Iodothyronine Deiodinase Activity Research Articles

Effects of selenium and iodine deficiency on thyroid hormone concentrations in the central nervous system of the rat

The effects of single and combined nutritional selenium and iodine deficiency on intracellular thyroid hormone concentrations and type II 5'-iodothyronine deiodinase (5'D-II) activity were examined in different regions of the adult rat brain. Four groups (n = 6) of weanling female Wistar rats proceeding from a breeding line fed a selenium-deficient or a selenium-replete diet for 3 generations, were fed selenium-deficient, iodine-deficient, combined selenium- and iodine-deficient or selenium- and iodine-replete diets for 2 months before they were killed. Tissue thyroxine (T4) and tri-iodothyronine (T3) concentrations were determined by highly sensitive RIAs after extraction of the iodothyronines from the tissue samples. The measurement of 5'D-II was based on the release of radioiodide from the 125I-labelled substrate. Selenium deficiency significantly decreased tissue T3 concentrations in the hippocampus, hypothalamus and striatum to 70-80% of controls, whereas no significant changes were found in the cerebellum, cerebral cortex and brain stem. Tissue T4 concentrations were only marginally affected with the exception of a 35% increase in the cerebral cortex. Iodine deficiency dramatically diminished serum T4 levels as well as intracellular T4 concentrations in all regions examined up to 10-30% of control. In spite of a threefold enhancement of 5'D-II, the iodine-deficient animals still had a significant reduction of tissue T3 concentrations (50-65% of controls) in all regions excepting the cerebellum. The combination of selenium and iodine deficiency did not significantly alter this pattern of changes. These findings suggest that prolonged selenium deficiency as well as iodine deficiency may compromise thyroid hormone homeostasis in the adult brain leading to tissue hypothyroidism and therefore to impaired brain function.

Evidence for circadian variations of thyroid hormone concentrations and type II 5'-iodothyronine deiodinase activity in the rat central nervous system.

The 24-h patterns of tissue thyroid hormone concentrations and type II 5'- and type III 5-iodothyronine deiodinase (5'D-II and 5D-III, respectively) activities were determined at 4-h intervals in different brain regions of male euthyroid rats entrained to a regular 12-h light/12-h dark cycle (lights on at 6:00 a.m.). Activity of 5'D-II, which catalyzes the intracellular conversion of thyroxine (T4) to 3,3',5-triiodo-L-thyronine (T3) in the CNS, and the tissue concentrations of both T4 and T3 exhibited significant daily variations in all brain regions examined. Periodic regression analysis revealed significant circadian rhythms with amplitudes ranging from 9 to 23% (for T3) and from 15 to 40% (for T4 and 5'D-II) of the daily mean value. 5'D-II activity showed a marked nocturnal increase (1.3-2.1-fold vs. daytime basal value), with a maximum at the end of the dark period and a minimum between noon and 4:00 p.m. 5D-III did not exhibit circadian patterns of variation in any of the brain tissues investigated. Our results disclose circadian rhythms of 5'D-II activity and thyroid hormone concentrations in discrete brain regions of rats entrained to a regular 12:12-h light-dark cycle and reveal that, in the rat CNS, T3 biosynthesis is activated during the dark phase of the photoperiod. For all parameters under investigation, the patterns of variation observed were in part regionally specific, indicating that different regulatory mechanisms may be involved in generating the observed rhythms.

Plasma Thyroid Hormone Levels and Iodothyronine Deiodinase Activity Following an Acute Glucocorticoid Challenge in Embryonic Compared with Posthatch Chickens

Embryonic chickens (Day 18 of incubation) and 8-day-old posthatch chicks were subjected to an acute glucocorticoid challenge by a single iv injection of corticosterone (B), dexamethasone, or porcine adrenocorticotropin. Plasma samples were analyzed for changes in T4, T3, rT3, α-subunit, LH, GH, and B levels; iodothyronine deiodinase activity was measured in liver, kidney, and hypothalamus at several time points after injection. The effects of the different treatments were broadly similar within one age group, but differed clearly between pre- and posthatch animals. In 18-day-old embryos glucocorticoids increased plasma T3and decreased plasma T4, rT3, and the calculated TSH index, within hours after injection. These changes were accompanied by an immediate (1–4–24 hr after injection) decrease in hepatic inner ring deiodinating type III enzyme (IRD-III) activity and a delayed (24–48 hr after injection) increase in hepatic outer ring deiodinating type I enzyme (ORD-I) activity. Glucocorticoid challenge in 8-day-old chicks similarly decreased plasma T4and the TSH index but tended to also lower plasma T3. Hepatic ORD-I activity decreased within 1 hr after injection, while the already very low hepatic IRD-III activity was for the most part unaffected. Acute increases in glucocorticoids lower thyroidal T4secretion in both pre- and posthatch chickens but have clearly different effects on peripheral thyroid hormone deiodination and hence on circulating T3at both developmental stages.

A method for the determination of type I iodothyronine deiodinase activity in liver and kidney using 125I-labelled reverse triiodothyronine as a substrate

Objectives: A detailed method for the determination of iodothyronine deiodinase type I (DI-I) activity is described. The objective of the present method development was to consolidate the effective procedures of previous methods and produce an efficient assay that can be easily reproduced. Design and methods: This method uses a 5′,- 125I-labelled rT3 as substrate and ion-exchange chromatography to separate released ionic iodine. Released 125I- collected in the eluate is counted, and the results used to calculate DI-I activity. Results: Results were found to be linear for tissue homogenates containing 3–11 mg protein · mL −1. Day-to-day coefficient of variation of liver homogenate was determined to be 13%. Conclusions: This method was found to be reliable, reproducible, and sample sizes as small as 10 μL could be readily assayed. The use of centrifuge filter units to contain the ion-exchange medium decreased handling of the material, and potential sources of error.

Only the combined treatment with thyroxine and triiodothyronine ensures euthyroidism in all tissues of the thyroidectomized rat.

We have recently shown that it is not possible to restore euthyroidism completely in all tissues of thyroidectomized rats infused with T4 alone. The present study was undertaken to determine whether this is achieved when T3 is added to the continuous sc infusion of T4. Thyroidectomized rats were infused with placebo or T4 (0.80 and 0.90 microgram/100 g BW.day), alone or in combination with T3 (0.10, 0.15, or 0.20 microgram/100 g BW.day). Placebo-infused intact rats served as euthyroid controls. Plasma and 12 tissues were obtained after 12 days of infusion. Plasma TSH and plasma and tissue T4 and T3 were determined by RIA. Iodothyronine deiodinase activities were assayed using cerebral cortex, pituitary, brown adipose tissue, liver, and lung. Circulating and tissue T4 levels were normal in all the groups infused with thyroid hormones. On the contrary, T3 in plasma and most tissues and plasma TSH only reached normal levels when T3 was added to the T4 infusion. The combination of 0.9 microgram T4 and 0.15 microgram T3/100 g BW.day resulted in normal T4 and T3 concentrations in plasma and all tissues as well as normal circulating TSH and normal or near-normal 5'-deiodinase activities. Combined replacement therapy with T4 and T3 (in proportions similar to those secreted by the normal rat thyroid) completely restored euthyroidism in thyroidectomized rats at much lower doses of T4 than those needed to normalize T3 in most tissues when T4 alone was used. If pertinent to man, these results might well justify a change in the current therapy for hypothyroidism.

Selenoenzyme expression in thyroid and liver of second generation selenium- and iodine-deficient rats.

The stimulation of thyroid hormone synthesis in iodine deficiency may increase the requirement for the selenoproteins which are involved in thyroid hormone synthesis in the thyroid gland. Selenoenzyme activity and expression were investigated in the thyroid and liver of second generation selenium-and/or iodine-deficient rats. Selenium deficiency caused substantial decreases in hepatic selenium-containing type I iodothyronine deiodinase (ID-I) and cytosolic glutathione peroxidase (cGSHPx) activities and mRNA abundances, but phospholipid hydroperoxide glutathione peroxidase (phGSHPx) activity was only 55% of selenium-supplemented control levels, despite the absence of change in its mRNA abundance. Selenoenzyme mRNA concentrations were maintained at control levels in thyroid glands from the selenium-deficient rat pups. Despite this, a differential effect was observed in selenoenzyme activities: ID-I activity was decreased to 61%, cGSHPx activity to 45% and phGSHPx to 29% of that in selenium-adequate controls. In iodine-deficient thyroid glands, mRNA levels were increased 2.2, 5.0 and 2.8 times for ID-I, cGSHPx and phGSHPx respectively. ID-I and cGSHPx enzyme activities were also increased but the activity of phGSHPx was decreased despite the high mRNA abundance. Thyroid selenoprotein mRNA levels were also increased in combined selenium and iodine deficiency but again there were differential effects on enzyme activities, with ID-I activity increased, cGSHPx unchanged and phGSHPx decreased. Thus, iodine deficiency may produce an oxidant stress on the thyroid gland, increasing the requirement for selenium to maintain selenoenzyme activity. When dietary supplies of selenium are limiting, thyroid selenoprotein mRNA levels are increased to compensate for overall lack of the micronutrient. Furthermore, there is a preferential supply of available selenium to ID-I and cGSHPx to allow maintenance of thyroid function.

Serum iodothyronine concentrations in intestinally decontaminated rats treated with a 5′-deiodinase type I inhibitor 6-anilino-2-thiouracil

Enteric bacteria have been postulated to have a role in thyroid economy by promoting the hydrolysis of thyroid hormone conjugates of biliary origin, thus permitting the absorption and recycling of thyroxine (T4) and triiodothyronine (T3). An enterohepatic circulation of T3 might be more pronounced under conditions in which type I iodothyronine deiodinase activity (5'D-I) is inhibited, because this augments the accumulation of T3 sulfate conjugates in bile. This potential of increased gut reabsorption of T3 might explain, at least in part, the failure of serum T3 values to decrease appreciably when marked reductions in peripheral 5'D-I activity are induced by selenium deficiency or 6-anilino-2-thiouracil (ATU) administration. Thus, studies were performed to determine the effect of intestinal decontamination, in the absence and in the presence of 5'D-I inhibition, on plasma T4 and T3 concentrations. Groups of adult male rats received either enteric antibiotics or no antibiotics for 12 days and then, in half of the rats in each group, treatment for 10 days with ATU, a 5'D-I inhibitor that does not affect thyroid hormone synthesis. The activity of intestinal arylsulfatase and arylsulfotransferase, enzymes that catalyze hydrolysis of thyroid hormone conjugates, was reduced markedly by approximately 87% in rats that received antibiotics, regardless of whether or not they also received ATU. The ATU treatment markedly inhibited liver 5'D-I activity in antibiotic-treated as well as in non-antibiotic-treated rats (control = 399 +/- 32 U/mg protein (mean +/- SEM); ATU = 152 +/- 17: antibiotics = 351 +/- 29; antibiotics + ATU = 130 +/- 10; p < 0.01) and significantly increased plasma T4 and T3 sulfate (T4S, T3S) concentrations (control: T4S = 2.8 +/- 0.4 and T3S = 6.7 +/- 1.3 ng/dl; ATU: T4S = 6.2 +/- 1.4 and T3S = 10.6 +/- 2.1 ng/dl; antibiotics: T4S = 1.8 +/- 0.2 and T3S = 3.6 +/- 1.0 ng/dl; antibiotics + ATU: T4S = 6.8 +/- 0.7 and T3S = 9.7 +/- 1.8 ng/dl; p < 0.05). The ATU treatment was associated with a significant increase in plasma T4 and rT3 concentrations but did not affect plasma T3 concentrations, and intestinal decontamination did not alter these ATU-associated effects on circulating thyroid hormones. These results suggest that anaerobic enteric bacteria in the rat do not have an important role in recycling of thyroid hormones, either under normal conditions or in circumstances where 5'D-I activity is markedly reduced, and that increased gut absorption of T3 from T3S cannot explain the near-normal serum T3 values found when peripheral 5'D-I activity is markedly decreased.

Replacement therapy for hypothyroidism with thyroxine alone does not ensure euthyroidism in all tissues, as studied in thyroidectomized rats.

We have studied whether, or not, tissue-specific regulatory mechanisms provide normal 3,5,3'-triiodothyronine (T3) concentrations simultaneously in all tissues of a hypothyroid animal receiving thyroxine (T4), an assumption implicit in the replacement therapy of hypothyroid patients with T4 alone. Thyroidectomized rats were infused with placebo or 1 of 10 T4 doses (0.2-8.0 micrograms per 100 grams of body weight per day). Placebo-infused intact rats served as controls. Plasma and 10 tissues were obtained after 12-13 d of infusion. Plasma thyrotropin and plasma and tissue T4 and T3 were determined by RIA. Iodothyronine-deiodinase activities were assayed using cerebral cortex, liver, and lung. No single dose of T4 was able to restore normal plasma thyrotropin, T4 and T3, as well as T4 and T3 in all tissues, or at least to restore T3 simultaneously in plasma and all tissues. Moreover, in most tissues, the dose of T4 needed to ensure normal T3 levels resulted in supraphysiological T4 concentrations. Notable exceptions were the cortex, brown adipose tissue, and cerebellum, which maintained T3 homeostasis over a wide range of plasma T4 and T3 levels. Deiodinase activities explained some, but not all, of the tissue-specific and dose related changes in tissue T3 concentrations. In conclusion, euthyroidism is not restored in plasma and all tissues of thyroidectomized rats on T4 alone. These results may well be pertinent to patients on T4 replacement therapy.

Differential control of type-I iodothyronine deiodinase expression by the activation of the cyclic AMP and phosphoinositol signalling pathways in cultured human thyrocytes

The effects of TSH and the activation of the cyclic AMP (cAMP) and Ca(2+)-phosphatidylinositol (Ca(2+)-PI) cascades on the activity and expression of the selenoenzyme thyroidal type-I iodothyronine deiodinase (ID-I) have been studied using human thyrocytes grown in primary culture. Stimulation of ID-I activity and expression was obtained with TSH and an analogue of cAMP, 8-bromo-cAMP. In the presence or absence of TSH, the addition of the phorbol ester, phorbol 12-myristate 13-acetate (PMA) together with the calcium ionophore A23187, caused a decrease in ID-I activity; a decrease in ID-I expression was also observed as assessed by cell labelling with [75Se]selenite. PMA alone had no effect on ID-I activity in the presence or absence of TSH. A23187 alone produced a small but significant reduction in ID-I activity, but only in TSH-stimulated cells. These data provide evidence that the expression of thyroidal ID-I is negatively regulated by the Ca(2+)-PI cascade, and positively regulated by the cAMP cascade.

Effect of selenium depletion on thyroidal type-I iodothyronine deiodinase activity in isolated human thyrocytes and rat thyroid and liver.

The effects of dietary selenium deficiency on hepatic and thyroidal type I iodothyronine deiodinase (ID-I) and selenium-dependent glutathione peroxidase (GPx) activities have been studied in weanling rats. In selenium-deficient animals hepatic ID-I activity was reduced to 11% of the activity found in the selenium-replete groups, whilst thyroidal ID-I activity increased by 42%. Hepatic and thyroidal GPx activities were also reduced by selenium deficiency to approximately 0.6 and 70%, respectively, of the values found in the selenium-replete animals. We have also studied the effects of thyrotropin (TSH), and selenium supply on the activity of IDI and GPx in human thyrocytes grown in primary culture. When thyrocytes were grown in selenium-deficient (< 1 nmol l-1 Se) medium in the absence of TSH, addition of sodium selenite up to 1000 nmol l-1 had little or no effect on ID-I activity. In the absence of added selenite, TSH addition produced a significant increase in ID-I activity and this stimulation was increased further when selenite was added at concentrations of 50-1000 nmol l-1 with an optimal effect on ID-I activity being observed at a 500 nmol l-1. Selenium content and GPx activity in human thyrocytes grown in selenium-free media (selenium content < 1 nmol l-1) were not significantly lower than the corresponding measurements made in cells grown in media containing selenium at a concentration of 5.4 nmol l-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid function and deiodinase activities in rats with marginal iodine deficiency.

The hypothesis tested was whether marginal iodine deficiency for a period of 6 wk affects iodothyronine deiodinase activities in liver and brain of rats. Male rats were fed purified diets either deficient or sufficient in iodine; the diets were fed on a restricted basis (60% of ad libitum intake). Body weight gain of the two groups was comparable. Iodine deficiency was evidenced by increased thyroid weight (26%), reduced urinary iodine excretion (80%), and reduced plasma T4 concentrations (22%). Activities of liver type I and brain type III deiodinase were unchanged, but the activity of type II deiodinase in brain was increased (28%) in the iodine-deficient rats. Food restriction per se significantly lowered T3 (30%) and T4 (22%) concentrations in plasma and decreased type III deiodinase activity in brain (30%). These results indicate that in marginal iodine deficiency the activities of hepatic type I deiodinase and brain type III deiodinase are unchanged, whereas that of brain type II deiodinase is increased.

Thyroid hormones in tissues from fetal and adult rats.

Concentrations of T4 and T3 were recently measured in rat fetal tissues, and the reported values were found to be more than 10-fold higher than those found by us. The differences have been explained by the assumption that previous analytical procedures, neither avoid deiodination during autopsy of the animals or during extraction and purification, because phloretin [(3'),4',4,6-(tetra)trihydroxyaurone], a potent inhibitor of 5'-iodothyronine deiodinase activity in vitro, had not been used to prevent such problems. We here show that perfusion with phloretin during autopsy does not affect 5'-iodothyronine activity or T4 and T3 concentrations in liver, kidney, or brain. Evidence is also provided that the addition of phloretin during the homogenization process is superfluous, as the use of 80% ethanol and 0.02 M NaOH for this step results in undetectable deiodinase activity. Data are presented showing that during the final sample drying, no losses or degradation of T4 and T3 occur, confirming the adequacy of the individual recovery corrections using radiolabeled iodothyronines as internal tracers. We also present quantitative information on the intralaboratory variability of the T4 and T3 concentrations found in tissues from normal fetuses and their mothers as well as in adult males and nonpregnant females. Results are comparable to those obtained by others using entirely different analytical procedures.

Quantification of Type III Iodothyronine Deiodinase Activity Using Thin-Layer Chromatography and Phosphor Screen Autoradiography

Quantification of Type III Iodothyronine Deiodinase Activity Using Thin-Layer Chromatography and Phosphor Screen Autoradiography

The role of thyroidal type-I iodothyronine deiodinase in tri-iodothyronine production by human and sheep thyrocytes in primary culture

We have studied the origin of tri-iodothyronine (T3) secreted by human and sheep thyrocytes in primary culture and also the expression of type-I thyroidal iodothyronine deiodinase (ID-I) in the thyroid and liver of man and various other animals. Inhibitors of ID-I reduced T3 secretion from human but not sheep thyrocytes. In contrast, inhibitors of de-novo thyroid hormone synthesis reduced both thyroxine (T4) and T3 production in sheep thyrocytes, but had no effect on the T3 secreted by human thyrocytes. Human thyrocytes did not produce T4 under the culture conditions used, although some endogenous T4 was present in the cells following their isolation. Although thyrotrophin (TSH) stimulated T3 production in both human and sheep thyrocytes, iodine in the form of potassium iodide was only essential for T3 and T4 production by the sheep cells. Although 125I from Na125I was incorporated into T3 and T4 in TSH-stimulated sheep thyrocytes, no 125I incorporation into T3 or T4 was detected in TSH-stimulated human thyrocytes. Using activity measurements and affinity labelling, ID-I was present in the livers of all species studied, but ID-I could not be detected in thyroid tissue from cattle, pigs, sheep, goats, rabbits, deer or llamas. In contrast, thyroid tissue from man, mice, guinea-pigs and rats had significant ID-I activity and expressed an affinity-labelled protein with a molecular mass of approximately 28.1 kDa on SDS-PAGE. These data show that under the culture conditions used, sheep thyrocytes produced T3 by de-novo synthesis, whilst human thyrocytes produced T3 by deiodination of endogenous T4. We conclude that thyroidal ID-I shows marked species difference in its expression and that, in those species which express the enzyme (man, mice, guinea-pigs and rats, in this study), it appears that it may make an important contribution to thyroidal T3 production.

Increased glucuronidation of thyroid hormone in hexachlorobenzene-treated rats

Metabolism of thyroid hormones was investigated in WAG/MBL rats that had been exposed to hexachlorobenzene (HCB). Serum thyroxine (T4) levels were lowered by 35.5%, whereas triiodothyronine (T3) levels were not changed. Bile flow, as well as T4 excretion in bile were increased by HCB treatment. Analysis of bile by HPLC revealed a more than 3-fold increase of T4 glucuronide (T4G) and a concomitant reduction of non-conjugated T4. T4 UDP-glucuronyltransferase activity (T4 UDPGT) activity in hepatic microsomes was increased more than 4.5-fold in animals exposed to HCB. p-Nitrophenol (PNP) UDPGT showed a comparable increase by HCB. Both T3 and androsterone UDPGT activities were low in WAG/MBL rats compared with normal Wistar rats. T3 UDPGT activity was increased 2.5-fold by HCB, but androsterone UDPGT activity was unchanged. These results suggest that T4 is a substrate for HCB-inducible PNP UDPGT and T3 for androsterone UDPGT. In the absence of the latter, T3 is also glucuronidated to some extent by PNP UDPGT. Type 1 iodothyronine deiodinase activity was decreased by HCB treatment. It is concluded that decreased T4 levels in serum of animals after exposure to HCB may be due to a combined effect of displacement of T4 from carriers, an increased glucuronidation of T4 and enhanced bile flow.

Effects of combined iodine and selenium deficiency on thyroid hormone metabolism in rats

This paper compares the effects of combined iodine and selenium deficiency, of single deficiencies of these trace elements, and of no deficiency on thyroid hormone metabolism in rats. In rats deficient in both trace elements, thyroidal triiodothyronine (T3), thyroidal thyroxin (T4), thyroidal total iodine, hepatic T4, and plasma T4 were significantly lower, and plasma thyroid-stimulating hormone (TSH) and thyroid weight were significantly higher than in rats deficient in iodine alone. Plasma and hepatic T3 concentrations were similar in the dietary groups. Hepatic type I iodothyronine deiodinase (ID-I) activity was inhibited by selenium deficiency irrespective of the iodine status. Type II deiodinase (ID-II) activity in the brain was significantly higher and in pituitary, significantly lower in combined deficiency than in iodine deficiency alone. These data show that selenium can play an important role in determining the severity of the hypothyroidism associated with iodine deficiency.

Ontogenesis of iodothyronine deiodinase activities in brain and liver of the chick embryo

The ontogeny of 5'-monodeiodinase activity (5'-MA) was analyzed in chick embryo brain and liver tissue. The enzymatic type predominant in this path and the activity of the deactivating pathway (5 MA-III) were determined during certain periods of neural development in both organs. Results show that T3 is predominantly formed in both organs during the first third of embryogenesis (from day 5) until neuroblast proliferation (day 13). Within this lapse, the slow type II enzyme (insensible to propylthiouracil) is present in the brain, whereas in the liver the predominating enzyme is type I (the rapid enzyme). Further along the synaptogenesis period (day 14-17), 5' deiodination virtually disappears in the brain, the hepatic type I enzyme switches to the slow autoconsume enzyme (type II), and 5 MA-III levels increase significantly in both organs. Finally, on days 18-20 (perinatal period) the 5' pathway reaches the highest levels observed throughout the study in both tissues. Associated to this increase, liver enzymatic activity returns to type I. During this period, 5 MA-III is reduced by 40% in the brain and disappears from the liver. Together, these data strengthen the notion of a protective mechanism against brain overexposure to T3 during synaptogenesis and suggest that the protective mechanism also involves the regulation of extraneural deiodinases.

Impairment of the selenoenzyme type I iodothyronine deiodinase in C3H/He mice

Type I iodothyronine deiodinase (ID-I) activity is impaired in C3H/He (C3H) mice compared with BALB/c and C57BL/6N (C57) mice. In this study we compared ID-I activity and protein labeling with N-bromoacetyl(-)[125I]T3 (BrAc[125I]T3) or 75Se in liver microsomes of C3H and C57 mice. Hepatic ID-I activity in C3H mice was highly variable with a median of only 18% of that in C57 mice. However, C3H mice had normal serum T4 and T3 levels, although serum reverse T3 was increased. The 28-kilodalton (kDa) ID-I protein was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of BrAc[125I]T3-labeled microsomes. Labeling of this protein was virtually undetectable in C3H samples with low enzyme activity. ID-I activity in liver microsomes was strongly decreased in Se-deficient mice, which was paralleled by a drastic decrease in BrAc[125I]T3-labeling of the 28-kDa band compared with control mice. Labeling of ID-I with 75Se was demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of liver microsomes of [75Se]selenite-injected mice. 75Se labeling of the 28-kDa band was markedly higher in Se-deficient than in control mice and was also markedly higher in C57 than in C3H mice. Finally, liver ID-I messenger RNA (mRNA) was measured on Northern blots using a rat ID-I complementary DNA probe. Messenger RNA levels correlated strongly with ID-I activity, showing a significant decrease in C3H mice. We conclude that in mice, like in rats and humans, ID-I is a selenoprotein. ID-I activity is impaired in C3H mice because of decreased transcription of the ID-I gene or reduced stability of the mRNA.

Type I Iodothyronine Deiodinase Activity after High Selenium Intake, and Relations between Selenium and Iodine Metabolism in Rats

Type I iodothyronine deiodinase (I-D), which catalyzes the production of the thyroid hormone 3,3´,5-triiodothyronine from thyroxine, has recently been identified as a selenoenzyme. It is therefore of interest to investigate the relationships between selenium and iodine metabolism. In the livers of Se-deficient rats I-D activity was inhibited; the production of 3,3´,5-triiodothyronine and 3,3´-diiodothyronine from added thyroxine was decreased by >95% relative to Se-adequate controls. The hepatic I-D activity was also reduced in rats fed a diet with a low iodine concentration. Unaltered glutathione peroxidase activities in liver and plasma of these rats suggest, however, that with normal Se intake this metabolic pathway of Se is not affected by iodine depletion. When rats were administered 75Se-labeled selenium at levels equal to the amounts ingested from diets with Se concentrations of 0.3 or 2 mg Se/kg, greater Se concentrations were found in the thyroid and liver of the animals receiving the higher dosage. The thyroidal 3,3´,5-triiodothyronine and thyroxine concentrations, however, were comparable in rats fed diets with 0.3 mg Se/kg diet as selenite and 2 mg Se/kg as selenite or L-selenomethionine. The measurement of the hepatic I-D and glutathione peroxidase activities in these animals showed that excessive Se supply does not elevate the activities of the two enzymes but might even have the opposite effect. At high Se intake tissue Se concentration cannot therefore be used as indicator of the selenoenzyme activities.

The rat placenta and the transfer of thyroid hormones from the mother to the fetus. Effects of maternal thyroid status.

We have studied the effects of maternal thyroid status on the effectiveness of the rat placenta near term as a barrier for the transfer of T4 and T3 to the fetus. Dams were given methimazole to minimize the fetal contribution to the T4 and T3 pools, so that the iodothyronines found in the conceptus are ultimately of maternal origin. The dams were infused with saline, or with T4 or T3 at doses ranging from 2.3-27.8 nmol T4 and from 0.77-20.7 nmol T3/100 g BW per day. A group of normal pregnant dams (C) was included. At 21 days of gestation T4, T3, and rT3 were measured by RIA in maternal and fetal plasma, and in maternal and fetal sides of the placenta. The total fetal extrathyroidal T4 and T3 pools were also determined. The dose-related changes in T4, T3, and rT3 levels in the placenta confirm the presence of both inner and outer ring iodothyronine deiodinase activities, and suggest increasing accumulation of the iodothyronines. Despite this, fetal extrathyroidal T4 and T3 increase progressively in T4-infused groups as a function of maternal circulating T4 levels. Fetal extrathyroidal T3 increases progressively in T3-infused groups as a function of maternal plasma T3. There was no evidence that the net maternal contribution of T4 or T3 would be proportionally less when the maternal pools became very high. It was concluded that the rat placenta is only a limited barrier for the transfer of T4 and T3 to the fetus.