Detoxification carboxylesterase

The in vivo metabolism of a homologous series of alkyl carbamates (7.2, Fig. 7.3) has yielded some informative results [13]. The hydrolysis of these esters liberates carbamic acid (7.3, Fig. 7.3), which breaks down spontaneously to C02 and NH3, allowing the extent of hydrolysis to be determined conveniently and specifically by monitoring C02 production. When such substrates were administered to rats, there was an inverse relationship between side-chain hydroxylation and ester-bond hydrolysis. Thus, for compounds 12 the contribution of hydrolysis to total metabolism (90 - 95% of dose) decreased in the series R=Et (ca. 85-90%), Bu (ca. 60-65%), hexyl (ca. 45 - 50%), and octyl (ca. 30%). Ethyl carbamate (urethane) is of particular toxicological interest, being a well-established carcinogen in experimental animals. In vitro studies of adduct formation have confirmed the competition between oxidative toxification mediated by CYP2E1 and hydrolytic detoxification mediated by carboxylesterases [14]. 

Natural (-)-cocaine (7.57, Fig. 7.8), which has the (2/ ,3S)-configuration, is a relatively poor substrate for hepatic carboxylesterases and plasma cholinesterase (EC 3.1.1.8), and also a potent competitive inhibitor of the latter enzyme [116][121], In contrast, the unnatural enantiomer, (+)-(2S,3/ )-cocaine, is a good substrate for carboxylesterases and cholinesterase. Because hydrolysis is a route of detoxification for cocaine and its stereoisomers, such metabolic differences have a major import on their monooxygenase-catalyzed toxification, a reaction of particular effectiveness for (-)-cocaine. 

The toxicity of the P-halidc anhydrides, like that of phosphoroxychloride (POCl3) and of other organophosphorus compounds discussed earlier in this section, is due to their high efficiency as irreversible inactivators of acetylcholinesterase [157]. The main target organs for the lethal effects of these chemical weapons are the brain and diaphragm. As for the detoxification of the P-halide anhydrides, it can occur by a number of biochemical mechanisms, namely chemical hydrolysis, enzymatic hydrolysis, and binding to hydrolases such as carboxylesterases, cholinesterases, and albumin [68][158][159]. 

An unusual case of intramolecular competition (chemoselectivity, see Chapt. 1 in [la]) between ester and oxirane occurs in the detoxification of (oxiran-2-yl)methyl 2-ethyl-2,5-dimethylhexanoate (10.49), one of the most abundant isomers of an epoxy resin. The compound is chemically very stable, i.e., resistant to aqueous hydrolysis, but is rapidly hydrolyzed in cytosolic and microsomal preparations by epoxide hydrolase and carboxylesterase, which attack the epoxide and ester groups, respectively [129], The rate of overall enzymatic hydrolysis was species dependent, decreasing in the order mouse > rat > human, but was relatively fast in all tissues examined (lung and skin as portals of entry, and liver as a further barrier). In mouse and rat lung microsomes, ester hydrolysis was 3-4 times faster than epoxide hydration, whereas the opposite was true in human lung microsomes. 

The well-known selectivities of some organophosphates may be explained by the balance of enzymatic events. The reduced toxicity of the insecticide malathion to mammals is largely the result of rapid activation by desulfuration in the insect and the more rapid detoxificaton by carboxylesterases and glutathione transferases in the mammal (3). Design of new pest bioregulators should exploit enhanced activation and decreased detoxification capabilities in the targeted pests. 

Fonnum, F., Sterri, S.H., Aas, P., Johnsen, H. (1985). Carboxylesterases, importance for detoxification of organophosphoms anticholinesterases and trichothecenes. Fundam. Appl. Toxicol. 5 S29-38. 

Sterri, S.H. (1989). The importance of carboxylesterase detoxification of nerve agents. In FOA Report C 40266-4.6,4.7 Proceedings of the Third International Symposium on Protection against Chemical Warfare Agents, pp. 235 0. FOA ABC-skydd, Umea. 

Pretreatment of rats with a carboxylesterase inhibitor enhances the respiratory irritation and lethality produced by the inhalation of methyl acrylate. This observation suggests that the toxicity of methyl acrylate becomes manifest when local detoxification/ defense mechanisms become overwhelmed. 

Both Aphis gossypii and Myras persicae are major agricultural pests, and insecticide resistance in these species is a significant problem. Organophosphate and carbamate tolerance in these aphids has been related to detoxification via increased carboxylesterase activity Suzuki el al., 1993 Devonshire and Moores, 1977). 

Metabolism also plays a critical role in the pharmacology of cocaine. The rapid hydrolysis of cocaine via two different pathways leads to its rapid inactivation/detoxification. This rapid metabolism has been a major determinant in the methods and modes of cocaine abuse. Identification and characterization of these hydrolytic enzymes would be useful in that selective induction of these enzymes offers a potential treatment strategy for dealing with cocaine overdose. It is conceivable that long-term elevation of the enzyme or enzymatic activity could be used in conjunction with maintenance therapy for cocaine addicts. Hydrolases or esterases are also responsible for the transesterfication of cocaine. The pharmacological effect of cocaine is prolonged and enhanced when cocaine is used in conjunction with ethanol. A carboxylesterase catalyzes an ethyl transeterification of cocaine to cocaethylene, which is biologically active. 

The hydrolysis of esters and amides (including peptides) is important in medicinal chemistry (see Chapters 36 and 38). Here, two drugs are presented to illustrate these two chemical classes. (-)-Cocaine (5) has two ester groups whose hydrolysis (Figure 32.7a) is a route of detoxification, which accounts for as much as 90% of the dose in humans. Three human enzymes are now known to be involved in the hydrolysis of cocaine. One is the liver carboxylesterase hCE-1 which catalyzes the hydrolysis of the methyl ester group. As for the benzoyl ester goup, it is hydrolyzed by the liver carboxylesterase hCE-2 and serum cholinesterase. 

Glutathione conjugation. The involvement of glutathione transferases in OP metabolism was realized in the early 1960 s (35. 361. It was difficult to establish this fact because of similarities between glutathione transferase-and carboxylesterase-produced metabolites. Induction of glutathione transferase activity in the fall armyworm caused a 2- to 3-fold decrease in the toxicity of diazi-non, methamidophos, and methyl parathion (37.) This shows indirectly the importance of glutathione transferase activity in the detoxification of these OPs. 

The carboxylesterase superfamily catalyzes the hydrolysis of ester- and amide-containing compounds. These enzymes are found in both the ER and cytosol of many cell types and are involved in detoxification or metabolic activation of drugs, environmental toxins, and carcinogens. Car-boxylesterases also catalyze the activation of prodrugs to their respective free acids. For example, the prodrug and cancer chemotherapeutic agent irinotecan is bioactivated by plasma and intracellular carboxylesterases to the potent topoisomerase inhibitor SN-38. 

Malathion (chemathion, mala-spray) requires conversion to malaoxon (replacement of a sulfur atom with oxygen in vivo, conferring resistance to mammalian species). Malathion can be detoxified by hydrolysis of the carboxyl ester linkage by plasma carboxylesterases, and plasma car-boxylesterase activity dictates species resistance to malathion. The detoxification reaction is much more rapid in mammals and birds than in insects. Malathion has been employed in aerial spraying of relatively populous areas for control of Mediterranean fruit flies and mosquitoes that harbor and transmit viruses harmful to human beings (e.g.. West Nile encephalitis virus). Evidence of acute toxicity from malathion arises only with suicide attempts or deliberate poisoning. 

Age-related differences in kinetic parameters of detoxification can partially explain the increased sensitivity of the young to acute exposure to chlorpyrifos and other OPs (Mortensen et ai, 1996 Moser et ai, 1998 Padilla et ai, 2000, 2004). Thus, in vitro assays show that differences in vivo are not due to intrinsic differences in sensitivity of the target enzyme (Mortensen el ai, 1998). Furthermore, differences in liver microsomal metabolism, which mediates activation and/or inactivation of some pesticides, do not adequately explain the increased sensitivity (Benke and Murphy, 1975 Brodeur and DuBoks, 1%7). Differences in detoxification pathways correlate better with age sensitivity. B-esterases (e.g., carboxylesterases) and A-esterase.s bind to and/or hydrolyze, and thus detoxify, some cholinesterase-inhibiting pesticides (Jokanovic et al., 1996 Maxwell, 1992). These pathways are much less well developed in the young, and maturation of these systems tracks the decreasing sensitivity to acute exposure to chlorpyrifos and other OPs (Attcrberry el ai, 1997 Benke and Murphy, 1975 Brodeur and DuBois, 1967 Chanda et ai, 1997, 2002 Mendoza, 1976 Mortensen et ai, 1996, 1998 ... 

Fonnum F, Sterri SH, Aas P, Johnsen H (1985) Carboxylesterases, importance for detoxification of organophosphorus anticholineserases and trichothecenes. Fundam Appl Toxicol 5 S29-S38. 

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