58
Richsrd H. Shimsbukuro et s/.
Table 2.2. Generalized Summary of Pesticide Behavior and Fate in Plants
Characteristics
Reactions
Solubility
Transport
Phytotoxicity
Bioavailability"
Phase I
Phase II
Phase ill
I
Oxidation;
:
Secondary conjugation
Pesti 'cl'de _+-_~ d t' -'--C'
. ---4-1.- or incorporation
• re uc IOn; --r onJugation
.. into biopolymers
hydrolysis
Lipophilic
Amphophilic
Selective
Modified or
mobility
reduced
mobility
Toxic
Modified or less
toxic
+++
+++
Hydrophilic
(insoluble residues)
Hydrophilic and
insoluble
Limited or immobile I Immobile
I
I
I
Greatly reduced or : Nontoxic
nontoxic
:
+ +
I + or Unavailable
I
• + + +, Readily absorbed in GI tract of animals; + +, less absorption in OI tract of animals; +, limited
absorption.
biological organisms, including plants, animals, and microorganisms, appear to
metabolize foreign compounds or xenobiotics in the same general way. The
common objective in all biological organisms appears to be the conversion of
xenobiotics into some innocuous forms that may be excreted or stored as inactive
materials in the organism. Pesticides and other xenobiotics appear to be metabolized in two phases in animals (Table 2.2) (Williams, 1959). Phase I metabolism
involves oxidation, reduction or hydrolysis reactions followed by phase II, in
which products from phase I are conjugated and detoxified before excretion. In
plants, excretion of the conjugated pesticide metabolites is not significant. Therefore, these metabolites must either be compartmentalized within plant cells or
removed from further metabolic activity by other mechanisms. This may be
accomplished by a phase III-type metabolism that appears to be peculiar to plants
(Table 2.2).
Phase I metabolism is probably the most important phase of pesticide metabolism in plants. Phase I reactions often decrease or modify the biological
activity of pesticides and predispose the compounds to phase II conjugation
reactions which yield nontoxic, water-soluble metabolites that are relatively
immobile in the plant. Secondary conjugations or incorporation into insoluble
pesticide residues with limited bioavailability are common in phase III reactions.
REFERENCES
Akram, M., Ahmad, S., and Forgash, A. J., 1978, Metabolism of phosphorothioic acid, 0,0dimethyl-0-(6-ethoxy-2-ethyl-4-pyrimidinyl) ester (etrimfos), in bean and corn plants, J. Agric.
Food Chern. 26:925.
Richsrd H. Shimsbukuro et s/.
Table 2.2. Generalized Summary of Pesticide Behavior and Fate in Plants
Characteristics
Reactions
Solubility
Transport
Phytotoxicity
Bioavailability"
Phase I
Phase II
Phase ill
I
Oxidation;
:
Secondary conjugation
Pesti 'cl'de _+-_~ d t' -'--C'
. ---4-1.- or incorporation
• re uc IOn; --r onJugation
.. into biopolymers
hydrolysis
Lipophilic
Amphophilic
Selective
Modified or
mobility
reduced
mobility
Toxic
Modified or less
toxic
+++
+++
Hydrophilic
(insoluble residues)
Hydrophilic and
insoluble
Limited or immobile I Immobile
I
I
I
Greatly reduced or : Nontoxic
nontoxic
:
+ +
I + or Unavailable
I
• + + +, Readily absorbed in GI tract of animals; + +, less absorption in OI tract of animals; +, limited
absorption.
biological organisms, including plants, animals, and microorganisms, appear to
metabolize foreign compounds or xenobiotics in the same general way. The
common objective in all biological organisms appears to be the conversion of
xenobiotics into some innocuous forms that may be excreted or stored as inactive
materials in the organism. Pesticides and other xenobiotics appear to be metabolized in two phases in animals (Table 2.2) (Williams, 1959). Phase I metabolism
involves oxidation, reduction or hydrolysis reactions followed by phase II, in
which products from phase I are conjugated and detoxified before excretion. In
plants, excretion of the conjugated pesticide metabolites is not significant. Therefore, these metabolites must either be compartmentalized within plant cells or
removed from further metabolic activity by other mechanisms. This may be
accomplished by a phase III-type metabolism that appears to be peculiar to plants
(Table 2.2).
Phase I metabolism is probably the most important phase of pesticide metabolism in plants. Phase I reactions often decrease or modify the biological
activity of pesticides and predispose the compounds to phase II conjugation
reactions which yield nontoxic, water-soluble metabolites that are relatively
immobile in the plant. Secondary conjugations or incorporation into insoluble
pesticide residues with limited bioavailability are common in phase III reactions.
REFERENCES
Akram, M., Ahmad, S., and Forgash, A. J., 1978, Metabolism of phosphorothioic acid, 0,0dimethyl-0-(6-ethoxy-2-ethyl-4-pyrimidinyl) ester (etrimfos), in bean and corn plants, J. Agric.
Food Chern. 26:925.