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

Reactions of semi-oxidized radicals derived from 3,5-diiodotyrosine (I2TyOH, 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 (I2TyO.) is low, the average bimolecular rate constant, k being 1.7 +/- 0.22 x 10(6) dm3 mol-1s-1. On the other hand, superoxide anion (O2-) reactivity towards I2TyO. is close to the diffusion controlled limit, the k being 5 +/- 1 x 10(9) dm3 mol-1s-1. The major reaction channel in this case (approximately 60%) leads to the reformation of the parent compound by one-electron transfer. Under similar experimental conditions, ascorbate (As-) completely reduces I2TyO. to the parent compound with k = 3 +/- 0.5 x 10(9) and < or = 1 x 10(9) dm3 mol-1s-1 at pH 7.4 and 12 respectively. The propensity of these reactions are not dependent on the primary .OH/.O- or secondary N3. radicals used. These results suggest that the superoxide anion may actively interact at the cellular level, in the Thyroid during the course of I2TyOH oxidation, and the observed in vitro reaction mechanism implies its participation in a new role.