Hydrolases selective toxicity

Another example of insecticide selectivity is shown in Figure 9.12. Acephate is a proinsecticide that has to be converted to methamidophos, an active insecticide, by a carboxy-lamidase. It has been shown that this hydrolase is much more active in insects than in mammals. This explains why acephate is very toxic to insects but not to mammals. 

Recently, sterically differently substituted epoxides, such as reactive metabolites of the pesticides vinclozolin, rotenone, and phenothrin, were investigated in terms of their ability to inhibit epoxide hydrolase. Mono-and di-substituted oxiranes were good substrates and strong noncompetitive inhibitors of hepatic epoxide hydrolase, whereas tri-substituted epoxides were virtually inactive in this regard (Cova et al 1986). Obviously, selective inhibition of epoxide hydrolase by epoxide metabolites could interfere with the natural protection of the organism against other toxic epoxides. 

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. 

Blocking of the hydrolytic enzymes by selective hydrolase-inhibitors such as tetraethyl pyrophosphate (TEPP [1215]) or diethyl p-nitrophenylphosphate (paraoxon). However, aU of these inhibitors are highly toxic and have to be handled with extreme caution. 

Fluoronoraristeromycin (43, Fig. 1) was investigated as an inhibitor of S-adenosyl-L-homocysteine (SAH) hydrolase in Plasmodium falciparum, and showed 100-fold enhanced selectivity for toxicity to the parasite versus mammalian cells when compared to the unfluorinated noraristeromycin [67]. Analogues of SAH inhibit human DNA methyltransferases, which use S-adenosyl-L-methionine as the cofactor to transfer methyl groups to DNA [18]. The addition of a 2-fluoro substituent to SAH was found to confer selectivity for inhibition of the DNMTl isoform of enzyme over the DNMT3b2 subtype, while unfluorinated SAH was a better inhibitor of the DNMT3b2 enzyme. 

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