Effect of UV light on biological activity of tyrosinase in buffer solution

It has been reported previously that the UV irradiation of mushroom tyrosinase resulted in a logarithmic loss of biological activity due to the conformational changes in the enzyme molecule. The purpose of the present study was to investigate the factors influencing the radiosensitivity of enzymatic activity. A similar rate of the catalytic activity loss at different pH supported the attribution to the conformational change of the enzyme molecule but not to the active site damage. The presence of potassium chloride and the absence of oxygen resulted in only a little protection against enzyme photoinactivation. Activity survival curve as a function of radiation dose at a higher enzyme concentration showed to have a shoulder which indicate the mutual protection of the enzyme molecules. Copper (II) protected the loss in catalytic activity of enzyme on irradiation. This was explained in terms of scavenging of the hydrated electron by copper (II). Therefore, it was concluded that photoinactivation of this enzyme was mainly due to conformational changes caused by the damage of constitutional aromatic amino acid residues but also partially due to inactivation of copper of enzyme with hydrated electron. (author)