A-esterase

There are marked species differences in A-esterase activity. Birds have very low, often undetectable, levels of activity in plasma toward paraoxon, diazoxon, pirimi-phos-methyl oxon, and chlorpyrifos oxon (Brealey et al. 1980, Mackness et al. 1987, Walker et al. 1991 Figure 2.10). Mammals have much higher plasma A-esterase activities to all of these substrates. The toxicological implications of this are discussed in Chapter 10. Some species of insects have no measurable A-esterase activity, even in strains that have resistance to OPs (Mackness et al. 1982, Walker 1994). These include the peach potato aphid (Myzus persicae Devonshire 1991) and the... 

FIGU RE 2.10 Plasma A-esterase activities of birds and mammals. Activities were originally measured as nanomoles product per milliliter of serum per minute, but they have been converted to relative activities (male rat = 1) and plotted on a log scale. Each point represents a mean value for a single species. Substrates , paraoxon , pirimiphos-methyl oxon. Vertical hues indicate limits of detection, and all points plotted to the left of them are for species in which no activity was detected. (Activities in the male rat were 61 4 and 2020 130 for paraoxon and pirimiphos-methyl oxon, respectively.) (From Walker 1994a in Hodgson and Levi 1994.)... 

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... 

The microsomal fraction consists mainly of vesicles (microsomes) derived from the endoplasmic reticulum (smooth and rough). It contains cytochrome P450 and NADPH/cytochrome P450 reductase (collectively the microsomal monooxygenase system), carboxylesterases, A-esterases, epoxide hydrolases, glucuronyl transferases, and other enzymes that metabolize xenobiotics. The 105,000 g supernatant contains soluble enzymes such as glutathione-5-trans-ferases, sulfotransferases, and certain esterases. The 11,000 g supernatant contains all of the types of enzyme listed earlier. 

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. 

Brealey, C.J., Walker, C.H., and Baldwin, B.C. (1980). A Esterase activities in relation to differential toxicity of pirimiphos-methyl. Pesticide Science 11, 546-554. 

Mackness, M.I., Thompson, H.M., and Walker, C.H. (1987). Distinction between A esterases and arylesterases and implications for esterase classification. Biochemical Journal 245, 293-296. 

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). 

We must stress that organo-phosphorus compounds are not specific inhibitors for the cholinesterases, but are rather inhibitors for enzymes possessing carboxylic esterase activity. All the enzymes mentioned below will hydrolyse carboxylic esters. However, not all esterases are inhibited, for example, A-esterase which hydrolyses phenyl acetate is not inhibited by organo-phosphorus compounds. 

Arylesterase Aryl-ester hydrolase, A-esterase Aromatic 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],... 

The mechanism by which A-esterases hydrolyze organophosphates is not completely understood. Involvement of a phosphorylated active-site cysteine and displacement of an activated H20 molecule are two possible hypotheses (see Sect. 3.7.1) [56], A-Esterases comprise enzymes that hydrolyze aryl esters, paraoxon (2.2) and related organophosphate pesticides, and diisopropyl-fluorophosphate (DFP, diisopropyl phosphorofluoridate, 2.3) and related compounds, including nerve gases. These enzymes are found in the current nomenclature listed under arylesterases, aryldialkylphosphatase, and diisop-ropyl-fluorophosphatase. 

The A-esterases now classified as diisopropyl fluorophosphatases (diiso-propyl-fluorophosphate fluorohydrolase, DFPase, somanase, EC 3.1.8.2) were previously listed under EC 3.8.2.1. These enzymes, which hydrolyze P-F and P-CN bonds such as those of nerve gases, should be described as organophosphorus acid anhydrolases rather than phosphatases [56]. Diisopropyl-fluoro-phosphatases exist in different forms with contrasting substrate specificities. One form is able to hydrolyze paraoxon at a low rate, while others have no paraoxonase activity. The different forms differ in their molecular weights and in their requirements for bivalent cations for activity [56]. 

Aryldialkylphosphatase (aryltriphosphate dialkylphosphohydrolase, para-oxonase, PON, EC 3.1.8.1) is an A-esterase that cleaves aryldialkylphosphate... 

Y. S. Huang, L. Woods, L. G. Sultatos, Solubilizaion and Purification of A-Esterase from Mouse Hepatic Microsomes , Biochem. Pharmacol. 1994, 48, 1273-1280. 

A. L. Pond, C. P. Coyne, H. W. Chambers, J. E. Chambers, Identification and Isolation of Two Rat Serum Proteins with A-Esterase Activity toward Paraoxon and Chlorpyrifos-Oxon , Biochem. Pharmacol. 1996, 52, 363-369. 

Of great interest is the recent finding that human serum paraoxonase (EC 3.1.8.1, which belongs to the class of A-esterases), is very active in hydrolyzing a range of four-, five-, six-, and seven-membered lactones [167], Some cyclic carbonates (see Sect. 7.6.3) were also substrates. 

The inactivation and detoxification of paraoxon and congeners are catalyzed by the so-called A-esterases, which, as discussed, comprise aryleste-rase (sometimes still called paraoxonase, EC 3.1.1.2) and phosphoric triester hydrolases (phosphotriesterases, EC 3.1.8) subdivided into aryldialkylphos-phatase (organophosphate hydrolase, paraoxonase, EC 3.1.8.1) and organophosphorus acid anhydrolases (EC 3.1.8.2 see Sect. 9.3.7) [65][69][106-108], These activities, which occur mostly in the mammalian liver and... 

As anhydrides, such compounds are subject to spontaneous hydrolysis, which may contribute to detoxification [160]. Thus, soman hydrolysis at pH 7.5 and 37° occurs with a rate constant of 0.003 - 0.004 min-1 and an activation energy of ca. 55 kJ mol 1 [161]. However, most of the published data refer to enzymatic hydrolysis. Enzymes hydrolyzing P-X anhydride bonds are now known as organophosphorus acid anhydrolases (OPA anhydrolases) classified as EC 3.1.8.2 (also known as diisopropyl-fluorophosphatase, DFPase, tabunase, somanase), an activity related to EC 3.1.8.1 (aryldialkyl-phosphatase, paraoxonase, A-esterase) and formerly classified as EC 3.8.2.1 [64] [65] [69], Much public information on these enzymes can be found in [106],... 

J. A. Vitarius, L. G. Sultatos, Kinetic Mechanism of the Detoxification of the Organo-phosphate Paraoxon by Human Serum A-Esterase , Drug Metab. Dispos. 1994, 22, 472-478. 

R. Traverso, A. Moretto, M. Lotti, Human Serum A -Esterases. Hydrolysis of o,o-Dimethyl-2,2-dichlorovinyl Phosphate , Biochem. Pharmacol. 1989, 38, 671-676. 

In normal milk, the ratio of A B C esterase activity is about 3 10 1 but the level of A-esterase activity increases considerably on mastitic infection. A and C esterases are considered to be of little technological significance in milk. 

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. 

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