Active site enzymes carboxylesterases

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). 

The transesterification of cocaine to cocaethylene is an enzymatic reaction catalyzed by microsomal carboxylesterases and blocked by inhibitors of serine hydrolases [124][125], In Chapt. 3, we have discussed the mechanism of serine hydrolases, showing how a H20 molecule enters the catalytic cycle to hydrolyze the acylated serine residue in the active site of the enzyme. In the case of cocaine, the acyl group is the benzoylecgoninyl moiety (Fig. 7.9,d ), which undergoes esterification with ethanol according to Steps e and/ (Fig. 7.9). 

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. 

Mechanisms of Serine Hydrolases. Typical to enzymatic reactions, the enzyme (E) first binds its substrate (S) at the active site as an enzyme-substrate complex (E S). For the formation of the product P, the enzyme-catalyzed reaction then takes place through the mechanism typical of the enzyme. At the active site of serine hydrolases (lipases, carboxylesterases, and serine proteases), the catalytic machinery is called a cataljdic triad consisting of amino acid residues Ser, His, and either Asp or Glu (Fig. 5). In the E S complex, imidazole of His serves as a general acid/base catalyst, catalyzing the addition of the alcoholic hydroxyl of the serine residue to the carbonyl carbon of the acyl donor (R C02R, the first substrate S ). This leads both to the liberation of the first product P (R OH) and to the formation of the so-called acyl-enzyme intermediate. This ester intermediate then reacts with the second substrate (R OH), which leads to the... 

Drugs may also undergo hydrolysis by intestinal esterases (hydrolases), more specifically carboxylesterases (EC 3.1.1.1) in the intestinal lumen and at the brush border membrane [58, 59]. It has been shown that intestinal hydrolase activity in humans was closer to that of the rat than the dog or Caco-2 cells [60]. In these studies, six propranolol ester prodrugs and p-nitrophenylacetate were used as substrates, and the hydrolase activity found was ranked in the order human > rat Caco-2 cells > dog for intestinal microsomes. The rank order in hydrolase activity for the intestinal cytosolic fraction was rat > Caco-2 cells = human > dog. The hydrolase activity towards p-nitrophenylacetate and tenofovir disoproxil has also been reported in various intestinal segments from rats, pigs and humans. The enzyme activity in intestinal homogenates was found to be both site-specific (duodenum > jejunum > ileum > colon) and species-dependent (rat > man > Pig)-... 

In case of CarE there was lack of significant differences between the mean activity in the midgut of males from both sites. Reversibly, females from both sites showed a stimulation of CarE activity in this organ (Fig. 8). Ovaries of females from Szopienice had carboxylesterases unaffected by cadmium, with higher activity level than ovaries of females from Tychy. Induction of this enzyme by cadmium was observed in testes, with more pronounced effects in males from Szopienice (Fig. 8). 

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