Regulation of Superoxide Anion Radical–Superoxide Dismutase System in the Avian Thyroid by TSH with Reference to Thyroid Hormonogenesis

Abstract

This study shows that superoxide dismutase is present in the thyroid gland of pigeons as a constitutive enzyme serving as an antioxidant against oxygen toxicity. Exogenous administration of thyrotropin induced thyroidal superoxide dismutase with a simultaneous burst in superoxide anion radical levels during the initial phase of hormone treatment. The superoxide radical generated was completely scavenged by SOD during the late phase of TSH-treatment, presumably as an adaptive measure to check the oxygen burst. TSH failed to augment serum T3levels, although the thyroxine level in the serum was elevated. The peak level of SOD activity profile in the thyroid gland correlated very well with the peak level of thyroxine concentrations in the serum of pigeon. It is reasonable to postulate that the thyroidal SOD in homeotherms serves a dual role, firstly as a strategic antioxidant enzyme to protect the thyroid gland against the degenerative influence of toxic oxyradicals and secondly to provide H2O2for thyroid hormone biosynthesis. Our results confirm the previous observations that TSH is mainly thyrotropic in birds and that it has no influence on the peripheral activation of thyroxine to triiodothyronine by stimulating the extra thyroidal 5′-deiodinase activity.