Interactions of growth inhibitory factor with hydroxyl and superoxide radicals.

To show the effects of growth inhibitory factor (Cu₄Zn₃MT-III) involved in the scavenging of reactive oxygen species (ROS), a pulse radiolytic study was employed using N₂O-saturated Cu₄Zn₃MT-III aqueous solutions. It was demonstrated that the oxidizing OH^· radical efficiently reacted with Cu₄Zn₃MT-III by forming a thiyl radical RS^· with a second-order constant of 1.46×10¹¹ mol l⁻¹s⁻¹, which was determined by competition kinetics against KSCN. The thiyl radical RS^· reacted rapidly and reversibly with a thiolate in Cu₄Zn₃MT-III to form radical anion RSSR^{· −} with a constant of 1.65×10⁹ mol lL⁻¹s⁻¹ per thiolate, while the constant of the decay of this radical anion was 2.72×10⁵ s⁻¹, and the equilibrium constant of the formation for RSSR^{· −} was 6.08×10³ mol⁻¹ l. These values were close to those of Cd₅Zn₂MT-II. The SOD activity of Cu₄Zn₃MT-III to quench O₂^{· −} was assayed by the riboflavine-methionine-nitrobluetetrazolium (NBT) method which catalyzed the dismutation of superoxide (O₂^{· −}) at pH 7.8 with an IC₅₀ value of 1.50×10⁻⁶ M for Cu₄Zn₃MT-III and 1.62×10⁻⁶ M for Cd₅Zn₂MT-II. Additionally, the down-regulation of GIF may be a main factor in the decrease of the scavenging ability for the free OH^· and O₂^{· −} radicals, which is possibly associated with the pathogenesis of neurodegenerative disease. [ABSTRACT FROM AUTHOR]