Metabolic aspects of pesticide

Machin, A.F. et al. (1975). Metabolic aspects of the toxicology of diazinonl Hepatic metabolism in the sheep, cow, guinea-pig, rat, turkey, chicken, and duck. Pesticide Science 6, 461 73. 

Another aspect of pesticide-soil interactions that is very important in predicting the effect of pesticides on environmental quality is the degradation rate. It is not the purpose of this discussion to give an in-depth discussion of pesticide degradation in soils. Numerous reviews are available on transformations, metabolic pathways, persistence, and tl/2 values of... 

In general, studies of pesticide metabolism in cell cultures have shown that metabolism is qualitatively similar to that of the whole plant, but quantitative differences do exist. Whole plants or plant parts need to be used to confirm the quantitative aspects of pesticide metabolism observed in plant cell cultures. However, cell cultures can be used to estimate the phytotoxicity and metabolic fate of chemicals that exhibit poor uptake and mobility in whole plants. Thus, they provide an inq>ortant adjunct to whole plant studies. In addition, higher yields of minor or transitory metabolites can usually be achieved in cell cultures, allowing the determination of a sequence of metabolic steps in a reaction. 

Walker, C.H. (1974). Comparative aspects of the metabolism of pesticides. Environmental Quality and Safety 3, 113-153. 

The fundamental chemistry, especially of the newer economic poisons, is of primary importance. The mechanism of action of the various types of economic poisons and the relation of structure to toxicity of insects are of fundamental interest. Chemical versus biological methods of evaluation should be presented. Performance methods of evaluation of these chemicals have been given careful consideration by several workers. Emphasis was placed by several workers on the need for much additional information on various aspects of the problem regarding the use of DDT, 2,4-D, and other pesticides. There is direct importance in studies on the metabolism of DDT. 

Many xenobiotic compounds, such as pesticides, are esters, amides, or organophosphate esters, and hydrolysis is a very important aspect of their metabolic fates. Hydrolysis involves the addition of H20 to a molecule accompanied by cleavage of the molecule into two species. The two most common types of compounds that undergo hydrolysis are esters... 

Such information has led to explosive growth in the understanding of biochemical processes. Knowledge of metabolism, biosynthetic processes, neurochemistry, regulatory mechanisms, and many other aspects of plant, animal and insect biochemistry has provided a basis on which the mode of action of a pesticide may often be more clearly understood. The exploitation of biological information can lead to the synthesis of a new molecule designed to act at a particular site or block a key step in a biochemical process. 

This chapter provides an overview of factors affecting dermal absorption. Factors influencing absorption are among others related to the skin (e.g. anatomical site, difference between species, metabolism, etc.) and the exposure conditions (e.g. area dose, vehicle, occlusion and exposure duration). In order to provide relevant information for the risk assessment of pesticides, dermal absorption studies should take these aspects into account. With respect to the methods being used nowadays for the assessment of dermal absorption, it is important to realize that neither in vitro nor in vivo animal studies have been formally validated. Available data from various in vitro studies, however, indicate that the use of the total absorbed dose (i.e. the amount of test substance in the receptor medium plus amount in the skin) could be used in a quantitative manner in risk assessment. Tape stripping of the skin can be adequate to give a good indication of test chemical distribution, and hence its immediate bioavailability. 

An attempt has been made to give an overview of all chemical aspects of plant protection, with the exception of analytical chemistry. We are fully aware that the designation chemistry of pesticides does not cover an unequivocally defined, uniform branch of science, because the fundamental sciences on which it is built, particularly organic chemistry and biochemistry, have maintained their independence and their original scope also within the frame of this special field. The chemistry of pesticides integrates these fundamental sciences only functionally, and not with respect to their methods. Our book attempts to achieve this functional unity. In the discussion of individual compounds and types of compound our aim has been to cover preparative and organic chemical and biochemical aspects, metabolism, activity-structure relationships, fields of application, and environmental and toxicological problems. 

A previous study for the evaluation of the organochlorine pesticides burden in the human body, of a non-occupational-exposed population (WHO Project European Cooperation on Environmental Health Aspects of the Control of Chemicals ) indicated that human milk levels in the range of 11-12 mg kg HCH and 2.8 mg kg DDT, were about 5 times higher than in other European countries. The adipose tissue and fat sampled from humans, indicated a mean content in DDT plus DDE in the range of 8-17 mg kg, the DDE p entages demonstrated over a long period involving the metabolization of DDT and a constant intake of DDE in food. 

All of the comparative metabolism studies have shown that qualitatively there is little difference in pesticide metabolism in plants and cell cultures. Quantitative differences, either in the rate of conversion to a single product or relative inportance of one pathway over another, do occur. Some of these differences may be of biological significance and would require the use of whole plants or plant parts to confirm the quantitative aspects of metd)olism. 

However, a problem with these chemicals is their indiscriminate toxicity in the presence of light and molecular oxygen. This toxicity precludes their use as pesticides. Safe alternatives are to treat target organisms with chemicals that either are selectively metabolized to photodynamic compounds or cause the target organism to produce toxic levels of natural photodynamic compounds with its own biochemical machinery. This review examines one aspect of the latter alternative - treatment of plants with compounds that cause the accumulation of herbicidal levels of photodynamic porphyrins. 

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