Metal ion dependence of the binding of triiodothyronine by cytosol proteins of bullfrog tadpole tissues

Effect of coenzymes and thyroid hormones on the dual activities of Xenopus cytosolic thyroid-hormone-binding protein (xCTBP) with aldehyde dehydrogenase activity

A cytosolic thyroid-hormone-binding protein (xCTBP), predominantly responsible for the major binding activity of T3 in the cytosol of Xenopus liver, has been shown to be identical to aldehyde dehydrogenase class 1 (ALDH1) [Yamauchi, K., Nakajima, J., Hayashi, H., Horiuchi, R. & Tata, J.R. (1999) J. Biol. Chem. 274, 8460-8469]. Within this paper we surveyed which signaling, and other, compounds affect the thyroid hormone binding activity and aldehyde dehydrogenase activity of recombinant Xenopus ALDH1 (xCTBP/xALDH1) while examining the relationship between these two activities. NAD+ and NADH (each 200 microm), and two steroids (20 microm), inhibit significantly the T3-binding activity, while NADH and NADPH (each 200 microm), and iodothyronines (1 microm), inhibit the ALDH activity. Scatchard analysis and kinetic studies of xCTBP/xALDH1 indicate that NAD+ and T3 are noncompetitive inhibitors of thyroid-hormone-binding and ALDH activities, respectively. These results indicate the formation of a ternary complex consisting of the protein, NAD+ and thyroid hormone. Although the in vitro studies indicate that NAD+ and NADH markedly decrease T3-binding to xCTBP/xALDH1 at approximately 10-4 m, a concentration equal to the NAD content in various Xenopus tissues, photoaffinity-labeling of [125I]T3 using cultured Xenopus cells demonstrates xCTBP/xALDH1 bound T3 within living cells. These results raise the possibility that an unknown factor(s) besides NAD+ and NADH may modulate the thyroid-hormone-binding activity of xCTBP/xALDH1. In comparison, thyroid hormone, at its physiological concentration, would poorly modulate the enzyme activity of xCTBP/xALDH1.

Characterization of Xenopus cytosolic thyroid-hormone-binding protein (xCTBP) with aldehyde dehydrogenase activity

Multiple cytosolic thyroid-hormone-binding proteins (CTBPs) with varying characteristics, depending on the species and tissue, have been reported. We first purified a 59-kDa CTBP from Xenopus liver (xCTBP), and found that it is responsible for major [125I]T(3)-binding activity in Xenopus liver cytosol. Amino acid sequencing of internal peptide fragments derived from xCTBP demonstrated high identity to the corresponding sequence of mammalian aldehyde dehydrogenases 1 (ALDH1). To confirm whether or not xCTBP is identical to xALDH1, we isolated cDNAs encoding xALDH1 from an adult Xenopus hepatic cDNA library. The amino acid sequences deduced from the two isolated xALDH1 cDNAs were very similar to those of mammalian ALDH1 enzymes. The recombinant xALDH1 protein exhibited both T(3)-binding activity and ALDH activity converting retinal to retinoic acid (RA), which were similar to those of xCTBP purified from liver cytosol. The T(3)-binding activity was inhibited by NAD, while the ALDH activity was inhibited by thyroid hormones. Our results demonstrate that xCTBP is identical to ALDH1 and suggest that this protein might modulate RA synthesis and intracellular concentration of free T(3). Communications between thyroid hormone and retinoid pathways are discussed.

Xenopus cytosolic thyroid hormone-binding protein (xCTBP) is aldehyde dehydrogenase catalyzing the formation of retinoic acid

Amino acid sequencing of an internal peptide fragment derived from purified Xenopus cytosolic thyroid hormone-binding protein (xCTBP) demonstrates high similarity to the corresponding sequence of mammalian aldehyde dehydrogenase 1 (ALDH1) (Yamauchi, K., and Tata, J. R. (1994) Eur. J. Biochem. 225, 1105-1112). Here we show that xCTBP was co-purified with ALDH and 3,3',5-triiodo-L-thyronine (T3) binding activities. By photoaffinity labeling with [125I]T3, a T3-binding site in the xCTBP was estimated to reside in amino acid residues 93-114, which is distinct from the active site of the enzyme but present in the NAD+ binding domain. The amino acid sequences deduced from the two isolated xALDH1 cDNAs (xALDH1-I and xALDH1-II) were 94.6% identical to each other and very similar to those of mammalian ALDH1 enzymes. The two recombinant xALDH1 proteins exhibit both T3 binding activity and ALDH activity converting retinal to retinoic acid (RA), which are similar to those of xCTBP. The mRNAs were present abundantly in kidney and intestine of adult female Xenopus. Interestingly, their T3 binding activities were inhibited by NAD+ and NADH but not by NADP+ and NADPH, whereas NAD+ was required for their ALDH activities. Our results demonstrate that xCTBP is identical to ALDH1 and suggest that this protein might modulate RA synthesis and intracellular level of free T3.

Tissue-dependent and developmentally regulated cytosolic thyroid-hormone-binding proteins (CTBPs) in Xenopus

Xenopus cytosolic thyroid-hormone-binding proteins (CTBPs) were examined by a photoaffinity labeling and [125I]T3-binding assay. An affinity-labeled protein of 59 kDa, which was responsible for the major T3-binding activity in adult tissues, was predominant in liver. The 59-kDa CTBP first appeared in significant amounts at the metamorphic climax stage in liver cytosol and continued to be expressed after metamorphosis. Another affinity-labeled CTBP of 38 kDa appeared at the metamorphic climax stage in cytosol from head region, but disappeared after this stage. T3-binding assay using whole cytosol showed that a distinct CTBP, although not photoaffinity-labeled, was present in cytosol from hindlimb bud and gradually disappeared as the hindlimb grew. The cytosol from liver, head region, and hindlimb bud contained high affinity binding sites for T3 with Kd values ranging from 10(-9) to 10(-8) M. These results suggest that there are at least three distinct CTBPs in Xenopus cytosol, which are expressed in a tissue-dependent and developmentally regulated manner.

Purification and characterization of a cytosolic thyroid-hormone-binding protein (CTBP) in Xenopus liver

A variety of cytosolic thyroid-hormone-binding proteins with different characteristics have previously been reported. Here, we first describe the thyroid-hormone-binding characteristics of adult Xenopus liver cytosol, then a novel procedure for purifying cytosolic thyroid-hormone-binding protein (CTBP) from Xenopus liver (xCTBP). The procedure consists of combining preparative isoelectrofocusing, FPLC cation-exchange chromatography, HPLC hydrophobic-interaction chromatography and ultraviolet light cross-linking of 125I-labeled 3,3'5-triiodo-L-thyronine (T3). The isolated xCTBP thus prepared retained all the characteristics of the major thyroid- hormone-(TH)-binding component of the unfractionated cytosol. It is a monomeric protein of approximately 59 kDa with an isoelectric point of 7.0 +/- 0.1, binds T3 with a higher affinity than its analogs with a Kd of approximately 9 nM, and is sensitive to sulfhydryl agents but not to NADPH. In several respects, xCTBP differs from most CTBP-like preparations from other sources described hitherto. Microsequencing of a 23-amino-acid peptide generated from xCTBP by cyanogen bromide digestion revealed 92-100% identity of a 23-amino-acid sequence of several mammalian (amino acids 236-258) and avian (amino acids 245-267) cytosolic aldehyde dehydrogenases (ALDH); xCTBP also exhibited significant similarity of amino acid composition with rat ALDH. This novel finding of sequence identity between a CTBP and ALDH, and the diversity of CTBPs from different sources, suggest that a variety of cytosolic proteins, depending on the species and tissue, can function as thyroid-hormone-binding proteins.

Triiodothyronine increases translatable albumin messenger RNA in Rana catesbeiana tadpole liver

The effects of both 3,5,3'-triiodo-L-thyronine and spontaneous metamorphosis on Rana catesbeiana liver mRNA were studied using in vitro translation of isolated liver poly(A)+ RNA in a rabbit reticulocyte lysate system. Conventional phenol extraction methods yielded degraded RNA due to high levels of endogenous ribonucleases released upon homogenization of Rana catesbeiana liver. Isolation of intact total RNA was achieved using the potent ribonuclease denaturant, guanidinium thiocyanate. Adult bullfrog serum albumin was purified to homogeneity and a monospecific antibody was elicited against it. A serum protein of 23,000 daltons that migrated near serum albumin on a 6% native gel was also purified to homogeneity. A monospecific antibody was also raised against this protein. Both antibodies were used to quantitatively immunoprecipitate the in vitro translation products of poly(A)+ RNA isolated at intervals following a single injection of triiodothyronine or during various stages of spontaneous amphibian metamorphosis. Triiodothyronine caused a sevenfold increase in translatable albumin mRNA and a threefold increase in translatable mRNA for the 23,000 dalton protein. These increases are consistent with a nuclear initiated mechanism for thyroid hormone action during amphibian metamorphosis.

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.

Physicochemical properties of the cytoplasmic triiodothyronine binding protein from tadpole liver and tail fin

The sedimentation velocity, gel filtration properties and pattern of elution from ion exchange gels of the cytoplasmic triiodothyronine (T3) binding protein from Rana catesbeiana liver and tail fin cytosol were determined. The T3 binding protein in liver cytosol had a sedimentation coefficient of 4.4S on sucrose gradients and a Stokes radius from Sephadex gel filtration of 38.2 +/- 4.2 A. From these two values a molecular weight of 71,700 +/- 4,100 and a frictional ratio of 1.28 +/- 0.05 for the T3 binding protein from tadpole liver has been calculated. The liver T3 binding protein was eluted from DEAE--Sephadex gels at a salt concentration of 0.233 +/- 0.017 M NaCl. The sedimentation coefficient, Stokes radius and salt requirement for elution from DEAE--Sephadex of the T3 binding protein from tail fin cytosol were essentially identical (4.4S, 35.2 +/- 4.2 A, nd 0.213 +/- 0.015 M NaCl respectively. The calculated molecular weight is 66,100 +/- 7,900 and the frictional ratio is 1.21 +/- 0.10 for for the tail fin T3 binding protein. The great similarity in the physicochemical properties of the T3 binding protein from the liver and tail fin implies that the T3 binding protein in each tissue is similar if not identical. The possible reason for the differences in the dissociation constants previously reported for the binding of T3 in the two tissues is discussed.

Binding characteristics of L-triiodothyronine to isolated rat liver chromatin

The binding characteristics of [125I]-labeled L-triiodothyronine (T3) to chromatin isolated from rat liver nuclei were investigated. Binding of T3 to chromatin showed temperature-, incubation time-, and DNA concentration-dependence. According to Scatchard analysis, the apparent equilibrium dissociation constant was 225 pM, with a maximum binding capacity of about 0.2 pmoles per mg DNA. Displacement studies with unlabeled thyroxine (T4) and T3 showed that the binding sites for T4 might be the same as T3 but the binding affinity of the former was less than that of the latter. The binding was completely inhibited by the eukaryotic RNA polymerase inhibitor, rifampicin AF/021, but not by the prokaryotic inhibitor, rifampicin and alpha-amanitin. These observations indicate that the receptors for T3 have certain properties in common with RNA polymerase or other enzyme proteins which are sensitive to the rifampicin derivative. The hormone--chromatin fragments complex was solubilized from residual chromatin by digestion with DNase, but not with RNase, suggesting that the T3 receptors localize in the DNase-sensitive regions of DNA in the chromatin. This provides a useful method to use to investigate the localization of the receptor proteins in the chromatin and the interaction of the hormone-receptor complex with DNA.