Strand scission in DNA induced by quercetin and Cu(II): role of Cu(I) and oxygen free radicals

Abstract

The naturally occurring flavonoid, quercetin, in the presence of Cu(II) and molecular oxygen caused breakage of calf thymus DNA, supercoiled pBR322 plasmid DNA and single-stranded M13 phage DNA. In the case of the plasmid, the product(s) were relaxed circles or a mixture of these and linear molecules depending upon the conditions. For the breakage reaction, Cu(II) could be replaced by Fe(III) but not by other ions tested [Fe(II), Co(II), Ni(II) and Ca(II)]. Structurally related flavonoids, rutin, galangin, apigenin and fisetin, were ineffective or less effective than quercetin in causing DNA breakage. In the case of the quercetin--Cu(II) reaction, Cu(I) was shown to be an essential intermediate by using the Cu(I)-sequestering reagents, neocuproine and bathocuproine. By using Job plots we established that, in the absence of DNA, five Cu(II) ions can be reduced by one quercetin molecule; in contrast, two ions were reduced per quercetin molecule in the DNA breakage reaction. Equally neocuproine inhibited the DNA breakage reaction. The involvement of active oxygen in the reaction was established by the inhibition of DNA breakage by superoxide dismutase, iodide, mannitol, formate and catalase (the inhibition was complete in the last case). From these data we propose a mechanism for the DNA strand scission reaction of quercetin and related flavonoids.

Energy dissipation and radical scavenging by the plant phenylpropanoid pathway

“…Copper(II) catalyses the oxidation of phenolic compounds in a redox cycling process that can cause oxidative damage to DNA and other biological macromolecules (Rahman et al 1989;Li & Trush 1994;Yamanaka et al 1997). Addition of Cu(II) to a solution containing ca¡eic acid stimulates oxygen uptake (¢gure 4a).…”

Strand scission in DNA induced by l‐DOPA in the presence of Cu(II)

“…In addition, the mechanism of action of several anti-tumor antibiotics such as bleomycin and adriamycin [23,24] is considered to involve the generation of ROS. Several known antioxidants of plant origin such as flavonoids, generate ROS in the presence of transition metal ions such as copper, and lead to DNA cleavage [12,15,25]. A number of molecules present in human extracellular fluids are considered to have antioxidant function and these include ascorbic acid and uric acid.…”

“…It is recognized by most workers that hydroxyl radical reactions with DNA are preceded by the association of a complex with DNA, followed by the production of hydroxyl radicals at that particular site [29]. As shown above, L-DOPA is capable of binding to DNA, and if copper ions are available a ternary complex of L-DOPA-Cu(II)-DNA may be formed as in the case of several other ROS generating and DNA cleavage systems [12,30]. Halliwell and co-workers [10] have shown that copper ion concentrations in human brain tissue damaged in Parkinson's disease are at a level that could promote oxidative DNA damage.…”

“…Incubation at room temperature was normally for 2 h. All solutions were sterilized before use. S~ nuclease digestion was performed as described previously [12]. Acid-soluble deoxyribonucleotides were determined either colourimetrically [13] or by determining the absorbance at 260 nm.…”

“…DNA by L-DOPA and Cu(II) c-DOPA and Cu(II) generated St-sensitive sites in calf thymus DNA [12]. The reaction was assessed by recording the proportion of double-stranded DNA converted to acid-soluble nucleotides by S~ nuclease.…”