Solvent deuterium isotope effect on the oxidation of o-diphenols by tyrosinase.

A solvent deuterium isotope effect on the catalytic affinity (K(m)) and rate constant (k(cat)) of tyrosinase in its action on 4-tert-butylcatechol (TBC) was observed. Both parameters decreased as the molar fraction of deuterated water in the medium increased, while the k(cat)/K(m) ratio remained constant. In a proton inventory study, the representation of k(cat)(f(n))/k(cat)(f(0)) and K(m)(f(n))/K(m)(f(0)) vs. n (atom fractions of deuterium) was linear, indicating that, of the four protons transferred from the two molecules of substrate and which are oxidized in one turnover, only one is responsible for the isotope effects. The fractionation factor of 0.64+/-0.02 contributed to identifying the possible proton acceptor. Possible mechanistic implications are discussed.