In vitro triiodothyronine binding to cytoplasmic proteins from human red blood cells

Factors influencing the steady-state distribution and exchange of thyroid hormones between red blood cells and plasma of rainbow trout, Oncorhynchus mykiss

We studied effects of in vitro conditions on the steady-state distribution and exchange of thyroid hormones (TH) between red blood cells (RBC) and plasma of rainbow trout. At steady state at 12 degrees C the RBC contained 5-11% of L-thyroxine (T4), 14-23% of 3,5,3'-triiodo-L-thyronine (T3), and 23-24% of 3,3',5'-triiodo-L-thyronine (rT3) present in whole blood. The steady-state distribution was (i) higher in immature than in mature trout for T3, (ii) increased by incubation temperature from 0 to 22 degrees C for both T4 and T3, (iii) unaltered by blood T4 or T3 concentration (0-40 ng/ml), and (iv) increased by O2 gassing and decreased by N2 gassing for T4 and rT3, but negligibly for T3. The exchange between RBC and plasma was more rapid for T3 (50% of maximal influx or efflux at 30-40 s) than rT3 (14 min) than T4 (30 min). Efflux of T4 and T3 was greatly reduced in the absence of plasma protein. Incubation with 10% bovine serum albumin extracted > 98% of labeled T4 and T3 from RBC. We conclude that for trout (i) steady-state distribution and exchange kinetics between RBC and plasma differ greatly for T4, T3, and rT3 and vary with the in vitro conditions, (ii) almost all TH in RBC are reversibly bound to intracellular sites, (iii) efflux is strongly influenced by plasma binding sites, (iv) T3 exchange is rapid and may allow T3 access to RBC TH receptors, buffer plasma T3 levels, or aid T3 delivery to tissues, (v) T4 exchange is slow and this may prevent oxygenation state from altering T4 uptake into RBC, and (vi) rT3 uptake into RBC may contribute to low rT3 levels in trout plasma.

The monomer of pyruvate kinase, subtype M1, is both a kinase and a cytosolic thyroid hormone binding protein

Using a T7 expression system, the monomer of rat pituitary pyruvate kinase, subtype M1 (PKM1), was overexpressed in Escherichia coli and purified to homogeneity. The monomeric p58-M1 has intrinsic enzymatic activity with a Vmax of 79 +/- 20 units/mg and Km's for ADP and PEP of 1.43 +/- 0.76 and 0.14 +/- 0.07 mM, respectively. The monomer binds 3,3',5-triiodo-L-thyronine (T3) with Ka = 1.5 x 10(7) M-1. The order of analog specificity is L-T3 greater than L-thyroxine greater than D-T3 greater than 3'-isopropyl-3,5-diiodo-L-thyronine greater than or equal to 3',5',3-triiodo-L-thyronine. In contrast, tetrameric PKM1 lacks T3 binding activity. The kinase activity of p58-M1 is inhibited by T3 and its analogs in a concentration-dependent manner with the order of inhibitory activity similar to that of binding activity. This inhibition, however, is reversed by the addition of fructose 1,6-bisphosphate. p58-M1 is the second PK isoenzyme monomer to be identified as having thyroid hormone binding activity.

Binding of thyroid hormones to human hemoglobin and localization of the binding site

Radiolabeled thyroid hormones were allowed to bind to erythrocyte cytosol and the complex was fractionated by Sephadex G-100 or by high-performance liquid chromatography (HPLC). On Sephadex G-100, four radioactive peaks (P1-P4) were obtained, whereas HPLC gave only three radioactive peaks (P1-P3). Chromatographic studies with human adult Hb and non-Hb cytosol protein fractions, which had been reacted with radiolabeled thyroid hormones, and immune precipitation with specific antisera for the hormones, confirmed that the first peak of Sephadex G-100 radioactivity was a mixture of Hb and non-Hb proteins, while the second peak was Hb. The third peak was free 125I and the fourth peak was unbound 125I-T3 or 125I-T4. The third peak of HPLC was confirmed to be a mixture of free 125I and unbound radiolabeled thyroid hormones. Scatchard analysis of the interaction between T4 and apo-Hb, and the alpha- and beta-chains of human Hb suggested the presence of the specific binding site(s) for the hormone. Interaction between T4 and synthesized peptides, which constitute the heme pocket of the beta-chain of Hb (beta 61-75, beta 71-85, beta 81-95), indicated that the T4 binding site of Hb resides within the heme-binding cavity. It is concluded that human erythrocyte cytosol does not contain "receptor" for thyroid hormones and cannot be a model for studying functions of cytosol "receptor" for the hormones; rather, it contains binding protein with large binding capacity, including Hb and non-Hb proteins, which possibly constitute a large reservoir for the hormone in blood.

Solubilization and purification of a membrane-associated 3,3',5-tri-iodo-L-thyronine-binding protein from rat erythrocytes

3,3',5-Tri-iodo-L-thyronine (L-T3) binding sites from rat erythrocyte membranes were solubilized in an active form by using the zwitterionic detergent CHAPS or the anionic detergent lauroylsarcosine. The binding protein was successively purified by Sephadex G-200 and affinity chromatography. The purified material retained its binding activity and exhibited high affinity and specificity compared with those displayed in the original membrane. Yield was about 10% of the starting activity. The specific binding activity was enriched by approx. 100-fold, which represents a purity of only 0.1%. Analysis of the purified preparation on SDS/PAGE showed two major protein bands (Mr 64,000 and Mr 50,000), but these could not represent the binding protein since the purity obtained was low. However, affinity-labelling experiments with N-bromoacetyl-L-[125I]T3 in intact membranes showed that two proteins (also with Mr values of 64,000 and 50,000) bound the hormone specifically, suggesting a co-migration of hormone receptors and contaminants on gel electrophoresis.

Concentrations of thyroxine, 3,5,3'-triiodothyronine, 3,3',5'-triiodothyronine, 3,3'-diiodothyronine, and 3',5'-diiodothyronine in human red blood cells

A simple and rapid method for the estimation of cellular concentration of thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), 3,3'-diiodothyronine (3,3'-T2), and 3',5'-diiodothyronine (3',5'-T2) as well as their distribution between cytosol and membranes in human red blood cells (RBC) is presented. Concentrations of iodothyronines in RBC (RBC-T) were calculated by multiplying the total serum concentrations by the ratio of radioactivity in equal volumes of packed RBCs and serum, pre-incubated with 125I-labelled iodothyronines of high specific activity. Plasma and RBC were separated by centrifugation in capillary glass tubes. The separation of membranes and cystosol was performed by hypotone lysis and centrifugation. The median RBC-T of T4, T3, rT3, 3,3'-T2, and 3',5'-T2 from 17 euthyroid subjects were 360 pmol/l, 156 pmol/l, 2.77 pmol/l, 6.81 pmol/l, and 2.17 pmol/l, respectively. The cytosol/cytosol + membrane ration were 66%, 40%, 84%, 77%, and 97%, respectively. The differences in RBC-T were not similar to the differences in free serum concentrations. The ratio of RBC-T to free serum concentration differed considerably between T4 (16.6), T3 (24.4), and 3,3'-T2 (15.5) as compared to rT3 (5.8) and 3',5'-T2 (2.6). Data on three patients with thyroid diseases suggested that RBC-T values were increased in hyperthyroidism and decreased in hypothyroidism, whereas the cytosol/cytosol + membrane-ratio was unaltered.

Putative thyroid hormone receptors in red blood cells of some reptiles

Putative triiodothyronine (T3) receptors have been detected in the nuclei of red blood cells (RBC) in a number of reptile species. The binding characteristics of T3 receptors in vitro were dissociation constant (Kd) 9.1 to 28.58, 36.8 and 40, and 11.12 and 11.36 pM, and binding capacity (Bmax) 0.12 to 0.37, 0.17 and 0.24, and 0.19 and 0.28 fmol per million cells in the rat snake (Ptyas korros), soft-shelled turtle (Trionyx sinensis), and tokay gecko (Gekko gecko), respectively. These data were obtained in all species using in vitro incubation of whole cell according to current receptor studies on living cells. With modified technique in subsequent experiments, these values of the binding characteristics were seemingly low. The discrepancy was ascribed to the assessment of "free" fraction of hormone which would be used in subsequent calculation.

Solubilization of L-triiodothyronine binding site from human erythrocyte membrane

A thyroid binding peripheral membrane protein(s) has been characterized in human red cell. Two classes of affinity sites for triiodothyronine have been demonstrated. The high affinity, low capacity site showed values for dissociation constant of 2 X 10(-10)M. The binding activity depended on the presence of free -SH group and showed a high stereospecificity for L-triiodothyronine, L-thyroxine was less potent (about 1,000-fold) than L-triiodothyronine in competing for this site. The results are discussed with respect to their cellular significance.

A high affinity thyroid hormone binding protein in the cytosol of embryonic rat brain cells in primary cultures

A thyroid hormone binding protein(s) has been characterized in the cytosol of fetal rat brain cells in primary cultures. This protein is closely related to the one described in brain supernatants with respect to its electrophoretic mobility, binding kinetic parameters and estimated molecular weight (65 000 daltons). However, in contrast to the brain cytosolic binding protein, two classes of affinity sites for triiodothyronine (T3) and thyroxine (T4) have been demonstrated: a high affinity site (KA = 1.2-3.7(3) X 10(9) M-1 for T3 and KA = 3.7-5 X 10(8) M-1 for T4) and a low affinity site (KA = 0.8-1.4 X 10(8) M-1 for T3 and 1.6-2.9 X 10(7) M-1 for T4). The results are discussed with respect to their cellular significance.

Cytosolic thyroxine-binding protein and brain development

The properties of cytosolic thyroxine binding protein were studied in the cortex and cerebellum of the rat at different stages of postnatal development: (1) Polyacrylamide-gel electrophoretic analysis showed that rat-brain cortex and cerebellum contain the same cytosolic thyroxine-binding protein which is very similar to the liver-corresponding entity. No changes in the electrophoretic mobility were seen during development in the 2 brain regions. In contrast, no defined triiodothyronine-binding component could be observed by the same technique. (2) Kinetic analysis studies revealed that the equilibrium of binding is reached in approximately 10 min whatever the brain region, the concentration of cytosolic protein and the stage of development. In all these cases saturation was obtained with the same thyroxine concentration (approximately 5 x 10(-7) M). Scatchard analysis also showed that whatever the experimental conditions, brain cytosolic protein contains a single class of thyroxine-binding sites with a K A of approximately 8 x 10(7) M-1. (3) Comparison of the K A during development showed that this constant remains unchanged from day 3 after birth until day 35 in both the cortex and the cerebellum. In contrast the number of binding sites significantly decreases in the cortex (approximately 2-fold; p less than 0.001) from day 3 to 35 with an already significant decline from day 3 to 6 (p less than 0.001). In the cerebellum this decline was even more marked since almost no binding activity was left at adulthood. Comparison of cortex and cerebellum binding activities also showed that this latter region contains approximately half the binding sites (p less than 0.001) at every stage of development studied.

Binding of thyroid hormone by human erythrocyte cytosol proteins

Gel filtration (G-100, 0.01 M Tris, pH 7.4) of post-100,000 x g supernatant from lysate of washed human erythrocytes (RBC) revealed 3 fractions (R-1, R-2, R-3) which bound labeled T3 and T4. Major peak R-2 emerged with the mehoglobin fraction (A560 nm) and binding by this fraction was partially dissociable; the dissociable site bound D-T4, but not tetraidothyroacetic acid or reverse T3. Non-dissociable binding characterized peaks R-1 and R-3. R-1, R-2, and R-3 were pronase-digestible and R-1 binding was acid-unstable (pH 6.8 vs. 7.4). Evidence developed herein and elsewhere indicates that hemoglobin, itself, accounts for the binding within fraction R-2. Intact RBCs maintained for 72 hr at 4C in buffer enriched with T3 or T4 showed progressive incorporation with time of iodothyronines into the hemoglobin fraction.