Bovine thyroglobulin. 27 S iodoprotein interactions with thyroid membranes and formation of a 27 S iodoprotein in vitro.
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Low-molecular-weight iodoproteins in the congenital goiters of cog/cog mice.
Previously we described sedimentation and immunologic abnormalities of thyroglobulin (Tg) in a strain of mice with inherited congenital goiter and hypothyroidism (cog/cog). The goals of the present study were to determine the extent to which thyroid gland stimulation by TSH accounts for the abnormal properties of cog/cog Tg and to characterize further the abnormally small iodoproteins found in cog/cog mice. Cog/cog and control +/cog and BALB/c mice were fed with either normal or thyroid-hormone-containing diets and were injected with Na125I. Sucrose density gradient centrifugation of labeled thyroid extracts from cog/cog mice on normal diet showed that 82% of the iodine was in iodoproteins smaller than Tg, with sedimentation rates of 3-8S. No 12S and 19S peaks, characteristic of normal Tg, were present, but distinct and stable 12S and 19S peaks emerged after recentrifugation of the 12S and 19S areas. In contrast, in cog/cog mice treated with T4, a smaller (55%) amount of 3-8S iodoproteins and distinct 12S and 19S peaks were present. In both groups of mice, the labeled 3-8S iodoproteins were composed of three fractions: 15% precipitated by antirat Tg serum, 38% precipitated by antimouse albumin serum, and 47% not precipitated by either serum. The 3-8S iodoproteins contained labeled MIT and DIT and no T4. On sodium dodecyl sulfate polyacrylamide gel electrophoresis the 3-8S iodoprotein fraction that reacted with anti-Tg serum contained a distinct electrophoretic band at 49K. The 3-8S nonreactive iodoproteins resolved into several bands of lower molecular weight. We conclude that the 3-8S iodoproteins in cog/cog mice are heterogeneous and that TSH stimulation contributes to the production of these low-molecular-weight iodoproteins.
Amino Acids↗
9 S thyroid particulate iodoprotein.
Particulate iodoproteins have been studied in the rat thyroid gland using the isotopic 125I equilibration method. Pulse experiments were also performed with a second isotope, 131I. Labelled iodoproteins, both soluble and solubilized by digitonin from the thyroid particulate material, were analyzed by sucrose-gradient ultracentrifugation. 1. At isotopic equilibrium and irrespective of the iodide content of the diet two particulate iodoproteins, with sedimentation coefficients of 27 and 19 S, were solubilized by digitonin. In addition a 9 S iodoprotein was also present but its proportion varied markedly with the iodine content of the diet: it accounted for 50-60% of the label found at the particulate level when the dialy diet of iodine was high (75-500 mug/day) but was almost absent when the diet was only 2 mug/day. 2. Most of this 9 S protein was found in the 600-15 000 times g particulate pellet, i.e. a fraction enriched in lysosomes and phagolysosomes. 3. The iodoamino acid composition of the 9 S fraction was very similar to that of the 19 S particulate thyroglobulin: its hormone content was 19%. 4. The double precipitation technique showed that the 9 S fraction is immunochemically related to thyroglobulin. 5. Pulse experiments showed that the 9 S particulate iodoprotein was slowly labelled by 131I. 6. The amount of 9 S iodoprotein was increased by thyrotropin (30-40% increase versus control experiments 5 min after thyrotropin injection). These properties of the 9 S particulate iodoprotein are discussed in relation to the assumption that it might be a product of partial proteolysis of thyroglobulin after endocytosis and partial digestion by the phagolysosomes.
Amino Acids↗
Studies on the heterogeneity of labeled iodoprotein from iodine-replete and iodine-deficient rats as determined by susceptibility to proteolysis.
Thyroid iodoprotein from rats fed a high-iodine diet (HID) or a low-iodine diet (LID) were labeled with radioiodine in vivo for periods ranging from 4 hr to several days. Standardized aliquots of thyroid homogenate from rats with various treatments were digested for 4 hr with 1% pancreatin or with 0.003% pancreatin after 30 min pretreatment with beta-mercaptoethanol (ME-P). Four-hr labeled iodoprotein from both LID and HID rats was equally susceptible to digestion with 1% pancreatin; however, such iodoprotein from LID rats was more susceptible to digestion with ME-P than that from HID rats. With increasing intervals up to 7 days between administering radioiodine and removing the thyroids, there was a progressive rise in the resistance to digestion in iodoprotein from LID rats, but only a slight increase in resistance in iodoprotein from HID rats. If propylthiouracil was added to the diet beginning 24 hr after radioiodine administration, there was a marked increase in the rate of development of resistance of iodoprotein to digestion. Radioautographs showed that the radioiodine was localized primarily in the peripheral follicles after 2 days PTU. Similar differences in susceptibility to digestion were found in purified thyrogobulin prepared from HID and LID rats. No change in susceptibility to digestion of thyroid iodoprotein with time after labeling was seen in hypophysectomized LID rats in which thyroid secretion and thyroglobulin turnover is known to proceed at an extremely slow rate. The data indicate that there are at least two types of iodinated thyroglobulin in the rat thyroid. One is readily susceptible to digestion and has a rapid turnover in the thyroid. The other is more resistant to digestion, has a slow rate of turnover and is located primarily in the peripheral follicles.
Animals↗
Subunit structure of 27 S thyroid iodoprotein.
The dissociation of thyroid 27 S iodoprotein by sodium dodecyl sulfate (SDS) and by succinic anhydride was investigated by means of ultracentrifugation and polyacrylamide gel electrophoresis. The iodoprotein obtained from either a human or hog was dissociated into three kinds of subunits (S-19, S-17 and S-12) by SDS treatment. At increased concentrations of SDS, the S-12 subunit was predominant among the dissociation products. The succinylation of 27 S iodoprotein showed essentially the same dissociation pattern as in the case of SDS treatment. This dissociation products of the protein preparations of different animals were qualitatively the same as those of thyroglobulin of the respective animals, confirming the hypothesis that 27 S iodoprotein was composed of two molecules of thyroglobulin. However, the extent of dissociation of 27 S iodoprotein measured by S-12 formation showed higher resistancy of the protein to the dissociating agents than that of thyroglobulin. The contents of sialic acid and hexose as well as iodoamino acids of 27 S iodoprotein were found to be the same as, or not far from, those of thyroglobulin; The dissociability and chemical composition of 27 S iodoprotein was discussed with reference to the subunit structure of the protein.
Amino Acids↗
Subunit structure of 27 S thyroid iodoprotein.
The dissociation of thyroid 27 S iodoprotein by sodium dodecyl sulfate (SDS) and by succinic anhydride was investigated by means of ultracentrifugation and polyacrylamide gel electrophoresis. The iodoprotein obtained from either a human or hog was dissociated into three kinds of subunits (S-19, S-17 and S-12) by SDS treatment. At increased concentrations of SDS, the S-12 subunit was predominant among the dissociation products. The succinylation of 27 S iodoprotein showed essentially the same dissociation pattern as in the case of SDS treatment. The dissociation products of the protein preparations of different animals were qualitatively the same as those of thyroglobulin of the respective animals, confirming the hypothesis that 27 S iodoprotein was composed of two molecules of thyroglobulin. However, the extent of dissociation of 27 S iodoprotein measured by S-12 formation showed higher resistancy of the protein to the dissociating agents than that of thyroglobulin. The contents of sialic acid and hexose as well as iodoamino acids of 27 S iodoprotein were found to be the same as, or not far from, those of thyroglobulin. The dissociability and chemical composition of 27 S iodoprotein was discussed with reference to the subunit structure of the protein.
Anhydrides↗
Studies on the congenitally goitrous sheep. Iodoproteins of the goitre.
1. Congenitally goitrous thyroid tissue was obtained from South Australian Merino sheep. Ultrastructural studies of the secretory cells in this tissue showed active cells of normal appearance, containing apical protein droplets. 2. (125)I-labelling in vivo of goitre tissue was used to investigate the iodoproteins, in which the major proportion of (125)I appeared in the cell protein fraction soluble in 0.9% sodium chloride (average 62% in goitres from untreated sheep). 3. Ammonium sulphate fractionation showed two clear peaks of iodoprotein precipitation, one at 35-40% saturation and the other at 50-55% saturation. Both iodoprotein fractions contained iodotyrosines and iodothyronines, which were identified chromatographically after enzymic hydrolysis of the protein. 4. Polyacrylamide-gel electrophoresis at pH9.4, at either 7.5 or 5.0% acrylamide concentration, was used to characterize the iodoproteins. Two major fractions were observed, the fastest-migrating fraction coincident with serum albumin, and a slower-migrating, less-well-defined zone. This fraction migrated in 7.5% acrylamide gel, which excluded normal thyroglobulin. 5. Density-gradient (10-40% sucrose) centrifugation was used to determine the approximate sedimentation coefficients of the iodoproteins, which showed major components at s(20,w) 8-9S and s(20,w)<5S. 6. Immunoprecipitation with rabbit anti-(sheep thyroglobulin) failed to sediment (125)I-labelled proteins from goitre extracts. 7. Ouchterlony-type double diffusion in agar plates demonstrated immunoprecipitation lines between rabbit anti-(sheep thyroglobulin) and both the concentrated goitre extract and its Sephadex G-200-excluded fraction, which were confluent with that obtained on reaction with purified normal thyroglobulin. 8. It was concluded that both major iodoprotein fractions were capable of supplying thyroid hormones to the animal, and that the fraction of s(20,w)<5S was iodinated serum albumin. As (125)I-labelled thyroglobulin was not detected in goitre tissue from untreated or thyroxine-treated animals, it was possible that the genetic defect causing goitre resulted in an abnormal thyroglobulin, incapable of being iodinated but immunologically reactive.
Animals↗
Iodoprotein formation by rat mammary glands during pregnancy and early postpartum period.
Iodide organification by rat mammary glands was studied during the trimesters of pregnancy and early postpartum period. Organification was followed by measuring trichloroacetic acid (TCA) precipitation of delipidated tissue homogenates. The radiolabeled material was sensitive to proteolytic cleavage by a bacterial protease indicating that the 125I was protein-bound. Gel filtration column chromatography in the presence of sodium dodecyl sulfate (SDS) of delipidated mammary tissue homogenates of pregnant and postpartum rats reproducibly resolved several iodoproteins from free iodide. The Kav value for each iodoprotein peak was calculated and was used to estimate each subunit molecular weight which averaged 37,500, 25,100, and 8500. Another iodoprotein with a very large subunit molecular weight of greater than 300,000 was also detected in mammary tissue. Incorporation of 125I-iodide into the three smaller iodoproteins increased logarithmically from the start of the second trimester of pregnancy through the early postpartum period when approximately 20% of the total 125I uptake by mammary tissue was incorporated into protein. Hyperplasia, acinar development, and intracytoplasmic vacuolization of mammary tissue correlated with the increased incorporation of 125I-iodide into these iodoproteins. The characterization and quantitation of specific iodoproteins in mammary tissue may be important as organification of iodide is believed to be a marker for normal hormone-responsive cells.
Animals↗
Characterization of iodoproteins secreted by phagocytosing human polymorphonuclear leukocytes.
Several proteins, glycoproteins, and iodoproteins are secreted from human polymorphonuclear leukocytes during phagocytosis of inert latex particles. The amount of 125I-labeled proteins increases during 10 to 60 min of incubation. The 125I-iodoproteins secreted into the incubation medium during the phagocytosis were first separated on Sephadex G-150 column and then characterized by column chromatography on Sepharose 4B, Sephadex G-200, and G-100. Three 125I-iodoproteins were found with the molecular weights of 580,000, 100,000, and 22,000. The molecular mass of 15 protein subunits calculated after sodium dodecyl sulfate-polyacrylamide gel electrophoresis ranged from 11,000 to 86,000 daltons. Four of the protein subunits were labeled with 125I. Their molecular weights were 63,000 to 69,000, 44,000 to 49,000, 22,000, and 11,000. In addition to iodoproteins, several 125I-labeled peptides and compounds were found by column chromatography on Bio-Gel P-2 and P-10. The 125I-iodoproteins are not secreted from resting or boiled granulocytes, and their production is 92 to 98% inhibited by 1 mM KCN or 1 mM sodium azide. The double-labeling technique with [125I]- and [131I]-iodine suggests that the iodoproteins formed in the phagocytosing granulocytes or secreted into the incubation medium are not identical.
Granulocytes↗
Congenital goitre in sheep: isolation of the iodoproteins which replace thyroglobulin.
Immunological and chromatographic studies of proteins from the congenital goitre of South Australian Merino sheep revealed that normal thyroglobulin is absent from the thyroid glands of these sheep. However, a thyroglobulin-immunoreactive iodoprotein was isolated by affinity chromatography on agarose gel to which antibody against thyroglobulin had been covalently bound. Sucrose-gradient ultracentrifugation indicated that this iodoprotein had a sedimentation coefficient of 8S and a molecular weight of approximately 175000. This iodoprotein is therefore about one quarter the size of normal thyroglobulin (19S; 660000) and is similar in size to the subunit of thyroglobulin (3-8S; 165000) although this has usually been described as non-iodinated except when derived by reductive fission. In addition the goitre extract contained iodoproteins which had the immunological properties of serum albumin and immunoglobulin G. Determination of the iodine and iodoamino acid content of the hydrolysed iodoproteins revealed that they contained iodothyronines which were able to contribute to the production of thyroid hormones although the total iodothyronine content of the goitrous gland was less than that of the normal sheep thyroid gland.
Amino Acids↗
Immunochemical and immunohistochemical studies on the 27 S iodoprotein of dog thyroid with reference to thyroglobulin-like reaction of the parafollicular cells.
Our earlier finding that the thyroglobulin-like material responsible for the immunoreaction of parafollicular cells obtained in peak I fraction of Bio-Gel A-5m was followed up in the present study by an investigation of the immunochemical and immunohistochemical reactions of 27 S iodoprotein which was the most prominent material in the peak I fraction. The antibody was raised against completely purified 27 S iodoprotein which was obtained as follows: Thyroglobulin was extracted from dog thyroids and chromatographed initially on Bio-Gel A-5m and then on Bio-Gel A-50m. The area of 27 S migrated as a single bank on polyacrylamide gel slab electrophoresis. This was cut and eluted. Anti-27 S antiserum showed the same immunochemical patterns to 27 S and 19 S as anti-19 S antiserum with three different immunochemical methods: double diffusion test, one dimensional and two dimensional immunoelectrophoresis. The immunoperoxidase reactions of the anti-27 S antiserum and anti-19 S antiserum were restricted to follicular cells and luminal colloids. No reaction of the parafollicular cells was obtained by these antisera. Thus, 27 S iodoprotein shared common immunochemical and immunohistochemical properties with 19 S thyroglobulin. It was concluded that 27 S iodoprotein was not responsible for the thyroglobulin-like reaction of the parafollicular cells.
Animals↗
Electrophoretic analysis of thyroid iodoproteins in highly cross-linked polyacrylamide gels.
The electrophoretic behavior of thyroglobulin and larger thyroglobulin-like iodoproteins prepared from the hog thyroid was studied in polyacrylamide gels over a wide range of cross-linking degrees (C%). When examined at a constant total acrylamide monomer concentration (T%) of 5%, the mobility of 19S thyroglobulin decreased with increase in C below C = 5%, whereas above C = 5%, it increased markedly with increase in C, giving a minimum mobility at C = 5%. Similar biphasic mobility curves were obtained with 27S and 37S thyroid iodoproteins. Furthermore, in a region above C = 15%, at least two additional larger components which escaped detection in the gels with lower degrees of cross-linking appeared as separate bands. Ferguson plots constructed for thyroid iodoproteins at a higher constant C of 20% gave straight lines intersecting at a common point at T = 0%. From the calculated slopes of the Ferguson plots, it has been established that the thyroid contains a series of multimers of 19S thyroglobulin as constituent iodoproteins. Structural parameters of highly cross-linked gels were estimated under the assumption that the gels would be predominantly composed of a random meshwork of gel beads.
Animals↗
Isopycnic centrifugation of thyroid iodoproteins: selectivity of endocytosis.
Isopycnic centrifugation in RbCl was shown to be an effective method both for evaluating the iodine content of thyroglobulin labelled in vivo and for the fractionation of thyroglobulin molecules as a function of their iodine content. Iodination and degradation of thyroid iodoproteins were studied by this method and by zonal centrifugation in sucrose density gradients. Based on these methods it was shown that iodination in vivo is a selective process, 19-S and poorly iodinated thyroglobulin having a higher reactivity toward iodine than 27-S and iodine-rich thyroglobulin. The disappearance of iodoproteins from the thyroid was evaluated by equilibrium, labelling the iodoproteins, blocking iodine incorporation with thiourea derivatives and observing (by sucrose gradient and by RbCl isopycnic centrifugation) at different times the properties of the remaining molecules. Among molecules of different size (19-S and 27-S) and among molecules of the same size (19 S) but different iodine content it was shown that reabsorption from the thyroid gland occurred at the same rate. It was concluded, therefore, that the degradative pathway is essentially a random, non-selective process. Newly iodinated (pulse-labelled) iodoproteins were degraded faster than preexisting molecules. Among the pool of those newly iodinated thyroid proteins, 27-S molecules were reabsorbed faster than 19-S molecules and iodine-rich thyroglobulin molecules were reabsorbed faster than the iodine-poor ones. Since iodination in vivo occurs as repeated pulses of iodine incorporation, it is suggested that this latter phenomenon is a regulatory mechanism which minimizes degradation of molecules which are iodine-poor and have a lower hormonal content.
Animals↗
Thyroid hormone regulation of extrathyroidal iodoproteins.
The effect of thyroid status on plasma and tissue levels of labeled nonextractable iodine (NEI) derived from the metabolism of radioiodothyronines was examined in the rat. Concentrations of radioiodoprotein were substantially elevated in plasma, kidney, and liver in thyroidectomized animals 72 h postinjection of [125I]triiodothytonine ([125I]T3). Similarly, total rat concentrations of radioactive NEI were increased (52%) 72 h after injection of [125I]T3. NE125I concentrations from [125I]T3 in plasma, kidney, and liver were diminished progressively in thyroidectomized animals maintained on increasing doses of thyroxine replacement, demonstrating that iodoprotein levels were inversely related to thyroid state. The plasma disappearance rate of radioiodoprotein from [125I]T3 was markedly slowed in hypothyroid animals and accelerated in intact controls rendered hyperthyroid with daily injections of T4, 8 mug/100 g BW. Propylthiouracil (PTU) treatment of thyroidectomized rats maintained on T4, 2 mug/100 g BW per day resulted in increased NE125I from [125E]T3 in plasma, kidney, and liver. The results of the foregoing investigations suggest that thyroid hormone regulates levels of iodothyronine-derived iodoproteins by influencing the rate of degradation of iodoproteins. Moreover, the observed elevation of iodoprotein levels in T4-maintained thyroidectomized animals after PTU administration appears consistent with the modification of thyroid status due to the peripheral antithyroxine effect of PTU.
Animals↗
Radioactive iodoprotein in thyroid lymph and blood.
1. Samples of thyroid and non-thyroid lymph and of thyroid and peripheral venous blood were obtained from primates and cats that had previously been given radioactive iodine. The distribution of the organic radioiodine between the protein and non-protein fractions in these samples was determined. 2. The proportion of the organic radioiodine in the form of iodoprotein was assessed by paper chromatography, acid-ethanol precipitation, hot-butanol washing, column chromatography and separate estimation of iodotyrosines after enzymic hydrolysis. 3. In thyroid lymph the relative proportion of the organic radioiodine in the form of iodoprotein was 75-98%; in blood it was much lower, probably no more than 6-7%. The absolute concentration of iodoprotein radioactivity also was considerably greater in thyroid lymph than in blood. 4. Enzymic hydrolysis of the protein of the thyroid lymph yielded a pattern of iodoamino acids that corresponded closely with that obtained after hydrolysis of protein extracted from the thyroid gland itself. 5. It can be concluded that the iodoprotein in thyroid lymph consists mainly of thyroglobulin or a closely related compound.
Animals↗
Abnormal iodoprotein distribution and resistance to proteolysis in Gunn rat black thyroid. An ultrastructural and biochemical study.
Gunn rats have a marked deficiency in hepatic UDP-glucuronosyl transferase activity which results in hyperthyroxinemia and hyperbilirubinemia. Their thyroids show a brownish-black discoloration associated ultrastructurally with intracellular dense granules and intraluminal dense masses. In order to determine whether colloid composition and colloid proteolysis are altered in the thyroid of the Gunn rat compared with the Wistar rat, we studied the in situ resistance of thyroid proteins to in vitro proteolysis, the pattern of in vivo (125I) labeled thyroid iodoproteins and the proteolysis of isolated iodoprotein fractions in both strains of rats. For the cytochemical study, thin sections of aldehyde-fixed and plastic-embedded thyroid tissue were treated with 0.3 or 1% pronase in aqueous solution. With the low concentration of pronase, the secretory granules in C-cells and the apical vesicles in follicular cells were extensively digested in both strains of rats, whereas the colloid in the follicular lumen and the colloid droplets were only partially digested. With the high concentration of pronase, the colloid in the lumen and the colloid droplets were more markedly digested in both strains. In the presence of both concentrations of pronase, the dense granules and intraluminal dense masses were unchanged in the Gunn rats. The (125I) iodoprotein pattern was investigated 24 h after a single injection of (125I) iodide and by labeling at the isotopic equilibrium. It was found that the (125I) thyroglobulin fraction was reduced, whereas the (125I) 3-8 S fraction was increased in Gunn rats compared to Wistar rats. Pronase hydrolysis of the soluble (125I) iodine fraction showed similar pronase-resistant fractions in both strains with the single labeling procedure. At the isotopic equilibrium, the pronase resistant fraction was significantly increased in Gunn rats (Gunn 24.0 +/- 5.3%; Wistar: 13.7 +/- 3.1% of the soluble 125I) and a linear correlation was observed between the (125I) 3-8 S fraction of the soluble extract and the pronase-resistant fraction. These data suggest that iodocompounds of small molecular size and low turnover accumulate in the thyroid of the Gunn rat due to their strong resistance to in vivo hydrolysis. A local accumulation of 3-8 S iodocompounds may occur within the intracellular dense granules and intraluminal dense masses in the thyroid of Gunn rat.
Animals↗
A new mechanism for the reabsorption of thyroid iodoproteins: selective fluid pinocytosis.
Rat thyroid hemilobes were incubated in presence of exogeneous, heterologous and homologous thyroglobulins. The median density of the thyroglobulin molecules originally added to the medium was compared with that of the molecules remaining at the end of a three-hour incubation period (37 degrees C). A certain degree of specificity in the reabsorption process of thyroglobulin was found: the uptake of homologous molecules (rat) was higher than heterologous (hog) molecules. The median density of the iodoproteins remaining after the incubation did not change for the heterologous whereas it shifted towards lower density for the homologous thyroglobulins (equilibrium labelled, 35 iodine atoms/molecule). In addition, rat follicular cells display selectivity in the endocytosis of homologous thyroglobulin. Among the rat molecules, the normally iodinated are taken up more actively than the lowly iodinated or newly synthesized ones. Dissimilarity in the median density of the thyroglobulin molecules before and after endocytosis was only evident for the equilibrium-labelled, normally iodinated rat preparation: the disappearance from the medium of 25-30% of exogenous iodoproteins was sufficient to lower significantly the median density of the remaining molecules. This means that the thyroglobulin molecules having higher density are taken up preferentially by the tissue. A mechanism involving specific interactions between the iodoproteins and the surface of thyroid cell is suggested.
Animals↗
Isolation and characterization of hagfish thyroid iodoprotein by its non-thyroglobulin nature, very high iodine and carbohydrate contents and low hormone/iodine ratio.
We have characterized the thyroid iodoprotein of a hagfish, Eptatretus burgeri, one of the lowest marine vertebrates. The iodoprotein was not very homogeneous in its apparent molecular mass which decreased with the increase in hormone/iodotyrosine ratio. Four subfractions with an apparent molecular mass of about 400 kDa were purified from one major fraction by size-exclusion and Mono Q ion-exchange HPLC. The subfractions appeared to have the same peptide backbone, since they showed a single band with the same mobility as a 160-kDa protein in SDS/PAGE and the same amino acid composition. However they differed from each other in having increasing iodine contents (1.9% to 5.9% by mass of total amino acids) associated with the increase in hormonal iodine proportion (8.4% to 16.7% of total iodine) and carbohydrate content (35.6% to 53.5% by mass). These values are strikingly different from those of thyroglobulin with an iodine content of less than 1%, hormonal iodine of 20-40% and carbohydrate content of less than 10%. The amino acid composition of the hagfish iodoprotein, especially the cysteine content of less than 1%, was also entirely different from that of thyroglobulin. These results suggest that most, if not all, tyrosine residues of the hagfish thyroid glycoprotein with a less rigid structure are susceptible to an iodinating system, but hormone residues are formed by a much less efficient mechanism than those in thyroglobulin, when poorly iodinated.
Amino Acids↗