Perspective Cite This: J. Agric. Food Chem. 2018, 66, 9379−9383
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Pesticide Detox by Design John E. Casida*
J. Agric. Food Chem. 2018.66:9379-9383. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/12/18. For personal use only.
Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, California 94720, United States ABSTRACT: Detoxification (detox) plays a major role in pesticide action and resistance. The mechanisms involved are sometimes part of the discovery and development process in seeking new biochemical targets and metabolic pathways. Genetically modified and chemical-safener-modified crops are a marked exception and often involve herbicide detox by design to achieve the required crop tolerance. This perspective evaluates the role of detox by design or chance and target-site-based selectivity in insecticide, herbicide, and fungicide action and human health and environmental effects. KEYWORDS: pesticide, detoxification, crop tolerance, target site, selectivity, pest resistance
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INTRODUCTION Pesticide effectiveness and safety depend upon target site selectivity and detoxification (detox) at the right time and place, a very large topic involving nearly 1000 pesticides1 acting by around 100 mechanisms2 in a broad range of organisms. Pesticides are mostly detoxified by oxidation, reduction, or hydrolysis (phase I metabolism) to give modified functional groups that are conjugated for excretion or deposition (phase II metabolism) (Figure 1).3 Among the
DETOX-BASED SELECTIVITY AND PEST RESISTANCE (TABLE 1) Insecticides. Indoxacarb (1) is highly toxic to insects relative to mammals as a result of activation by an insect carboxyamidase to the decarbomethoxylated oxadiazine acting as a voltage-dependent blocker of insect Na+-dependent action potentials.6 Malathion (2)7 and tetramethrin (3)8 selectivity is attributable to their more rapid detox by hydrolysis in mammals than insects. Acephate (4) is activated by carboxyamidase hydrolysis followed by sulfoxidation in insect pests (house flies), whereas in mammals (mice), the deacetylated product (methamidaphos) inhibits the deacetylase, leading to detox favored over activation.9 Detox-based pest resistance involves hydrolysis of dimethoate (5) in melon aphids10 and CYP6CMIvQ oxidation of imidacloprid (6) to 5hydroxy-6 in whiteflies.11 Insecticide resistance in mosquitoes can be monitored by a microarray containing unique detox genes on “detox chips”.12 This ingenious approach is limited by knowledge of the mechanisms for the hundreds of resistant insect−insecticide combinations. Herbicides. Propanil (7) is less toxic to rice than to annual grasses and some broadleaf weeds because the crop has a detoxifying aryl acylamidase, which is very effective, except following organophosphate and methylcarbamate insecticide inhibition.13 Isoxaflutole (8) undergoes isoxazole conversion to the phytotoxic diketonitrile derivative faster in weeds than in rice, resulting in activation selectivity.14 Fenoxaprop-P-ethyl (9) resistance in some populations of black grass results from enhanced metabolism.15 Chlorotoluron (10) resistance in annual ryegrass involves N-methyl oxidation and then hydrolysis of the resulting urea.16 Fungicides. Dinobuton (11) is highly toxic to fungi as a result of hydrolase activation17 but less so to mammals1 that detoxify by metabolism at other substituents. Triadimenol (12) undergoes selective activation by oxidation to triadimefon in several species of fungi.18 Fungal resistance to carbendazim
Figure 1. Pesticide detox reactions. Other phase I reactions are hydration and dehalogenation. Other phase II reactions are acetylation, methylation, and conjugation with glucose and amino acids.
detox enzymes are cytochrome P450s (CYPs), which add oxygen at sensitive sites, hydrolases, which cleave esters and epoxides, glutathione (GSH) S-transferases (GSTs), which catalyze GSH addition to thiol-sensitive sites, and various glucosyl- or glucuronosyltransferases, which couple hydroxyl and carboxyl groups with polar glucoside or glucuronide substituents.4,5 The detox enzymes recognize substrates to oxidize, reduce, hydrolyze, or conjugate independent of the molecular features conferring toxicity. Four related topics are considered and illustrated with well-studied examples. The first is detox-based selectivity and pest resistance for insecticides (1−6), herbicides (7−10), and fungicides (11−14) (Table 1). The second is target-site-based selectivity and pest resistance, which are major goals in pesticide design. The third is crop tolerance involving expressed enzymes (15−18) or chemicalsafener-induced herbicide detox enzymes (20 for 19, 22 for 21, and 24 for 23), the only real detox by design (Table 2). The final topic relates detox to humans and the environment considering pesticide mechanisms and chemotypes (Figure 2). © 2018 American Chemical Society
Received: May 9, 2018 Accepted: August 16, 2018 Published: August 16, 2018 9379
DOI: 10.1021/acs.jafc.8b02449 J. Agric. Food Chem. 2018, 66, 9379−9383
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Journal of Agricultural and Food Chemistry Table 1. Pesticide Detox-Based Selectivity and Pest Resistanceb
b
Arrows on the chemical structures designate relevant metabolism sites.
with selectivity problems. Nerve and muscle poisons are the major insecticides working on the GABAergic, glutaminergic, cholinergic, and Na+ channel systems or the Ca2+-activated Ca2+ channel. Target-site-based resistance involves mutations in the specific binding sites (AChE, GABA-R, Glu-R, nAChR, and VGSC; see abbreviations), lowering sensitivity to whole classes of insecticides,21 or in the detox mechanisms. In each of these cases, some compounds have adequate insect/mammal target site selectivity, while safety for others comes from detox. Herbicides. Many target sites important in plants are not represented in mammals and other animals. The most obvious target sites are photosynthesis and carotenoid, amino acid, and phytosterol biosynthesis in plants only. The pathways of sterol and cell wall synthesis differ in insects, plants, and fungi, providing important targets for selectivity. Inhibitors of photosystem II and acetohydroxyacid synthase (also named as acetolactate synthase) act only in plants. The most important herbicide glyphosate (15) blocks 5-enolpyruvoylshikimate-3-phosphate (EPSP) synthase (EPSPS) in plants with no equivalent site in mammals. Despite this target site specificity, glyphosate is still of concern as a purported human carcinogen and restricted or banned in some countries.
(13) is usually due to selection for target site insensitivity, but a 13 hydrolase is described for Mycobacterium sp. SD-4.19 Fenhexamid (14) resistance in gray mold is based on CYP684 oxidation.20
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TARGET-SITE-BASED SELECTIVITY AND PEST RESISTANCE The goal of much of pesticide design is to achieve target site selectivity and avoid or circumvent the development of resistant pests. Insecticides. Most of the current insecticides have undergone pest selection for low sensitivity, leading to the continuing need for new target sites. Insect development is programmed by juvenile hormones and differentiation hormones (ecdysones) with no counterpart in mammals. Juvenoids and ecdysonoids provide excellent control of populations synchronized by the developmental stage, but the damage resulting from their slow action is often unacceptable. Inhibitors of chitin synthesis are ideal for selective toxicity but, again, are often unacceptably slow in their action. Respiratory poisons, such as rotenone, have been used for over a century in pest insect control but sometimes 9380
DOI: 10.1021/acs.jafc.8b02449 J. Agric. Food Chem. 2018, 66, 9379−9383
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Journal of Agricultural and Food Chemistry Table 2. Corn Herbicide Tolerance Induced by GM and Chemical Detox Safenersf
f
Arrows on the chemical structures designate relevant metabolism sites.
multisite (multiple target site) in their action, and less likely to undergo selection for resistance than the specific site fungicides.21
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CROP TOLERANCE BY DESIGN (TABLE 2) Herbicides are intended to kill weeds without damaging crops. This requires crop/weed selectivity in the target, detox, or a combination, which serves as the basis for most genetically modified (GM) crops. The far most important example is glyphosate (15) tolerance in corn, soybean, and some other crops conferred in part by glyphosate oxidase (GOX) but mostly by overexpression of a GM-modified EPSPS target.22 These “Roundup Ready” GM crops are accepted in the U.S., Canada, Brazil, and many other countries but not in Japan or Europe. Glufosinate (16)-tolerant crops express a N-acetyltransferase for detoxification.23 2,4-D (17) tolerance comes from an aryloxyalkanoate dioxygenase,24,25 and dicamba (18) tolerance comes from an expressed dicamba O-demethylase.26 Toxicological data on the GM herbicide-tolerant crops do not reveal any significant toxicity problem; therefore, the issue becomes natural versus GM foods, which continues as a heated debate. Crop tolerance without altered weed sensitivity can be achieved by the addition of herbicide safeners [dichlormid for EPTC (20 and 19),27 benoxacor for metolachlor (22 and 21),28 and isoxadifen-ethyl for nicosulfuron (24 and 23)29] that induce herbicide-detoxifying GSH synthesis and GST activity in corn, barley, and a few other crops.27,28 Although the induction mechanism and species selectivity are only partially understood for the chemical safeners,29 this is a more
Figure 2. Detox by design. Major parameters of pesticide mechanisms and chemotypes are considered in a detox by design process.
Fungicides. Target-site-based selectivity for fungicides involves sterol and chitin biosynthesis by systems somewhat different from those of other organisms. The four fungicides used in by far the largest amounts are sulfur, mancozeb, copper salts, and chlorothalonil, all thiol-reactive, categorized as 9381
DOI: 10.1021/acs.jafc.8b02449 J. Agric. Food Chem. 2018, 66, 9379−9383
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Journal of Agricultural and Food Chemistry
in the process of mineralization. Some residues however have negligible bioavailability. Challenges remain in finding suitable toxophores, detoxophores, and selectophores, so that the benefits of pest control can be more fully realized in a balance with minimal non-target toxicity and public perception.
acceptable approach for some biologists than crop protection by expression of foreign genes.
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DETOX RELATIVE TO PEOPLE AND THE ENVIRONMENT Maintaining a Balance. We have learned to produce food for an expanding population, live with much reduced insecttransmitted malaria, yellow fever, and plague, and enjoy the associated relief from hunger, disease, and anxiety. Each decade starting in the mid-20th century brought major changes in insecticides, with the chlorinated hydrocarbons of the 1940s, the organophosphates of the 1950s, the methylcarbamates of the 1960s, the pyrethroids of the 1970s, the phenylpyrazoles of the 1980s, the neonicotinoids of the 1990s, and the ryanodine receptor activators (phthalic and anthranilic diamides) of the 2000s.30 Similar changes have occurred for herbicides and fungicides. Many pesticides were used for years and then withdrawn as unfavorable toxicology became apparent or safer and more effective replacements were found. These superseded compounds are nearly equal in number to the currently used pesticides.1 Dichlorodiphenyltrichloroethane (DDT) taught us the lesson of too much pesticide is not a good thing. Four billion pounds, slow breakdown, and passage through food chains could not be tolerated. Pesticides must be regulated. They must be assumed dangerous until proven otherwise. The pesticide era and the “Silent Spring” of Rachel Carson31 opened our eyes to the need to maintain natural systems as sacred rights for the future. Some organisms and ecosystems are at particular risk. The honeybee and monarch butterfly are of major current concern. We seek a balance of nature that is sustainable for people and much of the environment. With an increasing number of people and fewer natural areas, a decision must be made on where to draw the line now and in the future. Mechanisms and Chemotypes. Target site resistance and pesticide replacements mean people will inevitably be exposed to pesticides acting on new targets already wellunderstood or potentially poorly defined. The list of pesticideselective targets has expanded little in recent years. New genomic, proteomic, and other omic techniques reveal new candidate targets as do novel biochemical mechanisms discovered in microorganisms, fruit flies, zebra fish, and genetic expression systems. The increased use of molecular modeling and precision of structural target site structures open new vistas in substituent replacements. Biodegradability then becomes an important consideration. Detox by Design (Figure 2). Detox by design can involve changing the chemical substituents of the pesticide or changing the organism. The classical substituent changes alter the toxicity (toxophore) or detoxification/activation (detoxophore). Further changes may attempt to overcome resistance or confer safety for crops, people, or the environment (selectophore). The organism can be changed by formulation additives to improve the penetration, synergists to increase the toxicity, or safeners to prevent the toxicity, such as of herbicides to crops. The largest change is to genetically modify the organism, such as the crop, by introducing a less sensitive overexpressed target site alone or with a detox enzyme. Detox by design also applies to the environment. Photodecomposition plays a major role in the fate of pesticides in the environment. Pesticides binding in soils and contaminating water are often difficult to remove. Microbial detox of polluted soils and water can supplement physical methods of removal, but there are often a series of metabolites
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 510-642-5424. E-mail:
[email protected]. ORCID
John E. Casida: 0000-0002-3181-7561 Notes
The author declares no competing financial interest. This paper was accepted for publication after John E. Casida passed away on June 30, 2018. Minor changes were made by Associate Editor Qing X. Li before acceptance. John E. Casida was a preeminent toxicologist and pesticide expert. His prominent publications in the Journal of Agricultural and Food Chemistry are acknowledged.
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ACKNOWLEDGMENTS The author gives special thanks to Thomas Zy Lin (Department of Molecular and Cell Biology, University of California, Berkeley) for outstanding contributions in searching, compiling, and presenting the information in this perspective.
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ABBREVIATIONS USED AChE, acetylcholinesterase; CYP, cytochrome P450; EPSP, 5enolpyruvoylshikimate-3-phosphate; EPSPS, 5-enolpyruvoylshikimate-3-phosphate synthase; GABA, γ-aminobutyric acid; GABA-R, γ-aminobutyric acid receptor; Glu-R, glutamate receptor; GM, genetically modified; GOX, glyphosate oxidase; GSH, glutathione; GST, glutathione S-transferase; nAChR, nicotine receptor; VGSC, voltage-gated Na+ channel
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REFERENCES
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