Related Experiment Videos
Quantitative evaluation of DNA binding data for risk estimation and for classification of direct and indirect
Abstract:
Investigation of covalent DNA binding in vivo provided evidence for whether a test substance can be activated to metabolites able to reach and react with DNA in an intact organism. For a comparison of DNA binding potencies of various compounds tested under different conditions, a normalization of the DNA lesion with respect to the dose is useful. A covalent binding index, CBI = (mumol chemical bound per mol DNA nucleotide)/(mmol chemical administered per kg body weight) can be determined for each compound. Whether covalent DNA binding results in tumor formation is dependent upon additional factors specific to the cell type. Thus far, all compounds which bind covalently to liver DNA in vivo have also proven to be carcinogenic in a long-term study, although the liver was not necessarily the target organ for tumor growth. With appropriate techniques, DNA binding can be determined in a dose range which may be many orders of magnitude below the dose levels required for significant tumor induction in a long-term bioassay. Rat liver DNA binding was proportional to the dose of aflatoxin B1 after oral administration of a dose between 100 micrograms/kg and 1 ng/kg. The lowest dose was in the range of general human daily exposures. Demonstration of a lack of liver DNA binding (CBI less than 0.1) in vivo for a carcinogenic, nonmutagenic compound is a strong indication for an indirect mechanism of carcinogenic action. Carcinogens of this class do not directly produce a change in gene structure or function but disturb a critical biochemical control mechanism, such as protection from oxygen radicals, control of cell division, etc. Ultimately, genetic changes are produced indirectly or accumulate from endogenous genotoxic agents. The question of why compounds which act via indirect mechanisms are more likely to exhibit a nonlinear range in the dose-response curve as opposed to the directly genotoxic agents or processes is discussed.
Insights
Covalent DNA binding in vivo indicates if a substance can form DNA-damaging metabolites. A low covalent binding index (CBI) suggests indirect carcinogenic mechanisms, not directly altering DNA.
Area of Science:
- Toxicology
- Molecular Biology
- Carcinogenesis
Background:
- Covalent DNA binding in vivo is a key indicator of a substance's potential to form DNA-reactive metabolites within an organism.
- Comparing DNA binding potencies across different compounds and conditions necessitates dose normalization, often using a Covalent Binding Index (CBI).
Purpose of the Study:
- To investigate the relationship between in vivo covalent DNA binding and carcinogenic potential.
- To establish a method for comparing DNA binding potencies of various compounds.
- To differentiate between direct and indirect mechanisms of carcinogenesis based on DNA binding evidence.
Main Methods:
- Determination of covalent DNA binding in vivo using normalized dose-response relationships.
- Calculation of the Covalent Binding Index (CBI) for various compounds.
- Correlation of DNA binding data with long-term carcinogenicity bioassays and mechanistic studies.
Main Results:
- All compounds tested that covalently bind to liver DNA in vivo have proven carcinogenic, though the liver is not always the target organ.
- DNA binding can be detected at doses significantly lower than those required for tumor induction.
- A lack of liver DNA binding (CBI < 0.1) in a carcinogenic, nonmutagenic compound strongly suggests an indirect mechanism of action.
Conclusions:
- In vivo covalent DNA binding is a critical factor in chemical carcinogenesis.
- The CBI provides a valuable tool for comparing the DNA binding potencies of different substances.
- Carcinogens acting via indirect mechanisms, which do not directly damage DNA, may exhibit nonlinear dose-response curves and disturb cellular control mechanisms.