Zilvinas Anusevicius

Structure-Activity Relationships in Two-Electron Reduction of Quinones

Publisher Summary This chapter analyzes the structure-activity relationships in two-electron reduction of quinines. Quinones may accept electrons from various flavoenzymes, iron-sulfur proteins and photosynthetic reaction centers. The energetics of the quinine reduction are studied extensively by pulse-radiolysis, electron spin resonance, and electrochemical techniques. The single-electron reduction of quinones by flavoenzyme dehydrogenases-electrontransferases may be treated according to an ‘‘outer-sphere electron transfer’’ model. In general, the reaction rates increase with an increase in quinine single-electron reduction potential. In this chapter, it is demonstrated that multiparameter regression analysis, by using redox potential and several simple structural parameters of quinones, may provide important information on the mechanisms of two-electron enzymatic reduction. In view of the simplicity of the single-electron reduction mechanism and the presumable involvement of single-electron transfers in two-electron reduction, the single-electron reduction of quinones is analyzed. Single-electron reduction of quinones by flavoenzymes is analyzed. The chapter presents the outer-sphere electron transfer model in single-electron reduction of quinones. Structure-activity relationships in single-electron reduction of quinones by NADPH:Cytochrome P-450 reductase and ferredoxin:NADP + reductase are explored. Two-electron reduction of quinones by flavoenzymes is explained in the chapter. The chapter describes the mechanism of two-electron (Hydride) transfer.

The toxicity of aromatic nitrocompounds to bovine leukemia virus-transformed fibroblasts: the role of single-electron reduction

Bovine leukemia virus-transformed lamb embryo fibroblasts (line FLK) possess activity of DT-diaphorase of ca. 260 U/mg protein and similar levels of other NADP(H)-oxidizing enzymes: NADH:oxidase, 359 U/mg; NADPH:oxidase, 43 U/mg; NADH:cytochrome-c reductase, 141 U/mg; NADPH:cytochrome-c reductase, 43 U/mg. In general, the toxicity of aromatic nitrocompounds towards FLK cells increases on increase of single-electron reduction potentials (E1(1)) of nitrocompounds or the log of their reduction rate constants by single-electron-transferring enzymes, microsomal NADPH:cytochrome P-450 reductase (EC 1.6.2.4) and mitochondrial NADH:ubiquinone reductase (EC 1.6.99.3). No correlation between the toxicity and reduction rate of nitrocompounds by rat liver DT-diaphorase (EC 1.6.99.2) was observed. The toxicity is not significantly affected by dicumarol, an inhibitor of DT-diaphorase. Nitrocompounds examined were poor substrates for DT-diaphorase, being 10(4) times less active than menadione. Their poor reactivity is most probably determined by their preferential binding to a NADPH binding site, but not to menadione binding site of diaphorase. These data indicate that at comparable activities of DT-diaphorase and single-electron-transferring NAD(P)H dehydrogenases in the cell, the toxicity of nitrocompounds will be determined mainly by their single-electron reduction reactions.

Enzymatic redox properties of novel nitrotriazole explosives implications for their toxicity.

The toxicity of conventional nitroaromatic explosives like 2,4,6-trinitrotoluene (TNT) is caused by their enzymatic free radical formation with the subsequent oxidative stress, the formation of alkylating nitroso and/or hydroxylamino metabolites, and oxyhemoglobin oxidation into methemoglobin. In order to get an insight into the mechanisms of toxicity of the novel explosives NTO (5-nitro-1,2,4-triazol-3-one) and ANTA (5-nitro-1,2,4-triazol-3-amine), we examined their reactions with the single-electron transferring flavoenzymes NADPH: cytochrome P-450 reductase and ferredoxin:NADP+ reductase, two-electron transferring flavoenzymes mammalian NAD(P)H:quinone oxidoreductase (DT-diaphorase), and Enterobacter cloacae NAD(P)H:nitroreductase, and their reactions with oxyhemoglobin. The reactivity of NTO and ANTA in the above reactions was markedly lower than that of TNT. The toxicity of NTO and ANTA in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) was partly prevented by desferrioxamine and the antioxidant N,N′-diphenyl-p-phenylene diamine, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea. This points to the involvement of oxidative stress in their cytotoxicity, presumably to the redox cycling of free radicals. The FLK cell line cytotoxicity and the methemoglobin formation in isolated human erythrocytes of NTO and ANTA were also markedly lower than those of TNT, and similar to those of nitrobenzene. Taken together, our data demonstrate that the low toxicity of nitrotriazole explosives may be attributed to their low electron-accepting properties.

Quantitative structure activity relationships for the conversion of nitrobenzimidazolones and nitrobenzimidazoles by DT-diaphorase: implications for the kinetic mechanism

Quantitative structure activity relationships (QSARs) for the conversion of nitrobenzimidazolones and nitrobenzimidazoles by rat liver DT‐diaphorase (EC 1.6.99.2) are described. The parameter used for description of the QSARs is the energy of the lowest unoccupied molecular orbital (E(LUMO)) of the nitroaromatic compounds. Interestingly, correlations with E(LUMO) were observed for both the natural logarithm of k cat, but also for the natural logarithm of k cat/K m. The minimal kinetic model in line with these QSARs is a ping‐pong mechanism that includes a substrate binding equilibrium in the second half reaction.

Quantitative structure activity relationships for the electron transfer reactions of Anabaena PCC 7119 ferredoxin‐NADP+ oxidoreductase with nitrobenzene and nitrobenzimidazolone derivatives: mechanistic implications

The steady state single electron reduction of polynitroaromatics by ferredoxin‐NADP+ oxidoreductase (EC 1.18.1.2) from cyanobacterium Anabaena PCC 7119 has been studied and quantitative structure activity relationships are described. The solubility of the polynitroaromatics as well as their reactivity towards ferredoxin‐NADP+ oxidoreductase are markedly higher than those for previously studied mononitroaromatics and this enabled the independent measurement of the kinetic parameters k cat and K m. Interestingly, the natural logarithm of the bimolecular rate constant, k cat/K m, and also the natural logarithm of k cat correlate with the calculated energy of the lowest unoccupied molecular orbital of the polynitroaromatic substrates. The minimal kinetic model in line with these quantitative structure activity relationships is a ping‐pong mechanism which includes substrate binding equilibria in the second half reaction.

Modulation of erythrocyte photohemolysis rate by glutathione reductase inactivating alkylating agents.

In order to determine the rote of glutathione reductase (GR) in protection against AI‐phtalocyanine tetrasulfonate‐sensitized human erythrocyte photolysis, we have studied the effects of antitumour alkylating agents that inactivate GR, on photohemolysis rate. The rates of inactivation of reduced GR decreased in order BCNU > pharanox (N‐p‐[bis‐(2‐chloroethyl)‐amino]‐phe‐nylacetic acid N‐oxide) > phenalol (N‐p‐[bis‐(2‐chloroethyl)‐amino]‐phenylacetyl‐L‐phenylalanine) > o‐F‐ and p‐F‐lophenal (o‐ and p‐isomers of N‐p‐[bis‐(2‐chloroethyl)‐amino]‐phenylacetyl‐D,L‐fluorophenylalanine) > D,L‐melphalan. As supposed, erythrocyte photolysis was accelerated by BCNU and pharanox, however, it was slowed down by phenylalanine mustards. The latter effect was explained by singlet oxygen quenching and/or photooxidation reactions of these compounds. This points out to a possibility of certain phenylalanine derivatives to neutralize the side‐effects of photodynamic therapy.