Structural similarities between the plasma membrane binding sites for L-thyroxine and 3,3',5-triiodo-L-thyronine in cultured cells

The kinetics of thyroid hormone transporters and their role in non-thyroidal illness and starvation

Kinetic tracer studies show that thyroid hormones are transported into target tissues by stereospecific, high-affinity, low-capacity transporters, both in animals and humans. The K(d) of binding to the transporter varies within the nanomolar range. The different thyroid hormones (T(4), T(3), and rT(3)) are transported via different transporters, except in the pituitary, where they share the same transporter. The molecular mass of the transport proteins varies between 52 and 65kDa. The transport mechanisms are dependent on the energy charge of the cell and -- often -- the sodium gradient over the plasma membrane. A relationship exists with the transport systems of the aromatic amino acids. In non-thyroidal illness and starvation T(4) transport into T(3)-producing tissues is decreased, resulting in a low plasma T(3) concentration, by some considered to be an energy saving mechanism in situations of stress.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Investigation of type I and type III iodothyronine deiodinases in rat tissues using N-bromoacetyl-iodothyronine affinity labels

In the present study the hypothesis was tested that N-bromoacetyl-3,3',5-[125I]triiodothyronine (BrAc[125I]T3) is a useful affinity label for both type I and type III iodothyronine deiodinases (ID-I and ID-III). Therefore, the microsomal fractions of various rat tissues were tested for ID-I and ID-III activities, and microsomal proteins were labeled with BrAc[125I]T3 and analyzed by SDS-PAGE. In agreement with previous observations, high ID-I activities were found in liver, kidney and thyroid, and high ID-III activities in brain, in particular fetal brain, and placenta. SDS-PAGE of BrAc[125I]T3-labeled microsomes showed a prominent radioactive approximately 27 kDa protein (p27) in liver, kidney and thyroid, which was previously identified as ID-I, and a approximately 32 kDa protein (p32) in brain, in particular fetal brain, and placenta. A good correlation was found between the affinity labeling of p32 and the inactivation of ID-III by BrAcT3, suggesting that p32 represents ID-III or a subunit thereof. After treatment of microsomes with 0.05% deoxycholate or carbonate buffer (pH 11.5) p32 was still labeled by BrAc[125I]T3, indicating that p32 is a transmembrane protein. Although 3,3',5'-triiodothyronine (rT3) is not a substrate for ID-III, p32 was readily labeled with BrAc[125I]rT3. Labeling of p32 in rat brain microsomes by BrAc[125I]rT3 was not affected by addition of 100 microM unlabeled thyroxine (T4) or T3, whereas deiodination of [125I]T3 by ID-III was inhibited by 91 and 96% in the presence of 1 microM T4 and T3, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and properties of N-bromoacetyl-L-thyroxine

A one-step bromoacetylation of L-thyroxine (T4) produces N-bromoacetyl-L-thyroxine (BrAcT4) in good yield. The reaction product is best purified by high-speed countercurrent chromatography. While HPLC is satisfactory only for purification of microgram and submicrogram quantities, amounts ranging from about 1 ng to 1 g of BrAcT4 can be processed by high-speed countercurrent chromatography (HSCCC), a method which we have previously used for the purification of N-bromoacetyl-3,3',5-triiodo-L-thyronine (BrAcT3). Operating conditions for the one-step synthesis of BrAcT4 and BrAcT3 differ due to differences in solubility and reactivity of the two hormones. BrAcT4 purified by HSCCC and shown to be pure by analytical HPLC has been characterized by alpha max and epsilon max in the near and far uv in several solvents, mass spectrum, 1H NMR spectrum, TLC in three solvent systems, retention time in reverse-phase HPLC (C18) in relation to the retention times of two internal standards, 3,3',5-triiodo-L-thyronine and T4, and melting point. Corresponding data for BrAcT3, not previously reported, have also been determined. The described procedure can provide not only substantial amounts of highly purified BrAcT4 for competition studies, but also 125I-labeled BrAcT4 of high specific activity for affinity labeling. Since solutions of BrAcT4 and of BrAcT3 undergo partial decomposition on evaporation to dryness, suitable procedures for the preparation of these hormones in solid form and for storage in solutions have been devised.

Protein disulphide isomerase, a multifunctional endoplasmic reticulum protein

Protein disulphide isomerase (E.C. 5.3.4.1) has been purified, cloned, and sequenced from a variety of vertebrate tissues. The enzyme and its isoforms have been assigned a role in four functional activities: (1) hydroxylation of proline residues in procollagen; (2) disulphide bond oxidation, isomerization, and reduction; (3) the major non-nuclear binding protein of the thyroid hormone 3,3',5-triiodo-L-thyronine; and (4) a component of oligosaccharide transferase. The concentration of the enzyme has been shown to be positively correlated with an endoplasmic reticulum network which is active in secreting disulphide-bonded polypeptides. The enzyme is directed into the endoplasmic reticulum by virtue of a 19 residue N-terminal signal peptide; a four amino acid C-terminal KDEL sequence prevents the enzyme from being secreted. Careful inspection of the sequence data of the isoforms from human tissues reveals a 97% similarity; whereas, analyses of the data from chick tissues reveals only a 80% level of similarity. Chromosomal localizations using human cDNA probes against different human isoforms have assigned the gene(s) to opposite ends of the long arm of chromosome 17. The compiled data suggest the presence of a family of related polypeptides, all of which reside within the lumen of the endoplasmic reticulum.

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

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

Modification of L-triiodothyronine binding sites from rat erythrocyte membrane by heating and by proteinase treatments

The number of binding sites for L-triiodothyronine in rat erythrocyte membranes was increased 2-fold by incubation at 37 degrees C for 60 min. An increase of approximately 3-fold was found when the incubation was carried out at 50 degrees C. The proteinase inhibitor phenylmethylsulfonyl fluoride abolished the effect. Similar increments in the number of binding sites were obtained by treatment of the membranes with proteinases. The Kd values (0.09 X 10(-10) M and 3.6 X 10(-10) M for the high-affinity and the low-affinity binding sites, respectively) remained unchanged after the treatment, as did the free-SH group requirements, storage stability and stereospecificity. Our results suggest that endogenous proteolytic activity could be involved in the increase of the number of membrane latent sites for L-triiodothyronine.