The Study of NADPH-Dependent Flavoenzyme-Catalyzed Reduction of Benzo[1,2-c]1,2,5-oxadiazole N-Oxides (Benzofuroxans)

Abstract

The enzymatic reactivity of a series of benzo[1,2-c]1,2,5-oxadiazole N-oxides (benzofuroxans; BFXs) towards mammalian single-electron transferring NADPH:cytochrome P-450 reductase (P-450R) and two-electron (hydride) transferring NAD(P)H:quinone oxidoreductase (NQO1) was examined in this work. Since the =N+ (→O)O− moiety of furoxan fragments of BFXs bears some similarity to the aromatic nitro-group, the reactivity of BFXs was compared to that of nitro-aromatic compounds (NACs) whose reduction mechanisms by these and other related flavoenzymes have been extensively investigated. The reduction of BFXs by both P-450R and NQO1 was accompanied by O2 uptake, which was much lower than the NADPH oxidation rate; except for annelated BFXs, whose reduction was followed by the production of peroxide. In order to analyze the possible quantitative structure-activity relationships (QSARs) of the enzymatic reactivity of the compounds, their electron-accepting potency and other reactivity indices were assessed by quantum mechanical methods. In P-450R-catalyzed reactions, both BFXs and NACs showed the same reactivity dependence on their electron-accepting potency which might be consistent with an “outer sphere” electron transfer mechanism. In NQO1-catalyzed two-electron (hydride) transferring reactions, BFXs acted as more efficient substrates than NACs, and the reduction efficacy of BFXs by NQO1 was in general higher than by single-electron transferring P-450R. In NQO1-catalyzed reactions, QSARs obtained showed that the reduction efficacy of BFXs, as well as that of NACs, was determined by their electron-accepting potency and could be influenced by their binding mode in the active center of NQO1 and by their global softness as their electronic characteristic. The reductive conversion of benzofuroxan by both flavoenzymes yielded the same reduction product of benzofuroxan, 2,3-diaminophenazine, with the formation of o-benzoquinone dioxime as a putative primary reductive intermediate, which undergoes a further reduction process. Overall, the data obtained show that by contrast to NACs, the flavoenzyme-catalyzed reduction of BFXs is unlikely to initiate their redox-cycling, which may argue for a minor role of the redox-cycling-type action in the cytotoxicity of BFXs.

Identification of Novel Chemical Scaffolds Inhibiting Trypanothione Synthetase from Pathogenic Trypanosomatids

“…The compound library involves 144 chemical entities that are clustered by chemical scaffold as follow: (A) 6-arylpyrido[2,3- d ]pyrimidine-2,7-diamine derivatives ( APPDA ; S1 Table ) developed as ATP-competitive inhibitors of bacterial D-Alanine:D-Alanine ligase [ 43 ] and biotin carboxylase [ 44 ]; (B) 1-(benzo[ d ]thiazol-2-yl)-4-benzoyl-3-hydroxy-5-phenyl-1 H -pyrrol-2(5 H )-one derivatives ( BBHPP ; S2 Table ); (C) N , N' -bis(3,4-substituted-benzyl) diamine derivatives ( BDA ; S3 Table ) that display potent anti-malarial or -trypanosomal/leishmanial activity [ 45 , 46 ] and are simplified derivatives of compounds interfering with the parasite’s polyamine metabolism [ 45 , 47 – 50 ]; (D) benzofuroxan ( BZ ; S4 Table ) [ 51 – 55 ]; (E) 4,5-dihydroazepino[4,5- b ]indol-2(1 H ,3 H ,6 H )-one derivatives ( AI ; S5 Table ), some of which with reported anti-TryS or - T . b .…”

The QSAR study for antibacterial activity of structurally diverse nitroaromatics

“…(A) The energies of the lowest unoccupied molecular orbital (E LUMO ) and the highest occupied molecular orbital (E HOMO ), characterizing the electrophilic/electron-accepting and nucleophilic/ electron-donating potencies of the compounds, respectively, which could be approximated to the electron affinity (EA = -E LUMO ) and the ionization potential (IP = -E HOMO ). Note that E LUMO values of nitroaromatics have previously been observed to correlate well with single-electron reduction potentials as well as electrochemical peak potentials of the compounds [31,32];…”

Flavoenzyme-mediated reduction reactions and antitumor activity of nitrogen-containing tetracyclic ortho-quinone compounds and their nitrated derivatives

“…Since, in contrast to the unsubstituted quinoidals, the nitro-quinoid compounds contain two electron-accepting/redox-active sites, namely the quinone moiety and the nitro group, their regional electrophilic potencies were assessed in terms of the electrophilic Fukui function (FF) values grouped over C=O atoms of the quinone moiety ( f + q ) and N and O atoms of the nitro group ( f + -nitro ), which may reflect the propensity to accept nucleophile (an electron and/or an hydride ion) at the initial stage of the redox conversion of the compounds. It should be noted that for the assessment of the FF values, the single point frontier molecular orbital (FMO) approach (Contreras et al, 1999[ 16 ]) was used in this study as one of the most appropriate methods for redox active compounds which, in contrast to the frequently used finite-difference approximation methods, yields the positive FF values and obeys the normalization condition (Šarlauskas et al, 2014[ 46 ], and references therein). As expected, the highest fraction of the global electrophilic potency of the nitro-quinoidals resides upon the C=O atoms of the quinone moiety (the f + q values varied in the range of 0.42 (9-nitro-NPDO) - 0.56 (4-nitro-NPDO)) as the most preferential site for the reduction of compounds, while the f + -nitro values of the nitro groups were assessed to be highly dependent upon their position: the f + -nitro values of the coplanar (π-conjugative) nitro groups of 3-, 9-nitro-NPDO and 3-nitro-PNPDO compounds (0.100, 0.149 and 0.146, respectively) were much higher than those of the twisted nitro groups of 1- and 4-nitro-quinoidals, with f + -nitro of 0.003 and 0.004, respectively, implying that the nitro groups of 1- and 4-nitro-quinoidals, compared to 3- and 9-nitro-quinoidals, possess a much lower tendency to undergo reductive conversion.…”