Mechanism of action of iodothyronine-5'-deiodinase

Current concepts and challenges to unravel the role of iodothyronine deiodinases in human neoplasias

Thyroid hormones (THs) are essential for the regulation of several metabolic processes and the energy consumption of the organism. Their action is exerted primarily through interaction with nuclear receptors controlling the transcription of thyroid hormone-responsive genes. Proper regulation of TH levels in different tissues is extremely important for the equilibrium between normal cellular proliferation and differentiation. The iodothyronine deiodinases types 1, 2 and 3 are key enzymes that perform activation and inactivation of THs, thus controlling TH homeostasis in a cell-specific manner. As THs seem to exert their effects in all hallmarks of the neoplastic process, dysregulation of deiodinases in the tumoral context can be critical to the neoplastic development. Here, we aim at reviewing the deiodinases expression in different neoplasias and exploit the mechanisms by which they play an essential role in human carcinogenesis. TH modulation by deiodinases and other classical pathways may represent important targets with the potential to oppose the neoplastic process.

Kinetics and thiol requirements of iodothyronine 5'-deiodination are tissue-specific in common carp (Cyprinus carpio L.)

Iodothyronine deiodinases determine the biological activity of thyroid hormones. Despite the homology of the catalytic sites of mammalian and teleostean deiodinases, in-vitro requirements for the putative thiol co-substrate dithiothreitol (DTT) vary considerably between vertebrate species. To further our insights in the interactions between the deiodinase protein and its substrates: thyroid hormone and DTT, we measured enzymatic iodothyronine 5'-deiodination, Dio1 and Dio2 mRNA expression, and Dio1 affinity probe binding in liver and kidney preparations from a freshwater teleost, the common carp (Cyprinus carpio L.). Deiodination rates, using reverse T3 (rT3, 3,3',5'-triiodothyronine) as the substrate, were analysed as a function of the iodothyronine and DTT concentrations. In kidney rT3 5'-deiodinase activity measured at rT3 concentrations up to 10 μM and in the absence of DTT does not saturate appreciably. In the presence of 1mM DTT, renal rT3 deiodination rates are 20-fold lower. In contrast, rT3 5'-deiodination in liver is potently stimulated by 1mM DTT. The marked biochemical differences between 5'-deiodination in liver and kidney are not associated with the expression of either Dio1 or Dio2 mRNA since both organs express both deiodinase types. In liver and kidney, DTT stimulates the incorporation of N-bromoacetylated affinity labels in proteins with estimated molecular masses of 57 and 55, and 31 and 28 kDa, respectively. Although primary structures are highly homologous, the biochemistry of carp deiodinases differs markedly from their mammalian counterparts.

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.

Characterization of recombinant Xenopus laevis type I iodothyronine deiodinase: substitution of a proline residue in the catalytic center by serine (Pro132Ser) restores sensitivity to 6-propyl-2-thiouracil

In frogs such as Rana and Xenopus, metamorphosis does not occur in the absence of a functional thyroid gland. Previous studies indicated that coordinated development in frogs requires tissue and stage-dependent type II and type III iodothyronine deiodinase expression patterns to obtain requisite levels of intracellular T(3) in tissues at the appropriate stages of metamorphosis. No type I iodothyronine deiodinase (D1), defined as T(4) or reverse T(3) (rT3) outer-ring deiodinase (ORD) activity with Michaelis constant (K(m)) values in the micromolar range and sensitivity to 6-propyl-2-thiouracil (6-PTU), could be detected in tadpoles so far. We obtained a X. laevis D1 cDNA clone from brain tissue. The complete sequence of this clone (1.1 kb, including poly A tail) encodes an ORF of 252 amino acid residues with high homology to other vertebrate D1 enzymes. The core catalytic center includes a UGA-encoded selenocysteine residue, and the 3' untranslated region (about 300 nt) contains a selenocysteine insertion sequence element. Transfection of cells with an expression vector containing the full-length cDNA resulted in generation of significant deiodinase activity in the homogenates. The enzyme displayed ORD activity with T(4) (K(m) 0.5 microm) and rT3 (K(m) 0.5 microm) and inner-ring deiodinase activity with T(4) (K(m) 0.4 microm). Recombinant Xenopus D1 was essentially insensitive to inhibition by 6-PTU (IC(50) > 1 mm) but was sensitive to gold thioglucose (IC(50) 0.1 mum) and iodoacetate (IC(50) 10 microm). Because the residue 2 positions downstream from the selenocysteine is Pro in Xenopus D1 but Ser in all cloned PTU-sensitive D1 enzymes, we prepared the Pro132Ser mutant of Xenopus D1. The mutant enzyme showed strongly increased ORD activity with T(4) and rT3 (K(m) about 4 microm) and was highly sensitive to 6-PTU (IC(50) 2 microm). Little native D1 activity could be detected in Xenopus liver, kidney, brain, and gut, but significant D1 mRNA expression was observed in juvenile brain and adult liver and kidney. These results indicate the existence of a 6-PTU-insensitive D1 enzyme in X. laevis tissues, but its role during tadpole metamorphosis remains to be defined.

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.

Comparison of the effects of thyroxine and triiodothyronine on protein turnover and apoptosis in primary chick muscle cell cultures

Primary chick muscle cells were treated with physiological level of thyroxine (T4) or triiodothyronine (T3) to examine the effects of the hormones on growth, protein turnover, and apoptosis of the cells. Creatine kinase activity, as an index of differentiation, was increased by both T4 and T3. Even when the conversion from T4 to T3 was blocked by iopanoic acid, T4 increased creatine kinase activity. The rate of protein degradation estimated from [3H] tyrosine release was increased by T3 but not by T4. DNA cleavage and fragmentation, as indices of apoptosis, were induced by T3 but not by T4. These results show that T4 stimulates cell differentiation but not protein degradation and apoptosis in primary chick muscle cells, while all events are stimulated by T3.

Selenouracil derivatives are potent inhibitors of the selenoenzyme type I iodothyronine deiodinase

Type I iodothyronine deiodinase (ID-I) is a selenoenzyme, which is important for the conversion of the prohormone thyroxine (T4) to the bioactive thyroid hormone 3,3',5-triiodothyronine (T3). 2-Thiouracil derivatives inhibit ID-I by interaction with an enzyme form generated during catalysis. We have now tested the potential inhibitory effects of the selenocompounds 6-methyl- (MSU) and 6-propyl-2-selenouracil (PSU) in comparison with their thioanalogs 6-methyl- (MTU) and 6-propyl-2-thiouracil (PTU) on rat liver ID-I activity using 3,3',5-triiodothyronine (reverse T3, rT3) as substrate and dithiothreitol (DTT) as cofactor. All compounds showed dose-dependent inhibition of ID-I with IC50 values of 1, 0.5, 0.4 and 0.2 microM for MTU, MSU, PTU and PSU, respectively. Our results further suggest that these inhibitions are uncompetitive with substrate and competitive with cofactor. The high potency of selenouracils may be due to reaction with a substrate-induced enzyme selenenyl iodide intermediate under formation of a stable enzyme-selenouracil diselenide.

Evidence for the kidney as an important source of 5'-monodeiodination activity and stimulation by somatostatin in Oreochromis niloticus L

Radioimmunoassay of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) in the thyroidal region of mature male Oreochromis niloticus revealed stores of T4 but negligible levels of T3, yielding a very low T3/T4 ratio (0.3%). 5'-Deiodination (5-D) of T4 into T3 was examined in liver and kidney homogenates in vitro by radioimmunoassay of T3 with T4 as substrate. In both organs, the 5'-D activity was temperature dependent: at 4 degrees C, T3 production was below the level of detection and maximal in both tissues at 37 degrees C; and at 45 degrees C, the enzymatic activity was reduced. T3 production seemed to reach a plateau after 60 min of incubation. The reaction required exogenous thiol cofactor (dithiothreitol) and was inhibited partially or completely by propylthiouracil depending on the concentrations used. Hepatic and renal 5'-D activities were stimulated by somatostatin (SRIF) within 4 hr, but a subsequent increase in plasma T3 was observed only when SRIF was injected together with T4, while the magnitude of rT3 production decreased. It is concluded that almost all the circulating T3 is provided by peripheral T4 to T3 conversion since T3 RIA in thyroidal follicles demonstrated insignificant T3 production. The kidney may contain the large part of the functional deiodinase which converts T4 into T3. As in mammals and unlike in other fishes, there is not only 5'-D activity, but also 5-D activity, and both may be influenced by SRIF.

Properties of T4 5'-deiodinating systems in various tissues of the rainbow trout, Oncorhynchus mykiss

L-Thyroxine (T4) 5'-monodeiodinase (5'D) activity was examined in the microsomal fractions of liver, kidney, gill, white skeletal muscle, and red blood cells (RBC) of fed rainbow trout held in freshwater at 12 degrees. Two distinct 5'D systems were established and were examined at low (0.08-1.3 nM) or high (1.6-25 nM) T4 substrate ranges. The low substrate 5'D occurred in liver, gill, and muscle, but not in kidney or RBC. The pH optimum was 7.0 and the optimum dithiothreitol (DTT) level ranged from 7 to 10 mM. The Km values (nM) were liver, 0.098; muscle, 0.198; and gill, 0.168. The Vmax values (pmol.hr-1.mg protein-1) were liver, 3.74; muscle, 0.79; and gill, 0.62. DTT affected both the Vmax and the Km, and propylthiouracil (PTU) inhibited the Vmax. These data suggest a ping-pong type mechanism. In contrast, the high substrate 5'D occurred only in liver (pH 7 optimum, DTT optimum 15 mM) and in kidney (pH optima 6 and 8, DTT optimum 15 mM). The Km values (nM) were liver, 10.0; and kidney, 14.7; the Vmax values (pmol.hr-1.mg protein-1) were liver, 8.21; and kidney, 5.76. DTT affected the Vmax but not the Km and PTU did not inhibit, indicating a sequential type mechanism. In conclusion, in rainbow trout there are at least two types of 5'D which differ in their tissue distribution, T4 substrate affinity, and enzyme mechanism, and which do not resemble in their combined properties the 5'D forms established in higher vertebrate taxa.

Iodothyronine 5'-deiodinase activity as an early event of prenatal brown-fat differentiation in bovine development

Iodothyronine 5'-deiodinase activity appears to be a type I enzyme in bovine brown adipose tissue, on the basis of its high Km for 3,3',5'-tri-iodothyronine ('reverse T3') (in the micromolar range) and sensitivity to propylthiouracil inhibition. This enzyme activity is already detectable in perirenal adipose tissue of bovine fetuses in the second month of gestation, reaches peak values around the seventh month of fetal life, declines before birth, becomes lower after parturition and finally undetectable in the adult cow. Iodothyronine 5'-deiodinase activity is present in the pericardic, peritoneal and intermuscular adipose depots of the neonatal calf, but it is always undetectable in the subcutaneous adipose tissue. It is concluded that iodothyronine 5'-deiodinase is a specific feature of brown fat in the bovine species that is not shared by white adipose tissue. white adipose tissue. Peak values of 5'-deiodinating activity appear as an early event in the prenatal differentiation programme of bovine brown-fat cells as they occur when uncoupling-protein-gene expression first starts.

Kinetic characteristics of a thioredoxin-activated rat hepatic and renal low-Km iodothyronine 5'-deiodinase

The properties and kinetic characteristics of a non-GSH NADPH-dependent cofactor system activating rat hepatic and renal 5'-deiodinase (5'-DI), which we have previously demonstrated with partially purified cytosol Fractions A and B [Sawada, Hummel & Walfish (1986) Biochem. J. 234, 391-398], were examined further. Although microsomal fractions prepared from either rat liver or kidneys could be activated by crude cytosol Fractions A and B from those tissues as well as from rat brain and heart, a homologous hepatic or renal system was the most potent in producing 5'-deiodination of reverse tri-iodothyronine (rT3). At nanomolar concentrations both rT3 and thyroxine (T4) were deiodinated but with a much greater substrate preference for rT3 than for T4. However, at micromolar concentrations of these substrates no activation of 5'-DI could be detected. In this deiodinative system, T4 and tri-iodothyronine (T3) competitively inhibited 5'-deiodination of rT3. Dicoumarol, iopanoate, arsenite and diamide were also inhibitory to the activation of hepatic or renal 5'-deiodination by this cofactor system. Purification of cofactor components in hepatic crude cytosolic Fractions A and B to near homogeneity, as assessed by their enzymic and physical properties, indicated that these co-purified with and were therefore identical with thioredoxin reductase and thioredoxin respectively, and accounted almost entirely for the observed activation of rT3 5'-DI. When highly purified liver cytosolic thioredoxin reductase and thioredoxin were utilized to determine the kinetic characteristics of the reaction, evidence for a sequential mechanism operative at nanomolar but not micromolar concentrations of rT3 and T4 was obtained. The Km for rT3 was 1.4 nM. Inhibition by 6-n-propyl-2-thiouracil (Ki 6.7 microM) was competitive with respect to thioredoxin and non-competitive with respect to rT3, whereas inhibition by T4 (Ki 1.3 microM) was competitive. Since rT3 is a potent inhibitor of T4 5'-deiodination, this thioredoxin system activating deiodination of rT3 may play an important role in regulating the rate of intracellular production of T3 from T4.

Partial purification of the microsomal rat liver iodothyronine deiodinase. II. Affinity chromatography

Iodothyronine deiodinase has been solubilized and purified approximately 2400 times from liver microsomal fractions of male Wistar rats pretreated with thyroxine. The deiodinase was solubilized with 1% cholate, and stripped of adhering phospholipids by ammonium sulfate precipitation followed by solubilization with the non-ionic detergent Emulgen 911. The enzyme was further purified by successive ion-exchange chromatography on DEAE-Sephacel and Cellex-P and affinity chromatography on 3,3',5-triiodothyronine-Sepharose. Finally, the deiodinase was reacted with 6-propionyl-2-thiouracil-Sepharose, a derivative of the mechanism-based inhibitor 6-propyl-2-thiouracil. Covalent binding was observed only in the presence of substrate in agreement with the proposed mechanism of deiodination. The deiodinase was eluted from the affinity column by reduction of the enzyme-propylthiouracil mixed disulfide with 50 mM dithiothreitol. The enzyme was approximately 50% pure as judged by SDS-PAGE, exhibiting a subunit molecular weight of 25,000. This preparation was equally enriched in outer ring and inner ring deiodinase activities in keeping with the view that both are intrinsic to a single, type I deiodinase.

Direct assessment of brown adipose tissue as a site of systemic tri-iodothyronine production in the rat

Tri-iodothyronine (T3)production by interscapular brown fat was studied by measurements of arterio-venous differences and blood flow across the tissue in rats exposed to the following situations: controls, acute cold, chronic cold and starvation. Results demonstrate that brown adipose tissue is a source of systemic T3 in the rat and that the T3 release is modulated according to the physiological situation of the animal: increased in cold exposure and inhibited in starvation.

Iodothyronine 5'-deiodinase activity in rat brown adipose tissue during development

Iodothyronine 5'-deiodinase activity in rat brown adipose tissue has a characteristic pattern of developmental changes that is completely different from that of the liver. Fetal brown fat exhibits an extremely high iodothyronine 5'-deiodinase activity that is approx. 10-fold that in adult rats. Even though brown fat iodothyronine 5'-deiodinase activity falls suddenly at birth, there is a new peak in the activity around days 5-7 of life, whereas it remains very low afterwards. Just after birth, brown adipose tissue iodothyronine 5'-deiodinase activity is already capable of stimulation by noradrenaline. The postnatal peak in brown fat iodothyronine 5'-deiodinase correlates with the known increase in the thermogenic activity of the tissue in the neonatal rat, thus reinforcing the suggestion that local 3',3,5-triiodothyronine generation could be an important event related to thermogenesis in brown adipose tissue. However, the high fetal activity was only slightly related to the thermogenic activity of brown fat. Moreover, the increased iodothyronine 5'-deiodinase activity of brown adipose tissue during fetal and neonatal life suggests a substantial contribution by brown fat in the overall extrathyroidal 3',3,5-triiodothyronine production in these physiological periods.

Intermediate Mr cytosolic components potentiate hepatic 5'-deiodinase activation by thiols

The role in the activation of microsomal 5'-deiodinase (5'-DI) of rat hepatic cytosolic components of Mr approx. 13,000 (Fraction B) was studied in the presence of various concentrations of thiol compounds such as dithiothreitol (DTT), dihydrolipoamide (DHLA), GSH, and 2-mercaptoethanol (2-ME). Although Fraction B (which was prepared by gel filtration to exclude GSH and GSSG) had no intrinsic 5'-DI activity, could not stimulate microsomal 5'-DI activity in the absence of added thiol and did not contain GSH as a mixed disulphide, it could produce a 3-fold increase in the maximal deiodinase activity achievable with DTT as well as other thiols, with the order being the same as the activation potency of these thiols in the absence of Fraction B (i.e. DHLA greater than DTT greater than 2-ME greater than GSH). These observations suggest that: a component of cytosolic Fraction B, designated 'deiodination factor B' (DFB), operates as an efficient intermediary to enhance activation of microsomal 5'-DI by thiols through a mechanism independent of GSH; thiols may participate in a non-specific thiol-disulphide exchange with inactive (oxidized) DFB to convert it into an active form that contains one or more thiol groups and is more effective than GSH or other thiols in facilitating the re-activation of inactive (oxidized) microsomal 5'-DI thiol (ESI) to its active state (ESH).

Thyroxine 5'-monodeiodinase activity in hepatocytes of rainbow trout, Salmo gairdneri: distribution, effects of starvation, and exogenous inhibitors

L-Thyroxine (T4) 5'-monodeiodinase activity (MDA) of hepatocyte cell fractions of rainbow trout was evaluated by 125I- generation following incubation with [125I-3' or 125I-5']T4 at 12 degrees. Produced in approximately equal proportions, 3,5,[125I-3'] triiodo-L-thyronine and 125I- were the sole labeled products detected by gel permeation on G-25 Sephadex columns, confirming restriction of T4 deiodination in trout to removal of a single outer-ring iodine atom. T3 underwent no significant outer-ring deiodination. MDA activity, located mainly in the microsome fraction, was optimal at a pH of approximately 7.0 and was enhanced by dithiothreitol but not by reduced glutathione. Azide, thiocyanate, thiourea, and KCl exerted no significant influence on MDA, but MDA was inhibited by: 8-anilino-1-naphthalene sulfonic acid greater than N-ethyl maleimide greater than propylthiouracil greater than sodium salicylate greater than KI. Starvation for 2 weeks depressed MDA to 46% of the level of trout fed 1% of body wt once per day. This was due to a decreased Vmax of MDA. In conclusion, trout hepatic microsomal MDA is acutely and chronically susceptible to both exogenous and endogenous factors; as an enzyme responsible for extrathyroidal T3 generation, it may exert a key role in regulating peripheral thyroidal status under both natural and experimental conditions.

Avian hepatic T-3 production by two pathways of 5'-monodeiodination: effects of fasting and patterns during development

Two forms of iodothyronine 5'-monodeiodinase (5'-D) were studied in liver homogenates from adult and developing quail. The influence of fasting in adults and corticosterone treatment in embryonic quail on 5'-D also were examined. Liver homogenates were assayed for 5'-D activity in the presence of abundant substrate (T4) and cofactor (dithiothreitol; DTT). Generation of T3 during a 15 min incubation at 37 degrees C was assessed by an ethanol-based RIA. In adults, both Type I [the fraction of activity inhibited by propylthiouracil (PTU)] and a putative Type II (the PTU-insensitive fraction) were present in liver homogenates. Type II activity typically comprised about 30% of Total activity. Type I activity first appeared on day 15 of the 16.5 day incubation period, increased 20-fold to peak at hatching, then gradually declined to reach adult levels by 21 days of age. Type II activity was present at all developmental stages and was highest during the perinatal period. Corticosterone treatment in vivo on day 13 of development induced increases in both Type I and Type II activities in liver homogenates 24- and 48-h after treatment. This study demonstrates that in avian liver a putative Type II 5'-D activity (generally considered to be lacking in mammalian liver) is present and may be important in the maintenance of minimal concentrations of tissue T3 during fasting. Both types of 5'-D contribute to the developmental pattern of serum T3 concentrations. Type II comprises a large proportion of total activity during late embryonic life; Type I becomes predominant at the beginning of the perinatal period.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of cytosolic components activating rat hepatic 5' [corrected]-deiodination in the presence of NADPH

The effects of cytosol, NADPH and reduced glutathione (GSH) on the activity of 5'-deiodinase were studied by using washed hepatic microsomes from normal fed rats. Cytosol alone had little stimulatory effect on the activation of microsomal 5'-deiodinase. NADPH had no stimulatory effect on the microsomal 5'-deiodinase unless cytosol was added. 5'-deiodinase activity was greatly enhanced by the simultaneous addition of NADPH and cytosol (P less than 0.001); this was significantly higher than that with either NADPH or cytosol alone (P less than 0.001). GSH was active in stimulating the enzyme activity in the absence of cytosol, but the activity of 5'-deiodinase with 62 microM-NADPH in the presence of cytosol was significantly higher than that with 250 microM-GSH in the presence of the same concentration of cytosol (P less than 0.001). The properties of the cytosolic components essential for the NADPH-dependent activation of microsomal 5'-deiodinase independent of a glutathione/glutathione reductase system were further assessed using Sephadex G-50 column chromatography to yield three cytosolic fractions (A, B and C), wherein A represents pooled fractions near the void volume, B pooled fractions of intermediate Mr (approx. 13 000), and C of low Mr (approx. 300) containing glutathione. In the presence of NADPH (1 mM), the 5'-deiodination rate by hepatic washed microsomes is greatly increased if both A and B are added and is a function of the concentrations of A, B, washed microsomes and NADPH. A is heat-labile, whereas B is heat-stable and non-dialysable. These observations provide the first evidence of an NADPH-dependent cytosolic reductase system not involving glutathione which stimulates microsomal 5'-deiodinase of normal rat liver. The present data are consistent with a deiodination mechanism involving mediation by a reductase (other than glutathione reductase) in fraction A of an NADPH-dependent reduction of a hydrogen acceptor in fraction B, followed by reduction of oxidized microsomal deiodinase by the reduced acceptor (component in fraction B).

Stimulation of mouse liver glutathione S-transferase activity in propylthiouracil-treated mice in vivo by tri-iodothyronine

Female C57Bl/6J mice were given drinking water containing 0.05% propylthiouracil to induce a hypothyroid condition. Mitochondrial glycerol-3-phosphate dehydrogenase activity, used as an index of hypothyroidism, was 57.1 +/- 4.5 and 29.4 +/- 3.8 nmol/min per mg of protein for control and propylthiouracil-treated animals respectively. Administration of tri-iodothyronine resulted in an approx. 4.5-fold increase in dehydrogenase activity in propylthiouracil-treated animals. A dose-dependent increase in hepatic GSH S-transferase activity in propylthiouracil-treated animals was observed at tri-iodothyronine concentrations ranging from 2 to 200 micrograms/100 g body wt. This increase in transferase activity was seen only when 1,2-epoxy-3-(p-nitrophenoxy)propane was used as substrate for the transferase. Transferase activity with 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene as substrate was decreased by tri-iodothyronine. Administration of actinomycin D (75 micrograms/100 g body wt.) inhibited the tri-iodothyronine induction of transferase activity. Results of these studies strongly suggest that tri-iodothyronine administration markedly affected the activities of GSH S-transferase by inducing a specific isoenzyme of GSH S-transferase and suppressing other isoenzymic activities.

Iodothyronine 5'-deiodinase in rat kidney microsomes. Kinetic behavior at low substrate concentrations

The thiol-activated enzymatic outer-ring monodeiodination of iodothyronines by rat kidney microsomes at low (nanomolar) substrate concentrations shows an apparently sequential reaction mechanism and is further characterized by insensitivity to inhibition by dicoumarol, a moderate sensitivity to inhibition by propylthiouracil (Ki = 100 microM) and iopanoic acid (Ki = 0.9 mM), responsiveness to 5 mM glutathione (GSH), and a thermal activation profile that is concave downward with a Td of approximately 20 degrees C. In contrast, the activity at high (micromolar) substrate concentrations shows a ping-pong reaction mechanism, is inhibited by micromolar concentrations of propylthiouracil, iopanoic acid and dicoumarol, is unresponsive to 5 mM GSH, and shows a concave upward thermal activation profile. Analysis of the microsomal deiodinase reaction over a wide range of 3,3',5'-triiodothyronine (rT3) concentrations (0.1 nM to 10 microM) suggested the presence of two enzymatic activities, with apparent Michaelis constants (Km) of 0.5 microM and 2.5 nM. Lineweaver-Burk plots of reaction velocities at nanomolar substrate concentrations in presence of 100 microM propylthiouracil also revealed an operationally distinct enzymatic activity with Km's of 2.5 and 0.63 nM and maximum velocities (Vmax's) of 16 and 0.58 pmol/mg protein per h for rT3 and thyroxine (T4), respectively. These findings are consistent with the presence of a low Km iodothyronine 5'-deiodinase in rat kidney microsomes distinct from the well characterized high Km enzyme and suggest that at circulating levels of free T4 the postulated low Km enzyme could be physiologically important.

Inactivation and affinity-labeling of rat liver iodothyronine deiodinase with N-bromoacetyl-3,3',5-triiodothyronine

The thyroid hormone derivative N-bromoacetyl-3,3',5-triiodothyronine (BrAcT3) acts as an active site-directed inhibitor of rat liver iodothyronine deiodinase. Lineweaver Burk analysis of enzyme kinetic measurements showed that BrAcT3 is a competitive inhibitor of the 5'-deiodination of 3,3',5'-triiodothyronine (rT3) with an apparent Ki value of 0.1 nM. Preincubations of enzyme with BrAcT3 indicated that inhibition by this compound is irreversible. The inactivation rate obeyed saturation kinetics with a limiting inactivation rate constant of 0.35 min-1. Substrates and substrate analogs protected against inactivation by BrAcT3. Covalent incorporation of 125I-labeled BrAcT3 into "substrate-protectable" sites was proportional to the loss of deiodinase activity. The results suggest that BrAcT3 is a very useful affinity label for rat liver iodothyronine deiodinase.

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.

Modification of rat liver iodothyronine 5'-deiodinase activity with diethylpyrocarbonate and rose bengal; evidence for an active site histidine residue

Iodothyronine 5'-deiodinase activity of rat liver microsomes was rapidly and completely lost by treatment with diethylpyrocarbonate (DEP) and by photo-oxidation with Rose Bengal (RB). In both cases inactivation followed pseudo first order reaction kinetics. Inactivation by DEP was diminished in the presence of substrate or competitive inhibitors, and was reversed by hydroxylamine treatment. In addition to photo-oxidation, deiodinase activity was also inhibited by RB in the dark. This inhibition was reversible and competitive with substrate (Ki 60 nM). These results suggest the location of an essential histidine residue at or near the active site of rat liver iodothyronine deiodinase.

Differential sensitivity of brain iodothyronine 5'-deiodinases to sulfhydryl-blocking reagents

Propylthiouracil (PTU) and iodoacetate (IAc) partially inhibit the 5'-deiodination of 3,3',5'-triiodothyronine (rT3) in cerebral cortex, but PTU has no effect on the 5'-deiodination of thyroxine (T4). We now report that pretreatment of cerebral cortex microsomes with increasing concentrations of IAc or iodoacetamide inhibits the PTU-sensitive 5'-deiodination of rT3 with half-maximal inhibition at less than 1 microM IAc. PTU-insensitive 5'-deiodination of rT3 and T4 requires 100-1000-fold higher concentrations of IAc to achieve similar inhibition. Trypsinization decreases both 5'-deiodinating activities by greater than 90%. These data suggest that enzyme SH groups do not participate in the 5'-deiodination of T4 or rT3 by the PTU-insensitive pathway.

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.

Cerebral cortex responds rapidly to thyroid hormones

In rats subjected to thyroidectomy there was a two- to fourfold increase in cerebral cortex iodothyronine 5'-deiodinase activity within 24 hours. This increase was prevented by thyroxine replacement. The increased cortical 5'-deiodinase in chronically hypothyroid rats was normalized within 4 hours by a single intravenous injection of triiodothyronine. These results indicate that the adult central nervous system can give a very rapid biochemical response to thyroid hormone.

Inhibition of rat hepatic thyroxine 5'-monodeiodinase by propylthiouracil: relation to site of interaction of thyroxine and glutathione

When rat liver cytosol, dialyzed free of glutathione, was chromatographed on Sephadex G-100 after incubation with 35S-propylthiouracil, 2 peaks of bound radioactivity were observed, 1 of which contained nearly all the thyroxine 5'-monodeiodinase activity in rat liver cytosol. Binding of propylthiouracil to this peak was inhibited by glutathione but not by thyroxine. Approximately 25% of 35S-propylthiouracil initially bound to the thyroxine 5'-monodeiodinating activity peak remained bound after dialysis, precipitation with trichloroacetic acid, and multipe extractions with ethanol, methanol, and chloroform, suggesting that binding was at least in part covalent. Dialysis studies showed that the presumed covalent binding of 35S-propylthiouracil to the thyroxine 5'-monodeiodinase peak could be inhibited by glutathione, dithioerythritol, and unlabelled propylthiouracil but not by oxidized glutathione or thyroxine. Conversely, thyroxine binding was unaffected by thiol compounds. We studied the kinetics of thyroxine 5'-monodeiodination by radioimmunoassay techniques using rat liver homogenates as source of enzyme and observed the dependence of enzymic reaction upon glutathione (Km = 2.4 mM). Propylthiouracil inhibited the reaction and this inhibition could be overcome with increasing glutathione concentrations. We conclude that the thiol-dependent thyroxine 5'-monodeiodinase is inhibited by propylthiouracil through its covalent binding, probably as mixed disulfide, to s site on the enzyme at which glutathione interacts either as a cosubstrate or reducing agent. This binding site is separate from the site at which thyroxine binds.

Solubilization of a phospholipid-requiring enzyme, iodothyronine 5'-deiodinase, from rat kidney membranes

Enzymic activities catalyzing the reductive 5'-deiodination of thyroxine and 3,3',5'-triiodothyronine were solubilized from rat kidney microsomes by treatment with 0.2% deoxycholate. Deoxycholate reversibly inhibited the enzyme(s); removal of detergent restored activity and resulted in the formation of enzymatically active aggregates with a buoyant density of 1.17 g/ml resembling that of membranes. Fractionation of the solubilized membrane components in the presence of 0.2% deoxycholate by either gel filtration or sucrose gradient centrifugation inactivated the enzyme(s) and activity could be restored by the addition of partially purified soybean phospholipids; this allowed some of the physical properties of the enzyme(s) to be determined. 5'-Deiodinating activity of both thyroxine and 3,3',5'-triiodothyronine was associated with protein(s) with S20,W of 3.5 S, Stokes' radius of 32 A, and a calculated molecular weight of 49 900. A partial specific volume of 0.74 cm3/g was calculated from sedimentation in 2H2O and H2O sucrose gradients. Phospholipid reactivation of lipid-depleted enzyme preparations was concentration-dependent, with near maximal restoration when sufficient phospholipid was added to restore the phospholipid:protein ratio to that of thyroxine and 3,3',5'-triiodothyronine could not be resolved by sedimentation or molecular sieving and showed similar behavior toward deoxycholate solubilization and phospholipid reconstitution.

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

Preincubation of rat liver microsomal fraction with 1 microM 2-thiouracil and 0.01-1 microM 3,3',5'-triiodothyronine or 3',5'-diiodothyronine, 0.1-10 microM thyroxine or 3,5-diiodothyronine led to a progressive, irreversible and concomitant decrease in subsequently assayed 3,3',5'-triiodothyronine- and 3',5'-diiodothyronine-5'-deiodinase activity. Preincubation with thiouracil alone, with iodothyronines alone or with thiouracil and 10 microM thyronine or 3,5-diiodotyrosine had no or virtually no effect. The results indicate that (1) a previously proposed ping-pong mechanism for thyroid hormone deiodination, involving the formation of an enzyme-sulphenyl iodide intermediate, is correct; (2) thyroxine, 3,3',5'-triiodothyronine and 3',5'-diiodothyronine are substrates for a common 5'-deiodinase; (3) this 5'-deiodinase is not fully specific as regards the position of the iodine substituents in the substrate, since it also appears to catalyse the 5-deiodination of 3,5-diiodothyronine.

Study on the enzymatic 5'-deiodination of 3',5'-diiodothyronine using a radioimmunoassay for 3'-iodothyronine

A radioimmunoassay for 3'-iodothyronine has been developed. All iodothyronine analogues (except 3,3'-diiodothyronine) showed very litte (0.02% at most) cross-reactivity, and the assay was sensitive to 1 pg 3'-iodothyronine/tube. We have studied the 5'-deiodination of 3'-5'-diiodothyronine by rat liver microsomal fraction in the presence of dithiothreitol. Production of 3'-iodothyronine at 37 degrees C was found to be linear with time of incubation up to 30 min and with concentration of microsomal protein up to 100 microgram/ml. The reaction rate reached a limit on increasing 3',5'-diiodothyronine concentration to 10 microM. The effect of pH on 3'-iodothyronine production was found to depend on 3',5'-diiodothyronine concentration. Increasing 3,5'-diiodothyronine concentration from 0.1 to 10 microM resulted in a shjift of the pH optimum from 6-6.5 to 7.5. Similar effects on the 5'-deiodination of 3,3',5'-triiodothyronine were observed, supporting the hypothesis that these reactions are catalysed by a single enzyme (iodothyronine 5'-deiodinase).

Mechanism of inhibition of iodothyronine-5'-deiodinase by thioureylenes and sulfite

Previous studies have demonstrated that thiouracil inhibits the 5'-deiodination of 3,3',5'-triiodothyronine uncompetitively with respect to substrate and competitively with respect to cofactor (thiol compounds). This paper shows that sulfite is also a strong inhibitor of this reaction showing a dose-dependent effect between 1 microM and 1 mM. The mode of inhibition is similar to that described for thiouracil. Dose-dependent inhibition was also observed with thiosulfate (0.01-1 mM), iodide and thiocyanate (both greater than 1 mM). No effect was exerted by up to 10 mM cyanide and up to 100 mM azide. Methimazole and thiourea were weak inhibitors above 0.1 mM but inhibition did not reach completion. These experiments were carried out in the presence of 1 mM dithiothreitol. The effect of thiouracil was found to be competitively obviated by methimazole and thiourea. However, the effect of sulfite and that of methimazole or thiourea were additive. It is proposed that an enzyme-sulfenyl iodide is formed during deiodination (ping-pong mechanism). This sulfenyl iodide may be reduced by cofactor to yield native enzyme. It may also react with thioureylenes, yielding mixed disulfides, or with sulfite, yielding a thiosulfate. The enzyme-methimazole disulfide is apparently less stable than the enzyme-thiouracil complex. It is suggested that sulfite also reacts with the enzyme-thioureylene disulfide.