Phase II - Glutathione Conjugation
Glutathione Conjugation (Figure 18) - Glutathione conjugation involves the attachment of the small tripeptide, glutathione (γ-glutamylcysteinyl-ß-glycine) (Figure 19), to herbicides containing halogen, phenolate or alkyl sulfoxide groups (Figure 20). On a herbicide molecule, these groups can serve as leaving groups when reacted with the nucleophilic glutathione anion (GS-). In the reaction (equation 2), X is the herbicide and Z is the leaving group. These reactions are catalyzed by glutathione-S-transferases (GST). Conjugation proceeds as a nucleophilic displacement reaction with the glutathione anion (GS-) serving as the nucleophile. The sulfhydryl group (-SH) on the cysteine residue of the glutathione molecule is the reactive component required for this reaction. A simplified form of the reaction follows:
Equation 2: X-Z + GS- → GS-X + Z-
Fig. 18: Glutathione conjugation
Fig. 19: Glutathione (γ-glutamylcysteinyl-ß-glycine)
Fig. 20: Glutathione conjugation with herbicides (X) containing halogen, phenolate or alkyl sulfoxide groups (Z)
Pumps that recognize the glutathione conjugated with a xenobiotic (Figure 21) (GS-X) have been described in animal plasmamembranes and plant tonoplast membranes. These glutathione conjugates are thought to be transported into the plant vacuole (Figure 22). This sequestration is important to remove the glutathione conjugate from the cytoplasm for several reasons including the instability of glutathione conjugates. In some cases, vacuolar peptidases may cleave the glutamate and glycine of the original glutathione molecule, leaving the cysteinyl conjugate (cysteine-X) which may need further processing in Phase III reactions, such as incorporation into cell wall material.
Fig. 21: Glutathione conjugated with a xenobiotic
Fig. 22: Transported into the plant vacuole
Some safeners or antidotes protect corn, sorghum, and rice against soil-applied thiocarbamate and chloracetanilide herbicides by either increasing the levels of glutathione or by inducing activity of glutathione-dependent enzymes such as glutathione-S-transferase. This mechanism provides more crop protection while weeds remain susceptible.
Summary Note:
Crops can be protected from certain herbicides when safeners are applied to the crop because the safeners cause the crop to make more glutathione. Glutathione is attached to the herbicide in Phase II reactions to reduce herbicide toxicity.