On the free radical-induced aggregation of ribonuclease - a pulse radiolysis study using the light scattering...

The aggregation of bovine ribonuclease (RNase) induced by either .OH or Br(2) radicals has been studied. These radicals, which were generated by pulse radiolysis of aqueous solution with 16 MeV electrons, readily attack the enzyme thus producing RNase-radicals which subsequently combine. At initial radical concentrations low enough (less than 0.1 radical per enzyme molecule) to prevent multimerizations other than dimerization, rate constants for the latter process were determined from the increase of the light scattering intensity after the pulse: 2k2 = (22 +/- 0.3) 106 l/mol s (Br(2) initiated dimerization) and (5.4 +/- 0.4) 106 l/mol s (.OH initiated dimerization). The G-values for dimerization are: 1.3 (Br(2) and 0.85 (.OH). Transient optical absorption measurements revealed the existence of phenoxyl radicals of tyrosine (TyrO.) that decayed with a rate constant of 2 K2 (total) = (5.5 +/- 0.7) 106 l/mol s (Br(2)-case). The difference between k2 and k2 (total) presumably indicates the occurrence of disproportionation.