Tyrosinase-catalyzed hydroxylation of hydroquinone, a depigmenting agent, to hydroxyhydroquinone: A kinetic study

Hydroquinone (HQ) is used as a depigmenting agent. In this work we demonstrate that tyrosinase hydroxylates HQ to 2-hydroxyhydroquinone (HHQ). Oxy-tyrosinase hydroxylates HQ to HHQ forming the complex met-tyrosinase-HHQ, which can evolve in two different ways, forming deoxy-tyrosinase and p-hydroxy-o-quinone, which rapidly isomerizes to 2-hydroxy-p-benzoquinone or on the other way generating met-tyrosinase and HHQ. In the latter case, HHQ is rapidly oxidized by oxygen to generate 2-hydroxy-p-benzoquinone, and therefore, it cannot close the enzyme catalytic cycle for the lack of reductant (HHQ). However, in the presence of hydrogen peroxide, met-tyrosinase (inactive on hydroquinone) is transformed into oxy-tyrosinase, which is active on HQ. Similarly, in the presence of ascorbic acid, HQ is transformed into 2-hydroxy-p-benzoquinone by the action of tyrosinase; however, in this case, ascorbic acid reduces met-tyrosinase to deoxy-tyrosinase, which after binding to oxygen, originates oxy-tyrosinase. This enzymatic form is now capable of reacting with HQ to generate p-hydroxy-o-quinone, which rapidly isomerizes to 2-hydroxy-p-benzoquinone. The formation of HHQ during the action of tyrosinase on HQ is demonstrated by means of high performance liquid chromatography mass spectrometry (HPLC-MS) by using hydrogen peroxide and high ascorbic acid concentrations. We propose a kinetic mechanism for the tyrosinase oxidation of HQ which allows us the kinetic characterization of the process. A possible explanation of the cytotoxic effect of HQ is discussed.