2,3,7,8-Tetrachlorodibenzodioxin (TCDD)

TCDD is a colorless solid with no distinguishable odor at room temperature. It is usually formed as a side product in organic synthesis and burning of organic materials. TCDD and dioxin-like compounds act via a specific receptor present in all cells: the aryl hydrocarbon (AH) receptor. This receptor is a transcription factor which is involved in expression of genes; in fact it has been shown that high doses of TCDD either increase or decrease the expression of several hundred genes in rats. Genes of enzymes activating the breakdown of foreign and often toxic compounds are classic examples of such genes. TCDD increases the enzymes breaking down, e.g., carcinogenic polycyclic hydrocarbons such as benzo(a)pyrene. These polycyclic hydrocarbons also activate the AH receptor, but less than TCDD and only temporarily. Even many natural compounds present in vegetables cause some activation of the AH receptor.