Cadmium induced inhibition of type-I iodothyronine 5 '-monodeiodinase in young cockerel: The possible involvement of free radicals

The possible involvement of free radicals in the cadmium-induced thyroid dy sfunction with special reference to hepatic type-I iodothyronine 5'-monodei odinase (5'-D) enzyme activity has been studied in cockerels. Cadmium chlor ide (CdCl2) administration (i.m. 0.5, 1.0 and 2.0 mg/kg body weight for 30 days) enhanced hepatic lipid peroxidation (LPO) but, decreased 5'-D activit y and serum triiodothyronine (T-3) concentration. Serum thyroxine (T-4) was decreased only by the highest dose indicating that cadmium (Cd) is thyroid inhibitory only at higher doses. Liver appears to be highly sensitive to t he metal as disruption of membrane configuration evidenced by increased hep atic LPO and inhibition of 5'-D enzyme activity were observed by all the th ree doses. We suggest that, Cd-induced inhibition of extrathyroidal convers ion of T-4 to T-3, the major source of the generation of latter hormone, is dose dependent and the free radicals generated due to Cd intoxication coul d be one of the mechanisms involved in the inhibition 5'-D activity in hepa tic tissues.