Esterases pyrethroids

The organophosphorons insecticides dimethoate and diazinon are mnch more toxic to insects (e.g., housefly) than they are to the rat or other mammals. A major factor responsible for this is rapid detoxication of the active oxon forms of these insecticides by A-esterases of mammals. Insects in general appear to have no A-esterase activity or, at best, low A-esterase activity (some earlier stndies confnsed A-esterase activity with B-esterase activity) (Walker 1994b). Diazinon also shows marked selectivity between birds and mammals, which has been explained on the gronnds of rapid detoxication by A-esterase in mammals, an activity that is absent from the blood of most species of birds (see Section 23.23). The related OP insecticides pirimiphos methyl and pirimiphos ethyl show similar selectivity between birds and mammals. Pyrethroid insecticides are highly selective between insects and mammals, and this has been attributed to faster metabolic detoxication by mammals and greater sensitivity of target (Na+ channel) in insects. 

The metabolism of permethrin will be taken more generally as an example of the metabolism of pyrethroids (Figure 12.2). The two types of primary metabolic attack are by microsomal monooxygenases and esterases. Monooxygenase attack involves... 

Ghiasuddin, S.M. and D.M. Soderlund. 1984. Hydrolysis of pyrethroid insecticides by soluble mouse brain esterases. Toxicol. Appl. Pharmacol. 74 390-396. 

It was reported that the distribution and activities of esterases that catalyze pyrethroid metabolism using several human and rat tissues, including small intestine, liver, and serum, were examined [30]. The major esterase in human intestine was hCE2. //c/n.v-Permethrin was effectively hydrolyzed by pooled human intestinal microsomes (five individuals), while deltamethrin and bioresmethrin were not. This result correlated well with the substrate specificity of recombinant hCE2. In contrast, pooled rat intestinal microsomes (five animals) hydrolyzed trans-permethrin 4.5 times slower than the human intestinal microsomes. Furthermore, pooled samples of cytosol from human or rat liver were ca. half as hydrolytically active as the corresponding microsome fraction toward pyrethroids however, the cytosolic fractions had significant amounts (ca. 40%) of the total hydrolytic activity. Moreover, a sixfold interindividual variation in hCEl protein expression in human hepatic cytosols was observed. 

Both /nmv-permethrin and bioresmethrin were effectively cleaved by rat serum CES on the other hand, deltamethrin, esfenvalerate, a-cypermethrin, and cis-permethrin were slowly hydrolyzed. These results suggest that PBPK models of some pyrethroids may require the parameter of esterase activity to calculate the concentrations in the intestinal tract, liver, and serum if it is shown that the compounds in the model are appreciably hydrolyzed within these tissues. Such data for human and animal tissues will help to improve the accuracy of extrapolation between species (e.g., rats to humans) and thus enable better predictions of tissue and blood concentrations in humans following exposure to pyrethroids [30]. 

Godin SJ, Crow JA, Scollon EJ, Hughes MF, DeVito MJ, Ross MK (2007) Identification of rat and human cytochrome p450 isoforms and a rat serum esterase that metabolize the pyrethroid insecticides deltamethrin and esfenvalerate. Drug Metab Dispos 35 1664-1671... 

Many investigations of relevant enzymes in transformation of xenobiotics by aquatic species have shown that the similar enzymes observed in metabolism in soil, plant, and mammals play a role, such as esterases and oxidases [10, 159, 160]. Metabolism of pyrethroids has been most extensively studied in fish for cypermethrin (5) and permethrin (15). Aromatic hydroxylation at the 4 -position of the 3-phenoxybenzyl moiety followed by conjugation with glucuronic acid... 

The mechanisms of resistance fall into two main categories. Many insects produce an increased level of detoxifying enzymes, such as esterases, that modify the insecticides to inactive metabolites very rapidly. Such a system is seen in aphids that are resistant to OP insecticides. In other cases it is the target site that is modified such that the insecticide (the enzyme inhibitor) no longer binds to the target and is, therefore, ineffective. This has recently been shown to occur in some aphids that are resistant to OP insecticides but the classical example is knockdown resistance (kdr) and super-kdr to pyrethroid insecticides shown by many insects but particularly house flies Musca domes tied). This resistance is thought to result from a modification of... 

Mammalian toxicity to pyrethrins is quite low, apparently due to its rapid breakdown by liver microsomal enzymes and esterases. The acute LD50 to rats is about 1500 mg/kg. The most frequent reaction to pyrethrins is contact dermatitis and allergic respiratory reactions, probably as a result of other constituents in the formulation. Synthetic mimics of pyrethrins, known as the pyrethroids, were developed to overcome the lack of persistence. 

Blood and various organs of humans and other animals contain esterases capable of acetylsalicylic acid hydrolysis. A comparative study has shown that the liver is the most active tissue in all animal species studied except for the guinea pig, in which the kidney is more than twice as active as the liver. Human liver is least active the enzyme in guinea pig liver is the most active. The relatively low toxicity of some of the new synthetic pyrethroid insecticides appears to be related to the ability of mammals to hydrolyze their carboxyester linkages. Thus mouse liver microsomes catalyzing (+)-/runs-resin e 111ri n hydrolysis are more than 30-fold more active than insect microsomal preparations. The relative rates of hydrolysis of this substrate in enzyme preparations from various species are mouse > > milkweed bug > > cockroach > > cabbage looper > housefly. 

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). 

Induction of carboxylesterases and epoxide hydroloases would also affect the toxicity of insecticides. For example, host plant induction of 1-naphthyl acetate esterase would decrease the toxicity of certain insecticides containing an ester linkage such as organo-phosphates, pyrethroids, and some juvenile hormone analogs and, possibly, carbamates. 

Decreased renal excretion of penicillin when coadministered with probenecid, potentiating its therapeutic effect Ops (profenofos, sulprofos, DEF) potentiate the toxicity of fenvalerate and malathion by inhibiting esterase, which detoxifies many pyrethroid insecticides and malathion... 

Pyrethroid resistance in some strains of H. armigera is largely due to overproduction of specific esterase isoenzymes (Rni 0.24 0.33) which are thought to sequester and hydrolyse pyrethroids. The resistance factor is positively correlated to esterase litre, so that increasing resistance is accompanied by increasing esterase activity (Gunning et at.. 1996). Unfortunately, conventional esterase inhibitors, such as profenofos, have failed to synergize pyrethroids... 

In this chapter we present the results from studying the possible inhibitory effects of PBO on the hydrolysis of 1-naphthyl acetate by putative pyrethroid resistance-related esterases in H. armigera and the resistance-related esterases in A. gossypii and M. persicae. 

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