Decomposition of 2,4-dihalophenols by dehaloperoxidase activity and spontaneous reaction with hydrogen peroxide

The enzyme dehaloperoxidase (DHP) found in the marine worm Amphitrite ornata is capable of enzymatic peroxidation of 2,4-dichlorophenol (DCP) and 2,4-dibromophenol (DBP). There is also at least one parallel oxidative pathway and the major products 2-chloro-1,4-benzoquinone (2-ClQ) and 2-bromo-1,4-benzoquinone (2-BrQ) undergo aspontaneous secondary hydroxylation reaction. The oxidation and hydroxylation reactions have been monitored by UV-visible spectroscopy, High Performance Liquid Chromatography (HPLC), and mass spectrometry. Evidence from time-resolved UV-visible spectroscopy suggests that the hydroxylations of 2-ClQ and 2-BrQ in the presence of hydrogen peroxide (H₂O₂) are non-enzymatic spontaneous processes approximately ∼10 and ∼ 5 times slower, respectively, than the enzymatic oxidation of DCP or DBP by DHP in identical solvent conditions. The products 2-ClQ and 2-BrQ have λ_maxat 255 nm and 260 nm, respectively. Both substrates, DCP and DBP, react to form a parallel product peaked at 240 nm on the same time scale as the formation of 2-ClQ and 2-BrQ. The 240 nm band is not associated with the hydroxylation process, nor is it attributable to the catechol 3,5-dihalobenzene-1,3-diol observed by mass spectrometry. One possible explanation is that muconic acid is formed as a decomposition product, which could follow decomposition either the catechol or hydroxyquinone. These reactions give a more complete understanding of the biodegradation of xenobiotics by the multi-functional hemoglobin, DHP, in Amphitrite ornata. SYNOPSIS: The decomposition of 2,4-dihalophenols catalyzed by dehaloperoxidase was studied by UV-visible spectroscopy, High Performance Liquid Chromatography and Liquid Chromatography-Mass Spectrometry. Spectroscopic evidence suggests two major products, which we propose are 2-halo-1,4-benzoquinone and 2-halomuconic acid. These complementary techniques give a high-level view of the degradation of xenobiotics in marine ecosystems.