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 lambda maxapp (the apparent maximum inactivation constant) and r, the number of turnovers made by 1 mol of enzyme before being inactivated. The values of lambda maxapp and r were (8.2 +/- 0.1) x 10(-3) s(-1) and 35,740 +/- 2,548, respectively. (c) 2014 IUBMB Life, 66(2):122-127, 2014