Quantitative structure–activity relationships in prooxidant cytotoxicity of polyphenols: Role of potential of phenoxyl radical/phenol redox couple

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Abstract

The aim of this work was to characterize the role of the potential of phenoxyl radical/phenol redox couple, E72, in the cytotoxicity of polyphenols. The cytotoxicity of polyphenols in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK), and human promyelocytic leukemia cells (line HL-60) was partly inhibited by catalase, by the antioxidant N,N′-diphenyl-p-phenylene diamine and desferrioxamine, and potentiated by 1,3-bis-(2-chloro-ethyl)-1-nitrosourea, thus showing its prooxidant character. Dapsone, an inhibitor of myeloperoxidase, did not affect the cytotoxicity of polyphenols in HL-60 cells, whereas dicumarol, an inhibitor of DT-diaphorase, showed controversial effects on their cytotoxicity in FLK cells. Inhibitors of cytochromes P-450, α-naphthoflavone and izoniazide, decreased the cytotoxicity of several polyphenols, whereas 3,5-dinitrocatechol, an inhibitor of catechol-o-methyltransferase (COMT), increased it. The cytotoxicity of 13 polyhydroxybenzenes was described by the equations: logcL50(μM)=-0.67+5.46E72(V)-0.16logD(FLK), and logcL50(μM)=-1.39+6.90E72(V)-0.20logD(HL-60), where cL50 is compound concentration for 50% cell survival, and D is octanol/water distribution coefficient at pH 7.0. The flavonoids comprise a separate series of compounds with lower cytotoxicity. The correlations obtained quantitatively confirm the parallelism between the polyphenol cytotoxicity and the rates of their single-electron oxidation, and point to the leading role of formation of the reactive oxygen species in their cytotoxicity. Depending on the examined system, this parallelism may be distorted due to the cytochrome P-450 and COMT-catalyzed transformation of polyphenols.

Section snippets

Enzyme kinetic studies

All the compounds and enzymes were obtained from Sigma, and used without further purification. The concentration of horseradish peroxidase (HRP) and myeloperoxidase (MPO) was determined spectrophotometrically according to ε402 = 102 mM−1 cm−1, and ε430 = 91 mM−1 cm−1[45], respectively. The kinetic measurements were carried out spectrophotometrically, using a Hitachi-557 spectrophotometer, in 0.1 M K-phosphate buffer (pH 7.0) containing 1 mM EDTA at 25 °C. The rates of MPO-catalyzed oxidation of

Results

The concentrations of polyhydroxybenzenes and flavonoids causing 50% death of FLK and HL-60 cells (cL50) determined in the present study or partly taken from our previous work [24], their single-electron oxidation potentials, determined by pulse radiolysis at pH 7.0 (E72), and their calculated octanol/water distribution coefficients at pH 7.0 (log D) are given in Table 1. In reversible oxidation of hydroquinones and catechols to quinones, E72 refers to semiquinone/hydroquinone redox couple. For

Discussion

The data of our study demonstrate that polyphenols exert the prooxidant character of cytotoxicity in FLK and HL-60 cells (Fig. 3A), which is at least partly attributed to the autooxidation of polyphenols in cell growth media, i.e., the formation of extracellular H2O2[25], [26] (Figs. 3A and B). However, the cytotoxicity of polyphenols is also affected by their intracellular activation, as it is indicated by the effects of the inhibitors of cytochromes P-450 and COMT (Figs. 4A–D). Thus, their

Acknowledgments

This work was supported in part by the Lithuanian Science and Studies Foundation (program FUINMAKOSA). We thank Dr. Florence Lederer (CNRS, Gif-sur-Yvette, France) for the access to a stopped-flow spectrophotometer, and Drs. Jonas Šarlauskas and Žilvinas Anusevičius (Institute of Biochemistry), and Mr. Vladimir Koršunov (Institute of Immunology) for the excellent technical assistance.

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