title: Substitution of serine for proline in the active center of type 2 iodothyronine deiodinase substantially alters its in vitro biochemical properties with dithiothreitol but not its function in intact cells.
abstract: T(4) must be activated by its monodeiodination to T(3) by type 1 or 2 iodothyronine deiodinase (D1 and D2). Recent studies show that despite an approximately 2000-fold higher Michaelis constant (K(m); T(4)) for D1 than for D2 using dithiothreitol (DTT) as cofactor, D1 expressed in intact cells produces T(3) at free T(4) concentrations many orders of magnitude below its K(m). To understand the factors regulating D1 and D2 catalysis in vivo, we studied a mutant D2 with a proline at position 135 of the active center of D2 replaced with a serine, as found in D1. The P135S D2 enzyme has many D1-like properties, a K(m) (T(4)) in the micromolar range, ping-pong kinetics with DTT, and sensitivity to 6n-propylthiouracil (PTU) in vitro. Unexpectedly, when the P135S D2 was expressed in HEK-293 cells and exposed to 2-200 pm free T(4), the rate of T(4) to T(3) conversion was identical with D2 and conversion was insensitive to PTU. Using glutathione as a cofactor in vitro resulted in a marked decrease in the K(m) (T(4)) (as also occurs for D1), it showed sequential kinetics with T(4) and it was sensitive to PTU but was resistant when HEK-293 cytosol was used as a cofactor. Thus, the in vivo catalytic properties of the P135S D2 mutant are more accurately predicted from in vitro studies with weak reducing agents, such as glutathione or endogenous cofactors, than by those with DTT.
authors: Thyroid Section, Endocrine Division, Hospital de Clínicas de Porto Alegre, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS-90035-903, Brazil.
authors: Iuri Martin
authors: IM
authors: Goemann
authors: Balázs
authors: B
authors: Gereben
authors: John W
authors: JW
authors: Harney
authors: Bo
authors: B
authors: Zhu
authors: Ana Luiza
authors: AL
authors: Maia
authors: P Reed
authors: PR
authors: Larsen
mesh_terms: D019943:Amino Acid Substitution; D020134:Catalytic Domain; D002460:Cell Line; D004229:Dithiothreitol / Q000494:pharmacology; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D007453:Iodide Peroxidase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007668:Kidney; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D011392:Proline / Q000235:genetics*; D011441:Propylthiouracil / Q000494:pharmacology; D012694:Serine / Q000235:genetics*; D013974:Thyroxine / Q000378:metabolism; D014162:Transfection
chemical_list: D012694:Serine; D011441:Propylthiouracil; D011392:Proline; D007453:Iodide Peroxidase; D005978:Glutathione; D013974:Thyroxine; D004229:Dithiothreitol