Identification of a mutation in the coding sequence of the human thyroid peroxidase gene causing congenital goiter

Thyroid peroxidase (TPO) is the key enzyme in the synthesis of thyroid hormones, and the TPO defects are believed to be the most prevalent causes of the inborn errors of thyroid metabolism. We investigated an adopted boy with iodide organification defect, who presented with florid hypothyroidism at the age of 4 mo, poorly complied with thyroxine treatment, and developed a compressive goiter necessitating partial resection at the age of 12 yr. Biochemical studies revealed the absence of TPO activity in the resected tissue. Genomic DNA studies identified a 4 base-pair insertion in the eighth exon of the TPO gene, and showed that the patient was homozygous for this frameshift mutation. The direct genetic diagnosis of this mutation can be made by digestion of polymerase chain reaction products with NaeI restriction enzyme. This will help assessing its prevalence among the heterogenous genetic group of TPO defects.

Further studies on the antigoitrogenic action of iodoarachidonates

Previous studies have shown that iodoarachidonates (IAs) prevent goiter production in rats. In the present studies we show that both IL-d and IL-w (IAs bearing the iodine atom at the positions 6 and 14, respectively), cause a significant involution of preformed goiter. This effect was evident when IAs were administered either orally or via i.p., although the first one required larger doses to obtain the same degree of inhibition. No changes were observed in serum protein, urea, cholesterol, cholinesterase, T3 or T4. In vitro studies with FRTL-5 cells showed that both IAs inhibit iodide and alpha-AIB uptake, as well as ATPase activity.

Thyroid autoregulation: evidence for an action of iodoarachidonates and iodide at the cell membrane level

Iodolipids are the possible mediators of excess iodide in thyroid autoregulation. Previous work from our laboratory has shown that 14-iodo-15-hydroxy-5,8,11 eicosatrienoic acid (I-HO-A) and its omega lactone (IL-w) mimic the inhibitory action of excess iodide upon several parameters of thyroid metabolism. The present experiments were performed in order to study the mechanism of the inhibitory effect of I-HO-A and IL-w on 2-deoxy-D-glucose (DOG) and aminoisobutyric acid (AIB) uptake by calf slices. I-HO-A, IL-w and KI 0.1 mM caused a 33, 31 and 25% inhibition, respectively, of AIB uptake. The presence of 0.1 mM methimazole (MMI) only reversed the effect of KI. The transport of DOG was inhibited by both compounds: I-HO-A caused a 62% decrease, while IL-w produced a 64% inhibition; and MMI failed to relieve their action. On the contrary, the 33% inhibition caused by KI disappeared when MMI was present. Taking into account that AIB and DOG transport across the membrane requires energy, supplied by Na-K-ATPase, changes in its activity were studied. TSH (10 mU/ml) produced a 74% increase in the enzyme activity which was significantly blocked by KI (82%), I-HO-A (100%) and IL-w (100%). Basal enzyme activity was impaired by IL-w (33%), but not by KI. These results were correlated with the decrease of DOG uptake produced by 1 mM ouabain. Tissue specificity effect of iodoarachidonates was demonstrated by the absence of action on DOG transport in kidney and liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential levels of thyroid peroxidase and thyroglobulin messenger ribonucleic acids in congenital goiter with defective thyroglobulin synthesis

The biosynthesis of thyroid hormones requires iodide, thyroid peroxidase (TPO), thyroglobulin (Tg) and H2O2. We have studied two sisters with congenital large goiters and hypothyroidism. Perchlorate tests were negative. Serum T3 and T4 were decreased, TSH was increased and Tg was within the lower limit of normal. Biochemical and molecular studies were performed on goiter samples obtained after surgery. Tg content in both tissues was negligible. Paper chromatography of labeled iodocompounds showed a decrease in T4, and the presence of a pronase/pancreatin-resistant iodoprotein. TPO activity was normal in the tissues. Sephacryl S-300 gel filtration demonstrated labeled iodoalbumin-like protein and the absence of a Tg peak. Salting out studies of soluble protein fraction gave an abnormal pattern. Agarose gel electrophoresis showed the presence of an iodoalbumin-like protein and the absence of Tg in the tissues. This last finding was confirmed by immunoelectrophoresis. The Tg and TPO mRNAs levels were also analyzed. Dot-blot hybridization studies with pM5 (TPO cDNA) and phTgM2 (Tg cDNA) probes showed increased and decreased signals, respectively. The increase in TPO mRNA can be explained as a compensatory mechanism vis a vis an increase in serum TSH caused by decreased serum T3 and T4 due to the impairment in Tg mRNA. The Tg mRNA of both patients was further studied with four different probes covering 5' and 3' regions (phTgM1, phTgB1, phTgB2 and phTgB3). Hybridization was observed with all four probes, thus excluding a dramatic deletion defect. Northern transfer showed a clear signal of hybridization with the phTgB1 probe in the 8-9 Kb range. We may conclude that the biochemical and molecular abnormality of these patients is characterized by a decrease of Tg mRNA and of Tg translation.

Thyroid autoregulation. Inhibition of goiter growth and of cyclic AMP formation in rat thyroid by iodinated derivatives of arachidonic acid

Thyroid autoregulation has been related to intraglandular content of an unknown putative iodocompund. Data from different laboratories have shown that the thyroid is capable of producing different iodolipids, including iodinated derivatives of arachidonic acid; such as 5-hydroxy-6-iodo-8, 11, 14-eicosatrienoic-delta-lactone (IL-delta). Previous results from our laboratory showed that a semi-purified preparation of iodinated arachidonic acid exerts an inhibitory action in vitro on calf thyroid. In the present studies three purified iodinated derivatives of arachidonic acid were synthesized: IL-delta; 14-iodo-15-hydroxy-5, 8, 11-eicosatrienoic acid (I-OH-A) and its corresponding omega-lactone (IL-omega). Their action on MMI-induced goiter was studied in rats. Administration of MMI to rats during 10 days increased thyroid weight by 124%. This effect was significantly inhibited by the simultaneous injection of 5 micrograms/day of I-OH-A (57% inhibition of MMI action), IL-W (39%), IL-delta (33%) and T3 (95%), while arachidonic acid was without action. No inhibition was found with 1.25 micrograms/day Kl, a dose equivalent to that which could be originated from total dehalogenation of the iodocompounds. These results support the idea that these iodocompounds have an intrinsic biologic activity and that there is a correlation between action and chemical structure. Serum TSH was increased around 15-20 fold after MMI administration. Chronic or acute injection of I-OH-A failed to alter TSH levels, indicating that this iodocompound exerts its action directly on the gland, without altering TSH concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid autoregulation. Inhibitory effects of iodinated derivatives of arachidonic acid on iodine metabolism

Thyroid autoregulation has been linked to an organified iodocompound. Since several iodolipids are produced by the gland their possible role in thyroid autoregulation was examined. The following pure synthetic compounds were prepared: 1) 14-iodo-15-hydroxy-5,8,11-eicosatrienoic acid (I-OH-A); 2) its omega lactone (IL-omega); 3) 5-hydroxy-6-iodo-8,11,14-eicosatrienoic acid delta lactone (IL-delta). Their action on iodine metabolism was studied. Iodine uptake was measured in calf thyroid slices. At 10(-4)M I-OH-A caused a 64% decrease in the T/M ratio, while IL-omega inhibited it by 36% and IL-delta was without effect. At 10(-5)M the inhibition was 44% for I-OH-A and 19% for IL-omega, while T3 was without action. A possible isotopic dilution effect was excluded, and no change in iodine efflux was observed. The inhibition by I-OH-A of iodide uptake was observed after only 15 min preincubation. This compound also decreased 125I accumulation in rats. In calf thyroid slices, I-OH-A at 10(-4)M, inhibited PB125I formation by 80%, IL-omega by 62% and IL-delta by 37%. T3 and arachidonic acid were without action. I-OH-A also caused a dose-dependent inhibition of TSH-stimulated iodide organification. The present results demonstrate, for the first time, that iodinated derivatives of arachidonic acid inhibit thyroid function and mimic the effect of iodide on thyroid autoregulation.

The inhibition of PB125I formation in calf thyroid caused by 14-iodo-15-hydroxy-eicosatrienoic acid is due to decreased H2O2 availability

Previous work from our laboratory has shown that 14-iodo-15-hydroxy-5,8,11-eicosatrienoic acid (I-HO-A) is a potent inhibitor of iodine organification in calf thyroid slices. The present studies were performed in order to clarify the mechanism of this action. Incubation of thyroid slices with 10(-4)M I-HO-A caused a 47 and 53% decrease in PB125I formation after 30 and 60 min incubation, respectively. In a series of experiments an inverse relationship between the degree of inhibition caused by I-HO-A and total iodine content and basal iodoprotein formation was observed. Chromatographic analysis of the labeled compounds showed a significant decrease in 125I incorporation into MIT, DIT, T3 and total iodolipid. The site of the inhibitory effect of I-HO-A was then sought. TPO was measured by three different methods. When TPO was solubilized from I-HO-A treated slices, no change in enzymatic activity was observed. Moreover, the same lack of action was found when solubilized TPO was incubated with I-HO-A. The production and release of H2O2 into the incubation medium was measured by chemiluminiscence technique. In control slices the values increased during the first 10 min and reached a plateau. Pretreatment of the slices with 10(-4)M KI caused a 51% inhibition, while the same concentration of I-HO-A produced a 59% inhibition. The possibility that I-HO-A might exert its action through a putative protein inhibitor was also explored. Incubation of slices with 10(-5)M I-HO-A caused a 46% decrease in PB125I formation and addition of actinomycin D or puromycin failed to alter this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between nuclear ADP-ribosylation and RNA transcription in calf and human thyroid

ADP-ribosylation is a posttranslational modification of proteins that has been related to many cellular events, such as DNA replication and repair, cell proliferation and differentiation. The present studies were performed in order to explore the possible relationship between nuclear protein ADP-ribosylation and RNA transcription in the thyroid gland. Inhibition of RNA transcription by alpha-amanitin and actinomycin D caused a decrease in ADP-ribosylation of 27 and 17%, respectively. Nicotinamide caused a dose-related inhibition of ADP-ribosylation, which was highest at 2 mM (around 90%). At this dose nicotinamide inhibited total RNA transcription by 46%, while the activity due to RNA polymerase II decreased by 50% and that related to RNA polymerases I+III dropped by 24%. These results suggest that inhibition of total nuclear protein ADP-ribosylation is accompanied by a parallel decrease in RNA transcription. Since our previous work has shown that TSH stimulates both nuclear ADP-ribosylation and RNA transcription it may be concluded that these activities follow parallel changes within the thyroid. When the same activities were assayed in normal human and in glands bearing follicular adenoma, RNA polymerase II was increased 4 fold in the latter group, without change in nuclear ADP-ribosylation. These results would suggest that a mechanism, distinct from ADP-ribosylation, may also be involved in the regulation of RNA transcription. This latter might be altered under this pathologic condition.

Thyroid hormone binding and deiodination by pancreatic islets: relationship with the in vitro effect upon insulin secretion

Thyroxine dehalogenation by rat pancreatic islets was studied incubating isolated islets with labelled T4. [125I]T4 added to the incubation medium was deiodinated by the islets with the consequent production of T3, rT3 and iodide. This deiodination process showed a clear glucose-dependence, being significantly increased in the presence of 16.6 mmol/l glucose. The existence of high and low affinity binding sites for T3 was also demonstrated incubating [125I]T3 with islets under different experimental conditions. The properties of these binding sites were greatly influenced by the extracellular concentration of glucose. Addition of T3 to the incubation medium, significantly modified the insulin release, but its effect varied according to the glucose concentration in the medium, i.e. it enhanced the insulin release at a glucose concentration between 2 to 8 mmol/l; it has no effect at 12 mmol/glucose, and significantly inhibited the secretion of insulin in the presence of 16.6 mmol/l glucose. Our results suggest that thyroid hormones might play a direct regulatory effect on insulin secretion, probably mediated by its deiodination and interaction with specific receptors in the islet cell.

Subcellular localization and binding of 125I-triiodothyronine in calf thyroid

The subcellular distribution of 125I-T3 was studied in calf thyroid slices, under the same experimental conditions where T3 inhibits protein and RNA synthesis, labelled hormone was found mainly in the 20,000 X g supernatant. The specificity of each subcellular localization was determined by incubating the slices with 10(-5)M T3. Only in the purified nuclei a significant decrease was found, indicating a specific localization of the labelled hormone. When slices were incubated with 125I both labelled T3 and T4 were found in purified nuclei, indicating that endogenously synthesized hormones can reach thyroid nuclei. Purified thyroid nuclei were incubated with labelled T3 and increasing amounts of cold hormone. Specific binding reached a plateau after 90 min of incubation at 20 degrees C. When the displacement curves were analysed by a Scatchard plot a binding site with a Ka of 5.2 X 10(7) M-1 and a capacity of 3.0 X 10(-15) moles/microgram DNA was observed. Digestion of nuclei with trypsin and protease abolished completely the binding of 125I-T3 thus indicating the protein nature of the receptor. The hormone-receptor complex could be extracted with 0.4M KCI and eluted in the void volume after Sephadex G-25 column chromatography, similar to peripheral tissues nuclear T3 receptors. The present studies provide the first evidence for the existence of nuclear receptors for T3 in the thyroid, an event probably related to the autoregulatory mechanism.

Action of TSH on nuclear ADP-ribosylation in dog thyroid slices

Treatment of dog thyroid slices with thyrotropin (TSH) results in an increase in ADP-ribosylation in nuclei isolated thereafter. This increase is time-dependent and is observed with concentrations of TSH eliciting physiological responses. The technique described here does not involve permeabilization of cell membranes, thereby avoiding artefacts which could arise from hypotonic shock. Cyclic AMP mimicked the stimulatory action of TSH.

Biosynthesis and regulation of iodolipids in calf thyroid

The incorporation of 125I into iodolipids was investigated in calf thyroid slices. Time course studies showed that the iodination of lipid reaches a plateau after 30 min of incubation. A highly significant correlation was found between iodination of lipid and of protein (r = 0.906), suggesting that both reactions may be related. Addition of PTU or MMI caused a significant inhibition of lipid iodination, indicating that a peroxidase could be involved in this reaction. The iodinated lipids are not attached to protein, since they migrated in BEA chromatography and pancreatin digestion of the samples did not modify the percentage radioactivity. The iodolipid fraction observed in BEA chromatography was eluted and analysed by TLC. Iodinated free fatty acids and neutral lipids comprised most of the radioactivity. Although iodolipids are present in every subcellular fraction, the 20 000 X g pellet had the greatest proportion of iodinated fatty acids and neutral lipids.

Uptake, metabolism and subcellular distribution of [125I]T4 in calf thyroid slices

Previous work from our laboratory has shown that iodothyronines have a direct inhibitory action on the thyroid. In the present studies the uptake, metabolism and subcellular distribution of labelled thyroxine were analyzed. The entry of this hormone reaches a plateau after 15-20 min incubation and is temperature-dependent. The T/M values for T4 were much lower than those of labelled T3 and the curve was different from that obtained with [125I]. The influence of a series of compounds on the T/M values for thyroxine was studied. KI, T3, IOP, PTU and MMI were without effect. Absence of Ca++ in the buffer, or addition of KClO4 or ouabain caused a slight decrease, while TSH produced a stimulation in the T/M ratio. Calf thyroid slices dehalogenated thyroxine, producing both T3 and rT3. TSH increased the generation of these two compounds. Neither PTU nor IOP altered the production of T3 and rT3 significantly. The lack of effect of IOP on T3 and rT3 generation was confirmed in calf thyroid homogenates. However, and in agreement with previous reports, IOP significantly decreased production of both triiodothyronines in rat liver slices and in rat liver homogenates. When the subcellular distribution of labelled thyroxine was examined most of the radioactivity appeared in the soluble fraction and less than 1% was present in purified nuclei. The addition of unlabelled thyroxine to the slices only caused a significant displacement in the radioactivity of purified nuclei. The presence of labelled thyroxine in the nuclei was confirmed by paper chromatography.

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.

Biochemistry of thyroid regulation under normal and abnormal conditions

Perhaps in an oversimplified view, abnormal thyroid growth can be classified into two main categories: a) those cases due to excess of thyroid stimulators extrinsic to the gland; b) situations in which an intrinsic alteration in the gland occurs: Extrinsic (excess thyroid stimulation) Iodide deficiency with elevated TSH Goitrogens Graves' immunoglobulins Thyroid stimulating factors produced by tumors Dishormonogenesis with hypothyroidism Intrinsic (normal TSH) Increased sensitivity to TSH (iodine depletion) Altered autoregulation (?) Abnormal TSH receptor Other biochemical abnormalities From the studies performed in animals it can be concluded that since goiter appears before a detectable increase in serum TSH occurs, an intrinsic alteration in the thyroid gland would be responsible for the onset of growth. Under these conditions TSH would play a permissive role in promoting and maintaining the gland enlargement. In some aspects this situation is similar to that of certain endemic goiter areas. It may be postulated that under a mild iodine deficiency a decrease in thyroidal iodine concentration occurs (and/or in certain iodocompounds), thus rendering the gland more sensitive to the stimulatory action of TSH, and leading to the appearance of goiter. If this mechanism is able to maintain an euthyroid status no further alterations will occur. In more severely iodine deficient areas, or when additional factors such as dietary goitrogens are present, hypothyroidism develops and TSH is clearly elevated. A similar localized mechanism can be postulated for the development of nodular goiter. It is more difficult to explain the pathogenesis of goiter and tumors in nonendemic areas, since the biochemical findings so far reported are not conclusive. It seems likely that an alteration of the TSH receptor is a common factor to many tumors in man and animals. However, some contradictory results would preclude us from making a general statement. The wide variety of biochemical alterations reported would perhaps indicate, that there is not a single cause for the rise of abnormal thyroid growth and that different factors may play a role in the regulation of growth under such circumstances. It is to be hoped that future studies will provide a better comprehension of this problem.

Early effects of thyrotropin on ribonucleic acid transcription in the thyroid

Previous studies on the effects of TSH on the incorporation of labeled precursors into RNA have demonstrated an increase in the percentage of label in the phosphorylated nucleotide precursors of RNA. To avoid this nonspecific effect of TSH, we chose to measure RNA transcription in a system which uses exogenous phosphorylated precursors. Isolated nuclei were prepared from the thyroids of dogs which had been injected 90 min earlier with TSH. TSH caused a mean increase of 168% above control in total transcriptional activity, 193% in polymerase II-mediated (alpha-amanitin-sensitive) activity, and 155% in RNA polymerase I- and III-mediated (alpha-amanitin-resistant) activity. To determine whether part of this early effect of TSH was due to increased activity of RNA polymerase, incubations were also carried out in the presence of actinomycin D, which blocks transcription of endogenous template but does not inhibit transcription of added synthetic template poly deoxyadenylic-deoxythymidylic acid (dA.dT). Under these conditions, about half of the TSH-induced increase in the transcription of endogenous template could still be detected. Thus part of the early effects of TSH on RNA synthesis appears to be mediated through increased polymerase activity as well as enhanced template activity.

Action of KI and several iodocompounds on [3H]uridine incorporation into thyroid RNA

The present studies were performed in order to further clarify the action of iodine and iodocompounds on the incorporation of labelled uridine into thyroid RNA. KI decreased RNA labelling but did not alter total [3H]uridine uptake or [3H]inulin distribution space. KI also inhibited the increase in RNA labelling produced by 8 mM glucose. T4 was more potent on a molar basis than KI in impairing uridine incorporation. TETRAC, TRIAC and isopropyl-T3 also decreased RNA labelling, while T2 and isopropyl-T2 were ineffective. KI did not alter the distribution of the uridine derivatives, UMP, UDP and UTP, as determined by the distribution of [3H]uridine in these compounds by paper chromatography, suggesting that the action of KI does not take place at the step of uridine phosphorylation. Like its effect on TSH, KI also impairs the stimulatory effect of exogenous cAMP and cGMP on RNA labelling, suggesting that its action is exerted beyond the step of cyclic nucleotide formation. Iodine and iodocompounds may exert their inhibitory action on RNA labelling at the step of nucleotide polymerization.

Role of neurotransmitters, prostaglandins and glucose on precursor incorporation into the RNA of thyroid slices

The present studies were performed in order to investigate the role of neurotransmitters, prostaglandins and glucose on [3H] uridine incorporation into total RNA in beef thyroid slices. Carbamylcholine strongly stimulated RNA labelling from [3H uridine; atropine abolished this effect. NaF, at 1 and 5 mM, progressively increased this parameter while norepinephrine caused a similar effect at 10(-3) but not at 10(-6) M. Phentolamine (1 mM) blocked the stimulatory action of TSH; propranolol and atropine did not. Glucose at concentrations between 4 and 24 mM caused a progressive increase in RNA labelling from [3H] uridine. This effect was inhibited by dinitrophenol. Prostaglandins (E1, A1, F1alpha and F2alpha) assayed in concentrations between 5 and 25 microgram/ml, with or without caffeine, had no effect on RNA labelling. Moreover, neither aspirin nor indomethacin inhibited TSH stimulation. Under similar experimental conditions, PGE1 did simulate PB125I formation.

Action of cyclic nucleotides on protein and RNA synthesis in the thyroid

Thyrotrophin (TSH) regulates the biosynthesis of thyroid protein and RNA. This action is mediated by adenylate cyclase and cycl AMP. In the present study the action of cyclic GMP and cyclic CMP was investigated in beef slices. Both cyclic AMP and cyclic GMP significantly increased the incorporation of [3H]uridine into RNA. These effects were blocked by actinomycin D, suggesting that their action is located at a preor at a transcriptional step. The PCA soluble fraction radioactivity followed in parallel with tha variations observed in the RNA fraction, supporting the view that cyclic nucleotides may regulate RNA by modulating the nucleotide precursors pool. Cyclic CMP in concentrations between 0.35 to 1.5 mM progressively decreased the RNA labelling, and the values of the PCA soluble radioactivity again followed those of RNA. Furthermore, cyclic CMP also blocked the in vitro stimulatory action of cyclic AMP on the incorporation of [3H]leucine into protein, and of [3H]uridine into RNA. The present results provide the first information on the action of cyclic AMP on RNA synthesis and suggest that negative signals may also play a part in the regulation of thyroid function.

Action of KI, thyroxine and cyclic AMP on [3H]uridine incorporation into the RNA of thyroid slices

Potassium iodide (KI) has been shown to impair thyroid protein biosynthesis both in vivo and in vitro. The present study was performed in order to clarify its mechanism of action. Ribonucleic acid (RNA) synthesis was studied in beef thyroid slices with either [32P] or [3H]-uridine as labelled precursors. Both KI and thyroxine (T4) at 10(-5) M significantly decreased RNA labelling under our conditions. In other experiments RNA degradation was examined in pulse-labelled and actinomycin D-treated slices. KI did not modify the degradation of the [3H]-RNA thus indicating that it interferes with the biosynthesis rather than with the degradation of RNA. Taking the perchloric acid soluble radioactivity as a rough index of the precursor pool the present results would indicate an action at this level. Both KC1O4 and methylmercapto-imidazole relieved the gland from the inhibitory action of KI, supporting the view that an intracellular and organified form of iodine is responsible for this action. Since T4 also reproduced the effects of KI on RNA synthesis we would like to propose iodothyronines as the intermediates of this action. Cyclic AMP has been shown to stimulate thyroid protein biosynthesis. The present results demonstrate an action at the RNA level. Cyclic AMP increased both the PCA-soluble and RNA-linked radioactivity, thus suggesting an effect at the RNA precursor pool. KI at 10(-5) M blocked the action of 2 mM cyclic AMP.

Studies on the mechanism of action of potassium iodide on thyroid protein biosynthesis

Potassium iodide (KI) has been shown to have an antigoitrogenic action and to inhibit in vivo thyroid protein biosynthesis. Beef thyroid slices were used to clarify further the mechanism of action of KI. Incubations were performed in Krebs-Ringer-bicarbonate (KRB) buffer under 95%O2 and 5% CO2. KI caused a slight decrease in the uptake of [3H]eucine by the tissue. When labelled leucine incorporation into protein was measured it was found that 10(-6) M KI caused a marked inhibition. Increasing concentrations of KI up to 10(-3) M did not further increase this inhibition. This effect of KI was reduced by simultaneous addition of 0.5 mM KClO4 or 1 mM methylmercaptoimidazole (MMI). In several experiments it was found that equimolar amounts of thyroxine (T4) or triiodothyronine (T3) were more potent than KI in inhibiting thyroid protein biosynthesis. In double plabelled studies KI decreased [3H]leucine incorporation into thyroid soluble proteins and into immunoprecipitable thyroglobulin (T4) while it did not modify that of [14C]galactosamine. When tissue specificity was examined, KI failed to alter [3H]leucine incorporation into proteins either in the liver or in the submaxillary gland. The present results indicate that intracellular KI is necessary to exert its effect on protein synthesis, and that this effect is mediated through a organic form of iodine, probably iodothyronines. This action of KI is specific for the thyroid gland.

The role of reduced glutathione in thyroglobulin proteolysis iv vitro

During the process of secretion of the thyroid hormones, stored thyroglobulin (Tg) is digested within the lysosomes. An acid protease has been found, but this enzyme hydrolyses Tg at a relatively slow rate and has a low pH optimum (3.6). Since previous work has shown a stimulatory effect of reduced glutathione (GSH) on Tg digestion the following studies have been performed. Homogenates were obtained from dog thyroid and total homogenate or different subcellular fractions were used as enzyme source. Labelled thyroglobulin (125-I) was purufied from prelabelled dog thyroid and its hydrolysis, judged from the increase in butanol soluble radioactivity, was studied. The greatest stimulatory effect of GSH on Tg hydrolysis was found around pH 5.6. When butanol soluble radioactivity was considered as a function of incubation time and tissue concentration a linear relationship was found. GSH effect was evident at 2 mM. Subcellular distribution studies showed that the GSH-stimulated proteolytic activity was mainly found in the 15 000 times g pellet. Labelled Tg hydrolysis was progressively decreased with increasing amounts of non-labelled purified Tg. GSH also stimulated labelled insulin hydrolysis, but failed to alter haemoglobin or casein degradation. GSH could also be replaced by other reducing agents, like cysteine or dithiothreitol. The significance of these findings is discussed in relation to the process of thyroid hormone secretion.