The mechanism of action of this group of chemicals, which includes the interaction between the 17 congeners of PCDD and PCDF, the 12 dioxin-like action PCBs with the citosolic receptor Ah, is well distributed among species. When the ligand substance has a planar structure, it binds the receptor with different affinities forming a complex that translocates in the cell nucleus, where it binds to specific DNA sequences, and induces changes in gene expression. Several DNA sequences of this type can be found in animals, therefore many gene expressions could be influenced. The most toxic substance among the mentioned compounds so far is TCDD or 2,3,7,8-tetrachlorodibenzo-
/-dioxin. The relative toxic potency of the other compounds are expressed as proportions relative to TCDD and are referred to as toxicity equivalence factors (TEFs). Table 2 shows the TEFs for mammals, birds, and fish. The biological matrices analyzed usually present complex patterns of the above-mentioned chemicals. The toxic potency of these chemicals can be calculated by multiplying the concentration of each compound by its TEF value and summing the products.
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