Chapter 11
Pesticide Metabolites in Food 1
Larry G. Ballantine and Bruce J. Simoneaux
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Agricultural Division, CIBA-GEIGY Corporation, P.O. Box 18300, Greensboro, NC 27419 The definition of metabolic pathways of pesticides in plants and animals and the subsequent assay for toxicologically significant residues are essential to estimate dietary exposure. The composition of these residues is used as a basis to develop analytical methods to determine residue levels in food and to establish and enforce pesticide tolerances. Since the metabolism of pesticides by plants and animals may be very complex, this information needs to be evaluated carefully to determine which pesticide components to include in the tolerance expression. Metabolism information, in conjunction with toxicology information and analytical capabilities, dictates whether the parent compound, individual metabolites, or some other measure of total pesticide residues should be included in the tolerance expression. Results of studies conducted using atrazine are discussed as an example of an approach to address the metabolic component in the determination of pesticide residues in food. Atrazine, 2-chloro-4-ethylamino-6-isopropylamino-i-triazine, is a herbicide used to control many broadleaf and grass weeds in corn and sorghum as well as in several minor crops. Its maximum use rate on corn or sorghum varies from 2.0 to 3.0 lbs. a.i./A, depending on soil type. Results of field studies (i) on the metabolism of atrazine in plants show that atrazine residue uptake in plants is relatively low (Table I) and subsequent metabolism is rapid. The rapid metabolism of atrazine in plants may be demonstrated by the results of a greenhouse study (7) in which corn was treated pre-emergence at a rate of 2.0 lbs. a.i./A (Table II). Four-week corn contained 12.6% organic soluble C-radioactivity which would contain parent atrazine plus the chlorotriazine metabolites. By eleven weeks, the organic-soluble fraction made up 10% of the total radioactivity. The corresponding aqueous fraction accounted for 58% of the C-residues at four weeks and 77% at eleven weeks. 14
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Current address: Hazleton Wisconsin, 3301 Kinsman Boulevard, Madison, WI 53704 O097-6156/91/0446-O096$06.00/0 © 1991 American Chemical Society
In Pesticide Residues and Food Safety; Tweedy, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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11.
Table I.
Appl. Rate (Lbs. a.i./A) 3.0 3.0 4.0
Crop Corn
Sorghum
2.1 3.0
Table I I .
Organic Soluble
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Pesticide Metabolites in Food
BALLANTINE & SIMONEAUX
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PPM C-Atrazine Equivalent
Pre Pre Post
Stalks Foraae 1.2 (Silage) 0.76 1.07 (15 wks) 2.6 5.23 (5 wks) 5.4
Post Pre
1.76 (3 wks) 0.60 (5 wks)
ADDI .
0.64 1.2
Mature Cobs Grain 0.03 0.18 0.05 0.13 0.07 0.25 0.3
0.02 0.02
Partitioning Characteristics of Corn Metabolites
Plant Maturity (weeks) 4 11 15 Whole Whole Grain Cob Plant PI ant Stalks Percent of Total Radioactivity 12.6 10.4 6.6 9.2 9.0
Aqueous Soluble
58.2
76.5
64.1
44.5
57.3
Nonextractable
20.5
13.1
19.4
53.6
23.9
Total
91.3
100.0
90.1
107.3
90.2
In Pesticide Residues and Food Safety; Tweedy, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
98
PESTICIDE RESIDUES AND FOOD SAFETY
The metabolism of atrazine in plants is complex and involves at least 15 to 20 structures (Figure 1). Three metabolic pathways for atrazine metabolism in plants have been elucidated: 1. Dealkylation of the side-chain alkyl groups. 2. Enzyme-mediated s-glutathione conjugation with displacement of the chloro group. This glutathione pathway is complicated by the apparent availability of dealkylated metabolites as substrates for conjugation. 3. Hydrolysis of chlorotriazine to hydroxy triazines. The glutathione pathway has been shown only in studies involving young plants and evidently is not a major contributor to metabolites present in mature raw agricultural commodities. Examination of mature corn stalks and grain from C-atrazine-treated plots shows that most metabolites involve hydroxy triazines, their oxidation products, and corresponding conjugates (Figure 1). Present atrazine crop tolerances are based on individual analysis of the parent compound plus its metabolites that contain the chlorotriazine moiety. Based on atrazine plant metabolism study results, analyses of crops for the chlorotriazine moiety would account for a small percent of the total atrazine residue since the majority of the residue is comprised of the hydroxy moiety, either free or conjugated. An approach to estimate a worst-case dietary exposure to man for atrazine in plants and to avoid the potential need to develop residue methods to account for 15 to 20 individual atrazine metabolites, each of which would be present at very low levels, uses the total C radioactivity measured in the various plant substrates from the metabolism studies. The maximum radioactivity level of 0.07 ppm in grain, the only corn or sorghum commodity that provides a means of direct dietary exposure to man, can be used as a basis for the dietary exposure estimate. This value is increased further to 0.10 ppm to account for measurement and biological variability inherent in metabolism studies. Corn and sorghum forage, silage, stalks and grain are used as livestock feed and may contribute to the indirect exposure of man to atrazine metabolites through the consumption of cattle and poultry products. Studies have been conducted to determine the metabolism of atrazine in ruminants and poultry and to determine the potential transfer of plant metabolites to livestock (2). The profile of atrazine metabolites in animals is indicative of two pathways (5): dealkylation of the alkylamino side chains and glutathione conjugation at the chloro position with subsequent stepwise degradation of the glutathione moiety (Figure 2). Radioassay results indicate very little disposition of residues in animal tissues with the exception of liver, the primary site of metabolism. Results also demonstrate very rapid excretion of atrazine residues (Tables III and IV). To put these results in perspective, based on the plant metabolism studies, a worst-case level of 6.0 ppm in corn or sorghum forage and silage is a reasonable estimate of the maximum dietary level for beef or dairy cattle. Since forage and silage are not fed to poultry, dietary exposure to atrazine residues in poultry would be much less.
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In Pesticide Residues and Food Safety; Tweedy, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
11.
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99
Pesticide Metabolites in Food
BALLANTINE & SIMONEAUX
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Figure 1. Initial Pathway of Atrazine in Plants.
In Pesticide Residues and Food Safety; Tweedy, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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100
PESTICIDE RESIDUES AND FOOD SAFETY
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