24
Richard H. Shimabukuro et at.
very limited. Only a few of the plant enzymes involved in pesticide metabolism
have been investigated in any detail (Lamoureux and Frear, 1979).
2.3. 1. Oxidation
Oxidative reactions of pesticides in plants are frequently primary reactions
that result in either detoxication or activation. Comparative biochemistry indicates that several reactions known to occur in both plants and animals are possibly
mixed function oxidase-catalyzed reactions (mfo). These reactions include Ndealkylation, O-dealkylation, aromatic hydroxylation, alkyl oxidation, epoxidation, desulfuration, sulfur oxidation, ester hydrolysis, and nitrogen oxidation
(Lamoureux and Frear, 1979). However, direct evidence for the involvement of
cytochrome P-450 exists for only a few of the reactions demonstrated in plants
(Sandermann et al., 1977). In plants some of the oxidation reactions presumed
to be due to mixed function oxidases may be catalyzed by peroxidases or other
oxygenases (Lamoureux and Frear, 1979).
Aldrin epoxidation to dieldrin was reported to occur in excised plant tissues
(Table 2.1) (Fig. 2.1) (Oloff and Lichtenstein, 1969), but it is still not certain
whether aldrin epoxidase is an mfo system (Earl and Kennedy, 1975; Yu et al.,
1971). Cytochrome P-450 was not detected in pea aldrin epoxidase, and its
activity was stimulated by deoxycholate, Triton X-100, and other solubilizing
and chelating agents that inhibit cytochrome P-450 systems (Earl and Kennedy,
1975).
In plants, dieldrin does not appear to be the precursor to aldrin trans-diol
as it is in mammals (Brooks, 1974) and soil microorganisms (Matsumura et al.,
1968; Matsumura and Boush, 1967). Dieldrin was not metabolized to aldrin
trans-diol in cell-free preparations of pea and bean roots (McKinney and Mehendale, 1973; Yu et al., 1971). Aldrin trans-diol was formed in bean and potato
cell suspension cultures only when aldrin but not dieldrin was a substrate (Fig.
2.1) (Brain and Lines, 1977). Cell suspension cultures of bean shoots but not
bean roots converted small amounts of dieldrin to photodieldrin by a nonphotochemical pathway (Brain and Lines, 1977) similar to that in microorganisms
(Brooks, 1974). Plant enzymes that oxidize cyclodiene insecticides appear to
exhibit greater substrate specificity but are less widely distributed than corresponding animal enzymes. Aldrin, isodrin, and heptachlor were not metabolized
in com root homogenates, whereas aldrin and isodrin but not heptachlor were
epoxidized in bean and pea root homogenates (Yu et al., 1971).
Stable epoxide metabolites of herbicides have not been isolated in plants.
However, the hydroxylation of 2,4-D in bean plants to 2,5-dichloro-4-hydroxyphenoxyacetic acid (Hamilton et al., 1971; Montgomery et al., 1971) implicated
the formation of an epoxide intermediate (Fig. 2.2). Chlorine migration or the
NIH shift (Guroff et al., 1967) occurred in 2,4-D metabolism. The NIH shift
Richard H. Shimabukuro et at.
very limited. Only a few of the plant enzymes involved in pesticide metabolism
have been investigated in any detail (Lamoureux and Frear, 1979).
2.3. 1. Oxidation
Oxidative reactions of pesticides in plants are frequently primary reactions
that result in either detoxication or activation. Comparative biochemistry indicates that several reactions known to occur in both plants and animals are possibly
mixed function oxidase-catalyzed reactions (mfo). These reactions include Ndealkylation, O-dealkylation, aromatic hydroxylation, alkyl oxidation, epoxidation, desulfuration, sulfur oxidation, ester hydrolysis, and nitrogen oxidation
(Lamoureux and Frear, 1979). However, direct evidence for the involvement of
cytochrome P-450 exists for only a few of the reactions demonstrated in plants
(Sandermann et al., 1977). In plants some of the oxidation reactions presumed
to be due to mixed function oxidases may be catalyzed by peroxidases or other
oxygenases (Lamoureux and Frear, 1979).
Aldrin epoxidation to dieldrin was reported to occur in excised plant tissues
(Table 2.1) (Fig. 2.1) (Oloff and Lichtenstein, 1969), but it is still not certain
whether aldrin epoxidase is an mfo system (Earl and Kennedy, 1975; Yu et al.,
1971). Cytochrome P-450 was not detected in pea aldrin epoxidase, and its
activity was stimulated by deoxycholate, Triton X-100, and other solubilizing
and chelating agents that inhibit cytochrome P-450 systems (Earl and Kennedy,
1975).
In plants, dieldrin does not appear to be the precursor to aldrin trans-diol
as it is in mammals (Brooks, 1974) and soil microorganisms (Matsumura et al.,
1968; Matsumura and Boush, 1967). Dieldrin was not metabolized to aldrin
trans-diol in cell-free preparations of pea and bean roots (McKinney and Mehendale, 1973; Yu et al., 1971). Aldrin trans-diol was formed in bean and potato
cell suspension cultures only when aldrin but not dieldrin was a substrate (Fig.
2.1) (Brain and Lines, 1977). Cell suspension cultures of bean shoots but not
bean roots converted small amounts of dieldrin to photodieldrin by a nonphotochemical pathway (Brain and Lines, 1977) similar to that in microorganisms
(Brooks, 1974). Plant enzymes that oxidize cyclodiene insecticides appear to
exhibit greater substrate specificity but are less widely distributed than corresponding animal enzymes. Aldrin, isodrin, and heptachlor were not metabolized
in com root homogenates, whereas aldrin and isodrin but not heptachlor were
epoxidized in bean and pea root homogenates (Yu et al., 1971).
Stable epoxide metabolites of herbicides have not been isolated in plants.
However, the hydroxylation of 2,4-D in bean plants to 2,5-dichloro-4-hydroxyphenoxyacetic acid (Hamilton et al., 1971; Montgomery et al., 1971) implicated
the formation of an epoxide intermediate (Fig. 2.2). Chlorine migration or the
NIH shift (Guroff et al., 1967) occurred in 2,4-D metabolism. The NIH shift