Spectroscopy and kinetics of tyrosinase catalyzed trans-resveratrol oxidation

The spectroscopy and kinetics of the tyrosinase catalyzed trans-resveratrol oxidation were investigated by measuring both UV-vis absorption spectra over the 200-500 nm range and Raman spectra over the 600-1800 cm(-1) region. Room temperature UV-vis absorption spectra, as a function of time, showed the presence of two isosbestic points located at λ(1) = 270 nm and λ(2) = 345.5 nm delimiting two different regions: the reactant region around 300 nm, where the absorption decreased with time, and the product region over the low wavelength (λ < 260 nm) and high wavelength (λ > 390 nm) wavelength zone in which the absorption increased with time until, in both cases, constant values were achieved. A first-order kinetics was deduced with a rate coefficient of k(1) = (0.10 ± 0.001) min(-1), which turned out to be independent of substrate concentration over the 50-5 μM range; a feature that was rationalized by invoking the limiting case of the Michaelis-Menten scheme appropriate for substrate concentration much lower than the respective Michaelis constant. The observation of the distinct resonance enhanced Raman lines, specifically those peaking at 830 cm(-1), 753 cm(-1), and 642 cm(-1) together with their time evolution, permitted us to gain insight into some crucial features and steps of the catalytic reaction. Namely, that the formation of the so-called trans-resveratrol and tyrosinase (S)P complex with its O-O bridge plays a crucial role in the first steps of this enzymatic reaction and that the hydroxylation of the ortho C-H bond of the trans-resveratrol OH group occurs after O-O bond cleavage in the tyrosinase active site. The present study makes clear that a class of potential inhibitors of tyrosinase can be found in compounds able to bind the two Cu (II) ions of the enzyme bidentate form.