Substrate requirement for inactivation of iodothyronine-5'-deiodinase activity by thiouracil

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

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.

Substitution of cysteine for a conserved alanine residue in the catalytic center of type II iodothyronine deiodinase alters interaction with reducing cofactor

Human type II iodothyronine deiodinase (D2) catalyzes the activation of T(4) to T(3). The D2 enzyme, like the type I (D1) and type III (D3) deiodinases, contains a selenocysteine (SeC) residue (residue 133 in D2) in the highly conserved catalytic center. Remarkably, all of the D2 proteins cloned so far have an alanine two residue-amino terminal to the SeC, whereas all D1 and D3 proteins contain a cysteine at this position. A cysteine residue in the catalytic center could assist in enzymatic action by providing a nucleophilic sulfide or by participating in redox reactions with a cofactor or enzyme residues. We have investigated whether D2 mutants with a cysteine (A131C) or serine (A131S) two-residue amino terminal to the SeC are enzymatically active and have characterized these mutants with regard to substrate affinity, reducing cofactor interaction and inhibitor profile. COS cells were transfected with expression vectors encoding wild-type (wt) D2, D2 A131C, or D2 A131S proteins. Kinetic analysis was performed on homogenates with dithiothreitol (DTT) as reducing cofactor. The D2 A131C and A131S mutants displayed similar Michaelis-Menten constant values for T(4) (5 nM) and reverse T(3) (9 nM) as the wt D2 enzyme. The limiting Michaelis-Menten constant for DTT of the D2 A131C enzyme was 3-fold lower than that of the wt D2 enzyme. The wt and mutant D2 enzymes are essentially insensitive to propylthiouracil [concentration inhibiting 50% of activity (IC(50)) > 2 mM] in the presence of 20 mM DTT, but when tested in the presence of 0.2 mM DTT the IC(50) value for propylthiouracil is reduced to about 0.1 mM. During incubations of intact COS cells expressing wt D2, D2 A131C, or D2 A131S, addition of increasing amounts of unlabeled T(4) resulted in the saturation of [(125)I]T(4) deiodination, as reflected in a decrease of [(125)I]T(3) release into the medium. Saturation first appeared at medium T(4) concentrations between 1 and 10 nM.

IN CONCLUSION

substitution of cysteine for a conserved alanine residue in the catalytic center of the D2 protein does not inactivate the enzyme in vitro and in situ, but rather improves the interaction with the reducing cofactor DTT in vitro.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

3'-Monoiodothyronine degradation in rat liver homogenate: enzyme characteristics and documentation of deiodination by high-pressure liquid chromatography

Characteristics of 3'-monoiodothyronine (3'-T1) degradation were examined in vitro in rat tissue homogenates. In rat liver homogenates, 3'-T1 degradation was optimal at pH 7.4, and was dependent upon time, temperature, and tissue concentration. The Michaeli's constant (Km) = 0.84 mumol/L. 3'-T1 degradation was enhanced by dithiothreitol and inhibited by propylthiouracil, sodium ipodate, ANS, and sodium azide but not by methimazole. Animals that fasted for three days had significant reductions in both hepatic T4 to T3 conversion (199 +/- 12 v 116 +/- 12 pg T3 generated/mg protein; P less than 0.001) and 3'-T1 degradation (588 +/- 31 v 148 +/- 53 pg 3'-T1 degraded/mg protein; P less than 0.001). To document that 3'-T1 degradation was occurring by deiodination, both liver and kidney homogenates were incubated with 125I-3'-T1 (approximately 3 microCi; 13.1 nmol/L). The reaction products were separated on a reverse-phase high pressure liquid chromatography (HPLC) column. In both tissues an iodide peak was generated, and no other radiolabeled peaks appeared except for 125I-3'-T1. These data suggest that 3'-T1 is metabolized by phenolic-ring monodeiodination and is enzymic in nature.

Selective modification of the active center of renal iodothyronine 5'-deiodinase by iodoacetate

Pretreatment of renal iodothyronine 5'-deiodinase with sulfhydryl reagents, iodoacetate, iodoacetamide and N-alkylmaleimides, results in irreversible loss of catalytic activity. Iodoacetate and iodoacetamide were the most potent inhibitors, being 100- to 1000-times more potent than N-alkylmaleimides. Iodoacetate and iodoacetamide inactivation followed pseudo-first-order kinetics with maximum inactivation rate constants of 1.56 min-1 and 0.87 min-1, respectively. Thyroxine and 3,3',5'-triiodothyronine and the competitive inhibitor iopanoate, protected the enzyme against iodoacetate inhibition. Protection by 3,3',5'-triiodothyronine was competitive with iodoacetate with a dissociation constant (Kd) of 113 nM; in close agreement with the Km for rT3 of 190 nM determined under similar reaction conditions. [3H]Carboxymethylation of renal membranes in the absence and presence of 3,3',5'-triiodothyronine showed specific incorporation of iodo[3H]acetate into substrate-protected sites of 35-40% of total when non-essential residues were first blocked with excess unlabeled iodoacetate. ' Protectable ' [3H]acetate incorporation followed pseudo-first-order kinetics and the rate constant for incorporation was identical to the rate constant for inactivation. These results indicate that iodoacetate fulfills the minimum criteria for an active-site-directed reagent for renal 5'-deiodinase and that a sulfhydryl group is in close proximity to the iodothyronine-binding site.

Structural requirements of thiol compounds in the inhibition of human liver iodothyronine 5'-deiodinase

2-Thiouracil and a number of its alkyl derivatives are known to inhibit the enzymic 5'-deodination of thyroxine to 3,5,3'-tri-iodothyronine. The structural requirements for inhibition of iodothyronine 5'-deiodinase were investigated by using a washed postmitochondrial particulate fraction of human liver. A series of sulphur-containing derivatives of pyrimidine, pyridine, imidazole, benzene and urea, capable of existing in a thiol form, were incubated at several concentrations with the enzyme preparation in the presence of thyroxine and dithioerythritol (cofactor). The degree of inhibition by the respective compounds of the production of 3,5,3'-tri-iodothyronine was studied in relation to their structural features. The major observations were: (i) a free thiol group is essential; (ii) compounds that do not possess a polar hydrogen atom spatially configured so that it is proximal to the thiol group are poor inhibitors; (iii) aromatic characteristics in the presence of requirements (i) and (ii) lead to the expression of potent inhibitory properties; (iv) modification of potent inhibitors by the introduction of hydrophilic substituents reduces the inhibitory potency.

Properties of detergent-dispersed iodothyronine 5- and 5'-deiodinase activities from rat liver

In order to obtain more knowledge about the regulation and mechanism of thyroid hormone deiodination, some properties of detergent-solubilized iodothyronine deiodinase have been studied. Moreover, a starting point for its purification has been made. Several chromatography media were tested for their ability to purify the deiodinases. In some instances, a 4-fold purification was obtained. Treatment of cholate-solubilized microsomes with 35% ammonium sulphate resulted in quantitative precipitation of the deiodinase activities and concomitant removal of phospholipid. The pellet could be solubilized with 0.3% W-1 ether and the deiodinase in this ammonium sulphate extract exhibited approximately 10-fold higher apparent Km and Vmax values for its substrate compared with the cholate extract. Readdition of some phospholipids was shown to decrease enzyme activity. Isoelectric focusing of W-1 ether-solubilized microsomes resulted in a major activity peak around pH 6.4 and a minor peak at pH 5.2, while in the ammonium sulphate extract the deiodinase had an isoelectric point at pH 9.3. Refocusing of this activity peak yielded a preparation with a specific activity only 3-times higher than in the ammonium sulphate extract. However, after sodium dodecyl sulphate polyacrylamide gel electrophoresis only five bands could be detected. The elevated kinetic parameters as well as the higher isoelectric point of the deiodinase after ammonium sulphate treatment were caused by delipidation of the enzyme. Both the change in isoelectric point and the behaviour on several column materials were found to be similar for the 5- and 5'-deiodinase activities. These results suggest that a single enzyme is operative in the deiodination of iodothyronines in rat liver and that its activity may be regulated by phospholipids.

Kinetic evidence suggesting two mechanisms for iodothyronine 5'-deiodination in rat cerebral cortex

Enzymatic 5'-deiodination of 3,3',5'-triiodothyronine (rT3) and 3,3',5,5'-tetraiodothyronine (thyroxine, T4) was studied in microsomal preparations of rat cerebral cortex. Evidence was obtained for the existence of two thiol-dependent 5'-deiodinase entities. One of these predominates in tissue from euthyroid and long-term hypothyroid rats, is specific for rT3, follows "ping-pong" kinetics with dithiothreitol as the cosubstrate, and is inhibited by propylthiouracil (PrSUra) and iodoacetate. Inhibition by PrSUra is uncompetitive with rT3 and competitive with dithiothreitol. These properties are shared with the 5'-deiodinase activity of liver and kidney. The activity of a second type of 5'-deiodinase is highest in cerebral cortex from short-term hypothyroid rats, prefers T4 to rT3 as the substrate, is insensitive to PrSUra and iodoacetate, and follows "sequential" reaction kinetics. A similar PrSUra-insensitive 5'-deiodinase activity is also found in pituitary but is not detectable in liver and kidney; it seems, therefore, characteristic of tissues in which local T4 to 3,3',5-triiodothyronine (T3) conversion supplies a major portion of the total intracellular T3.

Evidence for a single enzyme in rat liver catalysing the deiodination of the tyrosyl and the phenolic ring of iodothyronines

The enzymic 5'-deiodination of 3',5'-di-iodothyronine and 5-deiodination of 3,3',5-tri-iodothyronine by rat liver microsomal fractions were found to be characterized by apparent Km values of 0.77 and 17.4 microM respectively, 3',5'-Di-iodothyronine was a competitive inhibitor of 3,3',5-tri-iodothyronine 5-deiodination (Ki 0.65 microM) and 3,3',5-tri-iodothyronine was a competitive inhibitor of 3',5'-di-iodothyronine 5'-deiodination (Ki 19.6 microM). In addition, several radiographic contrast agents and iodothyronine analogues inhibited both reactions competitively and with equal potencies (r = 0.999). These results strongly suggest the existence of a single hepatic deiodinase acting on both the tyrosyl and phenolic ring of iodothyronines.