Insecticides, carbamate, hydrolysis

Mode of Action. All of the insecticidal carbamates are cholinergic, and poisoned insects and mammals exhibit violent convulsions and other neuromuscular disturbances. The insecticides are strong carbamylating inhibitors of acetylcholinesterase and may also have a direct action on the acetylcholine receptors because of their pronounced stmctural resemblance to acetylcholine. The overall mechanism for carbamate interaction with acetylcholinesterase is analogous to the normal three-step hydrolysis of acetylcholine however, is much slower than with the acetylated enzyme. 

The pesticides methyl and ethyl parathion were determined in run-off water er preconcentration on XAD-2. This allowed analyses of these compounds at the parts per billion level (497). Parathion and paraoxon obtained from leaf extracts and orchard soil have also been determined (492). The separation of 30 carbamate pesticides by RPC has been described (493). Various modes of postcolumn fluorometric detection of carbamate insecticides have been reported including post-colun)n reaction between o-phthalaldehyde and methylamine, a carbamate hydrolysis... 

Sogorb, M.A. and Vilanova, E., Enzymes involved in the detoxication of organophosphorus, carbamate and pyrethroid insecticides through hydrolysis, Toxicol. Lett., 128, 315-228, 2002. 

Figure 7. Reaction scheme for the dansyl labeling of N-methylcarhamate insecticide (l) hydrolysis of the carbamate (2 and 3) labeling of the amine and phenol hydrolysis products (4) hydrolysis of the reagent by carbamate (29)...

AChE Organophosphorus insecticides Carbamates Inhibits the hydrolysis of ACh, causing overstimulation of the cholinergic synapses Nicotinic and muscarinic effects... 

The first synthetic pyrogaHol plant using hydrolysis of chlorinated cyclohexanol (2,2,6,6-tetrachlorocyclohexanone) was built by BFC Chemicals, Inc. (Muskegon, Michigan) and has been producing pyrogaHol for the carbamate insecticide Beniocarb since 1982 (8,19). SocifitH Fransaise Hoechst offers pyrogaHol for sale in the United States (American Hoechst Corp.), and Japan is also a source of this chemical. 

Organophosphates and carbamates containing a pyrazole ring, useful as insecticides as discussed earlier (Section 4.04.4.1.2), are metabolized mainly through hydrolysis of the ester function (B-80MI40406). 

The increased use of IV-methyl carbamate insecticides in agriculture demands the development of selective and sensitive analytical procedures to determine trace level residues of these compounds in crops and other food products. HPLC is the technique most widely used to circumvent heat sensitivity of these pesticides. However, HPLC with UV detection lacks the selectivity and sensitivity needed for their analysis. In the late 1970s and early 1980s, HPLC using post-column hydrolysis and derivatization was developed and refined with fluorescence detection to overcome these problems. The technique relies on the post-column hydrolysis of the carbamate moiety to methylamine with subsequent derivatization to a fluorescent isoindole product. This technique is currently the most widely used HPLC method for the determination of carbamates in water" and in fruits and vegetables." " ... 

Oxime carbamates are not directly amenable to gas chromatography (GC) because of their high thermal instability, which often leads to their breakdown at the injection port or in the column during analysis. Analysis of oxime carbamates by GC with sulfur detection or flame photometric detection involves oxidation of the intact insecticides or alkaline hydrolysis to form the more volatile but stable oxime compound. Enzymatic techniques have been reported for the analysis of these compounds. Enzyme-linked immunosorbent assay (ELISA) has been used to determine aldicarb and its sulfone and sulfoxide metabolites and methomyl in water, soil, and sediment samples. 

Phenylcarbamates, or carbanilates, generally exhibit low water solubilities, and thus they are almost immobile in soil systems. Chlorpropham and Propham are readily volatilized from soil systems, but Terbutol and Carbaryl (Fig. 10, Table 3) are not. Ester- and amide-hydrolysis, N-dealkylation and hydroxylation are among the chemical reactions that carbamates undergo. The N-methylcar-bamate insecticides (Fig. 10, Table 3) commonly used in soils are Carbaryl, Methiocarb,Aldicarb,and Carbofuran [74,173]. 

Aly, O.M. and El-Dib, M.A. Studies on the persistence of some carbamate insecticides in the aquatic environment. I. Hydrolysis of sevin, baygon, pyrolam and dimetilan in waters. Water Res., 5(12) 1191-1205, 1971. 

Most of the older methods of fluorimetric analysis of pesticides involved hydrolysis to form fluorescent anions. Co-ral (coumaphos) [147] was hydrolyzed in alkali to the hydroxybenzopyran, which was subsequently determined by means of its fluorescence. Guthion (azinphosmethyl) was hydrolyzed to anthranilic acid for fluorimetric analysis [148,149]. A method was developed [150] for Maretin (N-hydroxynaphthalimide diethyl phosphate) in fat and meat which involved hydrolysis in 0.5 M methanolic sodium hydroxide followed by determination of the fluorescence of the liberated naphthalimide moiety. Carbaryl (1-naphthyl N-methylcarbamate) and its metabolites have been determined by a number of workers using base hydrolysis and the fluorescence of the resulting naphtholate anion [151-153]. Nanogram quantities of the naphtholate anion could be detected. Zectran (4-dimethylamino-3,5-xylyl N-methylcarbamate) has been determined by the fluorescence of its hydrolysis product [154]. The fluorescence behaviour of other carbamate insecticides in neutral and basic media has been reported [155]. Gibberellin spray used on cherries has been determined fluorimetrically after treatment with strong acid [156]. Benomyl (methyl N-[l-(butylcarbamoyl)-2-benzimidazolyl]carbamate) has been analyzed by fluorimetry after hydrolysis to 2-aminobenzimidazole [157]. 

Methylcarbamate insecticides have been recently labeled with DNS-C1 [145]. The procedure involves the hydrolysis of the carbamates with 0.1 M sodium carbonate to form a phenol and methylamine [166]. The two hydrolysis products are labeled with DNS-C1 and subsequently detected and determined quantitatively by TLC on silica gel layers by scanning spectrofluorimetry in situ. The reaction conditions were examined, and optimum conditions for hydrolysis and labeling were established [167]. The overall reaction scheme is shown in Fig. 4.62. The phenol derivatives of a number of N-methylcarbamates have been separated by one- and two-dimensional TLC [168], and the fluorescence behaviour and stability of the derivatives have been examined [169]. Most of the DNS derivatives fluoresce at similar wavelengths (excitation, ca. 365 nm emission, ca. 520 nm). The fluorescence spectrum of a typical DNS derivative is shown in Fig. 4.63. The method has been applied successfully to the analysis of low concentrations of carbamates in water and in soil samples with little or no clean-up being required [170,171]. Amounts as low as 1 ng of insecticide can be detected instrumentally. Visual limits of detection are ca. 5-10 ng per spot. 

The OP and carbamate insecticides are relatively nonpersistent in the environment. They are applied to the crop or directly to the soil as systemic insecticides, and they generally persist from only a few hours to several months. Thus these compounds, in contrast to the organochlorine insecticides, do not represent a serious problem as contaminants of soil and water and rarely enter the human food chain. Being esters, the compounds are susceptible to hydrolysis, and their breakdown products are generally nontoxic. Direct contamination of food by concentrated compounds has been the cause of poisoning episodes in several countries. 

Figure 11.5 Hydrolysis of acetylcholine by the enzyme acetylcholinesterase and its inhibition by toxicants such as organophosphorus and carbamate insecticides.

Carbamates, widely used as pesticides, can undergo facile hydrolysis, depending on the substituents on the N atom. Carbaryl (Ar = napthyl), used as an insecticide, undergoes rapid alkaline hydrolysis at room temperature even at pH 7 (Equation (15)). 

Dansylation has also been used to detect N-phenyl-carbamate and urea herbicides by adding excess reagent on the applied spot directly on the tic (3 2. This in situ reaction proved to be cleaner and more practical than labelling in solution and was applied to the detection of Linuron (33) in agricultural crops at the 0.05 ppm level. Other applications involving hydrolysis and dansylation in solution prior to tic have been reported with organophosphorus insecticides... 

The mode of action of the carbamate insecticides is similar to that of the organophosphates. As shown in Figure 7.15, the reaction yields a carbamylated AChE, followed by decarbamylation via hydrolysis. Carbamates also attack the CNS system, and the symptoms of intoxication are similar to those with the organophosphates. However, unlike the organophosphates, decarbamylation of acetylcholinesterase is rapid, typically in minutes, and therefore carbamate insecticides are regarded as reversible acetylcholinesterase inhibitors. 

Insecticides such as organophosphates, carbamates, pyrethroids, and some juvenoids, which contain ester linkages, are susceptible to hydrolysis. Esterases are hydrolases that split ester compounds by the addition of water to yield an acid and an alcohol. 

There are two types of esterases that are important in metabolizing insecticides, namely, carboxylesterases and phosphatases (also called phosphorotriester hydrolases or phosphotriesterases). Carboxylesterases, which are B-esterases, play significant roles in degrading organophosphates, carbamates, pyrethroids, and some juvenoids in insects. The best example is malathion hydrolysis, which yields both a- and (i-monoacids and ethanol (Figure 8.10). 

The dependence of transformation rate on pH is not always consistent among pesticides within the same chemical class. For example, while the hydrolysis reactions of most OP (Konrad and Chesters, 1969 Konrad et al., 1969 Mabey and Mill, 1978) and carbamate insecticides (Wolfe et al., 1978) are primarily base-catalyzed, both diazinon (Konrad et al., 1967) and carbosulfan (Wei et al., 2000) are also subject to the acid-catalyzed reaction. Summaries of the pH dependence of hydrolysis rates for a variety of pesticides have been provided by Mabey and Mill (1978), Bollag (1982), Schwarzenbach et al. (1993), and Barb ash and Resek (1996). 

A situation opposite to that is seen with the carbamate insecticides. As shown in Table I, data from studies in our laboratory demonstrate that carbamate insecticides most toxic to rats are also hydrolyzed at a faster rate. Ester hydrolysis in these studies was measured by quantitation of radioactive carbon dioxide in the respiratory gases of animals treated orally with the carbamate radiolabeled on the carbonyl carbon. Yet, hydrolysis of... 

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