Chlorinated pesticides biotransformations

Separately living populations of wildlife species may show differences in patterns of environmental contaminants like PCBs. PCB and PCDD/F congener patterns may differ in the same species from various locations as a result of the difference in patterns already present in their (differing) food, as has been demonstrated for seals fed with fish.35 Such differences also relate to selective biotransformation capacities of the organisms involved.20 Large differences in metabolic capacity have been observed between pinnipeds and cetaceans (c/. biotransformation section). This does not only hold for CACs, but also for chlorinated pesticides. For instance, porpoises have been shown to have a... 

Certain substances are known to deplete chlorinated pesticides held in the body in the lipid phase. They induce greater biotransformation, thereby accelerating the depletion of toxicants in the lipid phase. For example, phenobarbital can induce biotransformation of dieldrin (Cook and Wilson 1971), while diphenyhydantoin may be effective against DDT residues (Davies et al. 1971). In addition, adsorbants such as activated carbon are used to treat pesticide poisoning. 

Besides chlorinated pesticides, some of the phosphorous pesticides are also chiral (see Table 2.1). The phosphorous pesticides also differ in their enantioselective toxicities. These pesticides were introduced in the 1950s to control insects in fruit, vegetables and other crops. Malathion is biotransformed into a racemic malaxon that has anti-acetylcholinesterate (insecticidal) activity. The /f-enantiomer has a 22 times greater inhibitory potency than the 5-enantiomer for bovine erythrocyte cholinesterase [39, 40]. The nerve agent, soman, has two chiral centres, and the two (—)-diastereoisomers are more potent inhibitors than their corresponding (-1-)-counteiparts for acetylcholinesterase and a-chymotrypsin. 

Hydrolytic reactions are slow for most of the chlorinated pesticides. Wolfe etal. (1977) estimated the hydrolytic half-lives of DDD, DDE, and DDT from rate constants calculated from structure-activity relationship and literature data. The estimated values are in good agreement with the experimental findings with distilled water or in raw river water (Eichelberger and Lichtenberg, 1971). DDE is resistant to hydrolysis since it is the end product in the hydrolysis of DDT between pH 3-11. The second-order rate constant for the hydroxide ion-catalyzed hydrolysis of DDT at pH 9 is 9.9 X 10 M sec , which corresponds to a half-life of 81 days. Both isomers of endosulfan hydrolyze slowly at pH 5 with the rate increasing with pH. The data reported by Greve and Wit (1971) were from direct hydrolytic rate measurements, whereas that of Martens (1976) was from one set of data points on controls in biotransformation studies. After 10 days, the loss of endosulfan in controls as a function of pH was as follows ... 

Table 6.3. Biotransformation and metabolites of some chlorinated pesticides.

Attempts have been made to apply the structure-activity concept (Hansch and Leo 1995) to environmental problems, and this has been successfully applied to the rates of hydrolysis of carbamate pesticides (Wolfe et al. 1978), and of esters of chlorinated carboxylic acids (Paris et al. 1984). This has been extended to correlating rates of biotransformation with the structure of the substrates and has been illustrated with a number of single-stage reactions. Clearly, this approach can be refined with the increased understanding of the structure and function of the relevant degradative enzymes. Some examples illustrate the application of this procedure ... 

Phosgene is widely used as a chemical intermediate. It is used in metallurgy and in the production of pesticides, herbicides, and many other compounds. It is a by-product of chloroform biotransformation and can be generated from some chlorinated hydrocarbon solvents under intense heats. Phosgene has been used as a chemical warfare agent. 

Biotransformation of certain chlorinated hydrocarbon insecticides results in their conversion to metabolites which are less polar than the parent chemical. Heptachlor and aldrin are converted to the more lipophilic compounds heptachlor epoxide and dieldrin, respectively, whereas DDT is converted to DDE. The primary residue of DDT, which persists to the present day in animals and humans after exposure over a decade ago, is DDE. Following biotransformation, these compounds distribute to tissues which are higher in neutral lipid content than are the major organs of metabolism and excretion, the liver and kidney. These lipid-rich tissues are relatively, deficient in the so-called mixed-function oxidase (MFO) enzyme systems necessary for biotransformation of the halogenated hydrocarbons to more polar and thus more easily excreted compounds. As a result, these lipophilic chemicals remain unchanged in adipose tissue with only limited amounts returning to the circulation for possible metabolism and excretion. Paradoxically, aldrin and heptachlor metabolism results in an increased rather than reduced body load. This is opposite of the pattern seen for most other pesticide classes. 

Chlorinated hydrocarbons are the most stable pesticides in the environment. Persistence of a compound is a measure of its resistance to degradation, but no pesticide is considered totally resistant to biological degradation. Table 6.3 lists the systems and metabolites in the biotransformation of chlorinated... 

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