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Comparative kinetic studies on aflatoxin B1-DNA binding and aflatoxin B1-glutathione conjugation with rat and hamster
Abstract:
Inhibition of microsome mediated aflatoxin B1 (AFB1) binding to exogenous or endogenous DNA by cytosolic glutathione (GSH) S-transferases is well established from our earlier studies. Correlation between inhibition of AFB1-DNA binding and AFB1-GSH conjugation in vitro using rat and hamster liver subcellular fractions is elucidated in this report. Even though hamster liver microsomes catalyzed AFB1 binding to exogenous DNA three times as much as the rat, hamster cytosol inhibited AFB1-DNA binding catalyzed by either microsomes severalfold more than the rat cytosol. AFB1-DNA binding is found to be inversely related to AFB1-GSH conjugation at all AFB1 concentrations (2-100 microM) studied. Presence of either styrene oxide or 3,3,3-trichloropropene oxide at 1 mM level diminished AFB1-GSH formation in vitro confirming some competition by these epoxides with AFB1-epoxide for cytosolic GSH S-transferases. In a reconstituted system with endogenous DNA, the ratio of AFB1-GSH to AFB1-DNA binding was found to be 10-15 times higher with the hamster in comparison with the rat indicating enhanced inactivation of the ultimate carcinogenic metabolite in the hamster. These results are discussed in relation to AFB1-DNA binding and AFB1 hepatocarcinogenicity in resistant and sensitive species.
Insights
Cytosolic glutathione S-transferases inhibit aflatoxin B1 (AFB1) DNA binding. Hamster cytosol showed stronger inhibition than rat cytosol, correlating with higher AFB1-GSH conjugation, suggesting enhanced detoxification in hamsters.
Area of Science:
- Biochemistry
- Toxicology
- Carcinogenesis
Background:
- Cytosolic glutathione S-transferases (GSTs) are crucial for detoxifying xenobiotics.
- Aflatoxin B1 (AFB1) is a potent hepatocarcinogen requiring metabolic activation.
- Previous studies established GSTs' role in inhibiting AFB1-DNA binding.
Purpose of the Study:
- To elucidate the correlation between AFB1-DNA binding inhibition and AFB1-GSH conjugation in vitro.
- To compare the protective capacity of rat and hamster cytosolic GSTs against AFB1-induced DNA binding.
- To investigate the role of species-specific detoxification mechanisms in AFB1 hepatocarcinogenicity.
Main Methods:
- In vitro assays using rat and hamster liver subcellular fractions.
- Measurement of AFB1 binding to exogenous and endogenous DNA.
- Quantification of AFB1-GSH conjugation.
- Competition studies with epoxide substrates.
Main Results:
- Hamster liver microsomes showed higher AFB1-DNA binding than rat microsomes.
- Hamster cytosol exhibited significantly greater inhibition of AFB1-DNA binding compared to rat cytosol.
- AFB1-DNA binding was inversely correlated with AFB1-GSH conjugation across tested AFB1 concentrations.
- A reconstituted system revealed a 10-15 times higher ratio of AFB1-GSH to AFB1-DNA binding in hamsters versus rats.
Conclusions:
- Hamster cytosolic GSTs are more effective in inhibiting AFB1-DNA binding than rat GSTs.
- Enhanced AFB1-GSH conjugation in hamsters contributes to their resistance to AFB1-induced hepatocarcinogenicity.
- Species differences in GST activity play a significant role in determining susceptibility to AFB1 toxicity.