Oxidation by mushroom tyrosinase of monophenols generating slightly unstable o-quinones

Tyrosinase can act on monophenols because of the mixture of mettyrosinase ( E-m) and oxytyrosinase (E-ox) that exists in the native form of the enzyme. The latter form is active on monophenols although the former is not. Howev er, the kinetics are complicated because monophenols can bind to both enzym e forms. This situation becomes even more complex as the products of the en zymatic reaction, the o-quinones, are unstable and continue evolving to gen erate o-diphenols in the medium. In the case of substrates such as 4-methox yphenol, 4-ethoxyphenol and 4-tert-butylphenol, tyrosinase generates o-quin ones which become unstable with small constants of approximately < 10(-3) s (-1). The system evolves from an initial steady state, reached when t --> 0 , through a transition state towards a final steady state, which is never r eached because the substrate is largely consumed. The mechanisms proposed t o explain the enzyme's action can be differentiated by the kinetics of the first steady state. The results suggest that tyrosinase hydroxylates monoph enols to o-diphenols, generating an intermediate E-m-diphenol in the proces s, which may oxidize the o-diphenol or release it directly into the medium. In the case of o-quinone formation, its slow instability generates o-diphe nol which activates the enzymatic system yielding parabolic time recordings .