Reactivity of the sulfhydryl groups of soluble succinate dehydrogenase

Computationally modeling mammalian succinate dehydrogenase kinetics identifies the origins and primary determinants of ROS production

Succinate dehydrogenase (SDH) is an inner mitochondrial membrane protein complex that links the Krebs cycle to the electron transport system. It can produce significant amounts of superoxide ([Formula: see text]) and hydrogen peroxide (H₂O₂); however, the precise mechanisms are unknown. This fact hinders the development of next-generation antioxidant therapies targeting mitochondria. To help address this problem, we developed a computational model to analyze and identify the kinetic mechanism of [Formula: see text] and H₂O₂ production by SDH. Our model includes the major redox centers in the complex, namely FAD, three iron-sulfur clusters, and a transiently bound semiquinone. Oxidation state transitions involve a one- or two-electron redox reaction, each being thermodynamically constrained. Model parameters were simultaneously fit to many data sets using a variety of succinate oxidation and free radical production data. In the absence of respiratory chain inhibitors, model analysis revealed the 3Fe-4S iron-sulfur cluster as the primary [Formula: see text] source. However, when the quinone reductase site is inhibited or the quinone pool is highly reduced, [Formula: see text] is generated primarily by the FAD. In addition, H₂O₂ production is only significant when the enzyme is fully reduced, and fumarate is absent. Our simulations also reveal that the redox state of the quinone pool is the primary determinant of free radical production by SDH. In this study, we showed the importance of analyzing enzyme kinetics and associated side reactions in a consistent, quantitative, and biophysically detailed manner using a diverse set of experimental data to interpret and explain experimental observations from a unified perspective.

Photoelectron spectroscopy and density functional theory studies of (FeS)mH- (m = 2-4) cluster anions: effects of the single hydrogen

Single hydrogen containing iron hydrosulfide cluster anions (FeS)_mH^- (m = 2-4) are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by Density Functional Theory (DFT) calculations. The structural properties, relative energies of different spin states and isomers, and the first calculated vertical detachment energies (VDEs) of different spin states for these (FeS)_mH^- (m = 2-4) cluster anions are investigated at various reasonable theory levels. Two types of structural isomers are found for these (FeS)_mH^- (m = 2-4) clusters: (1) the single hydrogen atom bonds to a sulfur site (SH-type); and (2) the single hydrogen atom bonds to an iron site (FeH-type). Experimental and theoretical results suggest such available different SH- and FeH-type structural isomers should be considered when evaluating the properties and behavior of these single hydrogen containing iron sulfide clusters in real chemical and biological systems. Compared to their related, respective pure iron sulfur (FeS)_m^- clusters, the first VDE trend of the diverse type (FeS)_mH_0,1^- (m = 1-4) clusters can be understood through (1) the different electron distribution properties of their highest singly occupied molecular orbital employing natural bond orbital analysis (NBO/HSOMO), and (2) the partial charge distribution on the NBO/HSOMO localized sites of each cluster anion. Generally, the properties of the NBO/HSOMOs play the principal role with regard to the physical and chemical properties of all the anions. The change of cluster VDE from low to high is associated with the change in nature of their NBO/HSOMO from a dipole bound and valence electron mixed character, to a valence p orbital on S, to a valence d orbital on Fe, and to a valence p orbital on Fe or an Fe-Fe delocalized valence bonding orbital. For clusters having the same properties for NBO/HSOMOs, the partial charge distributions at the NBO/HSOMO localized sites additionally affect their VDEs: a more negative or less positive localized charge distribution is correlated with a lower first VDE. The single hydrogen in these (FeS)_mH^- (m = 2-4) cluster anions is suggested to affect their first VDEs through the different structure types (SH- or FeH-), the nature of the NBO/HSOMOs at the local site, and the value of partial charge number at the local site of the NBO/HSOMO.

Catalytic mechanisms of complex II enzymes: a structural perspective

Over a decade has passed since the elucidation of the first X-ray crystal structure of any complex II homolog. In the intervening time, the structures of five additional integral-membrane complex II enzymes and three homologs of the soluble domain have been determined. These structures have provided a framework for the analysis of enzymological studies of complex II superfamily enzymes, and have contributed to detailed proposals for reaction mechanisms at each of the two enzyme active sites, which catalyze dicarboxylate and quinone oxidoreduction, respectively. This review focuses on how structural data have augmented our understanding of catalysis by the superfamily. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Thioredoxin and lipoic acid catalyze the denitrosation of low molecular weight and protein S-nitrosothiols

The nitrosation of cellular thiols has attracted much interest as a regulatory mechanism that mediates some of the pathophysiological effects of nitric oxide (NO). In cells, virtually all enzymes contain cysteine residues that can be subjected to S-nitrosation, whereby this process often acts as an activity switch. Nitrosation of biological thiols is believed to be mediated by N2O3, metal-nitrosyl complexes, and peroxynitrite. To date, however, enzymatic pathways for S-denitrosation of proteins have not been identified. Herein, we present experimental evidence that two ubiquitous cellular dithiols, thioredoxin and dihydrolipoic acid, catalyze the denitrosation of S-nitrosoglutathione, S-nitrosocaspase 3, S-nitrosoalbumin, and S-nitrosometallothionenin to their reduced state with concomitant generation of nitroxyl (HNO), the one-electron reduction product of NO. In these reactions, formation of NO and HNO was assessed by ESR spectrometry, potentiometric measurements, and quantification of hydroxylamine and sodium nitrite as end reaction products. Nitrosation and denitrosation of caspase 3 was correlated with its proteolytic activity. We also report that thioredoxin-deficient HeLa cells with mutated thioredoxin reductase denitrosate S-nitrosothiols less efficiently. We conclude that both thioredoxin and dihydrolipoic acid may be involved in the regulation of cellular S-nitrosothiols.

Purification and characterization of Plasmodium falciparum succinate dehydrogenase

Succinate dehydrogenase (SDH), a Krebs cycle enzyme and complex II of the mitochondrial electron transport system was purified to near homogeneity from the human malarial parasite Plasmodium falciparum cultivated in vitro by FPLC on Mono Q, Mono S and Superose 6 gel filtration columns. The malarial SDH activity was found to be extremely labile. Based on Superose 6 FPLC, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and nondenaturing-PAGE analyses, it was demonstrated that the malarial enzyme had an apparent native molecular mass of 90 +/- 8 kDa and contained two major subunits with molecular masses of 55 +/- 6 and 35 +/- 4 kDa (n = 8). The enzymatic reaction required both succinate and coenzyme Q (CoQ) for its maximal catalysis with Km values of 3 and 0.2 microM, and k(cat) values of 0.11 and 0.06 min(-1), respectively. Catalytic efficiency of the malarial SDH for both substrates were found to be relatively low (approximately 600-5000 M(-1) s(-1)). Fumarate, malonate and oxaloacetate were found to inhibit the malarial enzyme with Ki values of 81, 13 and 12 microM, respectively. The malarial enzyme activity was also inhibited by substrate analog of CoQ, 5-hydroxy-2-methyl-1,4-naphthoquinone, with a 50% inhibitory concentration of 5 microM. The quinone had antimalarial activity against the in vitro growth of P. falciparum with a 50% inhibitory concentration of 0.27 microM and was found to completely inhibit oxygen uptake of the parasite at a concentration of 0.88 microM. A known inhibitor of mammalian mitochondrial SDH, 2-thenoyltrifluoroacetone. had no inhibitory effect on both the malarial SDH activity and the oxygen uptake of the parasite at a concentration of 50 microM. Many properties observed in the malarial SDH were found to be different from the host mammalian enzyme.

A succinate dehydrogenase with novel structure and properties from the hyperthermophilic archaeon Sulfolobus acidocaldarius: genetic and biophysical characterization

The sdh operon of Sulfolobus acidocaldarius DSM 639 is composed of four genes coding for the 63.1-kDa flavoprotein (SdhA), the 36.5-kDa iron-sulfur protein (SdhB), and the 32.1-kDa SdhC and 14.1-kDa SdhD subunits. The four structural genes of the sdhABCD operon are transcribed into one polycistronic mRNA of 4.2 kb, and the transcription start was determined by the primer extension method to correspond with the first base of the ATG start codon of the sdhA gene. The S. acidocaldarius SdhA and SdhB subunits show characteristic sequence similarities to the succinate dehydrogenases and fumarate reductases of other organisms, while the SdhC and SdhD subunits, thought to form the membrane-anchoring domain, lack typical transmembrane alpha-helical regions present in all other succinate:quinone reductases (SQRs) and quinol:ifumarate reductases (QFRs) so far examined. Moreover, the SdhC subunit reveals remarkable 30% sequence similarity to the heterodisulfide reductase B subunit of Methanobacterium thermoautotrophicum and Methanococcus jannaschii, containing all 10 conserved cysteine residues. Electron paramagnetic resonance (EPR) spectroscopic studies of the purified enzyme as well as of membranes revealed the presence of typical S1 [2Fe2S] and S2 [4Fe4S] clusters, congruent with the deduced amino acid sequences. In contrast, EPR signals for a typical S3 [3Fe4S] cluster were not detected. However, EPR data together with sequence information implicate the existence of a second [4Fe4S] cluster in S. acidocaldarius rather than a typical [3Fe4S] cluster. These results and the fact that the S. acidocaldarius succinate dehydrogenase complex reveals only poor activity with caldariella quinone clearly suggest a unique structure for the SQR of S. acidocaldarius, possibly involving an electron transport pathway from the enzyme complex into the respiratory chain different from those for known SQRs and QFRs.

Sequence comparison between the flavoprotein subunit of the fumarate reductase (complex II) of the anaerobic parasitic nematode, Ascaris suum and the succinate dehydrogenase of the aerobic, free-living nematode, Caenorhabditis elegans

Complex II in adult mitochondria of the parasitic nematode, Ascaris suum, exhibits high fumarate reductase activity and plays a key role in the anaerobic electron-transport observed in these organelles. In the present study, cDNAs for the flavoprotein (Fp) subunits of complex II have been isolated, cloned and sequenced from both A. suum and the aerobic, free-living nematode, Caenorhabditis elegans. Additional sequence at the 3' end of the mRNAs was determined by the Rapid Amplification of cDNA Ends (RACE). Nucleotide sequence analysis of the A. suum cDNAs revealed a 22-nucleotide trans-spliced leader sequence characteristic of many nematode mRNAs, an open reading frame of 1935 nucleotides and a 3' untranslated region of 616 nucleotides including a poly (A) tail from a polyadenylation signal (AATAAA). The open reading frame encoded a 645 amino acid sequence, including a 30 amino acid mitochondrial presequence. The amino acid sequences for the Fp subunits from both organisms were very similar, even though the ascarid enzyme functions physiologically as a fumarate reductase and the C. elegans enzyme a succinate dehydrogenase. The ascarid sequence was much less similar to the Escherichia coli fumarate reductase. The sensitivity of other Fp subunits to sulfhydryl reagents appears to reside in a cysteine immediately preceding a conserved arginine in the putative active site. In both nematode sequences, this cysteine is replaced by serine even though the succinate dehydrogenase activity of both enzymes is still sensitive to sulfhydryl inhibition. A cysteine six residues upstream of the serine may be involved in the sulfhydryl sensitivity of the nematode enzymes. Surprisingly, in contrast to succinate dehydrogenase activity, the fumarate reductase activity of the ascarid enzyme was not sensitive to sulfhydryl inhibition, suggesting that the mechanism of the two reactions involves separate catalytic processes.

Inhibition of membrane-bound succinate dehydrogenase by fluorescamine

Fluorescamine rapidly inactivated membrane-bound succinate dehydrogenase. The inhibition of the enzyme by this reagent was prevented by succinate and malonate, suggesting that the group modified by fluorescamine was located at the active site. The modification of the active site sulfhydryl group by 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) did not alter the inhibitory action of fluorescamine. However, the protective effect of malonate against fluorescamine inhibition was abolished in the enzyme modified at the thiol.

Fumarate reductase activity of bovine heart succinate-ubiquinone reductase. New assay system and overall properties of the reaction

A simple system for aerobic assay of the quinol-fumarate reductase reaction catalyzed by purified soluble bovine heart succinate-ubiquinone reductase in the presence of NADH, NAD(P)H-quinone reductase (DT-diaphorase) and an appropriate quinone is described. The reaction is inhibited by carboxin, suggesting that the same quinone/quinol binding site is involved in electron transfer from succinate to ubiquinone and from ubiquinol to fumarate. The kinetic properties of the reaction in both directions and comparative affinities of the substrate binding sites of the enzyme to substrates (products) and competitive inhibitors are reported. Considerable difference in affinity of the substrates binding site to oxaloacetate was demonstrated when the enzyme was assayed in the direct and reverse directions. These results were taken to indicate that the oxidized dicarboxylate-free enzyme is an intermediate during the steady-state succinate-ubiquinone reductase reaction, whereas the reduced dicarboxylate-free enzyme is an intermediate of the steady-state ubiquinol-fumarate reductase reaction. No difference in the reactivity of the substrate-protected cysteine and arginine residues was found when the pseudo-first-order rate constants for N-ethylmaleimide and phenylglyoxal inhibition were determined for oxidized and quinol-reduced enzyme. Quinol-fumarate reductase activity was reconstituted from the soluble succinate dehydrogenase and low-molecular-mass ubiquinone reactivity conferring protein(s). No reduction of cytochrome b was observed in the presence of quinol generating system, whereas S-3 low temperature EPR-detectable iron-sulfur center was completely reduced by quinol under equilibrium (without fumarate) or steady-state (in the presence of fumarate). No significant reduction of ferredoxin type iron-sulfur centers was detected during the steady-state quinol-fumarate oxidoreductase reaction. The data obtained eliminate participation of cytochrome b in the quinol-fumarate reductase reaction and show that the rate limiting step of the overall reaction lies between iron-sulfur center S-3 and lower midpoint potential redox components of the enzyme.

Deleterious effects of disulfiram on the respiratory electron transport system of liver mitochondria

1. The mechanism of action of disulfiram on the respiratory electron transport system of the liver mitochondria was studied in vitro. 2. Disulfiram inhibited the respiration supported by malate-glutamate as well as succinate. 3. Mitochondrial respiration inhibition was dependent upon alteration of -SH groups. 4. The inhibitory action of disulfiram might be related to the crosslinking of several proteins of the inner mitochondrial membrane. 5. The effects described above could be attributed to disulfiram per se and not to the main metabolite diethyldithiocarbamate.

Effect of Ca2+ ions on the slow active/inactive transition of the mitochondrial NADH-ubiquinone reductase

Slow active/inactive transition of the membrane-bound mitochondrial NADH-ubiquinone reductase (Kotlyar, A.B. and Vinogradov, A.D. (1990) Biochim. Biophys. Acta 1019, 151-158) is sensitive to Ca2+ and other divalent cations. Millimolar concentrations of Ca2+ drastically reduce the rate of the turnover-dependent activation of NADH-ubiquinone reductase. When NADH oxidase, the rotenone-sensitive NADH-ubiquinone reductase or the succinate-supported delta mu H+-dependent NAD+ reduction were initiated by the deactivated enzyme preparations all the three activities were strongly inhibited by Ca2+; no sensitivity of these reactions to Ca2+ was observed when the assays were started by the activated enzyme preparations. The affinity of the deactivated enzyme to polyvalent cations was in the following order: Ni2+ greater than Co2+ greater than La3+ greater than Mn2+ greater than Ca2+ approximately Mg2+ greater than Ba2+. Monovalent metal cations had no effect on the slow turnover-dependent enzyme activation. The apparent affinity of the deactivated enzyme to Ca2+ was strongly pH-dependent. The KCa2+ values of 5.7 mM and 0.6 mM at pH 7.5 and 8.5 were determined from the presteady-state kinetics parameters. The spontaneous temperature-dependent deactivation of the enzyme was insensitive to Ca2+. Ca2+ increases the reactivity of the enzyme sulfhydryl group in the deactivated preparations towards N-ethylmaleimide. This effect was also used to quantitate Ca2+ affinity for the enzyme. The KCa2+ values of 1.2 mM and 0.4 mM at pH 8.0 and 9.0, respectively, were determined. The data obtained suggest that Ca2+ content in the mitochondrial matrix may play an important role in the control of NADH oxidation by the respiratory chain.

Inhibition of membrane-bound succinate dehydrogenase by disulfiram

The effect of disulfiram on succinate oxidase and succinate dehydrogenase activities of beef heart submitochondrial particles was studied. Results show that disulfiram inhibits both functions. Succinate and malonate suppress the inhibitory action of disulfiram when succinate dehydrogenase is stabilized in an active conformation. Disulfiram is not able to inhibit the enzyme when succinate dehydrogenase is inactivated by oxaloacetate. The inhibitory effect of disulfiram is reverted by the addition of dithiothreitol. From these results, it is proposed that disulfiram inhibits the utilization of succinate by a direct modification of an -SH group located in the catalytically active site of succinate dehydrogenase.

Study of the interaction of cadmium with membrane-bound succinate dehydrogenase

Cadmium ions inhibit membrane-bound succinate dehydrogenase with a second-order rate constant of 10.42 mM-1 s-1 at pH 7.35 and 25 degrees C. Succinate and malonate protect the enzyme against cadmium ion inhibition. The protection pattern exerted by succinate and malonate suggests that the group modified by cadmium is located at the active site. The pH curve of inactivation by Cd2+ indicates the involvement of an amino acid residue with pKa of 7.23.

New properties of Bacillus subtilis succinate dehydrogenase altered at the active site. The apparent active site thiol of succinate oxidoreductases is dispensable for succinate oxidation

Mammalian and Escherichia coli succinate dehydrogenase (SDH) and E. coli fumarate reductase apparently contain an essential cysteine residue at the active site, as shown by substrate-protectable inactivation with thiol-specific reagents. Bacillus subtilis SDH was found to be resistant to this type of reagent and contains an alanine residue at the amino acid position equivalent to the only invariant cysteine in the flavoprotein subunit of E. coli succinate oxidoreductases. Substitution of this alanine, at position 252 in the flavoprotein subunit of B. subtilis SDH, by cysteine resulted in an enzyme sensitive to thiol-specific reagents and protectable by substrate. Other biochemical properties of the redesigned SDH were similar to those of the wild-type enzyme. It is concluded that the invariant cysteine in the flavoprotein of E. coli succinate oxidoreductases corresponds to the active site thiol. However, this cysteine is most likely not essential for succinate oxidation and seemingly lacks an assignable specific function. An invariant arginine in juxtaposition to Ala-252 in the flavoprotein of B. subtilis SDH, and to the invariant cysteine in the E. coli homologous enzymes, is probably essential for substrate binding.

Oxidation of malate by the mitochondrial succinate-ubiquinone reductase

The purified succinate-ubiquinone reductase catalyzes the L- (or D-) malate: acceptor oxidoreductase reaction with Km for malate of about 2.10(-3) M and initial Vmax of 50 and 100 nmol per min per mg of protein for L- and D-stereoisomers, respectively (25 degrees C, pH 7.0). The reaction rate rapidly decreases both in the absence and presence of L-glutamate and L-glutamate-oxaloacetate transaminase added for trapping of oxaloacetate. Both keto and enol forms of oxaloacetate were found to be strong, slowly dissociating inhibitors of succinate dehydrogenase; the first-order rate constant for the enzyme inhibition by the enol form is about 3 times as high as that by the keto form. Oxidation of malate by succinate dehydrogenase in the presence of the oxaloacetate trapping system occurs at an indefinitely constant rate when enoloxaloacetate, which is an immediate product of the reaction, is rapidly converted into the keto isomer--a substrate for transaminase. A quantitative kinetic scheme for malate oxidation by succinate dehydrogenase which includes two kinetically distinct enzyme-oxaloacetate complexes is proposed, and the specific role of the mitochondrial oxaloacetate keto-enol-tautomerase (EC 5.3.2.2) in the regulation of succinate dehydrogenase is suggested.

Studies on the succinate dehydrogenating system. Interaction of the mitochondrial succinate-ubiquinone reductase with pyridoxal phosphate

The inhibitory effect of pyridoxal phosphate on the Triton X-100 solubilized purified bovine heart succinate-ubiquinone reductase (Choudhry, Z.M., Gavrikova, E.V., Kotlyar, A.B., Tushurashvilli, P.R. and Vinogradov, A.D. (1985) FEBS Lett. 182, 171-175) was studied. The kinetics of the enzyme inactivation by pyridoxal phosphate was found to be strongly dependent both qualitatively and quantitatively on the concentration of the protein-detergent complexes. In the diluted system the inactivation of the ubiquinone-depleted enzyme was completely prevented by the saturating concentrations of Q2, carboxin, thenoiltrifluoroacetone and pentachlorophenol, i.e., by the substrate and specific inhibitors of the enzyme. The protective effects of Q2 and the inhibitors was employed to quantitate the affinities of the ligands to their specific binding sites. Strong difference in the affinity of Q2 to the reduced and oxidized enzyme was found. When the soluble reconstitutively active succinate dehydrogenase was treated with pyridoxal phosphate, the reactivity of the enzyme towards low ferricyanide concentrations and its reconstitutive activity was significantly protected against aerobic inactivation.

Interaction of the membrane-bound succinate dehydrogenase with substrate and competitive inhibitors

The protective effect of dicarboxylates on the active-site-directed inhibition of the membrane-bound succinate dehydrogenase by N-ethylmaleimide, steady-state kinetics methods for Ki and Ks determinations, and equilibrium studies were employed to quantitate the relative affinities of succinate, fumarate, malonate and oxaloacetate to the reduced and oxidized species of the enzyme. A more than 10-fold difference in the relative affinities of the reduced and oxidized succinate dehydrogenase to succinate, fumarate and oxaloacetate is found, whereas the reactivity of the active-site sulphydryl group does not depend on the redox state of the enzyme. The redox-state-dependent changes in the affinity of the membrane-bound succinate dehydrogenase to oxaloacetate can be quantitatively accounted for by a 10-fold increase in the rate of dissociation of the enzyme-inhibitor complex which occurs upon reduction of the enzyme. The data obtained give no support for either the existence of a sulphydryl group other than the active-site one important for the catalysis or for the presence of a separate dicarboxylate-specific regulatory site in the succinate dehydrogenase molecule.

Sulphate ion-induced slow transformation of succinate dehydrogenase

A new type of slow change of succinate dehydrogenase (EC 1.3.99), activity which is induced by sulphate ion is described. After preincubation of submitochondrial particles or soluble succinate dehydrogenase with sulphate both preparations catalyze succinate:phenazine methosulphate reductase reaction with a significant lag. When added to the assay medium sulphate ion induces biphasic time-dependent competitive inhibition of the enzyme. The sulphate-induced inhibition is apparently due to a rapid interaction of the anion with an active site of the enzyme which is followed by a slow pH-dependent (pKa = 7.2) transformation of the enzyme-inhibitor complex. pH profiles of the overall succinate dehydrogenase reaction and of equilibrium between fast and slow enzyme-sulphate complexes suggest that the same protolytic equilibrium step is involved in the formation of an active intermediate and an inactive enzyme-sulphate complex.

Studies on the succinate dehydrogenating system. II. Reconstitution of succinate-ubiquinone reductase from the soluble components

1. A protein fraction containing three polypeptides (the major one with Mr < 13 000) was isolated by means of Triton X-100 extraction of submitochondrial particles specifically treated to remove succinate dehydrogenase. 2. The mixing of the protein fraction with the soluble reconstitutively active succinate dehydrogenase results in formation of highly active succinate-DCIP reductase which is sensitive to thenoyltrifluoroacetone or carboxin. 3. The maximal turnover number of succinate dehydrogenase in the succinate-DCIP reductase reaction revealed in the presence of a saturating amount of the protein fraction is slightly higher than that measured with phenazine methosulfate as artificial electron acceptor. 4. The protein fraction greatly increases the stability of soluble succinate dehydrogenase under aerobic conditions. 5. The titration of soluble succinate dehydrogenase by the protein fraction shows that smaller amounts of the protein fraction are required to block the reduction of ferrycyanide by Hipip center than that required to reveal the maximal catalytic capacity of the enzyme. 6. The apparent Km of the reconstituted system for DCIP depends on the amount of protein fraction; the more protein fraction added to the enzyme, the lower the Km value obtained. 7. A comparison of different reconstituted succinate-ubiquinone reductases described in the literature is presented and the possible arrangement of the native and reconstituted succinate-ubiquinone region of the respiratory chain is discussed.

Studies on the succinate dehydrogenating system. I. Kinetics of the succinate dehydrogenase interaction with a semiquindiimine radical of N,N,N',N'-tetramethyl-p-phenylenediamine

1. The activities of the soluble reconstitutively active succinate dehydrogenase (EC 1.3.99.1) measured with three artificial electron acceptors, e.g. ferricyanide, phenazine methosulfate and free radical of N,N,N',N'-tetramethyl-p-phenylenediamine (WB), have been compared. The values estimated by extrapolation to infinite acceptor concentration using double reciprocal plots 1/v versus 1/[acceptor] are nearly the same for ferricyanide and phenazine methosulfate and about twice as high for the WB. 2. The double reciprocal plots 1/v versus 1/[succinate] in the presence of malonate at various concentrations of WB give a series of straight lines intercepting in the third quadrant. The data support the mechanism of the overall reaction, in which the reduced enzyme is oxidized by WB before dissociation of the enzyme-product complex. 3. The dependence of the rate of the overall reaction on WB concentration shows that only one kinetically significant redox site of the soluble succinate dehydrogenase is involved in the reduction of WB. 4. Studies of the change of V and Km values during aerobic inactivation of the soluble enzyme suggest that only 'the low Km ferricyanide reactive site' (Vinogradov, A.D., Gavrikova, E.V. and Goloveshkina, V.G. (1975) Biochem. Biophys, Res. Commun. 65, 1264--1269) is involved in reoxidation of the reduced enzyme by WB. 5. The pH dependence of V for the succinate-WB reductase reaction shows that the group of the enzyme with the pKa value of 6.7 at 22 degrees C is responsible for the reduction of dehydrogenase in the enzyme-substrate complex. 6. When WB interacts with the succinate-ubiquinone region of the respiratory chain, the double reciprocal plot 1/v versus 1/[WB] gives a straight line. The thenoyltrifluoroacetone inhibition of succinate-ubiquinone reductase or extraction of ubiquinone alter the 1/v versus 1/[WB] plots for the curves with a positive initial slope intercepting the ordinate at the same V as in the native particles. The data support the mechanism of succinate-ubiquinone reduction, in which no positive modulation of succinate dehydrogenase by ubiquinone exist in the membrane.