Phase II- Amino Acid Conjugation

Amino Acid Conjugation (Figure 28) - Conjugation with amino acids is particularly common with the phenoxyacetic acid herbicides like 2,4-D. A peptide bond is formed between the herbicide’s carboxylic acid residue and the amino acid’s amino group. Some of the first evidence of amino acid conjugation with a herbicide was 2,4-D conjugation to aspartic acid (Figure 29). Other amino acids identified include glutamate, valine, leucine, phenylalanine and tryptophan. The most common amino acids conjugating with xenobiotics in plants are aspartate or glutamate. These conjugates can still be biologically active, but are relatively immobile. There is evidence that amino acid conjugates are excreted to the  cell wall.

Fig. 28: Amino acid conjugation

Fig. 29: 2,4-D conjugation to aspartic acid