Phase II - Sugar Conjugation
Sugar Conjugation (Figure 23) - The most commonly detected herbicide-sugar conjugates are glucosides and glucose esters.
Herbicides containing phenolic, N-arylamine, or carboxylic acid groups as well as those metabolized to phenols, anilines or acids during Phase I can be made into sugar conjugates. Uridine diphosphoglucose (UDPG) is the most common sugar donor. The glucose attaches to phenol (O-glucosylation) (Figure 24a) or N-arylamine (N-glucosylation) (Figure 25a) groups via a ß-glucoside linkage. Specific examples can be seen in Figure 24b and 25b.
Fig. 23: Sugar conjugation
Fig. 24a: O-glucosylation
Fig. 24b: O-glucosylation
Fig. 25a: N-glucosylation
Fig. 25b: N-glucosylation
Glucosyltransferases, which catalyze the conjugation reaction, are very specific for UDPG although these enzymes are thought to possess broad substrate specificity for xenobiotics. Broad substrate specificity may facilitate the glucosylation of herbicides in plants by these enzymes.
Glucose esters are formed in herbicides possessing acid groups (Figure 26a). Figure 26b shows a specific example where glucose is attached to 2,4-D at its carboxylic acid group. This conjugation product is not a stable detoxification; glucose esters can be converted readily back to active herbicides by cytoplasmic esterases (Figure 27).
Fig. 26a: Glucose esters
Fig. 26b: Esterification of glucose to 2,4-D
Fig. 27: Glucose ester of 2,4-D de-esterified
Summary Note:
Glucose can be added to Phase I products to increase water solubility, reduce mobility, and reduce toxicity in a process termed ‘glucose conjugation’.