Malathion metabolites

The oxidation of OPs can bring detoxication as well as activation. Oxidative attack can lead to the removal of R groups (oxidative dealkylation), leaving behind P-OH, which ionizes to PO . Such a conversion looks superficially like a hydrolysis, and was sometimes confused with it before the great diversity of P450-catalyzed biotransformations became known. Oxidative deethylation yields polar ionizable metabolites and generally causes detoxication (Eto 1974 Batten and Hutson 1995). Oxidative demethy-lation (0-demethylation) has been demonstrated during the metabolism of malathion. 

Pesticides contaminate not only surface water, but also ground water and aquifers. By 1990 in the USSR, 15% of all pesticides used were detected in underground water [29]. Pesticides were detected in 86% of samples of underground water in Ukraine in 1986-87 (including DDT and its metabolites, HCH, dimethoate, phosalone, methyl parathion, malathion, trichlorfon, simazin, atrazine, and prometrin). In actual fact, the number of pesticides was apparently larger, but the laboratory was able to determine the content of only 30 of the 200 pesticides used at that time in Ukraine [29]. In the 1960s, in the Tashkent and Andizhan oblasts of Uzbekistan, the methylmercaptophos content in the water of studied well shafts was, by clearly underestimated data, 0.03 mg/l (MPC was 0.01 mg/l), of DDT was 0.6 mg/l (MPC was 0.1 mg/ I), and of HCH was 0.41 mg/l (MPC was 0.02 mg/l) [A49]. 

Fenske, R.A. (1988) Correlation of fluorescent tracer measurements of dermal exposure and urinary metabolite excretion during occupational exposure to malathion, Am. Ind. Hygiene Assoc. /., 49 438-444. 

Dieldrin, the oxidative metabolite of aldrin, was the most toxic of all insecticides in this study but was only slightly more toxic than its parent compound. The oxidative metabolites of parathion and malathion, paraoxon and malaoxon, were slightly less toxic than their parent compounds. 

Conjugation involves a reaction between a common intermediate in some natural metabolic pathway with a synthetic molecule. Products of the combination of a normal metabolite with a toxicant frequently are harmless. Malathion conjugation is shown in Fig. 8. 

Matsumura and Bousch (1966) isolated carboxy lest erase (s) enzymes from the soil fungus Trichoderma viride und a bacterium Pseudomonas sp., obtained from Ohio soil samples, that were capable of degrading malathion. Compounds identified included diethyl maleate, desmethyl malathion, carboxylesterase products, other hydrolysis products, and unidentified metabolites. The authors found that these microbial populations did not have the capability to oxidize malathion due to the absence of malaoxon. However, the major degradative pathway appeared to be desmethylation and the formation of carboxylic acid derivatives. 

Selective bioactivation (toxification) is illustrated in the case of the insecticide malathion (3.35). This acetylcholinesterase inhibitor is desulfurized selectively to the toxic malaoxon, but only by insect and not mammalian enzymes. Malathion is therefore relatively nontoxic to mammals (LDjg = 1500 mg/kg, rat p.o.). Higher organisms rapidly detoxify malathion by hydrolyzing one of its ester groups to the inactive acid, a process not readily available to insects. This makes the compound doubly toxic to insects since they cannot eliminate the active metabolite. 

Metabolites that are less reactive than suicide inhibitors may impact more distant enzymes, within the same cell, adjacent cells, or even in other tissues and organs, far removed from the original site of primary metabolism. For example, organopho-sphates (OPs), an ingredient in many pesticides, are metabolized by hepatic CYPs to intermediates, which, when transported to the nervous system, inhibit esterases that are critical for neural function. Acetylcholinesterase (AChE) catalyzes the hydrolysis of the ester bond in the neurotransmitter, acetylcholine, allowing choline to be recycled by the presynaptic neurons. If AChE is not effectively hydrolyzed by AChE in this manner, it builds up in the synapse and causes hyperexcitation of the postsynaptic receptors. The metabolites of certain insecticides, such as the phos-phorothionates (e.g., parathion and malathion) inhibit AChE-mediated hydrolysis. Phosphorothionates contain a sulfur atom that is double-bonded to the central phosphorus. However, in a CYP-catalyzed desulfuration reaction, the S atom is... 

AChE by malaoxon, the oxon metabolite of malathion, a phosphorothionate (OP). Modified from [43]). 

Esterase activity is important in both the detoxication of organophosphates and the toxicity caused by them. Thus brain acetylcholinesterase is inhibited by organophosphates such as paraoxon and malaoxon, their oxidized metabolites (see above). This leads to toxic effects. Malathion, a widely used insecticide, is metabolized mostly by carboxylesterase in mammals, and this is a route of detoxication. However, an isomer, isomalathion, formed from malathion when solutions are inappropriately stored, is a potent inhibitor of the carboxylesterase. The consequence is that such contaminated malathion becomes highly toxic to humans because detoxication is inhibited and oxidation becomes important. This led to the poisoning of 2800 workers in Pakistan and the death of 5 (see chap. 5 for metabolism and chap. 7 for more details). 

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. 

The OPP malathion is widely used because of its low persistence in the environment and its high insecticide activity. Pure malathion has moderate toxicity, but crude malathion and its formulations contain impurities, which are far more toxic to mammals. These impurities not only are formed during commercial production but can also develop in the grains during storage. The most toxic of these products is the oxidation product malaoxon. An isocratic HPLC method indicated the degradation of malathion in stored wheat (55) and the presence of malathion and malaoxon in maize and bean samples (56). Isomalation and malation monocarboxylic acid metabolites were also detected. 

Figure 9.17. Organophosphate inhibitors of acetylcholinesterase. a The catalytic mechanism, shown here for diiso-propylfluorophosphate(DFP).b Stmcturesof soman and tabun. Like DFP, these were developed during world war II as nerve gases , c Stractures of the insecticides parathion and malathion, and of paraoxon, which is the achve metabolite of parathion. (Malathion likewise requires conversion to malaoxon.) The arrow above the malathione stmcture indicates the esterase cleavage sites in its leaving group esterase cleavage occurs in human plasma and renders the molecule non-toxic.

The effect of proteins on pollutant toxicity includes both quantitative and qualitative aspects. Experiments show that animals fed proteins of low biological value exhibited a lowered microsomal oxidase activity when dietary proteins were supplemented with tryptophan, the enzyme activity was enhanced. Alteration of xenobiotic metabolism by protein deprivation may lead to enhanced or decreased toxicity, depending on whether metabolites are more or less toxic than the parent compound. For example, rats fed a protein-deficient diet show decreased metabolism but increased mortality with respect to pentobarbital, parathion, malathion, DDT, and toxaphene (Table 6.4). On the other hand, rats treated under the same conditions may show a decreased mortality with respect to heptachlor, CC14, and aflatoxin. It is known that, in the liver, heptachlor is metabolized to epoxide, which is more toxic than heptachlor itself, while CC14 is metabolized to CC13, a highly reactive free radical. As for aflatoxin, the decreased mortality is due to reduced binding of its metabolites to DNA. 

Cook, G.H., J.C. Moore and D.L. Coppage. The relation of malathion and its metabolites to fish poisoning. Bull. Environ. Contam. Toxicol. 16 283-290, 1976. 

Fenske RA, Leffingwell JT. Method for determination of dialkyl phosphate metabolites in urine for studies of human exposure to malathion. J Agric Food Chem 1985 37 995-8. 

At the present time, the biomarkers that correlate best with exposure are metabolite levels in the urine (Baselt 1980 Beauchamp et al. 1983 Campbell et al. 1985 Lieben 1974 WHO 1986). The iodine-azide and the TTCA tests, which measure the presence of urinary carbon disulfide metabolites, have been shown to correlate well with actual exposure. However, the iodine-azide test is nonspecific (Dox et al. 1992). TTCA is produced in humans after exposure to Antabuse and in rats after exposure to Captan (Cox et al. 1992). Moreover, other investigations are necessary in order to determine whether the interaction of carbon disulfide with other substances (such as hydrogen sulfide, drugs, carbon tetrachloride, malathion, and alcohol), disease states, and variations in diet and in individual metabolism, as well as other factors, could confound the results of the iodine-azide test and the TTCA test for carbon disulfide exposure. Baseline urine, breath, and blood samples are necessary to correct for non-workplace exposures. For exposures around hazardous waste sites, the influence of workplace exposures must also be corrected for in this manner. 

Advantage of species differences in metabolism was taken with the synthesis of malathion which is attacked by esterases in mammals and excreted rapidly as the diacid before conversion of the innocuous thio-phosphate to the toxic phosphate. Insects have very low levels of esterases, and metabolism to the lethal oxo metabolite can occur unimpeded (Figure 20). 

The metabolism of malathion in warm-blooded animals proceeds via another route. As the first metabolic step, a-malathion monocarbonic acid (91) is formed by hydrolysis of one of the ester bonds. This metabolite is virtually nontoxic to warmblooded animals (Krueger and O Brien, 1959 Chen et al., 1969). 

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