Catalysis and inactivation of tyrosinase in its action on hydroxyhydroquinone

Hydroxyhydroquinone (HHQ) was characterized kinetically as a tyrosinase substrate. A kinetic mechanism is proposed, in which HHQ is considered as a monophenol or as an o‐diphenol, depending on the part of the molecule that interacts with the enzyme. The kinetic parameters obtained from an analysis of the measurements of the initial steady state rate of 2‐hydroxy p‐benzoquinone formation were kcatapp= 229.0 ± 7.7 s−1 and KMapp,HHQ= 0.40 ± 0.05 mM. Furthermore, the action of tyrosinase on HHQ led to the enzyme's inactivation through a suicide inactivation mechanism. This suicide inactivation process was characterized kinetically by λmaxapp (the apparent maximum inactivation constant) and r, the number of turnovers made by 1 mol of enzyme before being inactivated. The values of λmaxapp and r were (8.2 ± 0.1) × 10−3 s−1 and 35,740 ± 2,548, respectively. © 2014 IUBMB Life, 66(2):122–127, 2014