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 x 10(11) mol l(-1) s(-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 x 10(9) mol lL(-1) s(-1) per thiolate, while the constant of the decay of this radical anion was 2.72 x 10(5) s(-1), and the equilibrium constant of the formation for RSSR*- was 6.08 x 10(3) 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 x 10(-6) M for Cu4Zn3MT-III and 1.62 x 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.