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Mutagenesis, Vol. 16, No. 3, 271-275, May 2001
© 2001 UK Environmental Mutagen Society/Oxford University Press

Fast repair of the radical cations of dCMP and poly C by quercetin and rutin

Chenyang Zhao, Yimin Shi, Weizheng Lin1,, Wengfeng Wang1,, Zhongjian Jia2,, Side Yao2,, Botao Fan3, and Rongliang Zheng4,

School of Life Science, Lanzhou University, Lanzhou, 730000, China, 1 Laboratory of Radiation Chemistry, Shanghai Institute of Nuclear Research, Chinese Academy of Sciences, Shanghai, 201800, China, 2 State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, 730000, China and 3 Institute de Topologie et de Dynamique des Systems, University Paris 7, Paris 75005, France


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
The effects of quercetin and rutin on the repair of the radical cations of dCMP and poly C were studied using the technique of pulse radiolysis. The radical cations of dCMP and poly C were formed by the reaction of dCMP and poly C with SO 4–. After pulse irradiation of nitrogen-saturated aqueous solutions containing dCMP, 20 mM K2S2O8, 200 mM t-BuOH and either rutin or quercetin, the initially formed radical cation of dCMP, detected spectrophotometrically, rapidly decayed with the concurrent formation of the phenoxyl radical of rutin or quercetin within 8–40 µs. The repair efficiencies of the tested compounds towards the poly C radical cation were also determined using the same procedure. The results indicate that dCMP and poly C radical cations can be rapidly repaired by quercetin and rutin. The rate constants of the repair reactions were determined to be 4.3–8.8x108 M/s and 1.5–3.6x108 M/s for dCMP and poly C radical cations, respectively. Together with findings from our previous studies, the present results demonstrate that non-enzymatic fast repair may be a universal form of repair involving phenolic antioxidants.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
DNA damage is involved in many pathological processes, including cancer and aging, and can be caused by environmental agents such as ionizing radiation, UV light and a variety of chemical agents. DNA damage is also produced by normal metabolism in which reactive oxygen species (ROS) are formed as by-products (Lesko, 1988; Floyd, 1990Go). Through direct and indirect effects, ionizing radiation primarily produces base radical cations, base radical anions and hydroxyl radical adducts of bases, which are able to induce strand breaks or to form stable base lesions (Steenken, 1989Go). Base radical cations can also result from the oxidation of DNA by oxidizing intermediates of chemical carcinogens (Miller, 1994Go). Generally, cells can efficiently repair the majority of DNA damage through effective enzymatic repair systems (Wallace, 1997Go). However, there is always some DNA damage that escapes enzymatic repair. Moreover, in the process of aging the steady-state levels of DNA damage may increase as the repair capacity decreases resulting in the accumulation of DNA damage, which may finally lead to an increase of the mutation frequency and dysfunction of cells through aging. Consequently DNA damage is an inevitable result of living, always existing prior to DNA replication. Therefore, the possibility of the formation of a mutation leading to degenerative diseases always exists. Thus, effort is required to look for an approach by which either ROS are scavenged prior to damaging DNA, or whereby the damaged DNA is repaired to supplement the inadequate repair capacity of cells.

The scavenging of ROS by endogenous and exogenous antioxidants has attracted much attention and seems to be a feasible approach to prevent DNA from being attacked by ROS (Shi et al., 1999bGo). However, the effectiveness of this approach is limited. The direct effect of ionization occurs within the DNA itself, so the DNA damage induced cannot be prevented by antioxidants through their scavenging activities. On the other hand, because of the very high reactivity of hydroxyl radicals (Steenken, 1989Go) and the much higher concentration of biomolecules than antioxidants in cells, reactions of hydroxyl radicals with biomolecules are also very difficult to prevent in vivo unless the concentration of antioxidants is sufficiently high. Furthermore, DNA damage induced by the transition metals, which associate with DNA through Fenton reactions cannot be prevented by scavenging ROS. Therefore, a more effective and feasible strategy to prevent damage of DNA should be fast chemical repair (Simic et al., 1989Go). O'Neill et al. (1983) reported that endogenous antioxidants, such as thiols and ascorbate can rapidly repair oxidizing hydroxyl radical adducts of dGMP and dG with high rate constants (3.6x107–8.4x108 M/s). Jiang et al. (1997) showed that hydroxycinnamic acid derivatives can rapidly repair the hydroxyl radical adduct of dGMP. The fast repair activities of phenylpropanoid glycosides (PPGs) and their analogues towards hydroxyl radical adducts of dGMP and dAMP (Li et al., 1996aGo, 1997bGo; Shi et al., 1999cGo, 1999dGo), thymine radical anions (Li et al., 1996bGo, 1997aGo), TMP radical anions (Shi et al., 2000Go) and radical cations of dAMP, dGMP and dCMP (Shi et al., 1999aGo) have been verified in our laboratory. The mechanisms have been elucidated as either reduction or oxidation processes involving DNA radicals. However, the establishment of the generality of fast repair of DNA damage by antioxidants requires more evidence.

Flavonoids are representatives of a large and complex group of phenolic compounds that occur throughout the plant kingdom and are synthesized in most plant tissues, providing colour, flavour, antifungal and anti-bacterial activities, and contributing to many aspects of plant physiology (Bors et al., 1987, 1990Go; Jovanovic et al., 1994Go; Gee et al., 1998Go). The pharmacological significance of flavonoids has been known for a long time. These polyphenols are known free radical scavengers (Bors et al., 1994Go), have beneficial action in cardiovascular disorders (Jovanovic et al., 1996Go), inhibit H2O2-induced V79 cell death and prevent DNA single-strand breakage (Piero et al., 1998Go). However, their potential to rapidly repair DNA damage has not been investigated.

The current study focuses on the repair of radical cations of dCMP and poly C by employing two representative flavonoids namely quercetin and rutin.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
Materials
Quercetin (Q), rutin (R), 2'-deoxycytosine-5'-monophosphate (dCMP) and polycytidylic acid (poly C) were purchased from Sigma. All other reagents were obtained from Shanghai Biochemical Co. (Shanghai, China). All solutions were buffered with phosphate (2 mM, pH 7.0), freshly prepared with triple distilled water before each experiment, and were used immediately. All experiments were carried out at room temperature.

Pulse radiolysis
Pulse radiolysis experiments were conducted using a linear accelerator providing 8 MeV electron pulse of 8 ns duration. A 2 cm Suprasil quartz cell was used for sample irradiation. The thiocyanate dosimeter was used for dose determination, assuming {varepsilon}(SCN)– = 7600 dm3.mol/cm at 480 nm in nitrous oxide saturated 10 mM KSCN aqueous solution. In these experiments, the average dose per pulse is 14 Gy. The analyzing light source was a 500W xenon lamp and its intensity was increased ~100-fold during the detection of transient optical absorptions. The transmitted light entered a 44W monochromator equipped with a IP28 photomultiplier. The signals were collected with a 100 MHz transient recorder and processed with a computer. The rate constants for one-electron oxidation of flavonoids were determined by analysis of the build-up of the optical absorption of phenoxyl radical with time. The repair system was designed according to the principles of competitive reaction.

Generation of radical cations of dCMP and poly C
The radical cations of dCMP (or poly C) were generated by pulse radiolysis aqueous solution containing 2 mM dCMP (or 4 mM poly C), 20 mM K2S2O8, 200 mM t-BuOH and saturated with N2. On pulse irradiation, hydrated electrons (eaq– ), OH• and hydrogen atoms (H•) were produced with G-value yields (µmol/J) of 0.29, 0.29 and 0.06, respectively from ionization of water (Asmus, 1983Go). OH• is scavenged by t-BuOH to form t-BuOH(-H)• , while H• and eaq– react with S2 O8 2– to produce SO4– :






   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
Transient optical absorption spectra of dCMP and poly C radical cations
On pulse irradiation of 2 mM dCMP aqueous solution containing 20 mM K2 S2 O8 , 200 mM t-BuOH and saturated with nitrogen at pH 7.0, a transient optical absorption spectrum arising from reaction of SO4 – with dCMP was observed after 1 µs, and was characterized by an optical absorption maximum at 420 nm (Figure 1AGo), which is identical to those obtained by O'Neill and Davies (1987). The optical absorption reached its maximum within 1 µs. This transient absorption spectrum was assigned to dCMP+ . The transient spectrum of poly C was also observed by pulse radiolysis under the same conditions (Figure 1BGo).




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  		Fig. 1. . Transient optical absorption spectra of DNA radical cations obtained by pulse radiolysis of nitrogen-saturated DNA aqueous solution containing 40 mM K2S2O8, 200 mM t-BuOH at pH 7.0. (A) 2 mM dCMP, at 1 µs. (B) 4 mM poly C, at 2 µs.

 
Transient absorption spectra of phenoxyl radicals of the tested flavonoids
On the pulse radiolysis of aqueous solution containing 0.1 mM rutin, 20 mM K2 S2 O8 , 200 mM t-BuOH and saturated with nitrogen, a transient optical absorption spectrum appeared and was characterized by a maximum absorption between 410 and 440 nm (Figure 2AGo). The optical absorption reached a maximum after 6 µs (Figure 2AGo, inset). This transient absorption spectrum was attributed to the phenoxyl radical of rutin formed in reaction 6:

A similar result was observed on pulse radiolysis of aqueous solution containing 0.1 mM quercetin with {lambda}max = 490 nm (Figure 2BGo).



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  		Fig. 2. . Transient optical absorption spectra upon pulse radiolysis of aqueous solution containing the flavonoids, 20 mM K2S2O8, 200 mM t-BuOH and saturated with N2 at pH 7.0. (A) 0.1 mM rutin, at 6 µs; (B) 0.1 mM quercetin, at 5 µs. Inset: trace showing the buildup of optical absorption at: (A) 440 nm; (B) 460 nm.

 
Repair reactions of the radical cations of dCMP and poly C with flavonoids
At 1 µs after pulse radiolysis of aqueous solution containing 2 mM dCMP, 0.1 mM quercetin, 20 mM K2 S2 O8 , 200 mM t-BuOH and saturated with nitrogen at pH 7.0, a transient optical absorption spectrum was observed (Figure 3BGo, a). This spectrum is the same as that of dCMP•+ and is therefore assigned to dCMP•+ . At 40 µs after pulse irradiation, the transient absorption spectrum of the phenoxyl radical of quercetin grows in concomitantly with the disappearance of that of dCMP•+ (Figure 3BGo, b). These alterations of the transient absorption spectrum result from an electron transfer reaction between dCMP•+ and quercetin.



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  		Fig. 3. . Transient optical absorption spectra after pulse radiolysis of nitrogen saturated 2 mM dCMP aqueous solution containing 20 mM K2S2O8, 200 mM t-BuOH and 0.1 mM flavonoids. Transient absorption spectra. (A) Rutin at: a, 1 µs; b, 20 µs. (B) Quercetin at: a, 1 µs; b, 40 µs. The buildup trace of absorption: (C) rutin at 420 nm; (D) quercetin at 460 nm.

 
In principle, there are two parallel reactions competing in the above repair system (equations 7 and 8GoGo):


The probability (P) of dCMP reacting with SO4 – is calculated as follows:

In this study, the probability of dCMP (poly C) reacting with SO4 – is >90%. Therefore, it may be assumed that in the above repair system, SO4 – reacts with dCMP (poly C) predominantly.

Figure 3DGo shows the growth of optical absorption of the quercetin phenoxyl radical at 460 nm on pulse radiolysis of aqueous solution containing 2 mM dCMP, 20 mM K2 S2 O8 , 200 mM t-BuOH, 0.1 mM quercetin and saturated with nitrogen. The growth of absorbance follows first order kinetics, from which the apparent rate constant for the formation of the quercetin phenoxyl radical by the repair reaction, kapp , was determined. From the linear dependence of kapp on the concentration of quercetin (0.04–0.12 mM) as shown Figure 4Go, the rate constants of reaction of the tested flavonoids with dCMP•+ were determined (Table IGo).



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  		Fig. 4. . The dependence of the apparent rate constant for the growth of optical absorbance on [quercetin] upon pulse irradiation of nitrogen saturated aqueous solution containing 2 mM dCMP, 20 mM K2S2O8, 200 mM t-BuOH and quercetin.

 

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  		Table I. . The rate constants for repair of the radical cations of dCMP and poly C by rutin and quercetin
 
The rate constants of the repair reactions of rutin towards the radical cation of dCMP (Figure 3A and CGo) and of quercetin and rutin with the radical cations of poly C (Figure 5Go) were determined (Table IGo).



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  		Fig. 5. . The result of pulse radiolysis of nitrogen saturated 4 mM poly C aqueous solution containing 20 mM K2S2O8, 200 mM t-BuOH and 0.1 mM rutin. Transient absorption spectra. (A) Rutin at: a, 0.8 µs; b, 25 µs. (B) Quercetin at: a, 1 µs; b, 20 µs, The buildup trace of absorption: (C) rutin at 420 nm; (D) quercetin at 460 nm.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
In the reaction of dCMP and SO4 – , the primary product is the dCMP radical cation, dCMP+ , which undergoes deprotonation to generate the neutral radical, dCMP(-H)• (O'Neill and Davies, 1987Go) (equation 9).

In contrast to the much greater yield of reducing radicals formed in the interaction of dCMP with hydroxyl radicals followed by the process of dehydration (Hazra and Steenken, 1983Go), the yield of reducing products formed on interaction of dCMP with SO4 – , is <10% (O'Neill and Davies, 1987Go). Therefore, the neutral radical, dCMP(-H)• can reacts with rutin or quercetin and is repaired (equation 10Go).

With poly C, it is assumed that the reaction of SO 4 – primarily forms the radical cations of poly C (poly C+ ). Following deprotonation, poly C+ converts to the neutral radical [poly C(-H)•], which should be oxidizing. As shown from the results, this neutral radical is repaired by rutin and quercetin.

A number of investigations have shown that DNA monobases tend to stack in aqueous solution (Sevilla et al., 1976Go). Base stacking favours electron transfer between base radicals and bases; therefore, it may impose a negative effect on the repair reactions of base radicals by phenolic antioxidants. In other words, electron transfer between a base radical and a base would compete with the repair reaction involving flavonoids. However, the results of the present study indicate that even in the presence of base stacking rutin and quercetin can still rapidly repair radical cation of poly C.

The mechanism for the fast repair of the radical cations of dCMP and/or poly C proceeds by electron transfer, in other words, it is a redox reaction. It is inferred theoretically that a chemical with an appropriate reduction potential should have the properties of a good antioxidant, and hence be able to rapidly repair the radical cations of dCMP and poly C. The results of this study are consistent with this assumption.

Both the very high rate constants of reaction of quercetin and rutin with ROS and the formation of stable phenoxyl radicals are important factors for quercetin and rutin to act as potent antioxidants. Similarly, these properties are also essential for fast repair of DNA radical cations. On the one hand, the appropriate reduction potentials enable quercetin or rutin to reduce DNA radical cations rapidly, but on the other, the high stabilities of Q-PhO• and R-PhO• prevent these phenoxyl radicals, products of the fast repair reaction, from reacting with other biomolecules.

For phenolic antioxidants, the antioxidative capacities depend on the number of phenolic hydroxyl groups. Our previous study showed that the values of the rate constants for repair reactions of purine deoxynucleotide radical cations with PPGs and their analogues are positively related to the number of phenolic hydroxyl groups (Shi et al., 1999dGo). However, by comparison of the values of the rate constants for repair of the radical cations of dCMP and poly C by rutin and quercetin with those for repair by PPGs and their analogues, it is apparent that the number of phenolic hydroxyl groups of the flavonoids tested (5–10) are greater than those with the PPGs and their analogues (1–4), although the rate constants with quercetin and rutin are lower than those for PPG's analogues. This fact demonstrates that the number of phenolic hydroxyl groups is an important factor in determining the rate constants for repair of DNA radical cations, but is not the sole factor.

Owing to the continuous generation of ROS in normal cells through metabolism and the inadequate scavenging activity, damage to biomolecules is inevitable. Damaged DNA can be repaired by enzymatic systems, but the enzymatic repair of DNA damage is carried out on a time scale of hours (Yakes and Houten, 1997Go). Further, the repair enzymes, being proteins, are also sensitive to ROS. However, the fast repair reactions are complicated on the microsecond time scale preventing reaction of the DNA radicals with other biological macromolecules. Fast repair processes involving general redox reactions opens a wide field of study to find more effective antioxidants to protect DNA against the attack of ROS, and will enrich our knowledge of how traditional antioxidants as well may improve the therapy of free radical related diseases.

The results of the present study, together with our previous studies, demonstrate that non-enzymatic, fast repair is a universal form of repair exerted by phenolic antioxidants.


   Acknowledgments
 
We thank Prof. Peter O'Neill and Dr Margaret Thomas for their kind help with this manuscript preparation. This project was supported in part by the National Natural Science Foundation of China, the Doctoral Programme of the Ministry of Education of China and the China–France Coorperative Research Sponsored by the Ministry of Education of China.


   Notes
 
4 To whom correspondence should be addressed. Tel: +86 931 8912563; Email: zhengrl{at}lzu.edu.cn Back


   References
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

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Received on May 19, 2000; accepted on January 4, 2001.


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