Modification of the Radiolytic Oxidation of Ribonuclease Induced by Bound Copper

The specific effect of cupric ions on the radiolytic oxidative inactivation of enolase and ribonuclease has been investigated. It has been shown that cupric ions bound to the enzymes enhance the radiolytic inactivation induced by OH radicals. The destruction of the amino acid residues was followed in parallel to enzymatic inactivation in the absence and in the presence of Cu₂₊. It is shown that complexing of Cu₂₊by RNAase does not alter the total yield of radiolytic destruction of amino acid residues, but induces a significant change in the radiolytic yield of individual amino acid residues sensitive to oxidation. A significant increase in the yield of radiolyzed histidyl residues on RNAase in the presence of Cu(II) was observed, accompanied by a corresponding decrease in damaged S-S bonds. A quantitative evaluation of the effect of OH radicals on RNAase in the presence and absence of copper suggests an intramolecular charge transfer in the protein molecule following its reaction with OH radicals. The radiobiological implications of the copper-induced “sensitization” of vital sites in a bipolymer are discussed.