Pesticide Metabolism in Plants
49
Minor 2,4-D conjugates of alanine, valine, leucine, phenylalanine, and
tryptophan have been identified in soybean callus tissue (Feung et al., 1973).
The glutamic and aspartic acid conjugates of IAA were minor metabolites in
crown gall callus (Feung et al., 1976). Major metabolites were conjugates of
glycine, alanine, and valine. The significance of the differences between species
and pesticides in the formation of amino acid conjugates has not been determined.
Amino acid conjugates of 2,4-D display typical auxin activity by stimulating
cell elongation in A vena coleoptile sections and growth of soybean callus tissue
(Feung et al., 1974). It appears that the conjugate 2,4-D-Glu and other amino
acid conjugates are biologically active as the conjugated molecule and not necessarily as the free acid 2,4-D. Both 2,4-D-Glu and 2,4-D-methionine stimulated
callus growth by more than 50% over that of 2,4-D at the optimum 2,4-D
concentration of 0.1 J.LM (Feung et al., 1974). Soybean callus tissue incubated
with [l4C]-2,4-D-Glu hydrolyzed only 4% of the absorbed [l4C]-2,4-D-Glu, but
55% was oxidized to biologically inactive 4-0H-2,5-D and 4-0H-2,3-D (Feung
et al., 1973). Amino acid conjugation may be an activation mechanism for the
expression of biological activity by 2,4-D.
2.3.4.3. Glutathione Conjugation
Glutathione conjugation is becoming recognized as a major metabolic reaction for pesticide degradation in plants. The significance of glutathione conjugation and biosynthesis of mercapturic acids in mammals has been recognized
for a long time. The importance of this reaction in all biological systems is
reflected in the numerous reviews on this subject (Arias and Jacoby, 1976;
Boyland and Chasseaud, 1969; Hutson, 1976; Jacoby, 1978; Lamoureux and
Frear, 1979; Shimabukuro et al., 1978a). Glutathione conjugation in plants is
extremely important because of (1) the wide range of potential substrates, (2)
its role as a detoxication mechanism and a major factor in herbicide selectivity,
and (3) its influence on the nature of terminal pesticide residues that remain in
plants.
Glutathione conjugation is the initial reaction that leads to the synthesis of
mercapturic acids in mammals (Boyland and Chasseaud, 1969). This is illustrated
in the formation of the mercapturic acid of 3,4-dichloronitrobenzene (Fig. 2.22).
Conjugation with reduced glutathione (GSH) is catalyzed by glutathione S-transferases with different substrate specificities (Boyland and Chausseaud, 1969;
Hutson, 1976; Jacoby, 1978). The initial reaction involves an enzyme-catalyzed
nucleophilic displacement with the formation of the GSH conjugate. Sequential
removal of the glutamic acid and glycine residues gives the S-cysteine conjugate
that is then N-acetylated to yield the mercapturic alld, the terminal excretion
product in mammals.
Few examples of pesticides metabolized by GSH conjugation are known in
49
Minor 2,4-D conjugates of alanine, valine, leucine, phenylalanine, and
tryptophan have been identified in soybean callus tissue (Feung et al., 1973).
The glutamic and aspartic acid conjugates of IAA were minor metabolites in
crown gall callus (Feung et al., 1976). Major metabolites were conjugates of
glycine, alanine, and valine. The significance of the differences between species
and pesticides in the formation of amino acid conjugates has not been determined.
Amino acid conjugates of 2,4-D display typical auxin activity by stimulating
cell elongation in A vena coleoptile sections and growth of soybean callus tissue
(Feung et al., 1974). It appears that the conjugate 2,4-D-Glu and other amino
acid conjugates are biologically active as the conjugated molecule and not necessarily as the free acid 2,4-D. Both 2,4-D-Glu and 2,4-D-methionine stimulated
callus growth by more than 50% over that of 2,4-D at the optimum 2,4-D
concentration of 0.1 J.LM (Feung et al., 1974). Soybean callus tissue incubated
with [l4C]-2,4-D-Glu hydrolyzed only 4% of the absorbed [l4C]-2,4-D-Glu, but
55% was oxidized to biologically inactive 4-0H-2,5-D and 4-0H-2,3-D (Feung
et al., 1973). Amino acid conjugation may be an activation mechanism for the
expression of biological activity by 2,4-D.
2.3.4.3. Glutathione Conjugation
Glutathione conjugation is becoming recognized as a major metabolic reaction for pesticide degradation in plants. The significance of glutathione conjugation and biosynthesis of mercapturic acids in mammals has been recognized
for a long time. The importance of this reaction in all biological systems is
reflected in the numerous reviews on this subject (Arias and Jacoby, 1976;
Boyland and Chasseaud, 1969; Hutson, 1976; Jacoby, 1978; Lamoureux and
Frear, 1979; Shimabukuro et al., 1978a). Glutathione conjugation in plants is
extremely important because of (1) the wide range of potential substrates, (2)
its role as a detoxication mechanism and a major factor in herbicide selectivity,
and (3) its influence on the nature of terminal pesticide residues that remain in
plants.
Glutathione conjugation is the initial reaction that leads to the synthesis of
mercapturic acids in mammals (Boyland and Chasseaud, 1969). This is illustrated
in the formation of the mercapturic acid of 3,4-dichloronitrobenzene (Fig. 2.22).
Conjugation with reduced glutathione (GSH) is catalyzed by glutathione S-transferases with different substrate specificities (Boyland and Chausseaud, 1969;
Hutson, 1976; Jacoby, 1978). The initial reaction involves an enzyme-catalyzed
nucleophilic displacement with the formation of the GSH conjugate. Sequential
removal of the glutamic acid and glycine residues gives the S-cysteine conjugate
that is then N-acetylated to yield the mercapturic alld, the terminal excretion
product in mammals.
Few examples of pesticides metabolized by GSH conjugation are known in