Organophosphorus insecticide metabolism

Methods for Determining Biomarkers of Exposure and Effect. Section 2.6.1 reported on biomarkers used to identify or quantify exposure to diazinon. Some methods for the detection of the parent compound in biological samples were described above. The parent chemical is quickly metabolized so the determination of metabolites can also serve as biomarkers of exposure. The most specific biomarkers will be those metabolites related to 2-isopropyl-6-methyl-4-hydroxypyrimidine. A method for this compound and 2-(r-hydroxy-l -methyl)-ethyl-6-methyl-4-hydroxypyrimidine in dog urine has been described by Lawrence and Iverson (1975) with reported sensitivities in the sub-ppm range. Other metabolites most commonly detected are 0,0-diethylphosphate and 0,0-diethylphosphorothioate, although these compounds are not specific for diazinon as they also arise from other diethylphosphates and phosphorothioates (Drevenkar et al. 1993 Kudzin et al. 1991 Mount 1984 Reid and Watts 1981 Vasilic et al. 1993). Another less specific marker of exposure is erythrocyte acetyl cholinesterase, an enzyme inhibited by insecticidal organophosphorus compounds (see Chapter 2). Methods for the diazinon-specific hydroxypyrimidines should be updated and validated for human samples. Rapid, simple, and specific methods should be sought to make assays readily available to the clinician. Studies that relate the exposure concentration of diazinon to the concentrations of these specific biomarkers in blood or urine would provide a basis for the interpretation of such biomarker data. 

Table 8.5 Toxicity of Some Organophosphorus Insecticides Where Metabolism Is Crucial for the Toxicity...

It seemed likely that the failure of dichlorvos to induce mutations or tumours in mammals was due to the limiting effect of the known rapid metabolic degradation of this compound on the extent of methylation of DNA in vivo. Certainly in the case of such insecticidal organophosphorus compounds. 

The expected continued use of famphur in the environment and its vehicular transport along roads that border navigable waters suggest a need for aquatic toxicity data. Famphur data — like those on other organophosphorus insecticides — should reflect the influence of dose, exposure duration, formulation, and other biological and abiotic variables on growth, survival, and metabolism of representative species of aquatic organisms. 

Rao SLN, McKinley WP. 1969. Metabolism of organophosphorus insecticides by liver homogenates from different species. Can J Biochem 47 1155-1159. 

Metabolism of Organophosphorus Insecticides in Aquatic Organisms, with Special Emphasis on Fenitrothion... 

Further, by virtue of their larger livers, the R fish have a greater xenobiotic biotransformation potential. However, the in vivo studies show few consistent differences in metabolism between the two populations. Biotransformation may be a major contributory factor in mosquitofish resistance to other pesticides, for example, organophosphorus and botanical insecticides, since the level of resistance to these chemicals is very low (4 fold or less) 08,20,21). However, biotransformation does not appear to play a major role in organochlorine insecticide resistance. 

Dichlorvos (9.50) is an insecticide of reportedly wide use, the metabolites of which in humans include dichloroethanol and dimethyl phosphate. Like paraoxon, dichlorvos is hydrolyzed by human serum. However, the enzyme activities hydrolyzing the two substrates were shown to differ by a number of criteria [114], Clearly large gaps remain in our understanding of the human metabolism of organophosphorus insecticides and other toxins. A bacterial phosphodiesterase appears as a promising tool to understand the catalytic mechanisms of organophosphoric acid triester detoxification [115-117],... 

A few organophosphorus insecticides are also phosphoramidates, hydrolysis of the P-N bond being considered a route of detoxification. This is exemplified by the metabolism of acephate (9.82, Fig. 9.15), whose mechanisms of activation and detoxification have recently been re-examined in mice to better understand the relative innocuity of the compound in mammals and its selective toxicity in insects [156],... 

Ahmed. M.K. and Casida, J.E. Metabolism of some organophosphorus insecticides by microorganisms, /. Econ. Entomol, 51(l) 59-63, 1958. 

It Is well known that phosphorothlonate insecticides such as parathlon (, 0-diethyl p-nitrophenyl phosphorothloate) and malathion [0, -dimethyl -(l,2 -dlcarbethoxy)ethyl phosphoro-dithioate] are Intrinsically poor inhibitors of acetylcholinesterase and in vivo activation to the respective anticholinesterases paraoxon and malaoxon is required before animals exposed to the phosphorothionates are intoxicated. Since metabolic activation is essential to the biological activity of these thiono sulfur-containing organophosphorus insecticides, compounds of this type may be considered as propesticides or, more specifically, prolnsectlcldes. 

Dispositional interactions are those in which one chemical affects the disposition of the other, usually metabolism. Thus, one chemical may increase or inhibit the metabolism of another to change its toxicity. For example, 2,3-methylenedioxynaphthalene inhibits cytochrome P-450 and so markedly increases the toxicity of the insecticide carbaryl to flies (potentiation) (see chap. 5). Another example, which results in synergy, is the increased toxicity of the organophosphorus insecticide malathion (see chap. 5) when in combination with another organophosphorus insecticide, EPN. EPN blocks the detoxication of malathion. Many chemicals are either enzyme inhibitors or inducers and so can increase or decrease the toxicity of other chemicals either by synergism or potentiation (see chap. 5). 

Other important enzyme inhibitors of this type are the organophosphorus compounds. Thus, after metabolism to the oxygen analogues, the insecticides parathion and malathion (chap. 4, Fig. 25) (Fig. 5.12) form complexes with the enzyme acetylcholinesterase as described in more detail in chapter 7. 

The onset of symptoms depends on the particular organophosphorus compound, but is usually relatively rapid, occurring within a few minutes to a few hours, and the symptoms may last for several days. This depends on the metabolism and distribution of the particular compound and factors such as lipophilicity. Some of the organophosphorus insecticides such as malathion, for example (chap. 5, Fig. 12), are metabolized in mammals mainly by hydrolysis to polar metabolites, which are readily excreted, whereas in the insect, oxidative metabolism occurs, which produces the cholinesterase inhibitor. Metabolic differences between the target and nontarget species are exploited to maximize the selective toxicity. Consequently, malathion has a low toxicity to mammals such as the rat in which the LD50 is about 10 g kg-1. 

A. Modes of Toxic Action. This includes the consideration, at the fundamental level of organ, cell and molecular function, of all events leading to toxicity in vivo uptake, distribution, metabolism, mode of action, and excretion. The term mechanism of toxic action is now more generally used to describe an important molecular event in the cascade of events leading from exposure to toxicity, such as the inhibition of acetylcholinesterase in the toxicity of organophosphorus and carbamate insecticides. Important aspects include the following ... 

A recent publication on PBTK/TD modeling to determine dosimetry and cholinesterase inhibition for a chemical mixture of 2 organophosphorus insecticides, chlo-rpyrifos and diazinon (Timchalk and Poet 2008), deserves some special discussion here. Based on the individual PBTK/TD models developed earlier by the same laboratory, Timchalk and Poet (2008) reported their development of a binary interaction PBTK/TD model for chlorpyrifos and diazinon. In their development of the model, Timchalk and Poet (2008) took into consideration a number of important metabolic steps (CYP450 mediated activation and detoxification of a number of esterases—B-... 

The O-dealkylation of organophosphorus triesters differs from the above reactions in that it involves the dealkylation of an ester rather than an ether. The reaction was first described for the insecticide chlorfenvinphos (Figure 10.2B), but is now known to occur with a wide variety of vinyl, phenyl, phenylvinyl, and naphthyl phosphates and the thionophosphate triesters. At least one phosphonate, O-ethyl O-p-nitrophenyl phenylphosphonate (EPNO), is also metabolized by this mechanism. 

The inhibition by other organophosphate compounds of the carboxylesterase which hydrolyzes malathion is a further example of xenobiotic interaction resulting from irreversible inhibition because, in this case, the enzyme is phosphorylated by the inhibitor. A second type of inhibition involving organophosphorus insecticides involves those containing the P=S moiety. During CYP activation to the esterase-inhibiting oxon, reactive sulfur is released that inhibits CYP isoforms by an irreversible interaction with the heme iron. As a result, these chemicals are inhibitors of the metabolism of other xenobiotics, such as carbaryl and fipronil, and are potent inhibitors of the metabolism of steroid hormones such as testosterone and estradiol. 

The organophosphorus insecticides are all structurally related and undergo similar reactions. The chemical classification of the most widely used compounds of this type is given in Table V. These compounds can also be differentiated on the basis of whether they are largely effective per se or undergo oxidative conversions in plants or animals. All are inhibitors of the enzyme, cholinesterase. Their potency depends not only upon their intrinsic enzyme affinity but also on anticholinesterase properties acquired through in vivo metabolism. 

The carbamates, like the organophosphorus insecticides, are cholinesterase inhibitors. However, the reaction is rapidly reversible. Carbaryl has a half life in the soil of about 8 days and is decomposed by ultraviolet light. The carbamates metabolize rapidly in animals and show little, if any, propensity for storage in animal tissues. Additional properties and reactions of carbamates are discussed in the section on fungicides. 

N-Dealkylation This is a common reaction in the metabolism of xenobiotics, including organophosphorus and carbamate insecticides. The reaction is believed to proceed by an unstable a-hydroxy intermediate that spontaneously releases an aldehyde in the case of the primary alkyl group. For example, the carbamate insecticide propoxur is N-demethylated to 2-isopropoxyphenyl carbamate via 2-iso-propoxyphenyl N-hydroxymethyl carbamate. Microsomal N-dealkylation results in detoxification (Figure 8.5). 

Glutathione S-transferases are important in the metabolism of organophosphorus insecticides resulting in detoxification. For example, methyl parathion is dealkylated by glutathione S-transferase to form desmethyl parathion and methyl glutathione (Figure 8.21). 

In connection with the development of an analytical method (13) for the determination of organophosphorus pesticides in human blood and urine, mass spectral confirmation of a series of methylated and ethylated derivatives of the hydrolytic and metabolic products of these insecticides was required. The urine of an individual occupationally exposed to parathion was extracted with a 1 1 (v/v) solvent mixture of acetonitrile and diethyl ether. Simultaneously, the intact organophosphorus insecticides were hydrolyzed by adding a portion of 5N hydrochloric acid to... 

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