In vitro free radical scavenging capacity of thyroid hormones and structural analogues

It was reported that thyroid hormones decreased Cu2+ induced low-density li poprotein (LDL) oxidation in vitro. Here, we investigated free radical scav enging capacities of thyroid hormones (3,5,3'-tri-iodo-L-thyronine (T,), th ryoxine (T-4) and 3,3',5'-tri-iodo-L-thyronine (rT(3))) and structural anal ogues (L-thryonine (To), 3,5,3'-tri-iodothyroacetic acid (TA,) and 3,5,3',5 '-tetraiodothyroacetic acid (TA,)), using three different models of free ra dical generation. To, T, and TA, slowed down production of conjugated diene and thiobarbituric acid-reactive substances during LDL oxidation by 2,2'-a zobis[2-amidinopropane] (water-soluble), whereas rT(3), T4 and TA, had prac tically no effect. In this system, To was the more active compound. Using a 1,1-diphenyl-2-picrylhydrazyl (lipid-soluble) test, all compounds also rev ealed free radical scavenging capacities, but rT(3), T-4 and TA(4) were mor e active than T-0, T-3 and TA(3). T-3 was able to scavenge superoxide anion and hydroxyl radicals generated in an aqueous phase by a xanthine-xanthine oxidase system, as measured by electron paramagnetic resonance spectroscop y. It may be concluded that: (1) thyroid hormones and analogues with a 4'-h ydroxy diphenylether structure have free radical scavenging capacities, (2) this property is influenced by the number of iodines on the phenolic ring, and (3) thyroid hormone scavenging capacity should not be the only mechani sm explaining their protective effect on Cu2+-induced LDL oxidation. The: p hysiological significance of the findings is discussed.