40
Richard H. Shimabukuro et al.
analogous to the hydrolysis of 2,4-D esters. Hydrolysis of benzoylprop-ethyl to
benzoylprop occurred very rapidly in susceptible oats but not in resistant wheat
(Beynon et ai., 1974; Jeffcoat and Harries, 1973). Wheat coleoptiles were sensitive to benzoylprop but not to benzoylprop-ethyl. The carboxylesterase from
wild oat that catalyzed the hydrolysis of benzoylprop-ethyl was a stable enzyme,
required no cofactors, and had an apparent Km for benzoylprop-ethyl of 3.8 x
10-6 M (Hill et ai., 1978). A similar carboxylesterase was not detected in wheat
(Hill et al., 1978). Other analogs of benzoylprop-ethyl are known to act similarly
(Jeffcoat and Harries, 1975).
Not all pesticide esters appear to be activated by hydrolysis as with 2,4-D
esters and benzoylprop-ethyl. Both resistant wheat and susceptible wild oat and
cultivated oat hydrolyzed rapidly chlorfenprop-methyl and diclofop-methyl to
their acids (Fig. 2.14) (Fedtke and Schmidt, 1977; Shimabukuro et aJ., 1979).
However, very little of both acids and parent esters remained in either plant
species after 24-30 hr (Fedtke and Schmidt, 1977; Shimabukuro et aJ., 1979).
Chlorfenprop-methyl (Andreev and Amrhein, 1976; Collet and Pont, 1978) and
diclofop-methyl (Shimabukuro et aJ., 1978b) were stronger auxin antagonists
(approximately two times stronger) than their free acids. In in vitro membrane
experiments in which penetration into plant tissues by esters or acids was not
a factor, bound auxin was displaced more effectively by the ester, chlorfenpropmethyl, than by the acid, chlorfenprop (Andreev and Amrhein, 1976). Intact
susceptible plants were equally sensitive to the ester and acid forms when sprayed
or injected with chlorfenprop-methyl (Fedtke and Schmidt, 1977) and diclofopmethyl (Donald and Shimabukuro, 1980), respectively. More of the esters of
both herbicides were absorbed from leaf surfaces than the acids, but the absorption differences were minor (Fedtke and Schmidt, 1977; Shimabukuro and
Walsh, unpublished data, 1978). It appears that the esters and acids of both
herbicides are biologically active.
The organophosphorous insecticides, unlike the N-methyl and N,N-dimethylcarbamates, are readily cleaved in plants to form metabolites suggestive of
hydrolytic reactions (Bull, 1972; Cas ida and Lykken, 1969). The alkyl phosphate
and acid anhydride bonds of organophosphates are known to be cleaved in
animals and insects by phosphotriesterases, NADPH-dependent mfo systems,
and soluble glutathione S-transferases (Appleton and Nakatsugawa, 1977; Bull,
1972; Eto, 1974). It appears that cleavage by oxidative and glutathione conjugation mechanisms may be more significant than by a hydrolytic mechanism.
In plants the mechanisms for cleavage have not been characterized.
Hydrolysis of BAY NTN 9306@ (Fig. 2.4) yielded the free phenols that
were conjugated subsequently with glucose. Hydrolysis of the conjugates yielded
23% phenol sulfide fp-(methylthio)phenol], 41 % phenol sulfoxide fp-(methylsulfinyl)phenol], and 36% phenol sulfone fp-(methylsulfonyl)phenol] (Bull et
al., 1976). The pyrimidinyl bond of etrimfos was hydrolyzed to the pyrimidinol,
Richard H. Shimabukuro et al.
analogous to the hydrolysis of 2,4-D esters. Hydrolysis of benzoylprop-ethyl to
benzoylprop occurred very rapidly in susceptible oats but not in resistant wheat
(Beynon et ai., 1974; Jeffcoat and Harries, 1973). Wheat coleoptiles were sensitive to benzoylprop but not to benzoylprop-ethyl. The carboxylesterase from
wild oat that catalyzed the hydrolysis of benzoylprop-ethyl was a stable enzyme,
required no cofactors, and had an apparent Km for benzoylprop-ethyl of 3.8 x
10-6 M (Hill et ai., 1978). A similar carboxylesterase was not detected in wheat
(Hill et al., 1978). Other analogs of benzoylprop-ethyl are known to act similarly
(Jeffcoat and Harries, 1975).
Not all pesticide esters appear to be activated by hydrolysis as with 2,4-D
esters and benzoylprop-ethyl. Both resistant wheat and susceptible wild oat and
cultivated oat hydrolyzed rapidly chlorfenprop-methyl and diclofop-methyl to
their acids (Fig. 2.14) (Fedtke and Schmidt, 1977; Shimabukuro et aJ., 1979).
However, very little of both acids and parent esters remained in either plant
species after 24-30 hr (Fedtke and Schmidt, 1977; Shimabukuro et aJ., 1979).
Chlorfenprop-methyl (Andreev and Amrhein, 1976; Collet and Pont, 1978) and
diclofop-methyl (Shimabukuro et aJ., 1978b) were stronger auxin antagonists
(approximately two times stronger) than their free acids. In in vitro membrane
experiments in which penetration into plant tissues by esters or acids was not
a factor, bound auxin was displaced more effectively by the ester, chlorfenpropmethyl, than by the acid, chlorfenprop (Andreev and Amrhein, 1976). Intact
susceptible plants were equally sensitive to the ester and acid forms when sprayed
or injected with chlorfenprop-methyl (Fedtke and Schmidt, 1977) and diclofopmethyl (Donald and Shimabukuro, 1980), respectively. More of the esters of
both herbicides were absorbed from leaf surfaces than the acids, but the absorption differences were minor (Fedtke and Schmidt, 1977; Shimabukuro and
Walsh, unpublished data, 1978). It appears that the esters and acids of both
herbicides are biologically active.
The organophosphorous insecticides, unlike the N-methyl and N,N-dimethylcarbamates, are readily cleaved in plants to form metabolites suggestive of
hydrolytic reactions (Bull, 1972; Cas ida and Lykken, 1969). The alkyl phosphate
and acid anhydride bonds of organophosphates are known to be cleaved in
animals and insects by phosphotriesterases, NADPH-dependent mfo systems,
and soluble glutathione S-transferases (Appleton and Nakatsugawa, 1977; Bull,
1972; Eto, 1974). It appears that cleavage by oxidative and glutathione conjugation mechanisms may be more significant than by a hydrolytic mechanism.
In plants the mechanisms for cleavage have not been characterized.
Hydrolysis of BAY NTN 9306@ (Fig. 2.4) yielded the free phenols that
were conjugated subsequently with glucose. Hydrolysis of the conjugates yielded
23% phenol sulfide fp-(methylthio)phenol], 41 % phenol sulfoxide fp-(methylsulfinyl)phenol], and 36% phenol sulfone fp-(methylsulfonyl)phenol] (Bull et
al., 1976). The pyrimidinyl bond of etrimfos was hydrolyzed to the pyrimidinol,