Light-induced binding of riboflavin to lysozyme.

The photodynamic inactivation of lysozyme in air saturated H2O and D2O (phosphate buffer 0.05 M, pH 7.0) in the presence of methylene blue and riboflavin has been studied. When H2O was replaced by D2O a great increase in the rate of photoinactivation of lysozyme was observed. This finding, together with the fact that photooxidation is inhibited by singlet oxygen quenchers like NaN3, suggests that these reactions occur via a singlet oxygen mechanism. During the course of the studies of the riboflavin sensitized photoinactivation of lysozyme, it was found that riboflavin is strongly bound to the enzyme as a result of illumination. This finding would explain the higher quantum yield observed when riboflavin is used, although this dye is bleached during irradiation.