The Sensitization of the Radiolytic Inactivation of Ribonuclease by Cupric Ions

SummaryThe effect of complexed cupric ions on the radiation induced inactivation of ribonuclease in nitrous oxide saturated solutions has been studied. The effect of pH on the [Cu2+]/[RNase] ratio required for the maximum degree of sensitisation shows that more sites for binding become involved at higher pH. The variation of the inactivation efficiency with pH is due to histidine residues being involved in cupric binding to ribonuclease. At pH greater than 10, sensitisation occurs through hydroxyl radical reaction with tyrosine residues, as is shown by the effect of the selective (CNS)2− radical. Enzymatic inactivation involves formation of the CuIII state which oxidises an essential histidine residue. Oxygen and hydrated electrons exert a protective effect on the cupric ion sensitised inactivation.