Lysozyme Modification by the Fenton Reaction and Gamma Radiation

A comparative study was performed on lysozyme modification after exposure to Fenton reagent (Fe(II)/H 2 O 2 ) or hydroxyl radicals produced by n radiation. The conditions were adjusted to obtain, with both systems, a 50% loss of activity of the modified ensemble. n radiation modified almost all types of amino acid residues in the enzyme, with little specificity. The modification order was Tyr > Met=Cys > Lys > Ile+Leu > Gly > Pro=Phe>Thr+Ala>Trp=Ser>Arg>Asp+Glu, with 42 mol of modified residues per initial mole of native enzyme. In contrast, when the enzyme was exposed to the Fenton reaction, only some types of amino acids were modified. Furthermore, a smaller number of residues (13.5) were damaged per initial mole of enzyme. The order of the modified residues was Tyr > Cys > Trp > Met >His > Ile+Leu > Val > Arg. These results demonstrate that the modifications elicited by these two free radical sources follow different mechanisms. An intramolecular free radical chain reaction is proposed to play a dominant role in the oxidative modification of the protein promoted by n radiation.