Amidase, mammalian

Carboxylesterases and amidases catalyze hydrolysis of carboxy esters and carboxy amides to the corresponding carboxylic acids and alcohols or amines. In general those enzymes capable of catalyzing hydrolysis of carboxy esters are also amidases, and vice versa (110). The role of these enzymes in metabolsim of drugs and insecticides has been reviewed (111, 112). In addition to the interest in mammalian metabolism of drugs and environmental chemicals, microbial esterases have been used for enantioselective hydrolyses (113, 114). 

W. Junge, K. Krisch, The Carboxylesterases/Amidases of Mammalian Liver and Their Possible Significance , CRC Crit. Rev. Toxicol. 1975, 3, 371-434. 

Medicinal chemists are interested not only in hydrolysis of amides by mammalian amidases as exemplified above, but also in bacterial amidases as useful biosynthetic tools. Of particular interest is the enantioselective hydrolysis of chiral amides by various bacterial amidases. Some of these... 

Amidases can be found in all kinds of organisms, including insects and plants [24], The distinct activities of these enzymes in different organisms can be exploited for the development of selective insecticides and herbicides that exhibit minimal toxicity for mammals. Thus, the low toxicity in mammals of the malathion derivative dimethoate (4.44) can be attributed to a specific metabolic route that transforms this compound into the nontoxic acid (4.45) [25-27]. However, there are cases in which toxicity is not species-selective. Indeed, in the preparation of these organophosphates, some contaminants that are inhibitors of mammalian carboxylesterase/am-idase may be present [28]. Sometimes the compound itself, and not simply one of its impurities, is toxic. For example, an insecticide such as phos-phamidon (4.46) cannot be detoxified by deamination since it is an amidase inhibitor [24],... 

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. 

This family of enzymes is cytosolic and is widely distributed in a variety of mammalian tissues. There are also enzymes that hydrolyze N-substituted acetamides (i.e., amidases, as described previously) and the extent to which free versus acetylated amines are present in vivo depends on the relative rates of the acetylation and deacetylation reactions, on the physical and chemical properties of the two products, and whether or not the amine is metabolized by competing pathways. Some acetylated hydroxamic acids are chemically reactive and appear to be ultimate carcinogens. 

A chemoenzymatic synthesis of the P-a-methyl 2 -deoxynucleoside triphosphates 122 has been described which involves reaction of the 5 -0-(methylpho-sphonyl)-N-protected nucleosides with pyrophosphate in the presence of CDI. Removal of the base protection by treatment with penicillin amidase gave compounds 122 leaving the labile a-methylphosphonate intact. A number of 2 -deoxythymidine 5 -triphosphate and 3 -azido-2, 3 -dideoxythymidine 5 -tripho-sphate analogues (123) containing a hydrophobic phosphonate group have also been synthesised and evaluated as substrates for several viral and mammalian polymerases. Some y-ester (124) and y-amide (125) derivatives of dTTP and 3 -azido-2, 3 -dideoxythymidine 5 -triphosphate (AZTTP) were also synthesized and studied. The y-phenylphosphonate triphosphate 126 and its conjugation to biotin and fluorescein labels has also been described. 

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. 

Dimethoate is split by an amidase in the mammalian liver, whereas it is activated to oxon in insects. Dimethoate may also be detoxicated by glutathione transferases or isomerized to the more toxic derivatives by heating. Demeton-S-methyl is metabolized to highly toxic compounds, such as deme-ton-S-methyl sulfoxide and sulfone in plants and animals ... 

Dimethoate also has an intrinsic selectivity, for it is far more toxic to insects than to mammals. This favourable effect was found, surprisingly, to depend little on differences in S O conversion (as in malathion and diazinon), but to rely mainly on preferential operation of mammalian amidase (Krueger, O Brien and Dauterman, 1960). This discovery introduced an expanded feeling of latitude in designing selective organophosphate insecticides. 

The amide linkage is similar to the ester linkage and is hydrolyzed in plants by amidases. There has been speculation, however, that some car-boxylamidases may be carboxylesterases with extended specificity partially purified carboxylesterases from mammalian sources are known to utilize amide substrates. ... 

---