Esterase mammalian liver

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... 

Many pesticides are esters or amides that can be activated or inactivated by hydrolysis. The enzymes that catalyze the hydrolysis of pesticides that are esters or amides are esterases and amidases. These enzymes have the amino acid serine or cysteine in the active site. The catalytic process involves a transient acylation of the OH or SH group in serin or cystein. The organo-phosphorus and carbamate insecticides acylate OH groups irreversibly and thus inhibit a number of hydrolases, although many phosphorylated or carbamoylated esterases are deacylated very quickly, and so serve as hydrolytic enzymes for these compounds. An enzyme called arylesterase splits paraoxon into 4-nitrophenol and diethyl-phosphate. This enzyme has cysteine in the active site and is inhibited by mercury(ll) salts. Arylesterase is present in human plasma and is important to reduce the toxicity of paraoxon that nevertheless is very toxic. A paraoxon-splitting enzyme is also abundant in earthworms and probably contributes to paraoxon s low earthworm toxicity. Malathion has low mammalian toxicity because a carboxyl esterase that can use malathion as a substrate is abundant in the mammalian liver. It is not present in insects, and this is the reason for the favorable selectivity index of this pesticide. 

Carboxylesterases (EC 3.1.1.1) can be detected in most mammalian tissues. Besides organs with high carboxylesterase activity such as liver, kidney, and small intestine, esterase activity is present, e.g., in the brain, nasal mucosa, lung, testicle, and saliva. Compared to rat plasma, human plasma contains little carboxylesterase, its esterase activity being essentially due to cholinesterase [61][73][79][89-91],... 

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. 

The term carboxylesterase refers to a wide variety of enzymes with both esterase and amidase activity. They cleave carboxylesters, carboxylamides, and car-boxylthioesters, producing a carboxylic acid and an alcohol or phenol (Figure 8), amine, or mercaptan, respectively. There are many different esterases, some of which are important for the hydrolysis and detoxication of toxic organophosphate esters. In general, esterases are present in almost all mammalian tissues, occur as multiple isozymes, and are concentrated in the liver. The esterase activity present in plasma is normally due to the release of these enzymes from liver. 

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