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However, the researcher quite often will not routinely design his studies to ensure total accountability. Such an approach would require collection of respired gases from animals and plants, leachate from soils, etc. for isotope determinations. Procedures are of course available for such analyses, but they are often cumbersome and may place an inordinate stress on the study organism. The usual approach, which involves thorough extraction and analysis of excreta, tissues, or other matrices will in many cases be adequate to fully define the pesticide/organism interactions that occur. The extent of volatility losses, metabolism to CO^, etc. are quite often inferred on the basis of data obtained from the analysis of excreta and tissue samples.


A pesticide metabolism study usually involves a specific radiolabeled pesticide and a single species to be studied. In environments such as soil or water there is of course usually a rather large potential number of species (particularly microorganisms) that may interact with the compound under study.

Irrespective of the type of study under consideration, adequate replication in metabolism studies is important where quantitative data are required, but it is probably safe to say that replication is less crucial where one's emphasis is placed more on qua!itative aspects of metabolism, i.e. defining the nature of the metabolites generated by a specific organism or system. There are many examples in the literature, particularly with large animals such as cattle, where pesticide metabolism data are reported from a single animal.

Laboratory animals. Metabolism studies with radiolabeled pesticides in laboratory animals, usually rodents, are often done to gather data for extrapolation to man. In most cases, such studies involve oral administration of the radiochemical, which is representative of the major route of human exposure to most IAEA-SM-263/30 pesticides and which subjects the chemical to potential metabolic and absorption phenomena that may be specific to the gastrointestinal tract. Inhalation and dermal studies with radiolabeled pesticides in laboratory animals are much less common, but can provide very useful absorption data that may have toxicological relevance for humans. Intraperitoneal administration of radiolabeled pesticides, although having no direct environmental relevance, can be useful in comparative studies because it bypasses to a large extent the potential interactions with microflora of the upper gastrointestinal tract. In addition, IP administration can be used to facilitate the generation of rather large amounts of metabolites for isolation and characterization studies.

Livestock and poultry. Metabolism studies in livestock and poultry (cattle, sheep, chickens, etc.) are usually done to evaluate the potential for residue transfer into human foods.

Lactating or egg-producing animals are often used in such studies to allow collection of data related to the significance of potential milk or egg residues. The radiolabeled pesticides are usually administered orally, although a few dermal studies in livestock have been reported for pesticides whose use patterns involve direct dermal application to animals for ectoparasite control. In studies designed primarily to elucidate metabolic pathways and gain preliminary insights into patterns of residue retention and elimination, the radiochemical may be administered as a rather large single oral dose, often in the range of 5-100 mg/kg. Where data predictive of environmentally relevant exposures are needed, oral dosages are often multiple (1-2 dosages per day for several days) and are of relatively high specific activity and low total chemical dosage (1 mg/kg/day).

Such studies can provide data that are reasonably predictive of environmental exposures to low levels of a pesticide, particularly with respect to "steady-state" or "plateau" levels of residues that may be expected in tissues, milk, eggs, etc. as a result of continuous environmental exposure to a specific pesticide.

PI ants. Metabolism studies in plants may be conducted for a number of reasons, dependent upon the type of pesticide and use pattern involved. As discussed earlier, metabolism studies with herbicidal chemicals in plants may provide valuable insight into selective toxicity phenomena. Studies of pesticide fate in plants can be very important in evaluating the potential for residues appearing in plant products subsequently used either as human foods or as livestock feeds or forage. For obvious reasons, plant metabolism studies generally utilize routes of administration that are consistent with the actual or proposed use patterns of the specific pesticide. Thus, surface application of a radiolabeled pesticide to foliage is usually preferred when studying a 96 IVIE pesticide that is sprayed or dusted on plant foliage, and application of the radiochemical to soil for subsequent uptake by growing plants is preferred for those chemicals normally applied to soils. Quite often, radiolabeled pesticides are administered to plants through stem or petiole injection, or through root uptake from water, in studies designed to evaluate translocation phenomena. In plant metabolism studies, particularly involving foliage application of the radiochemical, it is often difficult if not impossible to distinguish between metabolic reactions that occur in the plant and photochemical or chemical reactions that occur on or within the plant tissues. From a toxicological standpoint, however, the inability to make such distinctions is usually not critical, if the studies are designed to be representative of normal environmental conditions.

Soil and aquatic studies. As with plant metabolism studies, pesticide degradation in soils and water may occur through a number of mechanisms, including metabolism by living organisms, chemical and photochemical reactions. Irrespective of the mechanisms involved, however, it is no doubt true that the environmental persistence of pesticides is governed primarily by their interactions with the soil and water environments.

Metabolic studies in soils may be done by surface application or by thorough mixing of the radiochemical with the soil itself.

Comparative studies with sterilized versus non-sterilized soils are often done to evaluate more precisely the role of soil flora and fauna in pesticide degradation. Studies utilizing sterilized soils that are subsequently inoculated with specific organisms (usually bacteria or fungi) can provide well-defined data relating to the role of individual organisms in pesticide/soil interactions.

Studies of pesticide metabolism by microorganisms that occur in natural waters are conducted quite often. To obtain environmentally relevant data, care must usually be taken in water studies not to exceed the solubility limits of the pesticide in question. The inclusion of appropriate controls may be needed to properly define the role of non-metabolic conversions that may be related to such factors as pH, photochemical reactions, etc.

Metabolism by pest species. Metabolism studies with pesticides in target pest organisms can be valuable for a number of reasons. Often, a clear understanding of the rate of metabolism or of the nature of metabolites generated will explain why a pesticide is or is not toxic to a particular pest species.

Metabolic considerations quite often form the basis for explaining the development of resistance in arthropods and other pest organisms. Certainly, a thorough knowledge of the potential of pest organisms to metabolize specific pesticides or pesticide types is an important consideration in the development of IAEA-SM-263/30 chemicals that are both efficacious and selective in toxicity toward the target species.

Because of the wide variety of pest species with which it may be appropriate to conduct pesticide metabolism studies, specific research approaches may vary considerably. With insects and other arthropods, exposure to the radiochemical may be topical or oral; studies with aquatic organisms (e.g. mosquito larvae) are usually done by adding the radiochemical to water;

soil pests (e.g. nematodes, fungi) may be treated in culture, etc.

Where possible, it is certainly preferable to use dosages and exposure routes that most closely approximate those likely to occur under anticipated environmental conditions.

Non-target species. All of the world's living organisms may be potentially exposed to pesticides in the environment, but it is neither possible nor appropriate to attempt to define the nature of these interactions in most instances. However, in circumstances where pesticide/organism interactions with significant toxic consequences have occurred or appear likely to occur, it may be appropriate to conduct both conventional toxicological and metabolic studies. Certainly, the potential of a given pesticide to reduce or adversely affect populations of non-target mammals, birds, fish and many other organisms is of sufficient seriousness to merit rather detailed analysis of the interactions that may occur. Appropriately designed metabolism studies in such organisms can be quite valuable in arriving at a determination of overall risk.

In vitro studies. Although I have emphasized the applicability of radioisotope techniques in evaluating the metabolic and residual behavior of pesticides in intact living organisms, radioisotopes are widely used and can be crucial to the success of in vitro studies as well. There is considerable basis for the belief that the toxic nature of pesticides and other chemicals is quite often attributable to highly reactive metabolites that are generated in trace quantities and that may be short lived. Appropriate in vitro studies with enzyme preparations and radiolabeled chemicals may demonstrate the occurrence of such reactive metabolic intermediates that cannot be detected by conventional in vivo studies. In vitro studies with radioisotopes that utilize subcellular, cellular, tissue, or organ preparations can provide invaluable insight into the metabolic mechanisms involved in pesticide metabolism, and such studies are uniquely suited to identify sites within the organism where major metabolic reactions occur.



In this discussion, I have attempted in a very general way to discuss the rationale that many of us as pesticide metabolism scientists use in the conception, design and execution of our radioisotope-aided metabolism studies. Our discipline is one that is both very broad and quite complex. The data we generate are of far more than academic interest. Our data often have great toxicological significance and direct implications with respect to the impact of pesticides on the environment or on human health and safety.

The nature and number of potential interactions that pesticides may have with living organisms is uncountable! We as individual researchers must therefore carefully apply our good judgment and scientific expertise to ensure that the limited resources available to us are focused in the most appropriate areas. We must carefully establish our research priorities, and design and conduct our studies such that the data we generate are scientifically sound and highly relevant to environmental and human health concerns. Radioisotopes as tracers for our studies have been in the past, and will no doubt continue to be in the future, one of the fundamental tools in our sometimes frustrating but always challenging endeavors.


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The radioisotope tracing technique has been a useful tool in obtaining extensive information on the fate of herbicides in the soil-plant system, including their uptake, transport and metabolism by plants, their photochemical, chemical and microbial degradation, their adsorption-desorption and translocation in soil, and their bioavailability to untreated crops.

A balance study under practical field conditions using radiolabelling can examine a number of factors affecting the fate of a compound at the same time and assess the magnitude of the major processes involved. On the basis of these results, more detailed studies are then formulated to be conducted under the precisely defined environment of a growth chamber or a laboratory. Use of tracer techniques in such studies is demonstrated, with results from experiments with different 14C-labelled herbicides.

FIG.4. Results of an outdoor lysimeter experiment: 14 С distribution in the treated spring wheat, rotational crop rye, and soil after application of [benzenering-14C] metabenzthiazuron.

(a) Spring wheat,,111 d after spray application: (b) Rotational crop rye, 503 d after spray application.

106 FÜHR

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FIG.5. Mineralization rates (%) of the different carbon atoms of methabenzthiazuron under constant conditions in the soil (1.1% C, pH 6.5) at 22°C, 65% WHC, 10 ppm methabenzthiazuron, 11 weeks. Total radioactivity applied = 100%.

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FIG. 7. Plant experiment to determine the bioavailability of the non-extractable radioactivity.

Test compound: [carbonyl-nC] methabenzthiazuron (МВТ); sandy soil, pH 6.4; 1.15% C.

Total radioactivity applied = 100%.

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