B-Esterase—

Cholinesterases are another group of B-esterases. The two main types are acetylcholinesterase (EC 3.1.1.7) and unspecific or butyrylcholinesterase (EC 3.1.1.8). Acetylcholinesterase (AChE) is found in the postsynaptic membrane of cholinergic... 

This process of aging is believed to be critical in the development of delayed neuropathy, after NTE has been phosphorylated by an OP (see Chapter 10, Section 10.2.4). It is believed that most, if not all, of the B-esterases are sensitive to inhibition by OPs because they, too, have reactive serine at their active sites. It is important to emphasize that the interaction shown in Fignre 2.11 occurs with OPs that contain an oxon group. Phosphorothionates, which contain instead a thion group, do not readily interact in this way. Many OP insecticides are phosphorothionates, but these need to be converted to phosphate (oxon) forms by oxidative desulfuration before inhibition of acetylcholinesterase can proceed to any significant extent (see Section 2.3.2.2). 

The reason for the contrasting behavior of A-esterases is not yet clearly established. It has been snggested that the critical difference from B-esterases is the... 

FIGURE 2.11 Interaction between organophosphates and B-esterases. R, alkyl group E, enzyme. 

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. 

OPs Myzus persicae Resistant clones (85-315) Enhanced B esterase Multiple copies of gene... 

In addition to ester bonds with P (Section 10.2.1, Figures 10.1 and 10.2), some OPs have other ester bonds not involving P, which are readily broken by esteratic hydrolysis to bring about a loss of toxicity. Examples include the two carboxylester bonds of malathion, and the amido bond of dimethoate (Figure 10.2). The two carboxylester bonds of malathion can be cleaved by B-esterase attack, a conversion that provides the basis for the marked selectivity of this compound. Most insects lack an effective carboxylesterase, and for them malathion is highly toxic. Mammals and certain resistant insects, however, possess forms of carboxylesterase that rapidly hydrolyze these bonds, and are accordingly insensitive to malathion toxicity. 

Mammalian esterases have been classified into three groups according to specificity for substates and inhibitors (110). In terms of overall hydrolytic activity in mammals, the most important class of esterases is that of the B-esterases, which are principally active with aliphatic esters and amides. A-Esterases are important for aromatic esters and organophosphorus esters, and C-esterases are active with acetyl esters. In general, the specificity of mammalian esterases is determined by the nature of substituent groups (acetyl, alkyl, or aryl) rather than the heteroatom (O, N, or S) that is adjacent to the carboxy group. That is, the same esterase would likely catalyze hydrolysis of an ester, amide, or thioester as long as the substituents were identical except for the heteroatom (110). 

Carboxylesterase Carboxylic ester hydrolase, nonspecific carboxylesterase, ali-esterase, B-esterase Aliphatic esters... 

A classification based on the interaction of esterases with organophosphates (2.1) has been introduced by Aldridge [64] class A-esterases hydrolyze organophosphate esters while B-esterases are irreversibly inhibited by them. Another, lesser-used criterion, is the effect of sodium 4-(hydroxymer-curio)benzoate or Hg2+, which inhibits A-esterases but has little effect on B-esterases. Class C-esterases, enzymes that do not interact at all with organophosphates, have been added to the classification system [64][65],... 

Organophosphates phosphorylate the OH group of the catalytic serine at the active site of B-esterases (see Sect. 3.3). The rate of dephosphorylation of the enzyme is very slow, thus, the organophosphate acts as a mechanism-based inactivator. B-Esterases are classified as carboxylesterases (EC 3.1.1.1). 

These enzymes [EC 3.1.1.1] (also referred to as ali-ester-ase, B-esterase, monobutyrase, cocaine esterase, methyl-butyrase, and procaine esterase) catalyze the hydrolysis of a carboxylic ester to yield a carboxylate anion and an alcohol. They exhibit a broad specificity, even acting on vitamin A esters. 

Various esterases exist in mammalian tissues, hydrolyzing different types of esters. They have been classified as type A, B, or C on the basis of activity toward phosphate triesters. A-esterases, which include arylesterases, are not inhibited by phosphotriesters and will metabolize them by hydrolysis. Paraoxonase is a type A esterase (an organophosphatase). B-esterases are inhibited by paraoxon and have a serine group in the active site (see chap. 7). Within this group are carboxylesterases, cholinesterases, and arylamidases. C-esterases are also not inhibited by paraoxon, and the preferred substrates are acetyl esters, hence these are acetylesterases. Carboxythioesters are also hydrolyzed by esterases. Other enzymes such as trypsin and chymotrypsin may also hydrolyze certain carboxyl esters. 

Bacillus subtilis /zNB esterase is a member of the a./(3 hydrolase fold family (Moore and Arnold, 1996 Ollis et al., 1992). The canonical a/j3 hydrolase fold consists of a mostly parallel eight-stranded [3 sheet surrounded on both sides by a helices (Nardini and Dijkstra, 1999). p B esterase contains 489 amino acids arranged in a central thirteen-stranded f3 sheet that is surrounded by fifteen a helices (Fig. 12, see color insert). Similar to the structure of acetylcholine esterase (Sussman et al., 1991), a large fraction of the pSB esterase structure has no defined secondary structure (52% random coil, 33% a helix, and 14% /3 sheet). This high degree of random coil structure is allowed in the a/(3 hydrolase fold, where large insertions in loops were found to be tolerated while still maintaining catalytic activity (Nardini and Dijkstra, 1999). 

Although several reports concerning esterases in milk have appeared in the literature, little detailed information on the individual enzymes is available. Arylesterase or A-esterase (EC 3.1.1.2), carboxylesterase or B-esterase (EC 3.1.1.1), and cholinesterase or C-esterase (EC 3.1.1.7 EC 3.1.1.8) have been identified (Forster et al., 1961 Kitchen, 1971 Nakanishi and Tagata, 1972 Deeth, 1978). 

Tissue esterases have been divided into two classes the A-type esterases, which are insensitive, and the B-type esterases, which are sensitive to inhibition by organo-phosphorus esters. The A esterases include the arylesterases, whereas the B esterases include cholinesterases of plasma, acetylcholinesterases of erythrocytes and nervous tissue, carboxylesterases, lipases, and so on. The nonspecific arylesterases that hydrolyze short-chain aromatic esters are activated by Ca2+ ions and are responsible for the hydrolysis of certain organophosphate triesters such as paraoxon (Figure 10.10B). 

New data from our experiments on enzyme changes with other chemicals now show that in rats, not only carbon tetrachloride but also ethylene dichloride, benzene, and methanol bring about a reduction in serum B-esterase during 4 days after a single oral dose at the acute level. These findings seem to indicate that the serum esterase activity response is considerably independent of the chemical structure and the molecular configuration of the stressing chemical, except that the stimulation of esterase activity occurs later than with the chlorinated hydrocarbon pesticides. 

Esterases that metabolize organophosphates can be divided into three groups A-esterases, which are not inhibited by organophosphates but hydrolyze them B-esterases, which are susceptible to organophosphate inhibition and C-esterases, which are uninhibited by organophosphates and do not degrade them. 

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). 

AChEs, BuChEs, and CaEs are specialized carboxylic ester hydrolases classed among the B esterases, enzymes that are inhibited by OPs. Another class of enzymes are the A esterases (e.g., paraoxonase and DFPase) that actively hydrolyze OPs, destroying their toxic potential... 

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