Repair of semi-oxidized 3,5-diiodotyrosine: radiation chemical studies in the presence of oxygen, ascorbate and superoxide anion

Abstract

Reactions of semi-oxidized radicals derived from 3,5-diiodotyrosine (I 2TyOH, a thyroid hormone precursor) have been studied using radiation chemical techniques. In buffered, aqueous medium at room temperature, molecular oxygen reactivity towards the phenoxyl radical (I 2TyO.) is low, the average bimolecular rate constant, k being 1.7+/- 0.22 106 dm3mol- 1s- 1. On the other hand, superoxide anion (O2-) reactivity towards I 2TyO. is close to the diffusion controlled limit, the k being 5+/- 1 109 dm3mol- 1s- 1. The major reaction channel in this case (~60%) leads to the reformation of the parent compound by one-electron transfer. Under similar experimental conditions, ascorbate (As-) completely reduces I 2TyO. to the parent compound with k = 3+/- 0.5 109 and 1 109 dm3mol- 1s- 1 at pH 7.4 and 12 respectively. The propensity of these reactions are not dependent on the primary. OH/. O- or secondary N 3. radicals used. These results suggest that the superoxide anion may actively interact at the cellular level, in the Thyroid during the course of I 2TyOH oxidation, and the observed in vitro reaction mechanism implies its participation in a new role.