THE PROTECTIVE ROLE OF SOME ANTIOXIDANTS AND SCAVENGERS ON THE FREE RADICALS-INDUCED INHIBITION OF THE LIVER IODOTHYRONINE 5'-MONODEIODINASE ACTIVITY AND THIOLS CONTENT

The influence of free radicals on iodothyronine 5'-monodeiodinase acti vity, the enzyme responsible for the deiodination of thyroxine to most active thyroid hormone 3,3',5-triiodothyronine (T-3), was examined in rabbit's liver. Incubation of the liver homogenate with the xanthine oxidase based free radical generating system (FRGS) caused a reduction in 5'-monodeiodinase activity to the 53.9% of initial value taken as 100%, and on increase (52.9% over the control value) in the level of l ipid peroxidation by-product malondialdehyde. The inhibitory effect of FRGS on 5'-monodeiodinase activity was blocked by free radical scaven gers: catalase (91.2%), thiourea (88.8%), superoxide dismutase (85%) a nd by some antioxidants; Trolox (the water soluble a-tocopherol analog , 81.4%) and glutathione (77.7%). These results suggest that oxygen ra dicals, hydrogen peroxide and hydroxyl radicals were involved in the i nhibition of the 5'-monodeiodinase activity. The same scavengers signi ficantly decreased the malondialdehyde formation. In the presence of t he FRGS the amount of total SH groups (the cofactor of the deiodinatio n reaction) was decreased in the liver homogenate to 51% of the initia l value, and a positive relationship between the total SH groups level s and the 5'-monodeiodinase activity in the presence of free radical s cavengers was observed. It suggests, that ac:ive oxygen radicals gener ated by FRGS may inactivate 5'-monodeiodinase, at least in part, by re duction of thiol cofactors.