CHARACTERIZATION OF DYE‐SENSITIZED PHOTOOXIDATION OF MUSHROOM TYROSINASE

Dye‐sensitized photooxidation was investigated as a nonthermal means to inactivate food quality‐related enzymes, using mushroom tyrosinase as a model. Illumination of tyrosinase in the presence of either rose bengal or riboflavin resulted in an apparent first‐order destruction of enzyme activity. Both dye and light were required, and photoinactivation was favored by increasing levels of dissolved oxygen. The rose bengal‐sensitized photooxidation was generally more rapid than that caused by riboflavin (at 0.01% dye), with k_i values ranging from 0.74 to 1.66 h⁻¹ and 0.25 to 1.23 h⁻¹, respectively. First‐order rate constants for photoinactivation decreased with decreasing temperatures (E_a was 8.4 to 12.2 kcal mol⁻¹ between 20° and 50°C), increasing protein concentration (0.175 to 1.40 mg⁻¹ ml), increasing sodium phosphate buffer concentration (10 to 200 mM) and increasing ionic strength (0.02 to 0.20). The dependence of enzyme photoinactivation rates on pH (between 6.0 and 9.0) resembled a titration curve with a pK of 7.5, and maximal rates were observed at pH 8.0–9.0.