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Studies on the interaction between Cd(2+) ions and DNA
1School of Biomedical Sciences, Faculty of Health Sciences C42, PO Box 170, The University of Sydney, East Street, Lidcombe, NSW 1825, Australia.
Abstract:
Cadmium is a potent carcinogen in rodents and has recently been accepted by the International Agency for Research on Cancer as a category 1 (human) carcinogen, but the molecular mechanism of its action remains largely unclear. It has however been suggested that cadmium-induced carcinogenesis may involve either direct or indirect interaction of Cd(2+) with DNA. Cd(2+) is believed to bind covalently with N7 centres of adenine and guanine. At low concentrations (< or =50 mM), Cd(2+) is found to react with plasmid DNA to produce a mixture of Form I and Form II bands whereas at higher concentrations (> or =100 mM), Cd(2+) causes extensive damage to DNA at a pH 5.8 solution of cadmium nitrate. Within the range 0-100 mM (when pH is adjusted to 7.4 by adding NaOH) an increase in concentration of Cd(2+) is found to cause a decrease in the gel mobility rate of plasmid and an increase in the intensity of the Form II band. When plasmid DNA is digested with BamH1, only the Form III band is observed both in the presence and absence of Cd(2+). However, the mobility of the band is found to decrease with the increase in the concentration of Cd(2+). When the enzyme Ssp1 which cuts plasmid DNA at the AT sites is used instead of BamH1, two bands are observed in the presence of cadmium as against one band in the absence of cadmium. These results suggest that Cd(2+) binds covalently with DNA (possibly at G, A and T centres) and can form intrastrand bifunctional AT adducts but not the GG adducts. It may also be that neither GG nor AT adducts are formed and yet Ssp1 digestion is prevented because of a structural modification introduced in adenine by its interaction with Cd(2+). In the presence of antioxidants such as cysteine, glutathione and ascorbate (especially cysteine and ascorbate), DNA damage is found to be greater than expected for the combined effects of the antioxidant and Cd(2+). The increased DNA damage is believed to be due to the formation of reactive oxygen species (ROS).
Insights
Cadmium (Cd2+) covalently binds to DNA, potentially forming AT adducts and altering DNA structure. This interaction, possibly involving reactive oxygen species, contributes to cadmium
Area of Science:
- Environmental Toxicology
- Molecular Biology
- Carcinogenesis
Background:
- Cadmium (Cd2+) is a known human carcinogen, but its molecular mechanisms remain unclear.
- Previous research suggests Cd2+ interacts with DNA, potentially through covalent binding.
Purpose of the Study:
- To investigate the molecular mechanism of cadmium-induced DNA damage.
- To elucidate the binding sites and adduct formation of Cd2+ with DNA.
Main Methods:
- Plasmid DNA was exposed to varying concentrations of Cd2+.
- DNA damage and structural changes were analyzed using gel electrophoresis after digestion with restriction enzymes BamH1 and Ssp1.
- The role of antioxidants and reactive oxygen species (ROS) was examined.
Main Results:
- Cd2+ binding decreased DNA gel mobility and increased Form II band intensity, indicating DNA damage.
- Ssp1 digestion (targeting AT sites) was inhibited by Cd2+, suggesting AT adduct formation or structural modification.
- Antioxidants, particularly cysteine and ascorbate, exacerbated DNA damage, indicating ROS involvement.
Conclusions:
- Cd2+ covalently binds to DNA, likely at G, A, and T bases, forming intrastrand bifunctional AT adducts.
- Structural modifications of DNA by Cd2+ can inhibit enzyme digestion.
- Cadmium-induced DNA damage may be mediated by reactive oxygen species (ROS).