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Research Letters in Physical Chemistry
Volume 2009 (2009), Article ID 614362, 4 pages
http://dx.doi.org/10.1155/2009/614362
Research Letter

Oxidation of Quercetin by Myeloperoxidase

Laboratory of Physical Chemistry, Vinča Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia

Received 3 February 2009; Accepted 12 March 2009

Academic Editor: Benjaram M. Reddy

Copyright © 2009 Tatjana Momić et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Study of effect of myeloperoxidase on quercetin at pH 6.0 indicated quercetin oxidation via the formation of the oxidation product. The stability of quercetin and oxidation product was investigated as a function of time by using spectrophotometric and HPLC techniques. The apparent pseudo first-order rate constants were calculated and discussed.

1. Introduction

Flavonol quercetin (Q) (3,5,7,3,4-pentahydroxyflavone) (Scheme 1) due to its phenolic structure is a strong antioxidant and free radical scavenger [1]. Oxidation of quercetin during its antioxidative functions is usually accompanied by the production of the quercetin radical anion, superoxide, and hydrogen peroxide [2]. Quercetin undergoes autoxidation—the nonenzymatic reaction with atmospheric oxygen. Also, it is known that one-electron oxidation of quercetin is catalyzed by different peroxidases like lactoperoxidase (LPO) [3] and horseradish peroxidase (HRP) [47] in the presence of H2O2.

614362.schm.001
Scheme 1: Oxidation of quercetin by MPO/H2O2 system (modified from [15]).

The heme enzyme myeloperoxidase (MPO) is oxidant enzyme in the process of inflammation and atherogenesis [8]. Myeloperoxidase is relatively nonspecific with respect to its reducing substrates. This enzyme is able to oxidize different substrates among which anilines and phenols [911]. This letter deals with quercetin oxidation by myeloperoxidase-H2O2 system.

2. Experimental

2.1. Chemicals

Quercetin dihydrate (Sigma-Aldrich) of the highest quality available (98%) was used without purification. 1×103 M stock solution of quercetin was prepared in methanol immediately before the experiments. For all experiments, freshly prepared solutions of quercetin were made by dilution of the appropriate amount of the stock solution with phosphate buffer at pH 6.0. Myeloperoxidase was purified from human neutrophils to a purity index (A430/A280) greater than 0.70 as described previously [12]. Its concentration was calculated using 𝜀430=91000M1cm1 per heme [13]. Hydrogen peroxide solutions were prepared daily by diluting a stock solution, and the concentration was determined using 𝜀240=43.6M1cm1 [14]. Redistilled water was used in all experiments.

2.2. Oxidation of Quercetin by Myeloperoxidase

Quercetin (5×105 M) was incubated in 50 mM phosphate buffer, pH 6.0, with various concentrations of MPO. Reaction was started by the addition of 50 μM H2O2, and UV absorbance changes were recorded. For the HPLC analysis, reaction was stopped after 30 minutes by adding catalase (100 μg/mL). The reaction mixture was centrifuged for 2 minutes at 10000 rpm. The clear supernatant was analyzed by HPLC.

2.3. HPLC Analysis

HPLC equipment consisted of an HP 1100 Series chromatograph coupled with a DAD. Chromatographic separations were run on a C18 Pinnacle ODS column (Restek, 250 mm × 4.6 mm, 5 μm) using an 80 : 20 mixture of 2 vol% H3PO4 (A) and acetonitrile (J.T. Baker) (B) as the eluent for the first 2 minutes. The linear gradient was applied from 20% to 45% B from 2 to 7 minutes. An isocratic 55 : 45 mixture was applied between 7 and 13 minutes. The eluent flow rate was 1.0 mL min−1, and the injection volume was 10 𝜇L. The elutions were monitored with DAD at different wavelengths between 200 and 450 nm.

2.4. UV-VIS Spectroscopic Studies

UV spectra were recorded on a Perkin Elmer Lambda 35 UV-Vis spectrophotometer equipped with thermostatted quartz cell. The temperature in the cell was kept at 25±0.05C with a water-thermostatted bath.

3. Results and Discussion

We investigated the effect of MPO on 5×105 M quercetin in phosphate buffer, pH 6.0 in the presence of 50 μM H2O2 at 25C, spectrophotometrically and by HPLC. The following concentrations of enzyme were used: 5, 11, 15, 20, 50, 100, 150, and 200 nM.

At the absorption spectra of samples in which concentration of MPO was ≤11 nM, two quercetin absorption bands (254 nm and 367 nm) were observed (Figure 1(a)). The same results were yielded when MPO or H2O2 was omited from the mixture.

fig1
Figure 1: Absorption spectra and HPLC chromatograms of 5×105 M quercetin at pH 6.0 in absence (a), (b) and presence (c), (d) of 20 nM MPO. 50 μM H2O2 was present in all reaction solutions. Spectra were recorded over a period of 30 minutes; arrows indicate the direction of the change. Chromatograms were recorded after 30 minutes (b) at 𝜆367 and (d) at 𝜆336.

The absorption spectra of samples in which concentration of MPO was ≥15 nM showed the decay of quercetin absorption bands at 254 nm and 367 nm. At the same time simultaneous rise of the absorption band at 336–342 nm, depended on MPO concentration, was observed (Figure 1(c)), typical of oxidation product formation [16]. Moreover, two well-defined isosbestic points at 290 and 364 nm indicate that there was no significant accumulation of intermediates in the reaction of oxidation product formation (Figure 1(c)). These spectral changes are analogous to those occurring upon autoxidation of quercetin in water [16] and UV irradiation [17] as well as oxidation with some oxidants in organic solvents [16].

The HPLC analysis of quercetin solution treated with MPO for 30 minutes (Figure 1(d)) revealed that concentration of quercetin (retention time 10 minutes) was negligible. Also, formation of major oxidation product (retention time 5.26 minutes) which was more polar than quercetin was detected (Figure 1(d)). From literature data and the obtained spectrophotometric and HPLC results in this work, we assume that the oxidation product detectable at 336–342 nm results from H2O addition on the p-quinonemethide formed by H-atom abstraction at 3-OH and 4-OH of quercetin and subsequent rearrangement of the central ring [16]. The complete sequence of quercetin autoxidation induced by oxidants and catalyzed by metal ions is described earlier [15, 16] (Scheme 1).

According to spectrophotometric and HPLC experimental data, quercetin degradation (1) and oxidation product formation (2) as the function of time followed the relation 𝐴Q,𝑡=𝐴0𝑒𝑘𝑡,𝐴(1)oxid,𝑡=𝐴1𝑒𝑘𝑡,(2) where 𝐴Q,𝑡 and 𝐴oxid,𝑡 are the absorbances proportional to the quercetin and its oxidation product concentrations, respectively, after an irradiation period of time t and k is the overall pseudo first-order rate constant. 𝐴0 and 𝐴 are the absorbance proportional to the initial concentration of quercetin and the concentration of the oxidation product on the plateau of the kinetics curve, respectively.

The kinetic curves that describe the quercetin degradation and the formation of the oxidation product in the presence of different MPO, concentrations are shown in Figure 2. In the presence of MPO concentration that was ≥15 nM oxidation product was formed at the first minute of reaction (Figure 2(a)). The overall apparent first-order rate constants for the oxidation product formation and quercetin transformation to its degradation products were determined from the kinetic curves and are presented in Table 1. For comparison, in Table 1 we also presented values for these constants obtained for quercetin oxidation by UV irradiation [17].

tab1
Table 1: Rate constants of quercetin oxidation.
fig2
Figure 2: Change of absorbance of quercetin in the presence of various MPO concentrations in the function of time, at pH 6.0 (a) formation of quercetin oxidation product at 𝜆336; Inset: 20 nM MPO and (b) degradation of quercetin oxidation product at 𝜆367.

The rate of quercetin oxidation by MPO at pH 6.0 is 3-fold faster compared with the rate of quercetin oxidation by UV irradiation at pH 10.0. Moreover, product obtained for quercetin oxidation by MPO at pH 6.0 is more stable than product of quercetin oxidation by UV irradiation at pH 10.0.

Acknowledgment

We wish to express our gratitude to the Ministry of Science and Technological Development of the Republic of Serbia supported this work through Project 142051.

References

  1. L. Magnani, E. M. Gaydou, and J. C. Hubaud, “Spectrophotometric measurement of antioxidant properties of flavones and flavonols against superoxide anion,” Analytica Chimica Acta, vol. 411, no. 1-2, pp. 209–216, 2000. View at Publisher · View at Google Scholar
  2. N. Cotelle, J.-L. Bernier, J.-P. Catteau, J. Pommery, J.-C. Wallet, and E. M. Gaydou, “Antioxidant properties of hydroxy-flavones,” Free Radical Biology and Medicine, vol. 20, no. 1, pp. 35–43, 1996. View at Publisher · View at Google Scholar
  3. D. Metodiewa, A. K. Jaiswal, N. Cenas, E. Dickancaité, and J. Segura-Aguilar, “Quercetin may act as a cytotoxic prooxidant after its metabolic activation to semiquinone and quinoidal product,” Free Radical Biology and Medicine, vol. 26, no. 1-2, pp. 107–116, 1999. View at Publisher · View at Google Scholar
  4. H. Yamasaki, Y. Sakihama, and N. Ikehara, “Flavonoid-peroxidase reaction as a detoxification mechanism of plant cells against H2O2,” Plant Physiology, vol. 115, no. 4, pp. 1405–1412, 1997.
  5. G. Galati, T. Chan, B. Wu, and P. J. O'Brien, “Glutathione-dependent generation of reactive oxygen species by the peroxidase-catalyzed redox cycling of flavonoids,” Chemical Research in Toxicology, vol. 12, no. 6, pp. 521–525, 1999. View at Publisher · View at Google Scholar · View at PubMed
  6. D. P. Makris and J. T. Rossiter, “An investigation on structural aspects influencing product formation in enzymic and chemical oxidation of quercetin and related flavonols,” Food Chemistry, vol. 77, no. 2, pp. 177–185, 2002. View at Publisher · View at Google Scholar
  7. U. Takahama, “Spectrophotometric study on the oxidation of rutin by horseradish peroxidase and characteristics of the oxidized products,” Biochimica et Biophysica Acta, vol. 882, no. 3, pp. 445–451, 1986. View at Publisher · View at Google Scholar
  8. S. J. Klebanoff, “Myeloperoxidase,” Proceedings of the Association of American Physicians, vol. 111, no. 5, pp. 383–389, 1999.
  9. J. K. Hurst, “Myeloperoxidase: active site structure and catalytic mechanism,” in Peroxidases in Chemistry and Biology, J. Everse, K. E. Everse, and M. B. Grisham, Eds., pp. 37–62, CRC Press, Boca Raton, Fla, USA, 1991.
  10. E. Shacter, R. L. Lopez, and S. Pati, “Inhibition of the myeloperoxidase-H2O2-Cl-system of neutrophils by indometacin and other non-steroidal anti-inflammatory drugs,” Biochemical Pharmacology, vol. 41, no. 6-7, pp. 975–984, 1991. View at Publisher · View at Google Scholar
  11. A. J. Kettle, C. A. Gedye, M. B. Hampton, and C. C. Winterbourn, “Inhibition of myeloperoxidase by benzoic acid hydrazides,” Biochemical Journal, vol. 308, no. 2, pp. 559–563, 1995.
  12. R. L. Olsen and C. Little, “Purification and some properties of myeloperoxidase and eosinophil peroxidase from human blood,” Biochemical Journal, vol. 209, no. 3, pp. 781–787, 1983.
  13. T. Odajima and I. Yamazaki, “Myeloperoxidase of the leukocyte of normal blood—I: reaction of myeloperoxidase with hydrogen peroxide,” Biochimica et Biophysica Acta, vol. 206, no. 1, pp. 71–77, 1970.
  14. R. J. Beers, Jr. and I. W. Sizer, “A spectrophotometric method for measuring the breakdown of hydrogen peroxide by catalase,” Journal of Biological Chemistry, vol. 195, no. 1, pp. 133–140, 1952.
  15. O. Dangles, C. Dufour, and S. Bret, “Flavonol-serum albumin complexation. Two-electron oxidation of flavonols and their complexes with serum albumin,” Journal of the Chemical Society Perkin Transactions, vol. 2, no. 4, pp. 737–744, 1999. View at Publisher · View at Google Scholar
  16. H. E. Hajji, E. Nkhili, V. Tomao, and O. Dangles, “Interactions of quercetin with iron and copper ions: complexation and autoxidation,” Free Radical Research, vol. 40, no. 3, pp. 303–320, 2006. View at Publisher · View at Google Scholar · View at PubMed
  17. T. Momić, J. Savić, U. Černigoj, P. Trebše, and V. Vasić, “Protolytic equilibria and photodegradation of quercetin in aqueous solution,” Collection of Czechoslovak Chemical Communications, vol. 72, no. 11, pp. 1447–1460, 2007. View at Publisher · View at Google Scholar