Parathion toxicity class

Strychnine and the multi-chlorinated hydrocarbons such as parathion. The six toxicity classes according to Gosselin, Smith, and Hodge are shown in table 6.4. 

According to VNIIGINTOKS, potent toxic agents (PTA), where the fated dose (FD, J is less than 50 mg/ kg, are not introduced into agriculture, are not produced in the USSR, and are not imported from abroad [12]. In actual fact, Risk Class I OPPs with an FD for laboratory animals of less than 50 mg/kg were not only actively used, but were also produced in the USSR for many years (including such OPPs as parathion, demeton, octamethyl pyrophosphoramide, methyl ethyl... 

Insecticides of the phosphoric acid triester class include paraoxon (9.49) and dichlorvos (9.50). The phosphorothioate derivative parathion is a relatively non-toxic insecticide that undergoes monooxygenase-catalyzed oxidative desulfuration to paraoxon [105] (see also Chapt. 7 in [59] see Sect. 9.3.6). Paraoxon itself, like its congeners and the P-halide nerve gases, is highly toxic through its potent inactivation of acetylcholinesterase [69]. 

The discovery in the early years of the 20 century that certain phosphate esters possess mammahan toxicity and insecticidal properties heightened interest in this class of compounds, both in agriculmre and as potential agents in chemical warfare. Parathion became the practical choice as a broad-spectrum insecticide because of its greater stability and lower mammalian toxicity compared to its P=0 analogue, paraoxon . 

Parathion is one of a class of phosphorothionate triesters widely used as insecticides. These compounds exert their toxic effects in insects and mammals by inhibiting the enzyme acetylcholinesterase. The phosphorothionates, in general, are relatively poor inhibitors of acetylcholinesterase but are converted by the cytochrome P-450-containing monooxygenase enzyme systems in insects and mammals to the corresponding phosphate triesters that are potent inhibitors of this enzyme. 

There seems to be no limit to the number of toxic organophosphorus compounds that can be synthesized and that exhibit insecticidal activity. While many compounds of this type have been marketed, only about 20 of them comprise the bulk of the total tonnage. Parathion, methyl para-thion, and malathion are perhaps the best known and most widely used of this class of insecticides. The relatively low cost of the first two, combined with their good performance against a broad spectrum of insects, probably accounts for their continuing popularity. The low order of toxicity of malathion to mammals has made it acceptable under many conditions where other, perhaps more insecticidally active, insecticides are restricted. 

Even though biochemical lesions have not been identified for this group, one class of insecticides (aldrin, DDT, etc.) can reduce the toxicity of another class (parathion, etc.) in rats, suggesting a possible reason why some exposed workers seem able to withstand high exposures to these toxic substances without apparent ill effects. 

The organophosphosphates represent another extremely important class of organic insecticides. They were developed during World War II as chemical warfare agents. Early examples included the powerful insecticide schradan, a systemic insecticide, and the contact insecticide parathion. Unfortunately, both of these compounds are highly poisonous to mammals and subsequent research in this field has been directed toward the development of more selective and less poisonous insecticides. In 1950, malathion, the first example of a wide-specUnm organophosphorus insecticide combined with very low mammalian toxicity, was developed. And at about the same time the phenoxyacetic acid herbicides were discovered. These systemic compounds ate extremely valuable for the selective control of broad-leaved weeds in cereal crops. These compounds have a relatively low toxicity to mammals and are therefore relatively safe to use. 

Although parathion is less toxic than its oxo analogue, paraoxon (15), it can nevertheless be classed among the strongly active phosphorus ester insecticides its oral LD50 for rats being 6.4 mg/kg. 

The phosphorothioates and phosphorodithioates, shown in Table 1 of Chapter 2, were developed when the nerve agents were found to be too toxic and volatile for use in agriculture. These OP insecticides contain a P-S-alkyl and/ or a P=S group in their structure. The best known member of this class is parathion, the most widely used insecticide at one time and responsible for more cases of accidental poisoning and death than any other OP compound. The activation and conversion of this weak AChEI (parathion) to the more active and potent form (paraoxon) was demonstrated to lake place in the liver (Diggle and Gage, 1951 Gage, 1953). 

Organophosphorus insecticides have been attributed to several cases of human poisonings and deaths. The first compound of this class, or tetraethylpyrophosphate (TEPP) [107-49-3], was synthesized during World War 11, which led to the development of extremely toxic nerve gases as chemical warfare agents (see Chapter 39). TEPP is unstable and rapidly hydrolyzed by moistures. It is also highly toxic to mammals. Development of other insecticides, including parathion [56-... 

Malathion is a widely used member of a class of pesticides known as organophospate derivatives. It is biodegradable and less toxic (LDg for female rats is 1000 mg/kg) than DDT. Several other organophosphorus compounds are modifications of this basic structure. They include dimethoate (LD = 387 mg/kg), chlorpyrifos (LD q = 95-270 mg/kg), and methyl parathion (LD o = 18 -50 mg/kg). These organophosphates often are used in the treatment of many pests that are found on vegetables, or, in the case of methyl parathion, on cotton plants. 

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