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 ki 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 (Ea 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.

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