Kinetics of redox interaction between substituted 1,4-benzoquinones and ascorbate under aerobic conditions: Critical phenomena

Abstract

Redox cycling is believed to be the most general molecular mechanism of quinone (Q) cytotoxicity. Along with redox cycling induced by a reductase, a similar process is known to occur via electron transfer from ascorbate (AscH-) to Q with formation of a semiquinone radical (Q.-): (1) Q + AscH- (k1)--> Q.- + Asc.- + H+ (2) Q.- + O2 --> Q + O2.-. The net effect of reactions (1) and (2) provides for the catalytic oxidation of AscH-, with Q serving as a catalyst. In this work, the kinetics of oxygen consumption accompanying this process were studied with several substituted 1,4-benzoquinones (BQ) at 37 degrees C in phosphate buffer, pH 7.40, using the Clark electrode technique. The value of k1 determined from the initial rate of oxygen consumption was typically found to increase when the one-electron reduction potential E(Q/Q.-) shifted to more positive values. With Q, for which E(Q/Q.-) is less than -100 mV, the rate of oxygen uptake (R(OX)) was found to be directly correlated with the [Q][AscH-] value independent of the concentration of individual reagents, remaining constant for a long period. With mono- and dialkyl-substituted 1,4-BQs, for which E(Q/Q.-) is higher than -100 mV, significant deviations from the above simple kinetic regularities were observed. In particular, R(OX) decreased dramatically with time and critical phenomena (the existence of certain concentrations of Q and/or AscH- above or below which the catalytic oxidation of AscH- ceased completely after a non-stationary period of short duration) were observed. These abnormalities can be explained on the basis of the kinetic scheme which contains, in addition to reactions (1) and (2), several side reactions including that between Q.- and AscH-. Implications of critical phenomena discovered in this study for the problems of Q toxicity and vitamin C avitaminosis are discussed.

Ortho-quinone-enhanced ascorbate oxidation. Combined roles of lipid charge and the magnesium cation

“…As shown in this figure, the constant slopes observed in these traces through almost oxygen depletion are consistent with the "standard" behavior of PHQ and NQ reported by Roginsky et al [29] in which the rate ascorbate oxidation is essentially independent of oxygen concentration and consistent with a two-step mechanism (Equations 4 and 5), being the first step rate-determining. A constant slope is also observed for the O 2 traces belonging to LQ, again in agreement with that type of behavior for other studied naphthoquinones [29]. The presence of MgCl 2 and/or lipids did not change the standard behavior feature of the quinones used in this work.…”

“…The presence of MgCl 2 and/or lipids did not change the standard behavior feature of the quinones used in this work. Initial rates of ascorbate oxidation, R ox , are enhanced by the presence of quinone, as reported elsewhere [29]. As observed previously, R ox is further enhanced if Mg +2 and an ortho-quinone (but not the para-quinone) are present (Table I) [7].…”

“…Thus, a more positive could be caused by a more positive E(Q/Q ·− ) value. Since quinones with more positive redox potentials produce larger R ox values [29], this ascorbate oxidation enhancement by o-quinones in the presence of a divalent cation is primarily caused by an increase in the semiquinone stabilization, which results in a more positive quinone redox potential, i.e. E (Q/Q ·− ) is replaced by E(Q, Mg +2 /Q ·− , Mg +2 ).…”

pH Dependent Electrochemistry of Anthracenediones at a Glassy Carbon Electrode

“…5 Most of the biological activities and inhibitory effects of such compounds are related to their redox behavior and hydrogen bonding characteristics. [6][7][8] Moreover, their redox properties are greatly dependent on the nature, number and position of different substituents. The synthesis, selection and application of new bicyclic and tricyclic electroactive compounds with more selective biological activity require the understanding of factors that could tune their redox characteristics.…”

Quinone-enhanced sonochemical production of nitric oxide from s-nitrosoglutathione

“…Quinones are known to catalytically increase the rate of oxygen consumption in the presence of reducing agents [12] and this effect has been found to be dependent on the quinone oneelectron reduction potential [13]. Riesz and Kondo have observed the formation of ubiquinol upon irradiating argon-saturated aqueous ubiquinone solutions with a 50 kHz ultrasonic generator [14].…”