Hydrogen peroxide-dependent 4-t-butylphenol hydroxylation by tyrosinase--a new catalytic activity

The aim of this work was to study the hydroxylation by tyrosinase of 4-t-butylphenol to 4-t-butylcatechol, in the presence of hydrogen peroxide. This hydroxylation reaction does not take place without the addition of hydrogen peroxide. Some properties of this new hydroxylating activity have been analysed. The kinetic parameters of mushroom tyrosinase for hydrogen peroxide (K(m) = 4.9 mM, V(m) = 48.1 microM/min) and 4-t-butylphenol (K(m) = 16 microM/min, V(m) = 6.7 microM/min) were evaluated. A lag period appeared, which was similar to the characteristic lag of monophenolase activity at the expense of molecular oxygen. The length of the lag phase decreased with increasing hydrogen peroxide concentrations but was longer with higher 4-t-butylphenol concentrations. The pH optimum for this hydroxylating activity was close to 5.5. The lag also varied with pH, reaching its highest value at pH 4.8. The lag was shortened by the addition of increasing amounts of 4-t-butylcatechol, and was abolished at 24.5 microM of 4-t-butylcatechol. 4-t-Butylphenol was oxidized by mushroom tyrosinase in the presence of 24.5 microM 4-t-butylcatechol and in the absence of hydrogen peroxide although the enzymatic activity tailed off. The presence of hydrogen peroxide is necessary to maintain a constant steady-state rate of 4-t-butylphenol oxidation by tyrosinase.