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Effect of induction by DDT on metabolism and mutagenicity of benzo[a]pyrene
Abstract:
Liver microsomal enzymes are essential for the detection of benzo[a]pyrene (B[a]P)-mediated mutagenesis in the Salmonella/mammalian microsome mutagenicity test and, furthermore, this mutagenicity is considerably enhanced by induction of hepatic enzymes involved with drug metabolism. Although Aroclor 1254 is most commonly used for induction of S9 enzymes, DDT is also capable of this induction. This paper reports a comparison of liver S9 fraction induced by the two agents: there is a marked difference in their concentration optima for metabolism of B[a]P; greater numbers of revertant colonies are seen with Aroclor-induced S9, which is optimal at a concentration of 10% (v/v), whereas DDT-induced S9 is optimal at 2.5% (v/v); Aroclor induces aryl hydrocarbon hydroxylase (AHH), cytochrome P-450 and epoxide hydrase while DDT induces only AHH, to about half the level detected in the Aroclor-induced S9 fraction. A comparison of metabolite distribution for Aroclor- and DDT-induced hepatic microsomes reveals quantitative differences only. DDT-induced microsomes yield a greater proportion of B[a]P-4,5-oxide and its metabolic product B[a]P-4,5-dihydrodiol than do Aroclor-induced microsomes. Time course studies on the mutagen half-life measured on the agar plate provides good evidence that metabolites responsible for mutagenicity were different for each inducer.
Insights
Aroclor and DDT induce liver enzymes differently, affecting benzo[a]pyrene (B[a]P) mutagenicity. Aroclor-induced enzymes show higher mutagenicity and induce more enzyme types than DDT-induced enzymes, suggesting different metabolic pathways.
Area of Science:
- Biochemistry
- Toxicology
- Genetics
Background:
- Liver microsomal enzymes are crucial for detecting benzo[a]pyrene (B[a]P) mutagenicity.
- Enzyme induction significantly enhances B[a]P-mediated mutagenesis.
- Aroclor 1254 and DDT are known inducers of hepatic enzymes.
Purpose of the Study:
- To compare the effects of Aroclor-induced and DDT-induced liver S9 fractions on B[a]P metabolism and mutagenicity.
- To investigate the differences in enzyme profiles and metabolite production between Aroclor- and DDT-induced S9 fractions.
- To understand the impact of inducer-specific metabolic pathways on B[a]P mutagenicity.
Main Methods:
- Salmonella/mammalian microsome mutagenicity assay.
- Enzyme induction using Aroclor 1254 and DDT.
- Quantification of B[a]P metabolites using chromatography.
- Measurement of enzyme activities including aryl hydrocarbon hydroxylase (AHH) and cytochrome P-450.
- Time course studies on mutagen half-life.
Main Results:
- Aroclor-induced S9 showed higher B[a]P mutagenicity, optimal at 10% (v/v), compared to DDT-induced S9, optimal at 2.5% (v/v).
- Aroclor induced aryl hydrocarbon hydroxylase (AHH), cytochrome P-450, and epoxide hydrase, while DDT induced only AHH at a lower level.
- DDT-induced microsomes produced a higher proportion of B[a]P-4,5-oxide and B[a]P-4,5-dihydrodiol.
- Mutagenicity half-life studies indicated distinct mutagenic metabolites depending on the inducer.
Conclusions:
- The choice of inducer (Aroclor vs. DDT) significantly impacts B[a]P mutagenicity due to differences in enzyme induction profiles.
- Quantitative differences in B[a]P metabolite profiles exist between Aroclor- and DDT-induced hepatic microsomes.
- Distinct metabolic pathways are responsible for B[a]P mutagenicity, influenced by the specific enzyme induction by Aroclor or DDT.