Carbaryl hydroxylation

From the identification of the degradation products by GC-MS, the degradation of die six carbamate pesticides demonstrated attack at the carbamate branch by hydroxyl radicals. In addition, for promecarb and carbaryl, hydroxyl radicals can also initiate attack at the alkyl site. These degradadve steps result in substituted phenols, which may or may not be the final products from AFT treatment. It can be demonstrated by measuring the BODs/COD of a solution of all six carbamates that the biodegradability of die solution is significandy... 

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]. 

Plant In plants, the A-methyl group may be subject to oxidation or hydroxylation (Kuhr, 1968). The presence of pinolene (P-pinene polymer) in carbaryl formulations increases the amount of time carbaiyl residues remain on tomato leaves and decreases the rate of decomposition. The half-life in plants range from 1.3 to 29.5 d (Blazquez et al., 1970). 

The oxidation of naphthalene was one of the earliest examples of an epoxide as an intermediate in aromatic hydroxylation. As shown in Figure 7.3, the epoxide can rearrange nonenzymatically to yield predominantly 1-naphthol, or interact with the enzyme epoxide hydrolase to yield the dihydrodiol, or interact with glutathione S-transferase to yield the glutathione conjugate, which is ultimately metabolized to a mercapturic acid. These reactions are also of importance in the metabolism of other xenobiotics that contain an aromatic nucleus, such as the insecticide carbaryl and the carcinogen benzo(a)pyrene. 

The mechanism of carbamate-resistance, formerly considered to be enhanced hydrolysis (e.g. carbaryl to 1-naphthol), was found to derive almost exclusively from hydroxylation at various points on the molecule, not only the aromatic leaving group but also the N-methyl on the carbamate (Fig. 3), as well as some desmethylation for good measure (14). Pyrethrin-resistance, at first considered to be due to hydrolysis of the alcohol-acid linkage, was also found to be due to an oxidation, occurring at the transmethyl group of the isobutenyl side-chain of the chrysanthemic acid (15). 

Hydroxylation Hydroxylation can occur with an aliphatic or aromatic carbon atom. DDT and the carbamate insecticide carbaryl are known to be hydroxylated by cytochrome P450 monooxygenases as shown in Figures 8.3 and 8.4. Microsomal hydroxylation usually results in detoxification. 

The reactions show some typical oxidations of insecticides catalyzed by CYP enzymes. Carbofuran is hydroxylated to another active compound carbaryl is demethylated or hydroxylated and aldrin is epoxidated to diel-drin, which also has insecticidal properties. Phosphorothionates must be oxidized to the phosphates by CYP enzymes in order to become inhibitors of acetyl cholinesterase. Parathion-methyl is transformed to the oxon analogue, paraoxon-methyl, which is the toxic compound. It also can be demethylated to the inactive desmethyl-parathion-methyl. 

Many CMs are metabolized in human liver by P450 enzymes. For example, at least CYPlAl/2, CYP2B6, and CYP3A4 participate in the metabolism of carbaryl (Tang et al., 2002). However, as depicted in Table 3, only CYPlAl is definitely demonstrated to be functionally expressed in human placenta and thus has a potential to hydroxylate carbaryl. This potential has not been studied experimentally. Chlorpyrifos is metabolized to an oxon metabolite by CYP2B6, but this enzyme is not expres.sed in human placenta thus, it is questionable whether chlorpyrifos is metabolized in the placenta. 

Phenols The term phenols is used to designate a series of hydroxyl derivatives of benzene, with phenol as the parent compound. These compounds come from paper mills and cellulose factories as well as from degradation of pesticides such as carbaryl. Disinfecting treatments of chlorine-containing waters produce chlorophenols, which cause a persistent taste. Analytical control of these compounds is commonly performed by chromatographic or photometric methods. 

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