Halogenated aromatic hydrocarbons Polychlorinated biphenyls

Halogenated aromatic hydrocarbons Polychlorinated biphenyls Polybrominated biphenyls Dioxins Pesticides Organophosphates Organochlorides Carbamates... 

Clearly, humans are exposed to a complex mixture of CDDs and other halogenated aromatic hydrocarbons such as chlorinated diben-zofurans (CDF) and polychlorinated biphenyls (PCBs). The toxicological concerns resulting from exposure to mixtures, as well as the gaps... 

For halogenated aromatic hydrocarbons like polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs), and polychlorinated dibenzo-p-dioxins (PCDDs) the binding to the aryl hydrocarbon (Ah) receptor regulates their toxicity [89]. The Ah receptor controls the induction of one of the cytochrome P450 enzymes in the liver. Toxic responses such as thymic atrophy, iveight loss, immu-notoxicity and acute lethality are associated ivith the relative affinity of PCBs, PCDFs and PCDDs for the Ah receptor [89]. The quantitative structure-activity relationship (QSAR) models predicting the affinity of the halogenated aromatic hydrocarbons ivith the Ah receptor describe the electron acceptor capability as well as the hydrophobicity and polarizability of the chemicals [89[. 

It is well known that immunosuppression is caused by a variety of hydrocarbons such as benzene, polycyclic aromatic hydrocarbons and halogenated aromatic hydrocarbons. Thus, human exposure to polychlorinated biphenyls (PCBs) in Japan (Yusho accident) and China has been associated with increased respiratory infections and decreased levels of immunoglobulins in serum. In animals exposed to these compounds there is atrophy of both primary and secondary lymphoid organs, lower circulating immunoglobulins and decreased antibody responses after exposure to antigens. Similarly, the exposure of both humans and farm animals to polybrominated biphenyls, which occurred in Michigan in 1973, resulted in depressed immune responses. 

The most extensively studied agonists are the halogenated aromatic hydrocarbons such as TCDD, polychlorinated biphenyls, and polychlorinated dibenzofurans as well as PAHs such as benzo[a]pyrene and 3-MC. One of the highest affinity ligands of the AHR and the most potent inducer of CYPlAl expression is TCDD. As the result of this ligand-receptor interaction, exposure to TCDD produces a wide variety of toxic effects that are species- and tissue-specific. The response to TCDD is due to the fact that TCDD has a remarkably high affinity for the AHR (on the order of 10 M, and that this ligand is... 

Polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs) are chemically classified as halogenated aromatic hydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclic aromatic compounds with similar physical and chemical properties, and both classes are similar structurally. Certain of the PCBs (the so-called coplanar or mono-ortho coplanar congeners) are also structurally and conformationally similar. The most widely studied of these compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This compound, often called simply dioxin, represents the reference compound for this class of compounds. The structure of TCDD and several related compounds is shown in Figure 4.1. 

Such xenobiotics as aliphatic hydrocarbons and derivatives, chlorinated ahphatic compounds (methyl, ethyl, methylene, and ethylene chlorides), aromatic hydrocarbons and derivatives (benzene, toluene, phthalate, ethylbenzene, xylenes, and phenol), polycyclic aromatic hydrocarbons, halogenated aromatic compounds (chlorophenols, polychlorinated biphenyls, dioxins and relatives, DDT and relatives), AZO dyes, compounds with nitrogroups (explosive-contaminated waste and herbicides), and organophosphate wastes can be treated effectively by aerobic microorganisms. 

This technology has been used to treat polychlorinated biphenyls (PCBs), halogenated and nonhalogenated solvents, semivolatile organic compounds (SVOCs), polynuclear aromatic hydrocarbons (PAHs), pesticides, herbicides, fuel oils, benzene, toluene, ethylbenzene, and xylenes (BTEX), and mercury. This system has also treated Resource Conservation and Recovery Act (RCRA) hazardous wastes such as petroleum refinery wastes and multisource leachate treatment residues to meet RCRA Land Disposal Restrictions (LDR) treatment standards. 

Aromatic compounds are especially stable and are, therefore, important persistent pollutants. They include the polyaromatic hydrocarbons (PAHs), and may be halogenated, such as the polychlorinated biphenyls (PCBs) and many pesticides. Also included are the substituted benzenes, such as phenol. A large body of literature has examined aromatic radiation chemistry [22— 38]. The discussion that follows examines benzene and substituted benzenes as a model for the radiolysis of more complicated aromatic compounds. 

From these inventories and data, it is clear that society is facing an enormous problem of contamination. Many of the polluting compounds that are continuously dispersed are products of industrial activities such as phenols and halogenated phenols, polycyclic aromatic hydrocarbons (PAH s), endocrine disruptive chemicals (EDC), pesticides, dioxins, polychlorinated biphenyls (PCB s), industrial dyes, and other xenobiotics. In this chapter, we critically review the literature information on the enzymatic transformation of these polluting xenobiotics. This work is focused on peroxidases as enzymes able to transform a variety of pollutant compounds with the aim to reduce their toxicity and their environmental impact. 

Polychlorinated dibenzo-y>-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) are chemically classified as halogen-ated (polychlorinated) aromatic hydrocarbons (PHAH). Dioxins are formed as a by-product of chemical processes, whereas PCBs are synthesized by direct chlorination of biphenyls. Due to their lipophilic character, they are concentrated in the food chain and both humans and wildhfe are exposed to them. The major source of human exposure is through the diet, as these substances are concentrated in fatty tissues of beef, poultry, pork and fish, and through cigarette smoking. Moreover, maternal milk contains considerable amounts of PCBs and dioxins. 

Chemicals Chlorinated pyridinal Organophosphate pesticides o-Phthalaldehyde Triclosan Polychlorinated biphenyls Chlorofluorocarbons Chlorinated aromatics hydrocarbons Halogenated biocides Atrazine Petroleum hydrocarbons Amitraz Bifenthrin Other pesticides, herbicides, biocides, fumigants, germicides, etc. 

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