Thyroid hormones and D-aspartic acid, D-aspartate oxidase, D-aspartate racemase, H2O2, and ROS in rats and mice.

Total concentrations of thyroid hormones T3 and T4, and of their free forms, FT3 and FT4, D-aspartic acid (D-Asp), D-aspartate oxidase (D-AspO), D-aspartate racemase, H2O2, and ROS (reactive oxygen species) were determined in rats and mice. T3 and T4 were 1 and 50 ng/ml, respectively, in serum, and 750 and 40000 ng/g, respectively, in thyroid. Concentrations of the free forms FT3 and FT4 were ca. 250 times lower than their respective total concentrations. The endogenous content of D-Asp in thyroid gland was ca. 100 nmol/g tissue, whereas the activity of D-AspO was ca. 80 units/mg thyroid, and that of D-aspartate racemase was ca. 15 units/mg thyroid. H2O2 Concentration in rat and mouse thyroid gland was ca. 290 pmol/g thyroid, and the concentration of ROS was ca. 10 pmol/DCF/min/mg protein. H2O2 is essential for the iodination of the tyrosyl residues to produce mono- and diiodotyrosine that are the precursors for the synthesis of T3 and T4. Production of H2O2 in thyroid glands occurs by oxidation of endogenous D-Asp by D-AspO (D-Asp+O2+H2O→α-oxaloacetate+NH3+H2O2). D-Aspartate racemase catalyzes the in vivo production of D-Asp from L-Asp. Thus, interaction of endogenous D-Asp, D-AspO, and D-aspartate racemase in thyroid gland constitutes an additional biochemical pathway for the production of H2O2 and consequently for the synthesis of thyroid hormones.