Metabolism of Insecticides in Living Organisms:

Work on the metabolism of insecticide chemicals innately has to wait till appropriate qualitative and quantitative methodology is available. Luckily, proper tools and theories are now available and as an outcome, the knowledge about the metabolism of insecticide chemicals is considerable and is continuing to rise at a quick pace.  

The significant enhancements in this field comprise:   

1) The utilization of radioactive compounds as tracers and for quantization. 

2) Employment of a broad range of chromatographic methods for resolution and isolation of the metabolic products.

3) Perfection of infrared, ultraviolet, nuclear magnetic resonance (NMR) and mass spectroscopy accessories for recognizing much small amounts of such products. 

4) Enhancements in the synthesis procedures for authentic compounds required to confirm the tentative characterization of the metabolic and degradation products.

5) Growth of the active mammalian, insect and plant in vitro enzyme systems which reproduce, at least in part, the reactions taking place in the living organism.

6) Accumulation of a body of associated literature giving analogies suitable for predicting the possible metabolic pathways comprised.   

The entire organic insecticide chemicals, to a differing degree, metabolize in the living organisms. The nature and extent of the biotransformation differ by the species and the chemical, the time of residence in the organism being a significant factor; in certain cases, a given amount of some chemicals metabolizes fully in a matter of minutes whereas other chemicals need hours or months. Therefore, some complex insecticide chemicals are readily metabolized in an organism whereas a few relatively simple ones are astonishingly resistant to the biodegradation.

The way and rate of metabolic attack on an insecticide chemical differ with the species, sex, strain, age and other features of an organism, by the presence or absence of certain additional  chemicals (which act as antagonists, synergists, inducers and so on.) and, specifically, with the diversity of functional or reactive groups accessible in the molecular structure of such chemicals. In the attack (which usually is enzymatic), ester and amide groups are hydrolyzed; ether groups are cleaved; hydroxylation takes place on aromatic rings and on C-alkyl and N-alkyl substituent; unsaturated bonds are modified through oxidation; thioethers are oxidized and phosphorothionates are transformed to phosphates having loss of sulfur. N-Alkyl- groups are eliminated through oxidation and hydroxylation; halogens and sulphur are eradicated or replaced; nitro-groups are reduced; isomerization is induced; rings are opened and alcoholic or acidic functions are conjugated. The attack is at times modified through small changes in the chemical structure of the insecticide; so, outcomes can't for all time be interpolated from one compound to a closely associated analog. The attack usually generates detoxification, however at times gives mount to degradation products that encompass toxicity equivalent to or greater than that of the parent compounds. Whenever hydrolysis of esters occurs then detoxification usually yields.

Botanicals:

i) Nicotine:

The fate of nicotine in mammals, comprising man, is familiar as an outcome of the requirement for this information in medicine, by means of health services and for safety purposes. 

The metabolic attack on nicotine by means of the mammalian liver comprises:

• Methylation of the pyridyl nitrogen, to make isomethylnicotinium ion. 
• Demethylation of the nitrogen in the 5-membered ring to make nornicotine.
• Hydroxylation of a carbon atom adjacent to the nitrogen. 

ii) Rotenone: 

Rotenone is a selective, non-specific insecticide having several acaricidal properties. It is mainly used in home gardens for insect control, for tick and lice control on pets and for fish abolitions as part of water body management. The utilization of the pesticide for control of fish and in cranberries is limited by the Environmental Protection Agency.

Rotenone is an extracted part of rotenoid plant from such species like barbasco, haiari, cub, nekoe and timbo. Such plants are members of the pea (Leguminosae) family. Rotenone having extracts are taken from the roots, leaves, seeds of the different plants. Rotenone is usually categorized as a botanical insecticide.

iii) Pyrethroids:

Pyrethrum is a compound which is basically extracted from Chrysanthemum flowers. It has been employed as an insecticide since from the first century. Pyrethroids are synthetic variations of naturally found pyrethrums. The benefit of preparing pyrethrins in the lab is that the compounds tend to be much potent and last longer in the environment, two properties desired for the control of pest. Pyrethroid pesticides are employed in mosquito control, agriculture, lawn and garden care and in the veterinary care. A number of representative pyrethroids are resmethrin, permethrin, cyfluthrin, fenvalerate, barthrin and sumethrin.

Methylenedioxyphenyl Synergists:

Methylenedioxyphenyl synergists are mostly used to improve the insecticidal activity of the pyrethrum, metabolize quickly in mammals and much slowly in houseflies. Tropital, Piperonyl butoxide and sulphoxide, in general, suffer initial hydroxylation most likely to give the unstable hydroxymethylenedioxyphenyl derivative which, in turn, decomposes to provide the corresponding catechol and formate, the subsequent oxidizing to carbon-dioxide. This path predominates over alternate pathways in living mice and in the mouse-liver microsome systems.

Chlorinated Hydrocarbons:

Chlorinated hydrocarbons are the organic molecules characterized through the presence of at least one chlorine atom bonded to the carbon atom. Compounds that comprise such molecules encompass a broad range of utilizations, from making cookware to creating industrial solvents. Many companies manufacture or work with such molecules, as well termed as chlorocarbons or organochlorides. Most of the people interact with products made with such chemicals on an everyday basis; however they might not be aware of this fact.

a) DDT and Methoxychlor:

DDT and Methoxychlor experience dehydrochlorination in the trichloroethane group making an ethylene bond like that in DDE. DDT as well suffers reductive dechlorination at the trichloroethylene group to form DDD (TDE) and hydroxylation at the tertiary carbon to make an alcohol (dicofol) that at times metabolizes to dichlorobenzophenone. In mammals, DDT and methoxychlor suffer a series of reactions, comprising dehydrochlorination, dechlorination and reduction, followed through hydration and oxidation to form the respective disubstituted acetic acids, like DDA (from DDT).

ii) Lindane:

Lindane (γ-BHC) degrades quickly in houseflies and mammals however only after initial conversion to a thiol conjugate. Glutathione (GSH) is most likely the thiol donor in this case. The thiol conjugate cleaves, making a pentachlorocyclohexene, or, alternatively, it in fact degrades to whole six isomers of dichlorophenyl mercapturic acid.  

iii) Chlorinated Methano-Bridged Cyclohydrocarbons (Cyclodienes):

Till recently, the epoxide forms of the so called chlorinated Cyclodienes were regarded to be stable in the metabolizing systems. Though, such compounds, too, undergo certain degree of biotransformation comprising hydroxylation, dechlorination and oxidation. Since some of the metabolic products have been recognized, a great many remain unknown, most of them being hydrophilic.

Organophosphorus Compounds:

Organophosphorus compounds are the chemical compounds in the molecules of which an atom of phosphorus is joined to the carbon atom of a hydrocarbon group. Such compounds encompass a number of industrial utilizations; however their principal application is as insecticides. Early Organophosphorus insecticides (example: parathion) were highly toxic to birds and mammals. Later compounds were less poisonous and dangers to wildlife were reduced, however careless or improper utilization can cause serious damage.

i) Organophosphates and Organophosphonates:

This group of compounds is vast, and mention is made here of just certain ones which are commercially significant or that describe different sites of metabolic attack. Insecticidal pyrophosphates comprise tepp, a tetraethyl derivative that hydrolyzes simply by enzymatic and non-enzymatic means, and Schradan, the much stable tetra (dimethylamidate) derivative. The metabolic attack on tepp is through hydrolysis of the pyrophosphate bond. Schradan suffers this hydrolysis as well however only after oxidative attack that comprises N-demethylation through N-oxide or N-hydroxymethyl intermediates.

ii) Organophosphorothionates and Organophosphonothionates:

The common comments made above in consider to the huge number of organophosphates and organophosphonates which have been investigated apply uniformly here. Thus, just a limited number of compounds are regarded in this group.

Methyl and Dimethyl Carbamates:

The metabolism of carbamic acid ester insecticide chemicals is distinct from that of phosphoric acid esters even although both act as cholinesterase inhibitors. This is due to the reason that the methyl and dimethylcarbamoyl groups are amazingly stable to hydrolysis in the living organisms. However hydrolysis comprised the main metabolic attack comprise of hydroxylation and oxidation followed through conjugation of the hydroxylation products. Just like phosphorylation of some tissue proteins takes place in mammals by means of organophosphates, there is proof that certain carbamoylation occurs in mammals in the presence of Carbamates.

Behavior of Pesticides in the Environment:

The extensive use and disposal of pesticides through farmers, institutions and the general public give numerous possible sources of pesticides in the environment. Following discharge into the environment, pesticides might have several different fates. Pesticides which are sprayed can move via the air and might finally end up in other portions of the environment, like in soil or water. Pesticides that are applied directly to the soil might be washed off the soil to close to bodies of surface water or might percolate via the soil to lower soil layers and groundwater. Pesticides that are injected to the soil might as well be subject to the latter two fates. The application of pesticides directly to bodies of water for weed control or indirectly as an outcome of leaching from boat paint, runoff from the soil or other routes, might lead not just to build up of pesticides in water, however as well might contribute to air levels via evaporation.

This partial list of possibilities proposes that the movement of pesticides in the environment is much complex with transfers taking place continually among various ecological compartments. In certain cases, such exchanges take place not only between areas which are close altogether (like a local pond receiving a few of the herbicide application on adjacent land) however as well might comprise transportation of pesticides over long distances.

Properties of Pesticides:

Moreover to resistance to degradation, there are a number of other properties of the pesticides that find out their behavior and fate. One is how volatile they are; that is, how simply they evaporate. The ones which are most volatile encompass the highest potential to go into the atmosphere and, when persistent, to move long distances. The other significant property is solubility in water; or how simply they dissolve in water. When a pesticide is much soluble in water, it is more simply taken off with rainwater, as runoff or via the soil as a potential groundwater contaminant (or leaching). Moreover, the water-soluble pesticide is more probable to stay mixed in the surface water where it can contain unfavorable consequences on fish and other organisms. When the pesticide is insoluble in water, it generally tends to stick to soil and as well settle to the bottoms of bodies of surface water, making it less accessible to organisms.

Fate of Pesticide in the Atmosphere:

Pesticides come into the atmosphere either through application drift, post-application vapor losses or by wind erosion of pesticide treated soil. They and their photodegradation products might be transported long distances prior to the removal methods of atmospheric wet and dry deposition return them to the earth's surface

a) Application drift:

The liquid sprays are applied via nozzles that give atomization, metering and uniform distribution of the pesticide mixture. The bulk of atomizers employ hydraulic pressure as the energy source for breaking the liquid to droplets. The proportion of the net spray volume contained in droplet sizes beneath 150μm can be employed as an indicator of the drift potential, as it is such small droplets which are most prone to the movement in windy conditions.  

b) Post-application vapor losses:

There are two kinds of applications. Pre-emergence applications are applied to the soil surface before the emergence of the crop, might be left undisturbed on the soil surface or incorporated through some form of soil disturbance to the upper layer of soil. Post-emergence applications are applied to the crop, a part of which will go through the crop and deposit on the soil surface.

c) Wind-erosion of pesticide-treated soil:

Pesticides on the soil surface might be vulnerable to transport via wind erosion of soil in which three methods are considered operative. Big soil particles can roll on the soil surface beneath the affect of wind and this movement is termed as surface creep. Smaller particles can become hanged in the air for short duration of time as they move laterally.

Fate of Pesticide in the Aquatic System:

The contagion of water bodies by pesticides can cause a significant threat to the aquatic ecosystems and drinking water resources. Pesticides can enter water bodies through diffuse or by means of point sources. Diffuse-source pesticide inputs into water bodies are the inputs resultant from the agricultural application on the field. Such are tile drain outflow, base flow seepage, surface and subsurface runoff and soil erosion from the treated fields, spray drift at application, and deposition after volatilization. In contrary, point-source inputs derive from the localized circumstance and enter a water body at a particular or limited number of positions. These are mostly sewage plants, farmyard runoff, sewer overflows and accidental spills. There are as well point sources of pesticides from non-agricultural use, example: from application on roads, railways or urban sealed surfaces like parking lots.

Most of the factors, like soil and pesticides properties and crop management practices, govern the potential for groundwater or surface water pollution through pesticides.

Fate of Pesticide in the Plant-Soil System: 

Research in the fields of metabolism and the ecological behavior of pesticides has build up into interdisciplinary cooperation, in which pesticide chemists assist with agricultural chemists, microbiologists, soil scientists and plant physiologists. All the method that occurs in the soil-plant system after the application of pesticides is of specific interest for agriculture. The ultraviolet fraction of sunlight is absorbed to a considerable level through numerous chemical compounds, giving the energy essential for photochemical reactions. Likewise decomposition reactions occur throughout the chemical and biochemical modifications in which the microorganisms, plant roots and soil macro fauna are comprised.

Absorption and fixation methods comprise an influence on the availability of active substances in the soil for treated cultures and following untreated crops.

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