hydroxyapatite. The molecular weight of the enzyme was estimated to be 124,000 by SDS-PAGE and 126,000 by gel filtration.

Inhibition kinetics of camel lens zeta-crystallin: multiple inhibition studies

The inhibition of camel lens zeta-crystallin by nitrofurantoin (NF) was uncompetitive with respect to co-factor NADPH, (Ki = 90 microM) and competitive with respect to the substrate 9,10-phenanthrenequinone (PQ), (Ki = 50 microM). Inhibition at micromolar concentrations was also observed with dicoumarol, NADP+ and cibacron blue (CB). Theorell-Yonetani double-inhibition analysis showed that NF and dicoumarol were mutually exclusive inhibitors against PQ. However, analysis of NF and NADP+ by a double-inhibition plot showed that they simultaneously bind to the enzyme molecule. These studies demonstrate that NF and dicoumarol share the same site so that both molecules are prevented from binding at the same time, while NF and NADP+ can bind simultaneously to different sites on the enzyme. Although CB was noncompetitive with respect to PQ, double inhibition analysis showed that CB and dicoumarol or NF were mutually exclusive inhibitors against PQ, implying a distinct mode of inhibition for CB.

Characterization of zeta-crystallin inhibition by juglone

Guinea pig lens zeta-crystallin showed hyperbolic saturation curves with 9,10-phenanthrenequinone (PAQ). 5-hydroxy-1,4-naphthoquinone (juglone) and NADPH. Whereas camel lens zeta-crystallin showed hyperbolic saturation curves only with PAQ and NADPH, but slightly segmoidal with juglone. For both enzymes PAQ was the preferred substrate. The catalytic center activity (Kcat) values indicated that camel zeta-crystallin catalyzed the reduction of PAQ more efficiently than the guinea pig lens zeta-crystallin, although the Km values of the two enzymes for this quinone were very similar. The guinea pig lens zeta-crystallin catalyzed the reduction of Juglone far more efficiently than that of the camel lens zeta-crystallin. Juglone did not serve as an efficient substrate for both zeta-crystallins compared to PAQ and appeared to act as a potent competitive inhibitor, with Kl values of 75 nM and 20 microM for guinea pig lens zeta-crystallin and camel lens zeta-crystallin, respectively. Thus, the camel lens zeta-crystallin was less active toward juglone as a substrate as well as less sensitive to its inhibitory action, when compared with guinea pig lens zeta-crystallin. The inhibition mechanism of guinea pig and camel lens zeta-crystallin by juglone is discussed.

Inhibition of camel lens zeta-crystallin/NADPH:quinone oxidoreductase activity by Cibacron blue

Camel lens zeta-crystallin/NADH:quinone oxidoreductase activity was inhibited by Cibacron blue 3GA (CB) with 9.10-phenanthrenequinone (PQ) as an electron acceptor and NADPH as an electron donor in a time-independent and concentration dependent manner. The IC50 value of CB was 50 nM. The Lineweaver-Burk plots and the secondary plots indicated that the inhibition was linear mixed type (partial competitive and pure noncompetitive) with respect to NADPH and noncompetitive with respect to PQ. The estimated inhibition constant (Ki) values were 26.0 nM for NADPH and 55.0 nM for PQ respectively, suggesting that CB has high affinity towards the NADPH binding site. The secondary plots of inhibition with respect to NADPH, also indicate a dissociation constant (Ki) value of 68.0 nM for the zeta-crystallin-NADPH-CB complex. This Ki being greater than the Ki value suggests that noncompetitive inhibition is predominant over competitive inhibition at the NADPH binding site.

Purification and properties of NAD(P)H: (quinone-acceptor) oxidoreductase of sugarbeet cells

NAD(P)H:(quinone-acceptor) oxidoreductase [NAD(P)H-QR], a plant cytosolic protein, was purified from cultured sugarbeet cells by a combination of ammonium sulfate fractionation, FPLC Superdex 200 gel filtration, Q-Sepharose anion-exchange chromatography, and a final Blue Sepharose CL-6B affinity chromatography with an NADPH gradient. The subunit molecular mass is 24 kDa and the active protein (94 kDa) is a tetramer. The isoelectric point is 4.9. The enzyme was characterized by ping-pong kinetics and extremely elevated catalytic capacity. It prefers NADPH over NADH as electron donor (kcat/Km ratios of 1.7 x 10(8) M-1 S-1 and 8.3 x 10(7) M-1 S-1 for NADPH and NADH, respectively, with benzoquinone as electron acceptor). The acridone derivative 7-iodo-acridone-4-carboxylic acid is an efficient inhibitor (I0.5 = 5 x 10(-5) M), dicumarol is weakly inhibitory. The best acceptor substances are hydrophilic, short-chain quinones such as ubiquinone-0 (Q-0), benzoquinone and menadione, followed by duroquinone and ferricyanide, whereas hydrophobic quinones, cytochrome c and oxygen are reduced at negligible rates at best. Quinone acceptors are reduced by a two-electron reaction with no apparent release of free semiquinonic intermediates. This and the above properties suggest some relationship of NAD(P)H-QR to DT-diaphorase, an animal flavoprotein which, however, has distinct structural properties and is strongly inhibited by dicumarol. It is proposed that NAD(P)H-QR by scavenging unreduced quinones and making them prone to conjugation may act in plant tissues as a functional equivalent of DT-diaphorase.

Metabolic activation of nitrodibenzofurans by rat liver in Salmonella/mutagenicity test

The effect of metabolic activation on the mutagenicity of nitrodibenzofurans (NDF) by rat liver S9 was evaluated with S. typhimurium tester strains. Except for 1-nitrodibenzofuran (NDF), five tested NDFs were mutagenic in strains TA98 and TA98/1,8-DNP6 without S9 mix but were not mutagenic in strain TA98NR. NDFs mutagenized strain TA98NR with S9 mix, and the NAD(P)H system plus 3-methylcholanthrene-induced S9 (3-MC-S9) was the most effective. The specificity of S9 enzyme(s) participating in the activation of NDFs was different from that of endogenous enzyme(s) in strain TA98, i.e., the order of mutagenic potency of NDFs in strain TA98 without S9 mix was 2,8- = 2,7-->3-->2-->4-->1-nitrated dibenzofuran and 2-NDF and 2,8-dinitrodibenzofuran (DNDF) were more mutagenic than 3-NDF and 2,7-DNDF, respectively, in strain TA98NR with S9 mix. The mutagenic potency of 2-NDF, 4-NDF, 2,7-DNDF and 2,8-DNDF in strain TA98NR with S9 mix was stronger than those in strain TA98 without S9 mix and the cytosolic fraction of the 3-MC-S9 accounted for more of the activation than the microsomal fraction. Studies with electron donors and inhibitors indicated that xanthine oxidase and/or NAD(P)H-quinone oxido-reductase (NQOR) participated in the activation of NDFs. The mutagenic potency of NDFs in strain TA98NR with S9 mix (3-MC-S9) was reflected in the induction of NQOR by pretreatment of rats with 3-MC.

NADH: ubiquinone oxidoreductase in obligate aerobic yeasts

The strictly aerobic yeasts Candida pinus, Cryptococcus albidus, Rhodotorula minuta, Rhodotorula mucilaginosa and Trichosporon beigelii possess mitochondrial NADH dehydrogenases with significant features of the NADH:ubiquinone oxidoreductase (complex I). These species show in all growth phases and under standard cultivation conditions, NADH dehydrogenases of approximately 700 kDa, which are sensitive to rotenone, a specific inhibitor of this complex. Identical results were obtained with the weakly fermenting C. pinus. The facultatively fermenting yeasts Saccharomyces cerevisiae and Kluyveromyces marxianus do not possess the 700 kDa-complex and are insensitive to rotenone. In S. cerevisiae, a rotenone-insensitive NADH dehydrogenase of about 500-600 kDa is detected only in stationary phase cells. As in Neurospora crassa, upon incubation of the obligately aerobic yeast R. mucilaginosa with chloramphenicol, an intermediate NADH dehydrogenase of approximately 350 kDa was formed, which was insensitive to rotenone.

Na(+)-translocating NADH-quinone reductase of marine and halophilic bacteria

The respiratory chain of marine and moderately halophilic bacteria requires Na+ for maximum activity, and the site of Na(+)-dependent activation is located in the NADH-quinone reductase segment. The Na(+)-dependent NADH-quinone reductase purified from marine bacterium Vibrio alginolyticus is composed of three subunits, alpha, beta, and gamma, with apparent M(r) of 52, 46, and 32 kDa, respectively. The FAD-containing beta-subunit reacts with NADH and reduces ubiquinone-1 (Q-1) by a one-electron transfer pathway to produce ubisemiquinones. In the presence of the FMN-containing alpha-subunit and the gamma-subunit, Q-1 is converted to ubiquinol-1 without the accumulation of free radicals. The reaction catalyzed by the alpha-subunit is strictly dependent on Na+ and is strongly inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), which is tightly coupled to the electrogenic extrusion of Na+. A similar type of Na(+)-translocating NADH-quinone reductase is widely distributed among marine and moderately halophilic bacteria. The respiratory chain of V. alginolyticus contains another NADH-quinone reductase which is Na+ independent and has no energy-transducing capacity. These two types of NADH-quinone reductase are quite different with respect to their mode of quinone reduction and their sensitivity toward NADH preincubation.

The enzymology of doxorubicin quinone reduction in tumour tissue

We have reported previously that enzymes present in the Sp 107 rat mammary carcinoma catalyse doxorubicin quinone reduction (QR) to 7-deoxyaglycone metabolites in vivo [Willmott and Cummings, Biochem Pharmacol 36: 521-526, 1987]. In order to provide insights into the role of QR in the antitumour mechanism of action of doxorubicin, we have attempted in this work to identify the enzyme(s) responsible. NAD(P)H: (quinone acceptor) oxidoreductase (DT-diaphorase) was the major quinone reductase in the tumour accounting for approximately 70% of all the activity measured in microsomes and cytosols (microsomal activity, 28.4 +/- 4.6 nmol/min/mg; cytosolic activity, 94.3 +/- 11.9 nmol/min/mg). Its presence was confirmed by western blot analysis. Low levels of NADH cytochrome b5 reductase (15.6 +/- 6.3 nmol/min/mg) and NADPH cytochrome P450 reductase (14.5 +/- 4.0 nmol/min/mg) were detectable in microsomes. The presence of the latter was confirmed by western blot analysis. Pretreatment of tumours with doxorubicin (48 hr) at a therapeutic dose decreased the level of activity of all the reductases studied by at least 2-fold (P < 0.01, Student's t-test). Doxorubicin was shown not to be a substrate for purified rat Walker 256 tumour DT-diaphorase with either NADH or NADPH as co-factor and utilizing up to 20,000 units of enzyme/incubation but was confirmed to be a substrate for purified rat liver cytochrome P450 reductase. 7-Deoxyaglycone metabolite formation by purified cytochrome P450 reductase had an absolute requirement for NADPH as co-factor, was inhibited by molecular oxygen and dicoumarol (IC50 approx. 50 microM), and modulated by specific reductase antiserum. Reductive deglycoslation of doxorubicin to 7-deoxyaglycones was localized to the microsomal fraction of the Sp 107 tumour, with negligible activity being found in cytosols (NADH, NADPH and hypoxanthine as co-factors) and mitochondria (NADH and NADPH). The tumour microsomal enzyme had an absolute co-factor requirement for NADPH, was inhibited by oxygen and dicoumarol, and modulated by cytochrome P450 reductase antiserum. These data indicate strongly that NADPH cytochrome P450 reductase is the principal enzyme responsible for catalysing doxorubicin QR in the Sp 107 tumour.

Sulfide quinone reductase (SQR) activity in Chlorobium

Membranes of the green sulfur bacterium, Chlorobium limicola f. thiosulfatophilum, catalyze the reduction of externally added isoprenoid quinones by sulfide. This activity is highly sensitive to stigmatellin and aurachins. It is also inhibited by 2-n-nonyl-4-hydroxyquinoline-N-oxide, antimycin, myxothiazol and cyanide. It is concluded that in sulfide oxidizing bacteria like Chlorobium, sulfide oxidation involves a sulfide-quinone reductase (SQR) similar to the one found in Oscilatoria limnetica [Arieli, B., Padan, E. and Shahak, Y. (1991) J. Biol. Chem. 266, 104-111].

Identification and characterization of the enzymatic activity of zeta-crystallin from guinea pig lens. A novel NADPH:quinone oxidoreductase

zeta-Crystallin is a major protein in the lens of certain mammals. In guinea pigs it comprises 10% of the total lens protein, and it has been shown that a mutation in the zeta-crystallin gene is associated with autosomal dominant congenital cataract. As with several other lens crystallins of limited phylogenetic distribution, zeta-crystallin has been characterized as an "enzyme/crystallin" based on its ability to reduce catalytically the electron acceptor 2,6-dichlorophenolindophenol. We report here that certain naturally occurring quinones are good substrates for the enzymatic activity of zeta-crystallin. Among the various quinones tested, the orthoquinones 1,2-naphthoquinone and 9,10-phenanthrenequinone were the best substrates whereas menadione, ubiquinone, 9,10-anthraquinone, vitamins K1 and K2 were inactive as substrates. This quinone reductase activity was NADPH specific and exhibited typical Michaelis-Menten kinetics. Activity was sensitive to heat and sulfhydryl reagents but was very stable on freezing. Dicumarol (Ki = 1.3 x 10(-5) M) and nitrofurantoin (Ki = 1.4 x 10(-5) M) inhibited the activity competitively with respect to the electron acceptor, quinone. NADPH protected the enzyme against inactivation caused by heat, N-ethylmaleimide, or H2O2. Electron paramagnetic resonance spectroscopy of the reaction products showed formation of a semiquinone radical. The enzyme activity was associated with O2 consumption, generation of O2- and H2O2, and reduction of ferricytochrome c. These properties indicate that the enzyme acts through a one-electron transfer process. The substrate specificity, reaction characteristics, and physicochemical properties of zeta-crystallin demonstrate that it is an active NADPH:quinone oxidoreductase distinct from quinone reductases described previously.

Selective inhibition of NADH-CoQ oxidoreductase (complex I) of rat brain mitochondria by arachidonic acid

The effects of arachidonic acid on the enzyme complexes in the electron transport system were investigated using submitochondrial particles from rat brain. Arachidonic acid irreversibly inhibited NADH-CoQ oxidoreductase (complex I) activity, but had no effect on the activities of succinate-CoQ oxidoreductase (complex II), CoQH2-cytochrome c oxidoreductase (complex III), cytochrome c oxidase (complex IV), ATPase (complex V), glutamate dehydrogenase, and malate dehydrogenase up to 50 microM. The inhibition was dose-dependent with an IC50 value of 110 nmol/mg protein. The Lineweaver-Burk plot revealed that the inhibition by arachidonic acid was noncompetitive against CoQ with a Ki value of 33 microM and uncompetitive against NADH with a Ki value of 22 microM.

Photodependent inhibition of rat liver NAD(P)H:quinone acceptor oxidoreductase by (A)-2-azido-NAD+ and (A)-8-azido-NAD

Two photoaffinity analogues of NAD+, (A)-2-azido-NAD+ [nicotinamide 2-azidoadenine dinucleotide] and (A)-8-azido-NAD+ [nicotinamide 8-azidoadenine dinucleotide], have been synthesized, and their reactivities with the rat liver NAD(P)H:quinone acceptor oxidoreductase have been investigated. The reduce nicotinamide nucleotide probes, (A)-2-azido-NADH and (A)-8-azido-NADH, were shown to be substrates of the quinone reductase. This enzyme was inhibited by (A)-8-azido-NADH, were shown to be substrates of the quinone reductase. This enzyme was inhibited by (A)-2-azido-NAD+ and (A)-8-azido-NAD+ in a photodependent manner, and the inhibition of the enzyme could be prevented by the presence of nicotinamide nucleotide substrates during photolysis. (A)-2-Azido-NAD+ was demonstrated to be a more potent inhibitor than (A)-8-azido-NAD+. In addition, the photodependent inhibition by (A)-8-azido-NAD+ increased when menadione, the substrate of the enzyme, was present during the photolysis, while menadione protected the enzyme from the photodependent inhibition by (A)-2-azido-NAD+. These results indicate that these two NAD+ analogues can be used to identify the nicotinamide nucleotide binding site of this quinone reductase and that they probably bind to the enzyme in different fashions.

Effect of exogenous and endogenous nitric oxide on mitochondrial respiration of rat hepatocytes

Although nitric oxide (.N = O) biosynthesis is inducible in rat hepatocytes (HC), the physiological significance of .N = O production by these cells is unknown. Short exposure of HC to authentic .N = O led to a concentration-dependent inhibition of mitochondrial aconitase, NADH-ubiquinone oxidoreductase, and succinate-ubiquinone oxidoreductase (complexes I and II of the mitochondrial electron transport chain). Most susceptible to .N = O inhibition was mitochondrial aconitase, in which a reduction in enzyme activity to 20.2 +/- 1.6% of control was observed. In contrast to mitochondrial aconitase, cytosolic aconitase activity was not inhibited by .N = O. After exposure to a maximal inhibitory concentration of .N = O, mitochondrial aconitase activity recovered completely within 6 h. Complex I did not fully recover within this incubation period. Endogenous .N = O biosynthesis was induced in HC by a specific combination of cytokines and lipopolysaccharide. After 18 h of incubation with these stimuli, a significant inhibition of mitochondrial aconitase activity to 70.8 +/- 2.4% of controls was detected. However, this was due only in part to the action of .N = O. A non- .N = O-dependent inhibition of mitochondrial function appeared to be mediated by tumor necrosis factor.

Induction of 8-hydroxydeoxyguanosine but not initiation of carcinogenesis by redox enzyme modulations with or without menadione in rat liver

Inducibility of oxidative stress in rat liver in vivo by menadione-associated redox cycling activation under redox enzyme modulating conditions was examined by monitoring hepatocyte injury and 8-hydroxydeoxyguanosine (8-OHdG) levels of liver DNA. In addition, the treatment-associated liver tumor initiating activity was assessed in terms of development of gamma-glutamyl-transpeptidase (GGT)- and glutathione S-transferase placental form (GST-P)-positive foci and hyperplastic nodules. With or without following menadione treatment (50 mg/kg, i.g.), redox enzyme modulations of increased cytochrome P450 reductase activity induced by phenobarbital (PB)-Na (100 mg/kg, i.p. for 5 days), inhibition of DT-diaphorase by dicumarol (25 mg/kg, i.p.) and depletion of glutathione by phorone (200 mg/kg, i.p.), with or without further supplement of iron EDTA-Na-Fe(III) (70 mg/kg, i.p.), caused both substantial hepatocyte necrosis and 8-OHdG production in Fischer 344 male rats. Subsequent feeding with a 0.05% PB diet for 64 weeks resulted in slightly increased development of GGT-positive foci but not GST-P positive lesions or hyperplastic nodules, suggesting a lack of tumor-initiating activity of the oxidative DNA damage associated with redox enzyme modulations with or without menadione.

Enhanced NAD(P)H:quinone reductase activity prevents glutamate toxicity produced by oxidative stress

Glutamate toxicity in the N18-RE-105 neuronal cell line results from the inhibition of high-affinity cystine uptake, which leads to a depletion of glutathione and the accumulation of oxidants. Production of superoxides by one-electron oxidation/reduction of quinones is decreased by NAD(P)H:quinone reductase, an enzyme with DT-diaphorase activity. Using glutamate toxicity in N18-RE-105 cells as a model of neuronal oxidative stress, we report that the degree of glutamate toxicity observed is inversely proportional to quinone reductase activity. Induction of quinone reductase activity by treatment with t-butylhydroquinone reduced glutamate toxicity by up to 80%. In contrast, treatment with the quinone reductase inhibitor dicumarol potentiated the toxic effect of glutamate. Measurement of cellular glutathione indicates that increases in its levels are not responsible for the protective effect of t-butylhydroquinone treatment. Because many types of cell death may involve the formation of oxidants, induction of quinone reductase may be a new strategy to combat neurodegenerative disease.

Effect of endogenous nitric oxide on mitochondrial respiration of rat hepatocytes in vitro and in vivo

Nitric oxide, a highly reactive radical, was recently identified as an intermediate of L-arginine metabolism in mammalian cells. We have shown that nitric oxide synthesis is induced in vitro in cultured hepatocytes by supernatants from activated Kupffer cells or in vivo by injecting rats with nonviable Corynebacterium parvum. In both cases, nitric oxide biosynthesis in hepatocytes was associated with suppression of total protein synthesis. This study attempts to determine the effect of nitric oxide biosynthesis on the activity of specific hepatocytic mitochondrial enzymes and to determine whether inhibition of protein synthesis is caused by suppression of energy metabolism. Exposure of hepatocytes to supernatants from activated Kupffer cells led to a 30% decrease of aconitase (Krebs cycle) and complex I (mitochondrial electron transport chain) activity. Using NG-monomethyl-L-arginine, an inhibitor of nitric oxide synthesis, we demonstrated that the inhibition of mitochondrial aconitase activity was due, in part, to the action of nitric oxide. In contrast, in vivo nitric oxide synthesis of hepatocytes from Corynebacterium parvum-treated animals had no effect on mitochondrial respiration. This suggests that inhibition of protein synthesis by nitric oxide is not likely to be mediated by inhibition of energy metabolism.

Synthesis of 3-(4-azido-5-iodosalicylamido)-4-hydroxycoumarin: photoaffinity labeling of rat liver dicoumarol-sensitive NAD(P)H: quinone reductase

A photoaffinity analog of 4-hydroxycoumarin containing an arylazido derivative at the 3-position has been synthesized and characterized. This compound, 3-(4-azido-5-iodosalicylamido)-4-hydroxycoumarin, serves as a strong competitive inhibitor of the dicoumarol-sensitive NAD(P)H: quinone reductase (DT-diaphorase) from rat liver, having an apparent inhibition constant of 4.2 10(-7) M. Irradiation of the reductase with ultraviolet light in the presence 10 microM of the photoprobe resulted in the covalent labeling of 2% of the reductase molecules. The enzyme is protected from labeling to greater than 99% by the inclusion of 3 microM dicoumarol, consistent with the specific labeling of the 4-hydroxycoumarin binding site of this enzyme. Furthermore, the quinone reductase was shown to specifically labeled by the probe even when contained within crude fractions rat liver cytosol.

Menadione partially restores NADH-oxidation and ATP-synthesis in complex I deficient fibroblasts

In this paper we report our studies on the effects of menadione in cultured fibroblasts treated with rotenone to block complex I. A normalization of the lactate to pyruvate ratio after incubation with glucose, an increased production of 14CO2 from [6-14C]glucose and an increased intra-cellular concentration of ATP was observed in the presence of micromolar concentrations of menadione. These results not only demonstrate the potential value of menadione in complex I deficient patients but also suggest that this system can be used advantageously for the in vitro assessment of therapeutic agents for disorders of the mitochondrial respiratory chain.

Inhibition by capsaicin of NADH-quinone oxidoreductases is correlated with the presence of energy-coupling site 1 in various organisms

The NADH-ubiquinone reductase activity of the respiratory chains of several organisms was inhibited by capsaicin and dihydrocapsaicin, which are the pungent principles of red pepper. This inhibition was correlated with the presence of an energy transducing site in this segment of the respiratory chain. Where the NADH-quinone oxidoreductase segment involved an energy coupling site (e.g., in Paracoccus denitrificans, Escherichia coli, and Thermus thermophilus HB-8 membranes and bovine heart mitochondria), capsaicin acted as an inhibitor of ubiquinone reduction by NADH. In contrast, where this energy coupling site was absent (e.g., in Saccharomyces cerevisiae mitochondria and Bacillus subtilis membranes), there was no inhibition of NADH-ubiquinone reductase activity by capsaicin. The capsaicin inhibition of Paracoccus membranes was reversed by washing the membranes with medium containing bovine serum albumin. In the E. coli and Paracoccus membranes and bovine submitochondrial particles, capsaicin acted as a noncompetitive inhibitor for ubiquinone-1 at lower concentrations of ubiquinone-1 (less than 20 microM) and as a competitive inhibitor at higher concentrations of ubiquinone-1 (greater than 50 microM). In addition, the concentrations of capsaicin required for 50% inhibition of NADH oxidase activity of bovine submitochondrial particles were increased when ubiquinone-10 was added to the particles. The mechanism by which capsaicin inhibits the energy-transducing NADH-quinone oxidoreductase is discussed.

New evidence for the dimeric nature of NADH:Q oxidoreductase in bovine-heart submitochondrial particles

The initial velocity of NADH oxidation by bovine-heart submitochondrial particles was measured at pH 8.0 after pretreatment of these particles with different amounts of the inhibitor piericidine A together with 0.035 mM NADH. The amount of piericidine A required to fully inhibit the NADH oxidation activity extrapolated to exactly 1.0 per Fe-S cluster 2 of NADH:Q oxidoreductase. When no reducing equivalents from NADH were present during the pretreatment, this ratio was 1.2. The difference is explained by assuming that NADH:Q oxidoreductase binds piericidine A more effectively in the reduced state than in the oxidized state. It was also found that after Q10-extraction and reincorporation of submitochondrial particles, the amount of piericidine A required to fully inhibit the NADH oxidation activity of the particles increased with the amount of Q10 present during reincorporation. This is explained by assuming that binding of piericidine A, to the inhibitory site of NADH:Q oxidoreductase requires Q10. When 0.035 mM NADPH instead of NADH was present during the pretreatment of submitochondrial particles with piericidine A, the amount of inhibitor per cluster 2 required to fully inhibit the initial NADH-oxidation activity extrapolated to 0.5. This result strongly suggests that NADH:Q oxidoreductase is a functional dimer.

Inhibition of rat liver NAD(P)H:quinone acceptor oxidoreductase (DT-diaphorase) by flavonoids isolated from the Chinese herb scutellariae radix (Huang Qin)

The glucuronide conjugates of oroxylin A and two other flavones, baicalein, and wogonin, were isolated from the methanol extract of the herb scutellariae radix (Huang Qin) and were found to be inhibitors of rat liver NAD(P)H:quinone acceptor oxidoreductase (EC 1.6.99.2). Baicalin (baicalein 7-O-glucuronide) and oroxylin-A 7-O-glucuronide are approximately 50-fold more potent than wogonin 7-O-glucuronide. The enzyme kinetic analysis revealed that oroxylin-A 7-O-glucuronide is a competitive inhibitor with respect to NADH (the electron donor), with a Ki value of 63 nM. Considering the similarities of their structures and inhibition kinetics to those of dicoumarol, it is thought that oroxylin-A 7-O-glucuronide and the other two flavonoids bind to an identical site and inhibit this quinone reductase in the same fashion as dicoumarol. The results also suggest that the inhibition of NAD(P)H:quinone acceptor oxidoreductase or another vitamin K reductase by oroxylin-A 7-O-glucuronide and the related flavonoids may be one of the steps associated with the anticoagulation action of the herb. These compounds are potentially useful anticoagulant drugs.

NAD(P)H: quinone oxidoreductase (DT-diaphorase) in chick embryo liver. Comparison to activity in rat and guinea pig liver and differences in co-induction with 7-ethoxyresorufin deethylase by 2,3,7,8-tetrachlorodibenzo-p-dioxin

NAD(P)H:quinone oxidoreductase (EC 1.6.99.2; DT-diaphorase) was present in the liver of 18- and 19-day-old chick embryos as assayed both by reduction of resorufin and by the more traditional assay, reduction of 2,6-dichlorophenolindophenol (DCPIP). Both reductions had the classic characteristics of DT-diaphorase: they were equally supported by NADPH and NADH and almost entirely inhibited by dicumarol. Chick embryo liver DT-diaphorase was entirely cytosolic. It was undetectable in the microsomal and mitochondrial fractions. Chick embryo liver cytosol and mitochondrial fractions contained an enzyme oxidizer of resorufin but not of DCPIP. The Km for NADPH for resorufin reductase was an order of magnitude higher in chick embryo than in rat or guinea pig cytosol (1 mM vs 0.1 mM). Resorufin reductase activity was higher for chick embryo than for rat or guinea pig cytosols: Vmax (nmol resorufin reduced per mg cytosolic protein per min +/- SEM) 355 +/- 28 for chick embryo, 159 +/- 10 for guinea pig and 68 +/- 28 for rat. The Vmax for DCPIP reduction was also twice as high in chick embryo as rat liver cytosol. In the chick embryo, 7 days after treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at 6.4 micrograms/kg egg (1 nmol/egg) mortality was increased 2.4-fold, hepatic DT-diaphorase 1.3-fold, and 7-ethoxyresorufin deethylase (7-EROD) 72-fold over control levels. At 32 micrograms/kg, mortality was increased 4.2-fold, DT-diaphorase 2.3-fold and 7-EROD 100-fold. In the guinea pig, 5 days after treatment with TCDD at 10 micrograms/kg, TCDD toxicity was also evident (loss of body weight and thymus weight); there was no change in DT-diaphorase as measured by resorufin reduction, confirming by a different assay the observation of Beatty and Neal (Biochem Pharmacol 27: 505-510, 1978) that TCDD does not induce DT-diaphorase in guinea pig liver, and 7-EROD was increased 8-fold. In contrast, in the rat, 7 days after exposure to TCDD at 10 micrograms/kg, there was no evidence of toxicity, DT-diaphorase was increased close to 7-fold and 7-EROD, 100-fold. The results demonstrate that avian liver contains DT-diaphorase and show that the extent to which DT-diaphorase is part of the pleiotypic response of the liver to an Ah (aryl hydrocarbon) receptor ligand is species dependent. They also suggest that DT-diaphorase induction and TCDD toxicity may be inversely related. The possibility that DT-diaphorase protects against TCDD toxicity and participates in species differences in sensitivity to TCDD toxicity warrants further investigation.

Effect of mitochondrial inhibitors on adenosinetriphosphate levels in Plasmodium falciparum

1. The effects of mitochondrial inhibitors on the ATP levels of intraerythrocytic Plasmodium falciparum have been studied. 2. Changes in parasite ATP or ADP levels with time in response to various mitochondrial inhibitors appear quite complex; ATP levels may be initially depressed and then elevated above normal, but the nature of the response depends upon the stage in the intraerythrocytic cycle and in some cases upon the concentration of the inhibitor used. 3. After ca 2 hr incubation of cultures with inhibitors ATP levels appear to be stabilized and are similar to those of untreated parasites. However, ADP levels of trophozoites show significant increases after a 2 hr incubation with inhibitors, particularly with oligomycin and to a lesser extent with antimycin A; increases in ADP levels however were not observed in ring-stages of the parasite. 4. Inhibition of red cell and parasite glycolysis leads to rapid decreases in parasite ATP levels which are not significantly affected by oligomycin. Incubation of in vitro cultures with oligomycin can result in a decreased, rather than increased rate of lactate production with a concomitant appearance of pyruvate in the growth medium. 5. This investigation would indicate that if there is a mitochondrial contribution to the parasite ATP pool it is relatively small, and that a short-fall in this contribution is quickly compensated for by ATP from other source(s), although this is not necessarily met by increased glycolysis.