Xenobiotics that contain either a carboxylic group or an aromatic hydroxylamine group are possible substrates for amino acid conjugation. Xenobiotics with a carboxylic group conjugate with an amino group of amino acids such as glycine, taurine and glutamine. The hydroxylamine group conjugates with the carboxylic group of amino acids such as proline and serine. The amino acid is first activated by an aminoacyl-tRNA-synthetase which then reacts with the hydroxylamine group to form a reactive N-ester. N-esters can degrade to form electrophilic nitrenium (R-N+-R') and carbonium (R-C+H2) ions. The pyrolysis product of tryptophan, an N-hydroxy intermediate, can potentially form these reactive electrophilic ions.