Type II thyroxine 5'-deiodinase in the rat thymus.
In the present study we have shown type II thyroxine 5'-deiodination (5'D) in the rat thymus. The enzyme activity was identified in crude extract homogenates by measuring the 125I released from [3',5'-125I]thyroxine which is used as a substrate of the reaction. The release of 125I is dependent on protein tissue concentration, time, temperature and pH, and is saturable by increasing the substrate concentration, indicating its enzymatic nature. Characteristics of the enzyme activity also include a low Km (9.1 nM), its dependence on dithiothreitol, and its inhibition by iopanoic acid, but not by propylthiouracil. Experiments to investigate the cellular location of the enzyme in the thymic gland showed that the enzyme is present in both stromal cells and thymocytes. At the subcellular level, 5'D activity was associated with cellular membranes. Thyroid status appears to regulate 5'D activity in rat thymus. Hypothyroidism caused an increase in thymus 5'D activity. The Km value remained unchanged (9.1 vs 10.5 nM) during hypothyroidism, but Vmax increased significantly from 17.7 fmol/mg protein per h in euthyroid rats to 53.5 fmol/mg protein per h in hypothyroid rats. 5'D activity was also modulated by catecholamines through beta-adrenergic receptors because isoproterenol, but not methoxamine or clonidine, could activate the enzyme. Because these characteristics define the type II iodothyronine-deiodinating pathway in other tissues, we suggest that the rat thymus also shares this pathway.
Molinero
P
P
Department of Medical Biochemistry and Molecular Biology, University of Seville School of Medicine, Spain.
Osuna
C
C
Guerrero
J M
JM
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D003000:Clonidine / Q000494:pharmacology; D005260:Female; D007037:Hypothyroidism / Q000201:enzymology; D007453:Iodide Peroxidase / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013154:Spleen / Q000201:enzymology; D013950:Thymus Gland / Q000201:enzymology*
D007453:Iodide Peroxidase; D008729:Methoxamine; D007545:Isoproterenol; D003000:Clonidine