Radiation-induced inactivation of flavocytochrome b2 in dilute aqueous solution.

Effect of gamma radiation on flavocytochrome b2 in dilute aqueous solution was studied. A study of the effect of the radiolytically produced inorganic free-radical anions such as I2.-, Br2.- and (SCN)2.- on the enzyme activity indicates the involvement of cysteine and tyrosine residues in the catalytic activity of flavocytochrome b2. The changes in kinetic parameters, i.e., Michaelis-Menten constant Km and maximal velocity Vmax, due to irradiation under different conditions suggest that radiation induced enzyme inactivation is the result of destruction of active-site residues as well as modification of the substrate binding site. Fluorescence studies of unirradiated and irradiated enzyme reveal that FMN (flavin mononucleotide) is inaccessible to water radicals.