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Metabolism of Herbicides or Xenobiotics in Plants

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Metabolism of Herbicides or Xenobiotics in Plants - Phase I - Hydrolysis Reactions

Hydrolysis (Figure 11) - Hydrolysis reactions are common Phase I reactions in plants and involve herbicides possessing ester, amide or nitrile groups:

Fig. 11: Hydrolysis

Ester Hydrolysis (Figure 12) - Ester hydrolysis or de-esterification is the conversion of an ester, using water, into a carboxylic acid and an alcohol.

Fig. 12: Ester hydrolysis

De-esterifications during Phase I reactions are usually catalyzed by esterases, although nonenzymatic conversion can occur at pH 6.0. The esterases involved are somewhat characterized, but relatively little is known about which ones modify xenobiotics. Many of these enzymes are non-specific and reside in cuticles and cell walls. Herbicides applied as esters are fairly lipophilic and mobile in the cuticle; however, de-esterification is required prior to entry of the herbicide (now an acid) into the phloem via ion trapping. De-esterification will also increase or maintain the concentration gradient because the ester is converted into an acid and therefore, the gradient is steeper for entry of additional ester. De-esterification can be viewed as a form of bioactivation because the herbicide will not be translocated as readily in the ester form. In some cases, the de-esterified form of the herbicide is more toxic as well (i.e. fenoxyprop is more toxic to grassy weeds than fenoxyprop-ethyl).
Amide Hydrolysis (Figure 13) - Amide hydrolysis is probably most important for the acylanilide group of herbicides. The major mechanism of selectivity for barnyardgrass control in rice using propanil, is that rice hydrolyzes propanil using the enzyme, acyl arylamidase. Barnyardgrass has about sixty-fold less acyl arylamidase in its tissue than rice does. Interestingly, some insecticides (carbamates and organic phosphates) act as synergists, by binding to the enzyme and inhibiting propanil hydrolysis resulting in more rice injury.

Fig. 13: Amide hydrolysis

Benzoxazinone-Mediated Hydrolysis of Chloro-S-triazines (Figure 14) - In roots of atrazine-tolerant corn, the natural product, DIMBOA (Figure 15) with water, performs a nucleophilic substitution at the Cl in position 2 of the triazine ring. This non-enzymatic reaction replaces the Cl with an OH at the same position resulting in hydroxyl-atrazine, a Phase I product with much less toxicity and which is now predisposed to Phase II reactions of conjugation.


Fig. 14: Benzoxazinone-mediated hydrolysis of chloro-S-triazines	Fig. 15: DIMBOA

Hydrolysis of Cyano Groups (Figure 16) - The cyano group (C=N) of herbicides such as cyanazine and bromoxynil can be metabolized via hydrolysis as well. The nitrile group is hydrolyzed to an amide and then to a carboxylic acid. The gene for the bacterial enzyme, nitrilase (Figure 17), has been moved from Klebsiella to crop plants to provide bromoxynil tolerance (e.g. cotton).