Exogenous Quinones Research Articles

The effect of ring substituents on the mechanism of interaction of exogenous quinones with the mitochondrial respiratory chain

In uncoupled pig-heart mitochondria the rate of the reduction of duroquinone by succinate in the presence of cyanide is inhibited by about 50% by antimycin. This inhibition approaches completion when myxothiazol is also added or British anti-Lewisite-treated (BAL-treated) mitochondria are used. If mitochondria are replaced by isolated succinate:cytochrome c oxidoreductase, the inhibition by antimycin alone is complete. The reduction of a plastoquinone homologue with an isoprenoid side chain (plastoquinone-2) is strongly inhibited by antimycin with either mitochondria or succinate:cytochrome c reductase. The reduction by succinate of plastoquinone analogues with an n-alkyl side chain in the presence of mitochondria is inhibited neither by antimycin nor by myxothiazol, but is sensitive to the combined use of these two inhibitors. On the other hand, the reduction of the ubiquinone homologues Q 2, Q 4, Q 6 and Q 10 and an analogue, 2,3-dimethoxyl-5- n-decyl-6-methyl-1,4-benzoquinone, is not sensitive to any inhibitor of QH 2:cytochrome c reductase tested or their combined use, either in normal or BAL-treated mitochondria or in isolated succinate:cytochrome c reductase. It is concluded that quinones with a ubiquinone ring can be reduced directly by succinate:Q reductase, whereas those with a plastoquinone ring can not. Reduction of the latter compounds requires participation of either center i or center o (Mitchell, P. (1975) FEBS Lett. 56, 1–6) or both, of QH 2:cytochrome c oxidoreductase. It is proposed that a saturated side chain promotes, while an isoprenoid side chain prevents reduction of these compounds at center o.

Studies on the nature of the water-oxidizing enzyme: III. Spectral characterization of the intermediary redox states in the water-oxidizing enzyme system Y

Spectral changes in the ultraviolet region (250–380 mn) that are caused by redox transitions within the water-oxidizing enzyme system Y were analyzed by measuring time-resolved (1–2 μs) absorption changes in dark-adapted PS II membrane fragments. In order to eliminate effects due to the interference with binary oscillation at the acceptor side the experiments were performed in trypsinized (at pH = 6.0) samples with 0.4 mM K 3[Fe(CN) 6] as exogenous oxidant. In the presence of 10 mM CaCl 2 PS II-membrane fragments trypsinized at pH = 6.0 are fully competent in oxygen evolution compared with the control (measured as average oxygen yield per flash). It was found: (1) Excitation of dark-adapted samples with a train of saturating laser flashes (7 ns full width at half maximum, FWHM) induces absorption changes with resolved kinetics in the micro- and millisecond range. Except for the kinetically unresolved initial amplitude the other transient compoents are strongly dependent (in extent and half-life time) on the flash number. (2) After elimination of the oxygen-evolving capacity by 1 mM hydroxylamine (NH 2OH) binary oscillation are completely lacking of absorption changes at 325 nm. The remarkably smaller absorption change in the first flash can be explained by partial oxidation of acceptor ‘C400’ due to K 3[Fe(CN) 6]. (3) A significantly different pattern of the absorption changes is observed if phenyl- p-benzoquinone is used instead of K 3[Fe(CN) 6]. Based on the oscillations of the initial amplitude of the absorption changes, exogenous quinones are assumed to give rise to a more complicated reaction pattern. (4) The spectral analysis performed on the basis of Kok's model (Kok, B., Forbush, B. and McGloin, P.M. (1970) Photochem. Photobiol. 11, 457–475) by the use of α, β and the apparent [ S 0] [ S 1] ratio experimentally determined via oxygen-yield measurement leads to the separation of the difference spectra Δε( S i → S i + 1 ) for the redox transitions in system Y. The obtained difference spectra are almost identical for the S 0 → S 1 and S 2 → S 3 transitions, while for S 1 → S 2 a markedly different spectrum has been observed. The deviations are especially pronounced in the range around 325 nm. (5) The relaxation kinetics of absorption changes at 325 nm that are characterized by a 1 ms half-life time oscillate in their amplitude synchronously with the oxygen yield in a flash train. The amplitude of these kinetics as a function of wavelength corrected for the Z ox Z difference spectrum closely resembles the calculated spectrum for the S 3 → S 0 transition. Based on the experimental data a model is proposed for the molecular mechanism of photosynthetic water oxidation that is an extension of our previous hypothesis (Renger, G. (1977) FEBS Lett. 81, 223–228). Accordingly, the catalytic site for water oxidation is assumed to be a binuclear manganese center which undergoes redox reactions at the manganese centers only during S 1 → S 2 and S 3 → S 0 transitions. The implications of this model are discussed.

Quinones in Biology: Functions in electron transfer and oxygen activation

Abstract The widespread occurrence and function of quinones in diverse biological systems with particular reference to their role in oxygen activation has been reviewed and discussed. The importance of quinones in such basic metabolic processes as respiration and photosynthesis has been well established. Less clear are the conditions which lead to loss of the normal metabolic functions of quinones, often resulting in the generation of active oxygen species. Exogenous quinones have been used extensively as tools to investigate physiological processes. In addition, they are increasingly applied to influence such reactions artificially. In particular, the loss of normal quinone function invariably has toxicological consequences, in which the activation of oxyqen hy quinone redox reactions plays a central role. The present review discusses hiological and toxicological aspects of quinone metabolism. Physical and chemical data are considered in order to attempt to explain the physiological effects.

Evidence for resistance of the microenvironment of the primary plastoquinone acceptor (Q A−·Fe 2+) to mild trypsinization in PS II particles

Mild trypsinization of PS II particles at pH 6.0, which almost completely blocks reoxidation of the primary plastoquinone acceptor, Q A −·Fe 2+, by endogenous plastoquinone (PQ) as well as by exogenous ( p-BQ) quinones, does not affect the shape or the amplitude of the EPR signal attributed to Q A −·Fe 2+. The effect of DCMU on the EPR signal [(1984) FEBS Lett. 105, 156-162] is completely eliminated in mildly trypsinized PS II particles. The lack of effect of mild trypsin treatment on the Q A −·Fe 2+ microenvironment is briefly discussed in relation to the functional and structural organization of the PS II acceptor side. Photosystem II Semiquinone-iron complex Trypsin Herbicide Photosythesis

Activation of site I redox-driven H+ pump by exogenous quinones in intact mitochondria.

The site I redox-driven H+ pump has been activated by the addition of exogenous quinones to antimycin A-KCN-inhibited mitochondria. The rate of quinone reduction and the degree of rotenone sensitivity increase in the order, duroquinone less than ubiquinone0 less than ubiquinone1. Apparent Km, Vmax, and degree of sigmoidicity during e- transfer in the absence and presence of rotenone have been determined for each quinone. The data support the view that the NADH dehydrogenase possesses two redox sites, one accounting for the rotenone-sensitive reduction and another accounting for the rotenone-insensitive reduction. The degree of activation of the redox H+ pump, which reflects the rotenone-sensitive e- transfer, depends, for each quinone, on the relative Km, Vmax, and sigmoidicity of the rotenone-sensitive and insensitive processes. The redox H+ pump activation is highest with ubiquinone1, where the rotenone-sensitive reaction has a lower Km than that of the rotenone-insensitive reaction, and lowest with duroquinone where the rotenone-insensitive reaction has a high Vmax and no sigmoidicity with respect to that of the rotenone-sensitive reaction. Using ubiquinone1 the stoichiometry of the site I redox-driven H+ pump has been determined on either the flow or the force ratios. The flow ratios approached values of 4 H+/2 e- under conditions close to stationary state for H+ pumping and to zero for H+ electrochemical gradient. The force ratio also approached values close to 4 H+/2 e- under static head conditions.

Elevation of quinone reductase activity by anticarcinogenic antioxidants

NAD(P)H:quinone reductase exhibitis broad specificity in the reduction of endogenous and exogenous quinones and quinone imines, such as those derived from polycyclic aromatic carcinogens, phenolic steroids, vitamin K, and numerous therapeutic drugs. This enzyme is found in several cell compartment and is widely distributed among tissues. In contrast to several other flavoprotein dehydrogenases, quinone reductase catalyzes obligatorily two electron reductions. Extensive studies by Huggins and by others have shown that the quinone reductase in liver and some other tissues of rats is inducible by various polycyclic hydrocarbons and aromatic amines, as well as by certain azo dyes. Huggins perceived that the relative effectiveness of such compounds in inducing quinone reductase correlated with their abilities to protect against toxicity and carcinogenesis. Certain antioxidants are also known to protect against the tumorigenic and toxic effects of carcinogens. Studies on the mechanisms underlying the protective effects of BHA, BHT, ethoxyquin, and disulfiram have revealed that these compounds alter the activity profiles of several enzymes which metabolize carcinogenic and toxic compounds. We have observed that quinone reductase specific activity is increased markedly in mouse liver and several extrahepatic tissues in response to dietary BHA, ethoxyquin, and disulfiram, whereas BHT has been shown by others to enhance this enzymatic activity in rat liver. These findings confirm and extend the correlation between the ability to elevate quinone reductase activity and to confer protection against carcinogenesis and toxicity. The broad specificity of quinone reductase, its apparent inability to catalyze one electron reductions of quinones, its widespread distribution, and its inducibility by a variety of structurally dissimilar protective compounds, suggest that quinone reductase may play a significant local protective role in varous regions of the cell.

Production of superoxide radicals and hydrogen peroxide by NADH-ubiquinone reductase and ubiquinol-cytochrome c reductase from beef-heart mitochondria

Complex I (NADH-ubiquinone reductase) and Complex III (ubiquinol-cytochrome c reductase) supplemented with NADH generated O 2 − at maximum rates of 9.8 and 6.5 nmol/min/mg of protein, respectively, while, in the presence of superoxide dismutase, the same systems generated H 2O 2 at maximum rates of 5.1 and 4.2 nmol/min/mg of protein, respectively. H 2O 2 was essentially produced by disproportionation of O 2 −, which constitutes the precursor of H 2O 2. The effectiveness of the generation of oxygen intermediates by Complex I in the absence of other specific electron acceptors was 0.95 mol of O 2 − and 0.63 mol of H 2O 2/mol of NADH. A reduced form of ubiquinone appeared to be responsible for the reduction of O 2 to O 2 −, since (a) ubiquinone constituted the sole common major component of Complexes I and III, (b) H 2O 2 generation by Complex I was inhibited by rotenone, and (c) supplementation of Complex I with exogenous ubiquinones increased the rate of H 2O 2 generation. The efficiency of added quinones as peroxide generators decreased in the order Q 1 > Q 0 > Q 2 > Q 6 = Q 10, in agreement with the quinone capacity of acting as electron acceptor for Complex I. In the supplemented systems, the exogenous quinone was reduced by Complex I and oxidized nonenzymatically by molecular oxygen. Additional evidence for the role of ubiquinone as peroxide generator is provided by the generation of O 2 − and H 2O 2 during autoxidation of quinols. In oxygenated buffers, ubiquinol (Q 0H 2), benzoquinol, duroquinol and menadiol generated O 2 − with k 3 values of 0.1 to 1.4 m − · s −1 and H 2O 2 with k 4 values of 0.009 to 4.3 m −1 · s −1.