An improved method for chromatography of iodothyronines

Type 1 and type 2 iodothyronine deiodinases in the thyroid gland of patients with huge goitrous Hashimoto's thyroiditis

PURPOSE

The serum free triiodothyronine (FT3)/free thyroxine (FT4) ratio in patients with huge goitrous Hashimoto's thyroiditis (HG-HT) is relatively high. We investigated the cause of high FT3/FT4 ratios.

METHODS

We measured the serum FT3, FT4, and thyrotropin (TSH) levels of seven patients with HG-HT who had undergone a total thyroidectomy. Eleven patients with papillary thyroid carcinoma served as controls. The activities and mRNA levels of type 1 and type 2 iodothyronine deiodinases (D1 and D2, respectively) were measured in the thyroid tissues of HG-HT and perinodular thyroid tissues of papillary thyroid carcinoma.

RESULTS

The TSH levels in the HG-HT group were not significantly different from those of the controls. The FT4 levels in the HG-HT group were significantly lower than those of the controls, whereas the FT3 levels and FT3/FT4 ratios were significantly higher in the HG-HT group. The FT3/FT4 ratios in the HG-HT group who had undergone total thyroidectomy and received levothyroxine therapy decreased significantly to normal values. Both the D1 and D2 activities in the thyroid tissues of the HG-HT patients were significantly higher than those of the controls. However, the mRNA levels of both D1 and D2 in the HG-HT patients' thyroid tissues were comparable to those of the controls. Interestingly, there were significant correlations between the HG-HT patients' D1 and D2 activities, and their thyroid gland volume or their FT3/FT4 ratios.

CONCLUSIONS

Our results indicate that increased thyroidal D1 and D2 activities may be responsible for the higher serum FT3/FT4 ratio in patients with HG-HT.

Type-2 iodothyronine 5'deiodinase in skeletal muscle of C57BL/6 mice. I. Identity, subcellular localization, and characterization

RT-PCR shows that mouse skeletal muscle contains type-2 iodothyronine deiodinase (D2) mRNA. However, the D2 activity has been hard to measure. Except for newborn mice, muscle homogenates have no detectable activity. However, we have reported D2 activity in mouse muscle microsomes. As the mRNA, activity is higher in slow- than in fast-twitch muscle. We addressed here the major problems in measuring D2 activity in muscle by: homogenizing muscle in high salt to improve yield of membranous structures; separating postmitochondrial supernatant between 38 and 50% sucrose, to eliminate lighter membranes lacking D2; washing these with 0.1 M Na(2)CO(3) to eliminate additional contaminating proteins; pretreating all buffers with Chelex, to eliminate catalytic metals; and eliminating the EDTA from the assay, as this can bind iron that enhances dithiothreitol oxidation and promotes peroxidation reactions. Maximum velocity of T(3) generation by postgradient microsomes from red muscles was approximately 1100 fmol/(h · mg) protein with a Michaelis-Menten constant for T(4) of 1.5 nM. D2-specific activity of Na(2)CO(3)-washed microsomes was 6-10 times higher. The enrichment in D2 activity increased in parallel with the capacity of microsomes to load (sarco/endoplasmic reticulum Ca(2+)-ATPase) and bind Ca(2+) (calsequestrin), indicating that D2 resides in the inner sarcoplasmic reticulum, close to the nuclei. The presence of D3 in the sarcolemma suggests that the most of D2-generated T(3) acts locally. Estimates from maximum velocity, Michaelis-Menten constant, and muscle T(4) content suggest that mouse red, type-1, aerobic mouse muscle fibers can generate physiologically relevant amounts of T(3) and, further, that muscle D2 plays an important role in thyroid hormone-dependent muscle thermogenesis.

Type 2 iodothyronine deiodinase is expressed in human preadipocytes

BACKGROUND

Type 2 iodothyronine deiodinase (D2) is an enzyme that catalyzes the production of triiodothyronine (T3) from thyroxine (T4) and plays a critical role in providing the local intracellular T3. Although D2 is highly expressed in brown adipose tissue, it was thought that D2 is hardly expressed in white adipose tissue. In the present study, we examined whether D2 is expressed in human preadipocytes, using human mesenteric and subcutaneous preadipocytes (HMPA and HSCPA, respectively).

METHODS

HMPA and HSCPA were purchased and cultured in the preadipocyte medium containing 10% fetal bovine serum. We measured D2 activity and mRNA level in HMPA and HSCPA incubated with or without dibutyryl cyclic adenosine monophosphate [(Bu)₂cAMP].

RESULTS

D2 activity and mRNA were detectable in human HMPA and HSCPA. The apparent Michaelis-Menten constant (K(m)) value for T4 in HMPA was 2.1 ± 0.2 nM, and the maximum velocity (V(max)) value was 333.3 ± 28.0 femtomols of I⁻ released/mg protein/hour, respectively. On the other hand, the apparent K(m) value for T4 in HSCPA was 2.0 ± 0.2 nM and the V(max) value was 91.2 ± 8.7 femtomols of I⁻ released/mg protein/hour, respectively. D2 activities in HMPA and HSCPA incubated with 1 mM (Bu)₂cAMP for 24 hours were 7-fold (HMPA) and 3-fold (HSCPA) higher than those without (Bu)₂cAMP, respectively. D2 mRNA levels in HMPA and HSCPA incubated with 1 mM (Bu)₂cAMP for 3 hours were 10-fold (HMPA) and 5-fold (HSCPA) higher than those without (Bu)₂cAMP, respectively.

CONCLUSIONS

In the present study, we have clearly demonstrated that D2 activity and mRNA are present in the human preadipocytes from both mesenteric and subcutaneous adipose tissue. Our experiments are the first ones that identify human preadipocytes as one of the sources of T3 production.

Type 1 and type 2 iodothyronine deiodinases in the thyroid gland of patients with 3,5,3'-triiodothyronine-predominant Graves' disease

OBJECTIVE

3,5,3'-triiodothyronine-predominant Graves' disease (T(3)-P-GD) is characterized by a persistently high serum T(3) level and normal or even lower serum thyroxine (T(4)) level during antithyroid drug therapy. The source of this high serum T(3) level has not been clarified. Our objective was to evaluate the contribution of type 1 and type 2 iodothyronine deiodinase (D1 (or DIO1) and D2 (or DIO2) respectively) in the thyroid gland to the high serum T(3) level in T(3)-P-GD.

METHODS

We measured the activity and mRNA level of both D1 and D2 in the thyroid tissues of patients with T(3)-P-GD (n=13) and common-type GD (CT-GD) (n=18) who had been treated with methimazole up until thyroidectomy.

RESULTS

Thyroidal D1 activity in patients with T(3)-P-GD (492.7±201.3 pmol/mg prot per h) was significantly higher (P<0.05) than that in patients with CT-GD (320.7±151.9 pmol/mg prot per h). On the other hand, thyroidal D2 activity in patients with T(3)-P-GD (823.9±596.4 fmol/mg prot per h) was markedly higher (P<0.005) than that in patients with CT-GD (194.8±131.6 fmol/mg prot per h). There was a significant correlation between the thyroidal D1 activity in patients with T(3)-P-GD and CT-GD and the serum FT(3)-to-FT(4) ratio (r=0.370, P<0.05). Moreover, there was a strong correlation between the thyroidal D2 activity in those patients and the serum FT(3)-to-FT(4) ratio (r=0.676, P<0.001).

CONCLUSIONS

Our results suggest that the increment of thyroidal deiodinase activity, namely D1 and especially D2 activities, may be responsible for the higher serum FT(3)-to-FT(4) ratio in T(3)-P-GD.

Cloning and characterization of a type 3 iodothyronine deiodinase (D3) in the liver of the chondrichtyan Chiloscyllium punctatum

Thyroid hormone bioactivity is finely regulated at the cellular level by the peripheral iodothyronine deiodinases (D). The study of thyroid function in fish has been restricted mainly to teleosts, whereas the study and characterization of Ds have been overlooked in chondrichthyes. Here we report the cloning and operational characterization of both the native and the recombinant hepatic type 3 iodothyronine deiodinase in the tropical shark Chiloscyllium punctatum. Native and recombinant sD3 show identical catalytic activities: a strong preference for T3-inner-ring deiodination, a requirement for a high concentration of DTT, a sequential reaction mechanism, and resistance to PTU inhibition. The cloned cDNA contains 1298 nucleotides [excluding the poly(A) tail] and encodes a predicted protein of 259 amino acids. The triplet TGA coding for selenocysteine (Sec) is at position 123. The consensus selenocysteine insertion sequence (SECIS) was identified 228 bp upstream of the poly(A) tail and corresponds to form 2. The deduced amino acid sequence was 77% and 72% identical to other D3 cDNAs in fishes and other vertebrates, respectively. As in the case of other piscivore teleost species, shark expresses hepatic D3 through adulthood. This characteristic may be associated with the alimentary strategy in which the protection from an exogenous overload of thyroid hormones could be of physiological importance for thyroidal homeostasis.

Expression of type 2 iodothyronine deiodinase in corticotropin-secreting mouse pituitary tumor cells is stimulated by glucocorticoid and corticotropin-releasing hormone

We identified the presence of iodothyronine deiodinase in AtT-20 mouse pituitary tumor cells that secrete corticotropin. Iodothyronine deiodinating activity in AtT-20 cells fulfills all the characteristics of type 2 iodothyronine deiodinase (D2), including the inhibition by thyroid hormones, the insensitivity to inhibition by 6-propyl-2-thiouracil, and the low Michaelis-Menten constant value for T4. Northern analysis using mouse D2 cRNA probe demonstrated the hybridization signal of approximately 7.0 kb in size in AtT-20 cells. D2 activity and D2 mRNA were stimulated by glucocorticoid in a dose-dependent manner but were not stimulated by testosterone or beta-estradiol. D2 expression was stimulated by (Bu)2cAMP, and CRH in a dose-dependent manner in the presence of dexamethasone. These results suggest the previously unrecognized role of local thyroid hormone activation by D2 in the regulation of pituitary corticotrophs.

Type 2 iodothyronine deiodinase expression is upregulated by the protein kinase A-dependent pathway and is downregulated by the protein kinase C-dependent pathway in cultured human thyroid cells

Type 1 and 2 iodothyronine deiodinases (D1 and D2) catalyze thyroxine (T4) activation. In human thyroid, unlike rodents', both D1 and D2 are expressed. We have investigated the effects of thyrotropin (TSH), dibutyryl cyclic adenosine monophosphate [(Bu)2cAMP] (an activator of protein kinase A [PKA]), 12-O-tetradecanoylphorbor 13-actate (TPA) (an activator of protein kinase C [PKC]), T4, and triiodothyronine (T3) on the D2 mRNA levels and activity in cultured human thyroid cells. D2 mRNA levels were increased by TSH and (Bu)2cAMP, and the increment was faster and greater than that of D1 mRNA levels. The increment of the maximum velocity (Vmax) value for D2 by (Bu)2cAMP stimulation was similar to that of D2 mRNA levels, suggesting that (Bu)2cAMP enhances D2 activity mainly at the pretranslational level. Cycloheximide, a protein synthesis inhibitor, partially inhibited the increase of D2 mRNA levels by (BU)2cAMP, suggesting that de novo protein synthesis-dependent pathways are involved. TPA suppressed the D2 mRNA levels in the presence of (Bu)2cAMP. However, T3 and T4 did not significantly change the D2 mRNA levels and activity. In conclusion, D2 expression in human thyroid cells is more rapidly and strongly upregulated by the PKA pathway than D1 expression, and is downregulated by the PKC pathway.

Iodothyronine deiodinases in a mammalian hibernator, the chipmunk (Tamias asiaticus)

We examined the activities of iodothyronine deiodinase, a key enzyme for thyroid hormone metabolism, in selected tissues of the chipmunk (Tamias asiaticus), a mammalian hibernator, of both sexes in the summer season. Reverse T3 5'-deiodinase (5'-D) activity was the highest in the liver followed by the kidney; T4 5'-D activity was the highest in brown adipose tissue (BAT) and T3 5'-deiodinase (5-D) activity was the highest in the testes followed by the brain. Distributions of three types of deiodinase activities in liver kidney BAT, and brain were comparable to other mammals reported, except that the type III deiodinase was unique in testes. The 5'-D activity of liver and kidney of chipmunks was 52% and 24%, respectively, of male rats and the 5-D activity of brain and testes of chipmunks was 227% and 567%, respectively of male rats. In addition, the cold exposure increased BAT 5'-D activity in chipmunks as reported in the ground squirrels. Our results indicated that tissue distribution of deiodinases and response to cold exposure in BAT in hibernators are similar to nonhibernators. However, there was a quantitative difference of rT3 5'-D and T3 5-D activities in some tissues between chipmunks and rats, indicating different local thyroid hormone metabolisms in hibernators and nonhibernators.

Impaired local deiodination of thyroxine to triiodothyronine in dogs with symmetrical truncal alopecia

Thyroid dysfunction causes certain dermatological alterations in dogs. Insufficient delivery of thyroid hormone to the skin may originate not only from inadequate thyroid function but also from impaired local activation of thyroxine in the target organ. Thyroid parameters and deiodination were investigated in healthy dogs (group C) and in dogs with cutaneous lesions associated with hypothyroidism (group H) or with a low-T3 syndrome (group LT). The ability of the skin to convert T4 to T3 was impaired in both groups H and LT but not in the controls. It is concluded that impaired local deiodination may contribute to skin problems in dogs.

Rapid alteration in circulating free thyroxine modulates pituitary type II 5' deiodinase and basal thyrotropin secretion in the rat

TSH secretion is decreased by both T4 and T3. This negative feedback control of TSH secretion has been correlated with an increase in pituitary nuclear T3 content, and it is not clear whether T4 exerts its effect directly on the thyrotroph or after its deiodination to T3. However, levels of the pituitary enzyme catalyzing T4 to T3 conversion, 5'D-II, are decreased in the presence of an increased amount of T4. Thus, it is unclear why the thyrotroph would have a mechanism for modulating the production of T3, if T3 is, in fact, the sole bioactive signal providing negative feedback inhibition. To examine this apparent paradox, we administered EMD 21388, a compound which inhibits the binding of T4 to transthyretin resulting in a rapid increase in circulating free T4 levels, to rats pretreated with radiolabeled T4 and T3. We observed increases in pituitary and liver T4 content of greater than 150%, without increases in the respective tissue T3 contents. The EMD 21388-treated rats also exhibited a 25% decrease in pituitary 5'D-II activity (103.8 +/- 15.8 fmol 125I released.mg protein-1.h-1, vs. control, 137.4 +/- 15.9, mean +/- SE), as did rats treated with sodium salicylate, another compound that inhibits T4-TTR binding (100.8 +/- 7.1). TSH levels significantly decreased 2 h after the administration of EMD 21388. These data demonstrate that despite a T4-mediated decrease in pituitary 5'D-II activity, an increase in T4 independently decreases TSH secretion.

The hepatic transcellular transport of 3,5,3'-triiodothyronine is reduced in aged rats

The mechanisms of age-related reduction in tissue-responsiveness to thyroid hormones is not clear. Since the first step in hormone action is the transport of the hormone from plasma to the tissues, tissue uptake of T3 was determined in three groups of male Fischer 344 rats: young (6 months old), aged (24-26 months old) and young rats pair-fed with aged rats. At steady state, total tissue uptake of T3 in the liver, heart and rectus abdominis muscle was reduced in aged rats while T3 uptake by cerebral tissues, femoris and soleus muscles was not altered with age. The subcellular distribution of T3 in the liver was determined and the binding power of cytosol and plasma was measured by equilibrium dialysis. In vitro saturation techniques provided estimates for the affinity (Ka) and binding capacity for L- and D-T3 of isolated hepatic nuclei at 37 degrees C. The plasma concentration of T3 was determined with radioimmunoassay. From these parameters the free cytosolic to plasma T3 ratio (Fc/Fp) and free nuclear to cytosolic ratio (Fn/Fc) were calculated. The Fc/Fp for L-T3 was significantly reduced in aged rats (2.34 +/- 0.15) compared to young rats (4.33 +/- 0.16) and young rats pair-fed with old (3.55 +/- 0.46). The Fc/Fp for D-T3 were 6.2 +/- 0.39, 24.3 +/- 2.3 and 10.1 +/- 1.5, respectively. The affinity constant (Ka) and the maximum binding capacity measured in isolated hepatic nuclei were not different in the three groups of rats. However, the nuclear uptake of L-T3 (T3N/P: percentage of dose per mg DNA per percentage of dose per ml plasma) was significantly reduced in aged rats (0.29 +/- 0.03) and young rats pair-fed with old (0.32 +/- 0.02) compared to young rats fed ad libitum (0.44 +/- 0.02). The corresponding values of D-T3 were 0.09 +/- 0.007, 0.13 +/- 0.006 and 0.22 +/- 0.01, respectively. The Fn/Fc of L- and D-T3 was not altered in aged rats or young rats pair fed with old. The liver uptake index (Ul) of L-[125I]T3 was determined in vivo with single injection tissue sampling technique. The liver uptake index in old rats (73.3 +/- 9.9%) was significantly reduced compared to young rats (107.5 +/- 9.4%). These results indicate that (1) cellular uptake of T3 is reduced in aged rat liver. These changes may be in part secondary to age-related reduction in food intake.(ABSTRACT TRUNCATED AT 400 WORDS)

Blood-brain transport of triiodothyronine is reduced in aged rats

An age-related decline in blood-brain barrier transport of thyroid hormones may contribute to the central nervous system changes with aging. To test this hypothesis, the brain uptake index (BUI) of levo (L) and dextro (D) triiodothyronine (T3) was determined in male Fischer 344 rats at 6 months of age (young) and 26 months of age (aged). Young rats pair fed with aged were included to control for reduced food intake in aged rats. The L-T3 BUI of aged rats (22.4 +/- 2.1%) was significantly reduced compared to young rats (29.5 +/- 2.0%) or young rats pair fed with aged rats (28.5 +/- 2.5%) (p less than 0.05). This could not be attributed to age-related changes in BBB permeability or to reduced cerebral blood flow. At steady state conditions, the brain uptake of either L-T3 or D-T3 was not altered with aging. There were no significant changes in plasma or brain binding of T3. These results indicate that the reduced BBB transport of T3 in aged rats is counterbalanced by a reduction in T3 clearance from the brain.

Phenolic and tyrosyl ring iodothyronine deiodination by the Caco-2 human colon carcinoma cell line

Thyroid hormone metabolism was studied in the human Caco-2 colon carcinoma cell line, which at confluence exhibits several functions of differentiated enterocytes. Cells were harvested two to 17 days after reaching confluence. Intact cells and homogenates were tested for deiodination of [125I]-labeled substrates. Small amounts of thyroxine (T4) were converted by homogenates to 3,3',5'-triiodothyronine (rT3), 3,3'-diiodothyronine (3,3'-T2), and 1-, with no detectable production of 3,5,3'-triiodothyronine (T3) by homogenates or cells. rT3 was converted to 3,3'-T2 and 1- with an apparent Michaelis constant (Km) for rT3 of 24 nmol/L; 6-n-propyl-2-thiouracil (PTU) had a 50% inhibitory concentration of 30 nmol/L and abolished rT3 5'-deiodination at 1 mmol/L in the presence of 20 mmol/L dithiothreitol (DTT). T3 was deiodinated to 3,3'-T2 and 3'-monoiodothyronine (3'-T1) with an apparent Michaelis constant (Km) for T3 of 5.7 nmol/L; this reaction was not inhibited by 1 mmol/L PTU. Phenolic and tyrosyl ring deiodinating activities were maximal four and six days, respectively, after the cells reached confluence. Homogenates of cells grown in standard medium containing fetal calf serum had fivefold higher rT3 5'-deiodinating activity than cells grown in a serum-free defined culture medium, reflecting a fivefold difference in the apparent Vmax with no difference in the apparent Km for rT3. There was no difference in T3 5-deiodination rates in homogenates of Caco-2 cells grown in the two media until 12 days postconfluence, when cells grown in standard medium had higher activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormones and 5'-deiodinase in rat brown adipose tissue during fetal life

Brown adipose tissue (BAT) iodothyronine 5'-deiodinase (5'D) activities are very high during fetal life but decrease 10-fold a few hours before birth. Accordingly, BAT 3,5,3'-triiodothyronine (T3) concentrations are also very high. The temporal patterns of changes in BAT 5'-D and fetal plasma insulin are similar (and differ from the pattern for catecholamines) but are not superimposable. A causal role for insulin in the activation of fetal BAT 5'-D is therefore not supported by the data. Maternal thyroidectomy leads to a decrease in the total and relative weight of fetal BAT and to a 30-50% increase in BAT 5'-D activities; BAT thyroid hormone concentrations are essentially unchanged. Fetal hypothyroidism was induced by giving methimazole and resulted in a marked decrease of BAT thyroxine (T4) and T3 concentrations. This treatment increased BAT 5'-D activity only on day 21 of gestation, but no effect was observed on day 20. The fetal 5'-D response to thyroid hormones infused into the methimazole-treated dams was studied at 21 days of gestation. The increase in BAT 5'-D induced by methimazole treatment was prevented by T4 infused into control dams but not by T3. In fetuses from thyroidectomized dams, the pattern of 5'-D regulation by thyroid hormones was impaired. It is suggested that the high concentrations of thyroid hormones present in fetal BAT might participate in the general maturation and development of fetal BAT.

Dibutyryl cAMP induction of type II 5'deiodinase activity in rat brain astrocytes in culture

Dibutyryl cAMP treatment of cultured rat astrocytes results in the rapid appearance of T4 to T3 conversion catalyzed by type II iodothyronine 5'deiodinase, without altering other deiodinating pathways. Induction of enzyme activity was time-dependent with a lag period of 60 min, reaching plateau levels after 6-8 hours, and required continued synthesis of mRNA and new protein. Isoproterenol also induced T4 to T3 converting activity through beta-adrenergic receptor mediated interactions. These data suggest that dibutyryl cAMP stimulated astrocytes provide an excellent model for the study of the molecular and cellular events modulating T4 to T3 conversion in the brain.

The regional hypothalamic distribution of type II 5'-monodeiodinase in euthyroid and hypothyroid rats

The brain topographical distribution of type II 5'-monodeiodinase (5'D-II), which converts thyroxine (T4) to triiodothyronine (T3), was studied in euthyroid and hypothyroid rats. Low levels of 5'D-II activity were detected in the median eminence, but not in any other brain regions of euthyroid rats. The arcuate nucleus and median eminence were also the sites of highest 5'D-II activity in brains of hypothyroid rats. Under these conditions, the paraventricular nucleus contained almost no detectable 5'D-II, while intermediate enzyme activity was present in other medial basal hypothalamic sites.

Type-II thyroxine 5'-deiodinase is present in the rat pineal gland

Thyroxine-5'-deiodinase has been identified in the rat pineal gland. The characteristics of the enzyme are compatible with a Type-II deiodinase which is tissue-specific and presumably related to generating a local action of thyroid hormone. Our data suggest there may be a previously unrecognized role of thyroid hormone in the regulation of pineal activity.

Potential of brown adipose tissue type II thyroxine 5'-deiodinase as a local and systemic source of triiodothyronine in rats

Previous reports suggest that a type II iodothyronine 5'-deiodinase may become the main enzymatic pathway for extrathyroidal triiodothyronine (T3) generation when the enzyme levels are sufficiently elevated and/or liver and kidney type I 5'-deiodinase activity is depressed. The present studies assessed the potential of brown adipose tissue (BAT) type II 5'-deiodinase to generate T3 for the plasma pool. BAT 5'-deiodination (BAT 5'D) was stimulated by either short- (4 h) or long-term (7 wk) cold exposure (4 degrees C). Long-term cold exposure increased thyroxine (T4) secretion 40-60% and extrathyroidal T3 production three-fold. In cold-adapted rats treated with propylthiouracil (PTU), extrathyroidal T3 production was 10-fold higher than in PTU-treated rats maintained at room temperature. Cold did not stimulate liver or kidney 5'D, but the cold-adapted rats showed a six- to eightfold higher BAT 5'D content. PTU caused greater than 95% inhibition of liver and kidney 5'D, but did not affect BAT 5'D. Thyroidectomized rats maintained on 0.8 micrograms of T4/100 g of body weight (BW) per day were acutely exposed to 4 degrees C. In rats given 10 mg of PTU/100 g of BW, 4 h of cold exposure still caused a 12-fold increase in BAT 5'D, a 2.3-fold increase in plasma T3 production, and a 4.8-fold increment in the locally produced T3 in BAT itself. All these responses were abolished by pretreatment with the alpha 1-antiadrenergic drug prazosin. Regardless of the ambient temperature, liver 5'D activity was greater than 90% inhibited by PTU. These results indicate that BAT can be a major source of plasma T3 under suitable circumstances such as acute or chronic exposure to cold. Furthermore, BAT 5'D activity affects BAT T3 content itself, suggesting that thyroid hormone may have a previously unrecognized role in augmenting the thermogenic response of this tissue to sympathetic stimulation. Such interactions may be especially important during the early neonatal period in humans, a time of marked thermogenic stress.

Characteristics of thyroxine 5'-deiodination in cultured human placental cells. Regulation by iodothyronines

Human and rat placental homogenates convert L-thyroxine (T4) to 3,5,3'-L-triiodothyronine (T3) via a pathway termed type II iodothyronine deiodination. To study regulation of this pathway, cell dispersions were prepared from human placental chorionic-decidual membrane. Dispersed cells deiodinated T4 and 3,3',5'-triiodothyronine (rT3), but not T3, at the 5' position. The reaction was only slightly inhibited by 1 mM 6-n-propylthiouracil, enhanced by dithiothreitol, and substantially inhibited by 50 nM iopanoic acid. Incubation of the cells in thyroid hormone-depleted medium induced a near doubling of T4 5'-deiodination in 36-48 h, with a significant rise seen as early as 12 h. Addition of T4, rT3, or T3 to hormone-depleted medium impaired the rise in type II deiodination in a dose-dependent fashion. T4 and rT3 were equipotent in this regard, and T3 was 2-3 times less potent. T4 was effective in physiological concentrations, 6.5-13 nM in medium containing 10% calf serum, and the effect of T4 was not due to its conversion to either T3 or rT3. In cells with deiodinase activity raised by 48 h incubation in thyroid hormone-depleted medium, addition of T4, T3, or rT3 reversed the increase in 8-24 h. Secretion of prolactin and beta hCG by the dispersed cells was not substantially affected by thyroid hormone deprivation. The increase in type II deiodination during thyroid hormone deprivation appears to depend on a signal from the thyroxine molecule, per se, and could potentially defend intracellular, and/or circulating, T3 pools in pathological states of mild-to-moderate hypothyroxinemia.

Regulation of iodothyronine 5'-deiodination in lean and obese (ob/ob) mice

Obese (ob/ob) mice exhibit impaired cold- and diet-induced thermogenesis, which may result in part from a defect in thyroid hormone action at the level of peripheral deiodination of thyroxine (T4) to 3,5,3'-triiodothyronine (T3). The latter possibility was examined by comparing kinetic parameters of 5'-deiodination (5'-D) in hepatic and renal microsomes of lean and obese mice during various physiological conditions. 5'-D Was adaptive to changes in age (1 to 8-10 wk), environmental temperature (14, 25, and 33 degrees C), and thyroid hormone status in both lean and obese mice. The magnitude of enzyme response, however, varied between phenotypes. 5'-D Response to age and environmental temperature was also dependent on tissue type, indicating that different isozymes for 5'-D may exist in liver and kidney. Under basal conditions at 25 degrees C, maximal hepatic and renal 5'-D was lower in obese mice than in lean mice. Differences in Vmax were observed as early as 1 wk of age. Km values for 5'-D were similar in lean and obese mice. These findings suggest that T3 availability to thermogenic target tissues may be impaired in obese mice, which may contribute to diminished thyroid hormone expression and heat production in these animals.

Binding of thyroid hormones by nuclei of target tissues during the chick embryo development

In the present studies we have compared the ontogeny of the binding of thyroxine (T4) and triiodothyronine (T3) to isolated nuclei from various target tissues of chick embryo. We observed a marked difference between the patterns of Satchard plots, maximal binding capacities (MBC) and association constants (Ka) of T4 and those of T3. Scatchard plots revealed that T4 and T3 had different binding sites. In liver, brain and lung MBCs and Kas of T3 and T4 were rather similar at day 9, but during the following days (12-19) T3 MBCs and Kas showed small changes, whereas T4 MBC markedly increased (4-5-fold) and T4 Ka significantly declined. In liver, for instance, T3 MBC = 395 +/- 19 (day 9) and 489 +/- 66 fmol/mg protein (day 19); T4 MBC = 631 +/- 6.5 (day 9) and 2201 +/- 516 fmol/mg protein (day 19); T4 Ka = 1.92 +/- 0.01 (day 9) and 0.56 +/- 0.21 X 10(8) M-1 (day 19). These data indicate that, during chick embryogenesis, nuclei of target tissues contain multiple T4 binding proteins, but only a single T3 binding site.

Decreased serum triiodothyronine in starving rats is due primarily to diminished thyroidal secretion of thyroxine

Although thyroxine (T4) 5'-deiodinase activity is diminished in liver homogenates of starved rats, no information is available regarding the effect of starvation on net T4 to triiodothyronine (T3) conversion in the intact rat. It appeared important to clarify this relationship since rat liver homogenates are widely used as a model for the study of the factors responsible for reduced circulating T3 in chronically ill and calorically deprived patients. In contrast to the expected selective decrease in circulating T3 levels in calorically restricted humans due to diminished T4 to T3 conversion, 5 d of starvation of two groups of male Sprague-Dawley rats resulted, paradoxically, in a greater decrease in serum T4 than in serum T3 levels. Kinetic studies show that starvation is associated with no change in the metabolic clearance rate (MCR) of T3, a 20% increase in the MCR of T4, a 67% reduction in turnover rate of T4, but only a 58% reduction in the turnover rate of T3. Moreover, in the first group of rats studied, direct chromatographic analysis of the isotopic composition of total body homogenates after the injection of 125I-T4 showed that 21.8% of T4 is converted to T3 in control rats and 28.8% in starved rats, suggesting that virtually all extrathyroidal T3 in starved and control rats is derived from the peripheral conversion of T4, and that there is little or no direct thyroidal secretion of T3. Our findings strongly point to a reduced thyroidal secretion of T4 as the primary cause of the observed reduction in circulating T3. Since the mechanisms leading to reduced levels of plasma T3 differ in humans and rats, it may be important to reexamine the use of liver homogenate preparations as models for study of the pathogenesis of the "low T3 syndrome" in humans.

Thyroid hormone metabolism in primary cultures of fetal rat brain cells

The metabolism of thyroxine 3,5,3',5'-tetraiodothyronine, (T4) and 3,5,3'-triiodothyronine (T3) was studied in primary cultures of dispersed fetal rat brain cells. Cultured brain cells actively metabolized both T4 and T3 by enzyme catalyzed deiodination reactions which increase (type II 5'-deiodinase) or decrease (type I 5'-deiodinase and type III 5-deiodinase) the bioactivity of thyroid hormone. Homogenates of cultured brain cells showed both type I and type II 5'-deiodinating activities and these two enzymes tended to differ in their time course of appearance. Cultures exposed to 10 microM cytosine arabinoside for 16 h showed up to a 70% reduction in type I activity without decreasing the type II enzyme suggesting that the type II enzyme is associated with non-dividing neuronal cells. The predominant pathway for T4 and T3 metabolism in situ was tyrosyl-ring or type III 5'-deiodination which followed first order kinetics with a t1/2 of 70 min. T4 to T3 conversion by the type II enzyme was consistently observed after correcting for the degradation of newly formed T3 by the type III enzyme. In situ, both type II and type II enzymes were thiol-dependent and both activities were inhibited by iopanoic acid. Type III 5-deiodination of T4 produced 34 fmol 3,3,5'-triiodothyronine (rT3)/h per 10(6) cells at 10 mM dithiothreitol (DTT) and 97 fmol of rT3/h per 10(6) cells at 50 mM DTT. T3 production by the type II enzyme was 1.2 and 4.4 fmol of T3/h per 10(6) cells at 10 and 50 mM DTT, respectively. Thyroid hormone deficient culture conditions increased type II enzyme activity by 4-5-fold within 48 h and this was prevented in a dose-dependent fashion by supplementing the media with increasing amounts of T3. These data indicate that primary cultures of dispersed brain cells mimic the intact cerebral cortex with respect to the metabolism of thyroid hormone and the regulatory mechanisms which defend cerebrocortical T3 levels. The vigorous metabolism of both T4 and T3 by these cultures may explain some of the difficulties in demonstrating thyroid hormone-dependent biochemical changes at physiologically relevant levels of thyroid hormone.

Stereospecific transport of triiodothyronine from plasma to cytosol and from cytosol to nucleus in rat liver, kidney, brain, and heart

We have investigated the transport of L- and D-triiodothyronine (T3) from plasma to cellular cytoplasm and from cytoplasm to nucleus by estimating the concentration of free hormone in these compartments in rat liver, kidney, brain, and heart. We assessed the distribution of T3 in various tissues and its metabolism by standard isotopic techniques and measured plasma and cytosolic tissue T3 by radioimmunoassay. In addition, we determined the fraction of radiosensitive T3 associated with the cytosol in individual tissues and estimated the cytosolic volume per gram of tissue. Equilibrium dialysis allowed us to determine the binding power of cytosols and plasma, and in vitro saturation techniques provided values for the affinity (ka) for L- and D-T3 of isolated nuclei in aqueous solution at 37 degrees C. We calculated the free cytosolic hormone from the product of cytosolic T3 and the binding power of cytosol for T3, and the free intranuclear T3 from the ka and previously determined ratio of occupied-to-unoccupied binding sites under steady state conditions in euthyroid animals. Our results showed that the free cytosolic/free plasma concentrations for L-T3 and D-T3, respectively, were: liver 2.8, 21.6; kidney 1.17, 63.3; heart 1.31, 1.58; brain 0.86, 0.24. The free nuclear/free cytosolic ratios for L-T3 and D-T3, respectively, were: liver 58.2, 3.70; kidney 55.9, 1.54; heart 80.6, 24.9; and brain 251, 108.6. Our findings suggest that stereospecific transport occurs both from plasma to cytosol and from cytosol to nucleus. The high gradients from cytosol to nucleus imply that there is an energy-dependent process and appear to account for the differences in the nuclear association constant determined in vivo and in vitro.

Production rates and turnover of triiodothyronine in rat-developing cerebral cortex and cerebellum. Responses to hypothyroidism

Local 5'-deiodination of serum thyroxine (T4) is the main source of triiodothyronine (T3) for the brain. Since we noted in previous studies that the cerebral cortex of neonatal rats tolerated marked reductions in serum T4 without biochemical hypothyroidism, we examined the in vivo T4 and T3 metabolism in that tissue and in the cerebellum of euthyroid and hypothyroid 2-wk-old rats. We also assessed the contribution of enhanced tissue T4 to T3 conversion and decreased T3 removal from the tissues to the T3 homeostasis in hypothyroid brain. Congenital and neonatal hypothyroidism was induced by adding methimazole to the drinking water. Serum, cerebral cortex (Cx), cerebellum (Cm), liver (L) and kidney (R) concentrations of 125I-T4, 125I-T3(T4), and 131I-T3 were measured at various times after injecting 125I-T4 and 131I-T3. The rate of T3 removal from the tissues was measured after injecting an excess of anti-T3-antibody to rats previously injected with tracer T3. In euthyroid rats, fractional turnover rates of T3 per hour were: Cx, 0.26 +/- 0.02 (SE); Cm, 0.20 +/- 0.02; L, 0.98 +/- 0.07; R, 0.97 +/- 0.12; and the calculated unidirectional plasma T3 clearance by these tissues were, in milliliters per gram per hour: Cx = 0.38, Cm = 0.32, L = 5.0, and R = 5.6. In hypothyroidism, the fractional removal rates and clearances were reduced in all tissues, in cortex and cerebellum by 70%, and in liver and kidney ranging from 30 to 50%. While greater than 80% of the 125I-T3(T4) in the brain tissues of euthyroid rats was locally produced, in hypothyroid cerebral cortex and cerebellum the integrated concentrations of 125I-T3(T4) were 2.7- and 1.5-fold greater than in euthyroid rats. In the Cx, this response resulted from an approximately sixfold increase in fractional conversion and an approximately fourfold decrease in T3 removal rate hampered by a decreased uptake of T4 from plasma, whereas in Cm the response resulted only from the reduced T3 removal rate. In euthyroid rats, the calculated production rate of T3 in nanograms per gram per hour by the Cx was 0.96 and 0.89 by the Cm, which on a per organ basis equals 15 and 2%, respectively, of the extrathyroidal production rate as assessed in the body pool exchanging with plasma. Several conclusions can be drawn: Production of T3 by developing brain is a very active process in agreement with the need of thyroid hormones during this period. (b) The brain-plasma exchange of T3 is slow compared with that of L or R. (c) This, along with the active local production, explains the predominant role of the latter as a source of T3 for the brain. (d) In hypothyroidism, the Cx is protected by an increase in the efficiency of T4 to T3 conversion and a prolong residence time of T3 in the tissue, whereas the Cm is protected only by the latter. Because of the large fraction of the T3 produced locally and the active turnover rate of T3 in the brain, reductions in T3 removal rate are of utmost importance for T3 homeostasis in these tissues.

Sources and quantity of 3,5,3'-triiodothyronine in several tissues of the rat

The local conversion of thyroxine (T4), which is an important source of intracellular 3,5,3'-triiodothyronine (T3) in several rat tissues, has been subject of recent investigations. In the present study the regulation of this phenomenon in vivo was investigated in various peripheral tissues of the rat. Intact euthyroid and radiothyroidectomized (Tx) rats received a continuous intravenous infusion of [125I]T4 and [131I]T3 until isotope equilibrium was attained. In addition to the labeled iodothyronines, Tx rats received a continuous intravenous infusion of 0.2 or 1.0 microgram carrier T4/100 g body wt per d, to create hypothyroid or slightly hypothyroid conditions, respectively. After the animals were bled and perfused the contribution of T3 derived from local conversion of T4 to T3 [Lc T3(T4)] to the total T3 in homogenates from several tissues and subcellular fractions from the liver, kidney, and anterior pituitary gland could be calculated. In all experiments T3 in muscle was derived exclusively from the plasma. In the cerebral cortex and cerebellum, however, most of the intracellular T3 was derived from the intracellular conversion of T4 to T3. It is demonstrated that for hypothyroid rats an increased relative contribution of Lc T3(T4) reduced the loss of total T3 in the brain. This phenomenon was also encountered for the anterior pituitary gland, although in this tissue the proportion of the total tissue T3, contributed by locally produced T3 was considerably lower than the values found for the cerebral cortex and cerebellum in all experiments. The present findings, regarding the source and quantity of pituitary nuclear T3 strongly suggest that both plasma T3 and T4 (through its local conversion into T3) play a role in the regulation of thyrotropin secretion. The contribution of Lc T3(T4) to the total pituitary nuclear T3 was of minor importance in euthyroid rats (approximately 20%), compared with that found for both groups in T4-supplemented athyreotic rats (approximately 40%). The total T3 concentration in the liver decreased from euthyroid to hypothyroid rats and was associated with a decrease in the tissue/plasma T3 concentration gradient. A minor proportion of hepatic T3 was contributed by Lc T3(T4), which in fact decreased significantly from the euthyroid to the hypothyroid state. In contrast to other subcellular fractions from the liver, no Lc T3(T4) could be demonstrated in the nuclear fraction. It is suggested that the liver plays an important role with respect to regulation of the circulating T3 concentration. In the kidney, a very small proportion of the total T3 was derived from locally produced T3 in all experiments (4-7%). As found in the liver, all nuclear T3 appeared to be derived from the plasma. In contrast to the liver, subcellular T3 pools in the kidney seemed to be exchangeable.

Formation of diiodotyrosine from thyroxine. Ether-link cleavage, an alternate pathway of thyroxine metabolism

Studies were performed to elucidate the nature of the pathway of hepatic thyroxine (T4) metabolism that is activated by inhibitors of liver catalase. For this purpose, the metabolism of T4 in homogenates of rat liver was monitored with T4 labeled with 125I either at the 5'-position of the outer-ring (125I-beta-T4) or uniformly in both the outer and inner rings (125I-U-T4). In homogenates incubated with 125I-beta-T4 in an atmosphere of O2, the catalase inhibitor aminotriazole greatly enhanced T4 degradation, promoting the formation of large proportions of 125I-labeled iodide (125I-I-) and chromatographically immobile origin material (125I-OM), but only a minute proportion of 125I-labeled 3,5,3'-triiodothyronine (125I-T3) (T3 neogenesis). In an atmosphere of N2, in contrast, homogenates produced much larger proportions of 125I-T3, and aminotriazole had no effect. In incubations with 125I-U-T4, under aerobic conditions, control homogenates degraded T4 slowly; formation of 125I-labeled 3,5-diiodotyrosine (125I-DIT) was seen only occasionally and in minute proportions. However, in homogenates incubated under O2, but not N2, aminotriazole consistently elicited the formation of large proportions of 125I-DIT, indicating that the ether link of T4 was being cleaved by an O2-dependent process. Formation of 125I-DIT in the presence of aminotriazole and O2 was markedly inhibited by the substrates of peroxidase, aminoantipyrine, and guaiacol. GSH greatly attenuated the increase in DIT formation induced by aminotriazole, whereas the sulfhydryl inhibitor N-ethylmaleimide (NEM) activated the DIT-generating pathway, even in the absence of aminotriazole. Activation of the in vitro formation of 125I-DIT from 125I-U-T4 was also produced by the in vivo administration of aminotriazole or bacterial endotoxin, an agent that reduces hepatic catalase activity. Studies with 125I-DIT as substrate revealed it to be rapidly deiodinated by liver homogenates under aerobic conditions. Recovery of 125I-DIT from 125I-U-T4 was increased by the addition of the inhibitor of iodotyrosine dehalogenase, 3,5-dinitrotyrosine. However, as judged from studies conducted in parallel with radioiodine-labeled DIT and 125I-U-T4 as substrates, none of the factors that altered the proportion of 125I-DIT found after incubations with 125I-U-T4 did so by altering the degradation of the 125I-DIT formed. The factors that influenced DIT formation from T4 in rat liver had opposite effects on T3 neogenesis. Thus, aminotriazole, endotoxin, NEM, and an aerobic atmosphere, all of which enhanced DIT formation, were inhibitory to T3 neogenesis. In contrast, anaerobiosis and GSH inhibited ether-link cleavage of T4, but facilitated T3 neogenesis. The foregoing results suggest that a pathway for the ether-link cleavage of T4 to yield DIT is present in rat liver. Activity of this pathway, which appears to be peroxidase mediated, is inversely related to activity of the pathway for the T3 neogenesis. It is further suggested that this reciprocity reflects a reciprocal relationship between hepatic GSH and H2O2, the former increasing T3 formation and inhibiting DIT formation, and the latter producing opposite effects.

Ether link cleavage is the major pathway of iodothyronine metabolism in the phagocytosing human leukocyte and also occurs in vivo in the rat

These studies were performed to test the hypothesis that ether link cleavage (ELC) is an important pathway for the metabolism of thyroxine (T(4)) in the phagocytosing human leukocyte. When tyrosyl ring-labeled [(125)I]T(4)([Tyr(125)I]T(4)) was incubated with phagocytosing leukocytes, 50% of the degraded label was converted into [(125)I]3,5-diiodotyrosine ([(125)I]DIT). Of the remaining [Tyr(125)I]T(4) that was degraded, two-thirds was recovered as [(125)I]-nonextractable iodine ([(125)I]NEI), and one-third as [(125)I]iodide. The production of [(125)I]DIT was not observed when phenolic ring-labeled [(125)I]T(4) ([Phen(125)I]T(4)) was used, although [(125)I]NEI and [(125)I]iodide were produced. None of these iodinated compounds were formed in leukocytes that were not carrying out phagocytosis. The fraction of T(4) degraded by ELC was decreased by the addition of unlabeled T(4) and by preheating the leukocytes, findings which suggested that the process was enzymic in nature. ELC was enhanced by the catalase inhibitor aminotriazole, and was inhibited by the peroxidase inhibitor propylthiouracil, suggesting that the enzyme is a peroxidase and that hydrogen peroxide (H(2)O(2)) is a necessary cofactor in the reaction. To test this hypothesis, studies were performed in several inherited leukocytic disorders. ELC was not observed in the leukocytes of patients with chronic granulomatous disease, in which the respiratory burst that accompanies phagocytosis is absent. ELC was normal in the leukocytes of two subjects homozygous for Swiss-type acatalasemia, and aminotriazole enhanced ELC in these cells to an extent not significantly different from that observed in normal cells. ELC was normal in the leukocytes of a patient with myeloperoxidase deficiency, but could be induced by the incubation of [Tyr(125)I]T(4) with H(2)O(2) and horseradish peroxidase in the absence of leukocytes. The in vivo occurrence of ELC in the rat was confirmed by demonstrating the appearance of [(125)I]DIT in serum from parenterally injected [(125)I]3,5-diiodothyronine, but no [(125)I]DIT was produced when [(125)I]3',5'-diiodothyronine was administered. FROM THESE FINDINGS WE CONCLUDE THE FOLLOWING: (a) ELC is the major pathway for the degradation of T(4) during leukocyte phagocytosis, and accounts for 50% of the disposal of this iodothyronine; (b) the NEI and iodide formed by phagocytosing cells are derived from the degradation of the phenolic and tyrosyl rings of T(4), although ELC per se accounts for only a small fraction of these iodinated products; (c) the process by which ELC occurs is enzymic in nature, and its occurrence requires the presence of the respiratory burst that accompanies phagocytosis; (d) the enzyme responsible for ELC is likely to be a peroxidase, although a clear role for myeloperoxidase as the candidate enzyme remains to be established; (e) iodothyronines are also degraded by ELC in vivo, and the quantitative importance of this pathway in various pathophysiological states requires further investigation.

Comparison of iodothyronine 5'-deiodinase and other thyroid-hormone-dependent enzyme activities in the cerebral cortex of hypothyroid neonatal rat. Evidence for adaptation to hypothyroidism

Recent studies have shown that approximately 75% of the nuclear 3,5,3'-triiodothyronine (T(3)) present in adult rat cerebral cortex (Cx) derives from 5'-deiodination of thyroxine (T(4)) within this tissue. The activity of iodothyronine 5'-deiodinase (I 5'D), the enzyme catalyzing T(4) to T(3) conversion, increases rapidly after thyroidectomy, suggesting that this could be a compensatory response to hypothyroxinemia. To evaluate this possibility during the period of central nervous system maturation, we studied several thyroid hormone-responsive enzymes (aspartic transaminase [AT], succinic dehydrogenase [S.D.], and Na/K ATPase) in the Cx of 2-, 3-, and 4-wk-old rats. The rats were made congenitally hypothyroid by placing 1, 2, 5, and 20 mg methimazole (MMI) in 100 ml of the mothers' drinking water from day 16 of gestation throughout the nursing period and to the litters after weaning. In addition, serum thyroid hormones, I 5'D, and, in some experiments, in vivo T(4) to T(3) conversion in Cx were measured in the same pups. Serum T(4) concentrations varied from <1 to 40 ng/ml and were generally inversely related to maternal MMI dose. Serum T(3) was less affected by MMI than was T(4). At 2 wk, decreases in AT, S.D., and ATPase were present in the 20-mg-MMI group but not in the 5-mg-MMI pups despite low serum T(4) (<10 ng/ml) in the latter. At 3 and 4 wk, both 5- and 20-mg-MMI groups had significant reductions in these cortical enzymes despite a normal serum T(3) in the 5-mg-MMI rats. Cortical I 5'D activity was 10-fold the control value in 5- and 20-mg-MMI animals at 2 wk but increased only three- to fivefold at 3 and 4 wk. I 5'D correlated inversely with serum T(4) (r >/= 0.96) at all ages, but the less marked elevation of this enzyme in 3- and 4-wk-old pups was not accompanied by an increase in serum T(4). Serum T(3) increased or remained the same between 2 and 3 wk. These results suggested that the 10-fold increase in I 5'D at 2 wk protected the 5-mg-MMI group from tissue hypothyroidism, but that the three- to fivefold increase at 3 and 4 wk could not. Injection of approximately 250 ng T(4)/100 g body weight to 2-wk-old, 20-mg-MMI pups (one-sixth the normal T(4) production rate) led to both a 1.8-ng/g cortical tissue increment in cortical T(3) and a significant increase in AT at 24 h, compared with a 0.38-ng/g cortical tissue T(3) increment and no change in AT in euthyroid controls. The larger increment in T(3) of the 20-mg-MMI pups was due in great part to increased fractional T(4) to T(3) conversion. Although the latter resulted in greater serum T(3) concentrations, three-fourths of the newly formed T(3) in the cortex was generated in situ, and it was blocked by iopanoic acid as was the increase in AT. We conclude that 70-80% of the T(3) in the Cx of the neonatal rat is produced locally. Serum T(4) appears to serve both as a precursor for T(3) and as a critical signal for increases in cortical I 5'D. The increased I 5'D can result in normal or near-normal cerebrocortical T(3) concentrations despite marked reductions in serum T(4). This mechanism seems to be particularly effective around 2 wk of age when many thyroid-hormone-dependent maturational changes occur in the rat Cx.

Uptake, metabolism and action of triiodothyronine in calf-thyroid slices

T3 and T4 at 10(-6)M caused a significant inhibition on [3H]uridine incorporation into RNA in calf-thyroid slices. This effect was not altered by addition of 10(-3) M PTU or MMI. The metabolism of 125I-T3 was studied under the same conditions. There was a very slight dehalogenation after 60 min (less than 5%) which was inhibited by PTU. The time course of the uptake of labeled T3 showed a temperature dependence, and this uptake was not altered by 10(-6)M KI, 1-T4, DIT or MIT, thus indicating specificity. The early phase of labeled T3 entrance into the cell was inhibited by the addition of colt T3 in a dose- dependent manner from 10(-9) to 10(-5)M. Slices previously stimulated by cAMP or CGMP showed a significant increase in the uptake of labeled T3. ATPase activity or ATP, protein or RNA synthesis does not seem to play a role in this process. The relationship between substitutions in the thyronine molecule and its biological action on RNA synthesis was also studied, and some preliminary conclusions were drawn. These studies demonstrate that T3 has a direct action on the thyroid.

Phenolic and tyrosyl ring deiodination of iodothyronines in rat brain homogenates

Conversion of thyroxine (T(4)) to 3,5,3'-triiodothyronine (T(3)) in rat brain has recently been shown in in vivo studies. This process contributes a substantial fraction of endogenous nuclear T(3) in the rat cerebral cortex and cerebellum. Production of T(4) metabolites besides T(3) in the brain has also been suggested. To determine the nature of these reactions, we studied metabolism of 0.2-1.0 nM [(125)I]T(4) and 0.1-0.3 nM [(131)I]T(3) in whole homogenates and subcellular fractions of rat cerebral cortex and cerebellum. Dithiothreitol (DTT) was required for detectable metabolic reactions: 100 mM DTT was routinely used. Ethanol extracts of incubation mixtures were analyzed by paper chromatography in t-amyl alcohol:hexane:ammonia and in 1-butanol:acetic acid. Rates of production of iodothyronines from T(4) and T(3) were greater at pH 7.5 than at 6.4 or 8.6 and greater at 37 degrees C than at 22 degrees or 4 degrees C. Lowering the pH, reducing the protein or DTT concentrations, and preheating homogenates to 100 degrees C all increased excess I(-) production but reduced iodothyronine production. In cerebral cortical homogenates from normal rats, products of T(4) degradation were as follows (percent added T(4)+/-SEM in nine experiments): T(3), 1.9+/-0.5%; 3,3',5'-triiodothyronine (rT(3)), 34.0+/-2.4%; 3,3'-diiodothyronine (3,3'-T(2)), 5.8+/-1.6%; 3'-iodothyronine (3'-T(1)), </=2.5%; and excess I(-), 4.7+/-1.2%. In the same experiments, products of T(3) degradation were 3,3'-T(2), 63.3+/-5.5%, and 3'-T(1), 12.6+/-1.4%. Cerebral cortical homogenates from hyperthyroid rats and normals were similar in regard to T(4) to T(3) deiodination. In contrast, in cerebral cortical homogenates from hypothyroid rats, phenolic ring deiodination rates were increased and tyrosyl ring deiodination rates were decreased compared with normals.T(4) to T(3) conversion rates in cerebellar homogenates were greater than rates in cerebral cortical homogenates from the same normal rats and less than rates in cerebellar homogenates from hypothyroid rats. T(4) and T(3) tyrosyl ring deiodination rates were greatly diminished in cerebellar homogenates compared with cerebral cortical homogenates in normal and hypothyroid rats. High-speed (1,000-160,000 g) pellets from cerebral cortical homogenates were enriched in phenolic and tyrosyl ring deiodinating activities relative to cytosol. Fractional conversion of T(4) to T(3) was inhibited by T(4), iopanoic acid, and rT(3), but not by T(3). Tyrosyl ring deiodination reactions were inhibited by T(3), T(4), and iopanoic acid, but not by rT(3). These studies demonstrate separate phenolic and tyrosyl ring iodothyronine deiodinase enzymes in rat brain. The brain phenolic ring deiodinase serves in vivo as a T(4) 5'-deiodinase and closely resembles anterior pituitary T(4) 5'-deiodinase in physiological and biochemical characteristics. The physiological significance of the tyrosyl ring iodothyronine deiodinase enzyme is unclear; it shares several properties with rat hepatic T(4) 5-deiodinase.

Changes in extrathyroidal conversion of thyroxine (T4) to 3,3',5-triiodothyronine (T3) in vitro during development and aging of the rat

Changes with age in the rate of in vitro conversion of thyroxine (T4) to triiodothyronine (T3) by organ homogenates were determined in female Wistar rats from 6 days to 18 months old. The conversion rates in liver and kidney homogenates of pups before day 13 are very low, and then rise to reach the maximum value at around day 23. During the period of sexual maturation the conversion activity suddenly decreases, and shows afterwards a gradual decline with age. The activity in both liver and kidney of male rats is higher than that of females except in the immature liver. A sudden rise was observed in the conversion activity of organ homogenates when the rats were released from a period of restricted food intake which started at an early age before maturity was achieved.

Exchange of triiodothyronine derived from thyroxine with circulating triiodothyronine as studied in the rat

At present it is widely assumed that T3 derived from T4 is rapidly and totally exchangeable within the volume of distribution of T3 secreted by the thyroid into the bloodstream. This concept is implied when conclusions are drawn from comparisons between a biological effect in a responsive tissue and circulating T3 and T4 levels. Such conclusions are often in conflict with those derived by comparing the biological effect with the concentrations of T3 and T4 in the responsive tissue itself. Thus, it appeared important to test the above assumption directly. Thyroidectomized rats have been treated for 4-4 1/2 days with a mixture of 131I labelled T4 (131T4) and 125I labelled T3 (125T3), which was either injected twice daily or administered by continuous i.v. infusion. The rats were bled, perfused, and their plasma and tissues submitted to extraction and paper chromatography. If the tested assumption were correct, the ratio between the T3 derived from T4 and the T3 injected as such (namely, the 131T3/125T3 ratio) should be the same in plasma, liver, kidney, heart, muscle, etc. It was evident that the 131T3/125T3 ratio was not the same for different tissues. The differences were not merely due to artefactual deiodinations. The presence of small amounts of 131I and 125I containing compounds in the T3 spot was considered as highly unlikely, though not totally excluded. The data thus suggest that T3 derived from T4 and the injected (or thyroidally secreted) T3 might not be totally exchangeable within an observation period which is considerably longer than the one for which complete equilibrium was previously assumed. If so, changes in the size of the T4 pool, or in the rate of T4 conversion to T3, might affect the concentration of T3 in a given tissue to an extent not disclosed from the circulating T3 levels alone. Several possible consequences of the present findings are discussed.

Synaptosomal [125I]triiodothyronine after intravenous [125I]thyroxine

We administered [125I]thyroxine intravenously to adult male rats and measured uptake and subcellular distribution of the hormone and its metabolites in brain. Fractional brain uptake decreased after a large dose of iodothyronine, providing evidence for saturability of the uptake mechanism. Well-defined patterns of regional and subcellular labeling were noted within 1 h after [125I]thyroxine injection. Radioactivity in synaptosomes was always greater than in any other particle separated per gram of brain, increasing linearly relative to radioactivity in brain cytosol during the 1st h. Although [125I]triiodothyronine derived from [125I]thyroxine was not identified in serum at any time interval, it was measurable in synaptosomes within 20 min and in brain cytosol within 1 h after labeled hormone administration. Concentrations of the radioactive metabolite were twofold greater and ratios of [125I]triiodothyronine to [125I]thyroxine concentration were threefold greater in synaptosomes than in cytosol. Therefore, thyroxine may be converted to triiodothyronine within nerve terminals. Synaptosomal localization of iodothyronines and their metabolites may be relevant to the marked central and peripheral adrenergic nervous system effects of these aromatic amino acid hormones.

Pituitary nuclear 3,5,3'-triiodothyronine and thyrotropin secretion: an explanation for the effect of thyroxine

An excellent correlation was observed between nuclear triiodothyronine (T3) and the ensuing suppression of thyrotropin (TSH) after a single intravenous injection of T3 to thyroidectomized (hypothyroid) rats. At 1 and 2 hours after injection of thyroxine (T4), in amounts equally potent to the administered T3 in terms of acute suppression of TSH, the same quantities of T3 were found in the pituitary nuclei. Virtually no nuclear T4 was present, and plasma T3 was negligible at these short intervals after T4 injection. These results suggest that suppression of TSH release in hypothyroid rats occurs by interaction of T3 with the nuclear receptor of the thyrotroph. After T4 injection, the T3 found in the nucleus is derived from rapid intrapituitary monodeiodination.

Radioimmunological measurement of 3,3'-diiodothyronine in serum and amniotic fluid

A radioimmunoassay for the measurement of 3,3'-diiodo-L-thyronine (rT2) in serum and amniotic fluid is described. Specific antisera to rT2 were produced by immunization of rabbits with rT2 conjugated to bovine serum albumin. The molar cross-reactivity tested for various iodothyronines and iodotyrosines was 0.3% for triiodothyronine and 0.001% for thyroxine. The sensitivity of the assay with a detection limit of 1.8 fmol/tube (0.94 pg/tube) was due to the high avidity of the antiserum and the use of 125I-labelled rT2 of maximum specific radioactivity. In most of 45 normal subjects, serum rT2 levels measured in evaporated ethanol extracts were below the detection limit of 0.018 nmol/1. Mean rT2 concentrations were 0.21 nmol/1 in newborn cord serum and 0.10 nmol/1 in amniotic fluid at 12 to 30 weeks of pregnancy. The molar concentration ratio of 3,3',5'-triiodothyronine (rT3) to rT2 was estimated to be 15.7 in cord serum of newborn. A similar rT3/T2 ratio was found in adults after intravenous application of 500 microgram rT3. Using these data and the known rT3 values, a hypothetical mean serum rT2 concentration in adult normal subjects of 0.015 nmol/1 was calculated. The radioimmunoassay described may be a useful analytical tool in studies of the synthesis as well as the metabolism of thyroid hormones.

Preparation of 125I-thyroxine and 125I-triiodothyronine of high specific activity for the radioimmunoassay of serum total T4 and T3

A simple and inexpensive procedure for the preparation of high specific activity 125I radioiodinated triiodothyronine and thyroxine is described. The specific activities achieved are approximately 350 Ci/g for 125I-T4, and either 200 Ci/g for 125I-T3 depending on the reagents employed. The high specific activity 125I-T4 and 125I-T3 are suitable for the routine radioimmunoassay of serum total T3 and T4. When properly stored they have a self life of at least seven weeks.

Studies on human thyroxine-binding globulin (TBG). IX. Some physical, chemical, and biological properties of radioiodinated TBG and partially desialylated TBG

Thyroxine-binding globulin (TBG) and partially desialylated or slow TBG (STBG) were purified from human serum by affinity chromatography. Purified TBG was identical to TBG present in serum by the criteria of electrophoretic mobility, affinity for thyroxine (T4), and heat-inactivation response. Purified STBG had slower electrophoretic mobility and lower affinity for T4. Both bound T4 in an equimolar ratio, were immunoprecipitable, and had similar inactivation t1/2 at 61 degrees C. TBG and STBG were iodinated by the chloramine-T-catalyzed reaction. An average of from 0.02 to 6 atoms I could be incorporated per molecule of the protein by adjusting the conditions of the reaction (time, protein and iodide concentrations). 125-I, 131-I, and 127-I were used. Iodination increased the anodal mobility of TBG but did not affect the reversible T4-binding, precipitation by antiserum, or the heat-inactivation properties. "Heavily" and "lightly" iodinated TBG had identical disappearance half-times from serum in the rabbit. 15 min after the intravenous administration of [131-I]-STBG and [125-I]TBG mixture to rats, more than 90% of the injected 131-I dose was in the liver, and the liver 131-I/125-I ratio was 32-fold that of serum. Selective uptake of STBG by the liver was also observed in the rabbit and in man. The serum [125-I]STBG/[131-I]TBG ratio declined from 1 to 0.2 in 10 min in the intact rabbit but remained unchanged for 1 h in the acutely hepatectomized animal. In the rabbit, t 1/2 was approximately 3 min for STBG and 0.8-3.4 days for TBG. The radioiodine derived from the iodinated proteins is partly excreted in bile but the bulk was precipitable with specific antibodies. Some isotope in the form of iodide appeared in blood and was excreted in the urine. Since radioiodinated TBG and STBG preserve their biologic and immunologic properties they are useful as tracer materials for metabolic studies. In rat, rabbit, and man STBG is rapidly cleared from serum by the liver. Conversion of TBG to STBG may be the limiting step in the regulation of TBG metabolism.

Estimation of thyroxine and triiodothyronine distribution and of the conversion rate of thyroxine to triiodothyronine in man

Studies on peripheral metabolism of simultaneously administered 125-I-labeled L-thyroxine ([125-I]T4) and 131-I labeled L-trilodothyronine ([131-I]T3) were performed in five normal subjects, in four patients with untreated hypothyroidism, and in 3 hypothyroid patients made euthyroid by the administration of T4. The fractional turnover rate (lambda 03) of thyroid hormones irreversibly leaving the site of degradation and the volumes of pool 1 (serum V1) of pool (interstitial fluid, V2), and of pool 3 (all tissues, V3)were obtained by using a three-compartment analysis. In addition to the turnover studies, the ratios for the in vivo T4 to T3 conversion were determined by paper chromatographic study in sera obtained 4, 7, and 10 daysafter the injection. The rate (K12) of the extrathyroidal conversion of T4 to T3 was also estimated by the compartment analysis. The T3 distribution volume (V3) of pool 3, in which T3 is utilized and degraded, was about 60% of totaldistribution volume (V=V1+V2+V3) in normal subjects, whereas only about 25% of the extrathyroidal T4 pool was in the intracellular compartment, indicating that T3 is predominantly an intracellular hormone..

Thyroid hormone action: in vitro characterization of solubilized nuclear receptors from rat liver and cultured GH1 cells

We previously reported that putative nuclear receptors for thyroid hormone can be demonstrated by incubation of hormone either with intact GH(1) cells, a rat pituitary tumor cell line, or with isolated GH(1) cell nuclei and rat liver nuclei in vitro. We characterized further the kinetics of triiodothyronine (T3) and thyroxine (T4) binding and the biochemical properties of the nuclear receptor after extraction to a soluble form with 0.4 M KCl. In vitro binding of [(125)I]T3 and [(125)I]T4 with GH(1) cell and rat liver nuclear extract was examined at 0 degrees C and 37 degrees C. Equilibrium was attained within 5 min at 37 degrees C and 2 h at 0 degrees C. The binding activity from GH(1) cells was stable for at least 1 h at 37 degrees C and 10 days at - 20 degrees C. Chromatography on a weak carboxylic acid column and inactivation by trypsin and Pronase, but not by DNase or RNase, suggested that the putative receptor was a nonhistone protein. The estimated equilibrium dissociation constants (K(d)) for hormone binding to the solubilized nuclear binding activity was 1.80 x 10(-10) M (T3) and 1.20 x 10(-9) M (T4) for GH(1) cells and 1.57 x 10(-10) M (T3) and 2.0 x 10(-9) M (T4) for rat liver. These K(d) values for T3 are virtually identical to those which we previously reported with isolated rat liver nuclei and GH(1) cell nuclei in vitro. The 10-fold greater affinity for T3 compared to T4 in the nuclear extract is also identical to that observed with intact GH(1) cells. In addition, the [(125)I]T3 and [(125)I]T4 high-affinity binding in the nuclear extract were inhibited by either nonradioactive T3 or T4, which suggests that the binding activity in nuclear extract was identical for T3 and T4. In contrast, the binding activity for T4 and T3 in GH(1) cell cytosol was markedly different from that observed with nuclear extract (K(d) values were 2.87 x 10(-10) M for T4 and 1.13 x 10(-9) M for T3). Our results indicate that nuclear receptors for T3 and T4 can be isolated in a soluble and stable form with no apparent change in hormonal affinity. This should allow elucidation of the mechanisms of thyroid hormone action at the molecular level.

Limited binding capacity sites for L-triiodothyronine in rat liver nuclei. Nuclear-cytoplasmic interrelation, binding constants, and cross-reactivity with L-thyroxine

Further studies have been performed to define the kinetic characteristics of nuclear triiodothyronine (T(3)) binding sites in rat liver (J. Clin. Endocrinol. Metab. 1972. 35: 330). Sequential determination of labeled T(3) associated with nuclei and cytoplasm over a 4-h period allowed analysis of the relationship of T(3) in nuclear and cytoplasmic compartments. A rapid interchange of hormone between nuclei and cytoplasm was demonstrated, and in vitro incubation experiments with nuclei yielded no evidence favoring metabolic transformation of T(3) by the nuclei. In vivo displacement experiments were performed by subcellular fractionation of liver (1/2) h after injection of [(125)I]T(3) with increasing quantities of unlabeled T(3). The nuclear binding capacity for T(3) could be defined (0.52 ng/mg DNA). Analysis of these experiments also allowed an estimation of the association constant of nuclear sites for T(3) (4.7 x 10(11)M(-1)). The affinity of these sites for T(3) was estimated to be 20-40 fold greater than for thyroxine (T(4)). Chromatographic analysis of the nuclear radioactivity after injection of labeled T(4) indicated that the binding of T(4) by the nucleus could not be attributed to in vivo conversion of T(4) to T(3) but reflected intrinsic cross-reactivity of the two iodothyronines at the nuclear binding sites.

Thyroid hormone action in cell culture: domonstration of nuclear receptors in intact cells and isolated nuclei

Triiodothyronine and thyroxine induce a 3-fold increase in the rate of growth of GH(1) cells in culture. To study further the action of these hormones, we examined the binding of [(125)I]triiodothyronine and purified [(125)I]thyroxine to cellular fractions after incubation with intact cells in serum-free medium. High-affinity, low-capacity binding sites for the hormones were demonstrated in nuclear but not in mitochondrial or cytosol fractions. Chromatographic analysis of the bound nuclear radioactivity from cells incubated with [(125)I]thyroxine demonstrated 97% thyroxine, 1% iodide, and 1% triiodothyronine. Apparent equilibrium dissociation constants, determined by Scatchard analysis, were 29 pM for triidothyronine and 260 pM for thyroxine. The maximal binding capacity was identical for both hormones, with about 5000 sites per cell nucleus. [(125)I]Thyroxine binding was competitively inhibited by triiodothyronine. These data suggest that triiodothyronine and thyroxine interact with identical nuclear receptors, and that conversion of thyroxine to triiodothyronine may not be a prerequisite for biologic activity. Similar high-affinity, low-capacity nuclear binding sites were also demonstrated by incubation of [(125)I]triidothyronine directly with isolated nuclei. Incubation of cells with increasing concentrations of nonradioactive triidothyronine results in a subsequent increase in binding when [(125)I]triiodothyronine is then incubated directly with isolated nuclei. This result suggests that nuclear receptors are not fixed, but increase after exposure of intact cells to hormone. This increase in nuclear receptor content may result from the transfer of an unstable cytosol receptor to the nucleus.