Thiol and Mn²⁺-mediated oxidation of veratryl alcohol by horseradish peroxidase

Horseradish peroxidase has been shown to catalyze the oxidation of veratryl alcohol (3,4-dimethoxybenzyl alcohol) and benzyl alcohol to the respective aldehydes in the presence of reduced glutathione, MnCl₂, and an organic acid metal chelator such as lactate. The oxidation is most likely the result of hydrogen abstraction from the benzylic carbon of the substrate alcohol leading to eventual disproportionation to the aldehyde product. An aromatic cation radical intermediate, as would be formed during the oxidation of veratryl alcohol in the lignin peroxidase-H₂O₂ system, is not formed during the horseradish peroxidase-catalyzed reaction. In addition to glutathione, dithiothreitol, L-cysteine, and β-mercaptoethanol are capable of promoting veratryl alcohol oxidation. Non-thiol reductants, such as ascorbate or dihydroxyfumarate (known substrates of horseradish peroxidase), do not support oxidation of veratryl alcohol. Spectral evidence indicates that horseradish peroxidase compound II is formed during the oxidation reaction. Furthermore, electron spin resonance studies indicate that glutathione is oxidized to the thiyl radical. However, in the absence of Mn²⁺, the thiyl radical is unable to promote the oxidation of veratryl alcohol. In addition, Mn³⁺ is unable to promote the oxidation of veratryl alcohol in the absence of glutathione. These results suggest that the ultimate oxidant of veratryl alcohol is a Mn³⁺-GSH or Mn²⁺-GS· complex (where GS· is the glutathiyl radical).