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 (I (2)TyOH, a thyroid hormone precursor) have been studied using radiatio n chemical techniques. In buffered, aqueous medium at room temperature , molecular oxygen reactivity towards the phenoxyl radical (I(2)TyO .) is low, the average bimolecular rate constant, k being 1.7 +/- 0.22 x 10(6) dm(3) mol(-1)s(-1). On the other hand, superoxide anion (O-2(.- )) reactivity towards I(2)TyO . is close to the diffusion controlled l imit, the k being 5 +/- 1x 10(9) dm(3)mol(-1)s(-1). The major reaction channel in this case (similar to 60%) leads to the reformation of the parent compound by one-electron transfer. Under similar experimental conditions, ascorbate (As-) completely reduces I(2)TyO . to the parent compound with k = 3 +/- 0.5 x 10(9) and less than or equal to 1x 10(9 ) dm(3)mol(-1)s(-1) at pH 7.4 and 12 respectively. The propensity of t hese 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(2)TyOH oxidation, and the observed in vitro reaction mech anism implies its participation in a new role.