Iodotyrosines Are Biomarkers for Preclinical Stages of Iodine-Deficient Hypothyroidism in Dehal1-Knockout Mice

Background: Iodine is required for the synthesis of thyroid hormone (TH), but its natural availability is limited. Dehalogenase1 (Dehal1) recycles iodine from mono- and diiodotyrosines (MIT, DIT) to sustain TH synthesis when iodine supplies are scarce, but its role in the dynamics of storage and conservation of iodine is unknown. Methods: Dehal1-knockout (Dehal1KO) mice were generated by gene trapping. The timing of expression and distribution was investigated by X-Gal staining and immunofluorescence using recombinant Dehal1-beta-galactosidase protein produced in fetuses and adult mice. Adult Dehal1KO and wild-type (Wt) animals were fed normal and iodine-deficient diets for 1 month, and plasma, urine, and tissues were isolated for analyses. TH status was monitored, including thyroxine, triiodothyronine, MIT, DIT, and urinary iodine concentration (UIC) using a novel liquid chromatography with tandem mass spectrometry method and the Sandell-Kolthoff (S-K) technique throughout the experimental period. Results: Dehal1 is highly expressed in the thyroid and is also present in the kidneys, liver, and, unexpectedly, the choroid plexus. In vivo transcription of Dehal1 was induced by iodine deficiency only in the thyroid tissue. Under normal iodine intake, Dehal1KO mice were euthyroid, but they showed negative iodine balance due to a continuous loss of iodotyrosines in the urine. Counterintuitively, the UIC of Dehal1KO mice is twofold higher than that of Wt mice, indicating that S-K measures both inorganic and organic iodine. Under iodine restriction, Dehal1KO mice rapidly develop profound hypothyroidism, while Wt mice remain euthyroid, suggesting reduced retention of iodine in the thyroids of Dehal1KO mice. Urinary and plasma iodotyrosines were continually elevated throughout the life cycles of Dehal1KO mice, including the neonatal period, when pups were still euthyroid. Conclusions: Plasma and urine iodotyrosine elevation occurs in Dehal1-deficient mice throughout life. Therefore, measurement of iodotyrosines predicts an eventual iodine shortage and development of hypothyroidism in the preclinical phase. The prompt establishment of hypothyroidism upon the start of iodine restriction suggests that Dehal1KO mice have low iodine reserves in their thyroid glands, pointing to defective capacity for iodine storage.

Synthetic Tyrosine tRNA Molecules with Noncanonical Secondary Structures

The L-shape form of tRNA is maintained by tertiary interactions occurring in the core. Base changes in this domain can cause structural defects and impair tRNA activity. Here, we report on a method to safely engineer structural variations in this domain utilizing the noncanonical scaffold of tRNA^Pyl. First, we constructed a naïve hybrid between archaeal tRNA^Pyl and tRNA^Tyr, which consisted of the acceptor and T stems of tRNA^Tyr and the other parts of tRNA^Pyl. This hybrid tRNA efficiently translated the UAG codon to 3-iodotyrosine in Escherichia coli cells, when paired with a variant of the archaeal tyrosyl-tRNA synthetase. The amber suppression efficiency was slightly lower than that of the “bench-mark” archaeal tRNA^Tyr suppressor assuming the canonical structure. After a series of modifications to this hybrid tRNA, we obtained two artificial types of tRNA^Tyr: ZtRNA had an augmented D (auD) helix in a noncanonical form and the D and T loops bound by the standard tertiary base pairs, and YtRNA had a canonical auD helix and non-standard interloop interactions. It was then suggested that the ZtRNA scaffold could also support the glycylation and glutaminylation of tRNA. The synthetic diversity of tRNA would help create new tRNA–aminoacyl-tRNA synthetase pairs for reprogramming the genetic code.

Towards the pre-clinical diagnosis of hypothyroidism caused by iodotyrosine deiodinase (DEHAL1) defects

DEHAL1 (also named IYD) is the thyroidal enzyme that deiodinates mono- and diiodotyrosines (MIT, DIT) and recycles iodine, a scarce element in the environment, for the efficient synthesis of thyroid hormone. Failure of this enzyme leads to the iodotyrosine deiodinase deficiency (ITDD), characterized by hypothyroidism, compressive goiter and variable mental retardation, whose diagnostic hallmark is the elevation of iodotyrosines in serum and urine. However, the specific diagnosis of this type of hypothyroidism is not routinely performed, due to technical and practical difficulties in iodotyrosine determinations. A handful of mutations in the DEHAL1 gene have been identified as the molecular basis for the ITDD. Patients harboring DEHAL1 defects so far described all belong to consanguineous families, and psychomotor deficits were present in some affected individuals. This is probably due to the lack of biochemical expression of the disease at the beginning of life, which causes ITDD being undetected in screening programs for congenital hypothyroidism, as currently performed. This worrying feature calls for efforts to improve pre-clinical detection of iodotyrosine deiodinase deficiency during the neonatal time. Such a challenge poses questions of patho-physiological (natural history of the disease, environmental factors influencing its expression) epidemiological (prevalence of ITDD) and technical nature (development of optimal methodology for safe detection of pre-clinical ITDD), which will be addressed in this review.

[Thyroid hormones and their precursors. II. Species-specific properties]

This paper surveys the species-specific physico-chemical parameters (basicity and lipophilicity) and related biological functions of thyroid hormones (thyroxine, liothyronine and reverse liothyronine) and their biological precursors (tyrosine, monoiodotyrosine and diiodotyrosine). The protonation macroconstants were determined by 1H NMR-pH titrations while the microconstants were determined by a multimodal spectroscopic-deductive methodology using auxiliary derivatives of reduced complexity. Our results show that the different number and/or position of iodine are the key factors to influence the phenolate basicity. The ionization state of the phenolate site is crucial in the biosynthesis and protein binding of thyroid hormones. The role of the protonation state in the receptor binding was investigated by an in silico docking method. Microspecies of thyroid hormones were docked to the thyroid hormone receptor isoforms. Our results quantitate at the molecular level how the ionization stage and the charge distribution influence the protein binding. The anionic form of the carboxyl group is essential for the protein binding, whereas the protonated form of the amino group loosens it. The protonation state of the phenolate plays a role of secondary importance in the receptor binding. The combined results of docking and microspeciation studies show that microspecies of the highest concentration at the pH of blood are not the strongest binding ones. The site-specific lipophilicity of our investigated molecules was determined with the measurement of distribution coefficients at different pH using carboxymethyl- and O-methyl-derivatives to mimic the partition of some of the individual microspecies. Correction factors were determined and introduced. Our data show that the iodinated aromatic ring system is the definitive structural element that fundamentally determines the lipophilicity of thyroid hormones, whereas the protonation state of the aliphatic part is essential in receptor binding. The membrane transport of thyroid hormones can be well interpreted in terms of the site-specific lipophilicity. At physiological pH these biomolecules are strongly amphipathic due to the lipophilic aromatic rings and hydrophilic amino acid side chains which can well be the reason why thyroid hormones cannot cross membranes by passive diffusion and they even become constituents of biological membranes. The site-specific physico-chemical characterization of the thyroid hormones is of fundamental importance to understand their (patho) physiological behavior and also, to influence the therapeutic properties of their drug candidate derivatives at the molecular level.

An exploratory evaluation of tyrosine hydroxylase inhibition in planaria as a model for parkinsonism

Planaria are the simplest organisms with bilateral symmetry and a central nervous system (CNS) with cephalization; therefore, they could be useful as model organisms to investigate mechanistic aspects of parkinsonism and to screen potential therapeutic agents. Taking advantage of the organism’s anti-tropism towards light, we measured a significantly reduced locomotor velocity in planaria after exposure to 3-iodo-l-tyrosine, an inhibitor of tyrosine hydroxylase that is an enzyme catalyzing the first and rate-limiting step in the biosynthesis of catecholamines. A simple semi-automatic assay using videotaped experiments and subsequent evaluation by tracking software was also implemented to increase throughput. The dopaminergic regulation of locomotor velocity was confirmed by bromocriptine, a drug whose mechanisms of action to treat Parkinson’s disease is believed to be through the stimulation of nerves that control movement.

[Thyroid hormones and their precursors I. Biochemical properties]

This paper and the following one (see the next issue of Acta Pharmaceutica Hungarica) survey the biological roles and the related site-specific physico-chemical parameters (basicity and lipophilicity) of the presently known thyroid hormones (thyroxine, liothyronine and reverse liothyronine) and their biological precursors (monoiodotyrosine and diiodotyrosine). Here the literature of the thyroid hormone biochemistry, biosynthesis, plasma- and membrane transport is summarized, focusing on the pH-dependent processes. Biosyntheses of the thyroid hormones take place by oxidative coupling of two iodotyrosine residues catalyzed by thyreoperoxidase in thyreoglobulin. The protonation state of the precursors, especially that of the phenolic OH is crucial for the biosynthesis, since anionic iodotyrosine residues can only be coupled in the thyroid hormone biosyntheses. In the blood more than 99% of the circulating thyroid hormone is bound to plasma proteins among which the thyroxine-binding globulin and transthyretin are crucial. The amphiphilic character of the hormones is assumed to be the reason why their membrane transport is an energy-dependent, transport-mediated process, in which the organic anion transporter family, mainly OATP1C1, and the amino acid transporters, such as MCT8 play important roles. Liothyronine is the biologically active hormone; it binds the thyroid hormone receptor, a type of nuclear receptor. There are two major thyroid hormone receptor (TR) isoforms, alfa (TRalpha) and beta (TRbeta). The activation of the TRalpha is associated with modifications in cardiac behavior, while activation of the TRbeta is associated with increasing metabolic rates, resulting in weight loss and reduction of blood plasma lipid levels. The affinity of the thyroid hormones for different proteins depends on the ionization state of the ligands. The site-specific physico-chemical characterization of the thyroid hormones is of fundamental importance to understand their (patho)physiological behavior and also, to influence their therapeutic properties at the molecular level.

Functional replacement of the endogenous tyrosyl-tRNA synthetase-tRNATyr pair by the archaeal tyrosine pair in Escherichia coli for genetic code expansion

Non-natural amino acids have been genetically encoded in living cells, using aminoacyl-tRNA synthetase-tRNA pairs orthogonal to the host translation system. In the present study, we engineered Escherichia coli cells with a translation system orthogonal to the E. coli tyrosyl-tRNA synthetase (TyrRS)-tRNA(Tyr) pair, to use E. coli TyrRS variants for non-natural amino acids in the cells without interfering with tyrosine incorporation. We showed that the E. coli TyrRS-tRNA(Tyr) pair can be functionally replaced by the Methanocaldococcus jannaschii and Saccharomyces cerevisiae tyrosine pairs, which do not cross-react with E. coli TyrRS or tRNA(Tyr). The endogenous TyrRS and tRNA(Tyr) genes were then removed from the chromosome of the E. coli cells expressing the archaeal TyrRS-tRNA(Tyr) pair. In this engineered strain, 3-iodo-L-tyrosine and 3-azido-L-tyrosine were each successfully encoded with the amber codon, using the E. coli amber suppressor tRNATyr and a TyrRS variant, which was previously developed for 3-iodo-L-tyrosine and was also found to recognize 3-azido-L-tyrosine. The structural basis for the 3-azido-L-tyrosine recognition was revealed by X-ray crystallography. The present engineering allows E. coli TyrRS variants for non-natural amino acids to be developed in E. coli, for use in both eukaryotic and bacterial cells for genetic code expansion.

The impact of iodine excess on thyroid hormone biosynthesis and metabolism in rats

Thyroid function ultimately depends on appropriate iodine supply to the gland. There is a complex series of checks and balances that the thyroid uses to control the orderly utilization of iodine for hormone synthesis. The aim of our study is to evaluate the mechanism underlying the effect of iodine excess on thyroid hormone metabolism. Based on the successful establishment of animal models of normal-iodine (NI) and different degrees of high-iodine (HI) intake in Wistar rats, the content of monoiodotyrosine (MIT), diiodotyrosine (DIT), T(4), and T(3) in thyroid tissues, the activity of thyroidal type 1 deiodinase (D1) and its (Dio1) mRNA expression level were measured. Results showed that, in the case of iodine excess, the biosynthesis of both MIT and DIT, especially DIT, was increased. There was an obvious tendency of decreasing in MIT/DIT ratio with increased doses of iodine intake. In addition, iodine excess greatly inhibited thyroidal D1 activity and mRNA expression. T(3) was greatly lower in the HI group, while there was no significant difference of T(4) compared with NI group. The T(3)/T(4) ratio was decreased in HI groups, antiparalleled with increased doses of iodine intakes. In conclusion, the increased biosyntheses of DIT relative to MIT and the inhibition of thyroidal Dio1 mRNA expression and D1 activity may be taken as an effective way to protect an organism from impairment caused by too much T(3). These observations provide new insights into the cellular regulation mechanism of thyroid hormones under physiological and pathological conditions.

Deciphering the peptide iodination code: influence on subsequent gas-phase radical generation with photodissociation ESI-MS

Iodination of tyrosine was recently discovered as a useful method for generating radical peptides via photodissociation of carbon-iodine bonds by an ultraviolet photon in the gas phase. The subsequent fragmentation behavior of the resulting odd-electron peptides is largely controlled by the radical. Although previous experiments have focused on mono-iodination of tyrosine, peptides and proteins can also be multiply iodinated. Tyrosine and, to a lesser extent, histidine can both be iodinated or doubly iodinated. The behavior of doubly iodinated residues is explored under conditions where the sites of iodination are carefully controlled. It is found that radical peptides generated by the loss of a single iodine from doubly iodinated tyrosine behave effectively identically to singly iodinated peptides. This suggests that the remaining iodine does not interfere with radical directed dissociation pathways. In contrast, the concerted loss of two iodines from doubly iodinated peptides yields substantially different results that suggest that radical recombination can occur. However, sequential activation can be used to generate multiple usable radicals in different steps of an MS(n) experiment. Furthermore, it is demonstrated that in actual peptides, the rate of iodination for tyrosine versus mono-iodotyrosine cannot be predicted easily a priori. In other words, previous assumptions that mono-iodination of tyrosine is the rate-limiting step to the formation of doubly iodinated tyrosine are incorrect.

Molecular characterization of iodotyrosine dehalogenase deficiency in patients with hypothyroidism

CONTEXT

The recent cloning of the human iodotyrosine deiodinase (IYD) gene enables the investigation of iodotyrosine dehalogenase deficiency, a form a primary hypothyroidism resulting from iodine wasting, at the molecular level.

OBJECTIVE

In the current study, we identify the genetic basis of dehalogenase deficiency in a consanguineous family.

RESULTS

Using HPLC tandem mass spectrometry, we developed a rapid, selective, and sensitive assay to detect 3-monoiodo-l-tyrosine and 3,5-diodo-l-tyrosine in urine and cell culture medium. Two subjects from a presumed dehalogenase-deficient family showed elevated urinary 3-monoiodo-l-tyrosine and 3,5-diodo-l-tyrosine levels compared with 57 normal subjects without thyroid disease. Subsequent analysis of IYD revealed a homozygous missense mutation in exon 4 (c.658G>A p.Ala220Thr) that co-segregates with the clinical phenotype in the family. Functional characterization of the mutant iodotyrosine dehalogenase protein showed that the mutation completely abolishes dehalogenase enzymatic activity. One of the heterozygous carriers for the inactivating mutation recently presented with overt hypothyroidism indicating dominant inheritance with incomplete penetration. Screening of 100 control alleles identified one allele positive for this mutation, suggesting that the c.658G>A nucleotide substitution might be a functional single nucleotide polymorphism.

CONCLUSIONS

This study describes a functional mutation within IYD, demonstrating the molecular basis of the iodine wasting form of congenital hypothyroidism. This familial genetic defect shows a dominant pattern of inheritance with incomplete penetration.

Mutations in the iodotyrosine deiodinase gene and hypothyroidism

DEHAL1 has been identified as the gene encoding iodotyrosine deiodinase in the thyroid, where it controls the reuse of iodide for thyroid hormone synthesis. We screened patients with hypothyroidism who had features suggestive of an iodotyrosine deiodinase defect for mutations in DEHAL1. Two missense mutations and a deletion of three base pairs were identified in four patients from three unrelated families; all the patients had a dramatic reduction of in vitro activity of iodotyrosine deiodinase. Patients had severe goitrous hypothyroidism, which was evident in infancy and childhood. Two patients had cognitive deficits due to late diagnosis and treatment. Thus, mutations in DEHAL1 led to a deficiency in iodotyrosine deiodinase in these patients. Because infants with DEHAL1 defects may have normal thyroid function at birth, they may be missed by neonatal screening programs for congenital hypothyroidism.

Site-specific incorporation of non-natural amino acids into proteins in mammalian cells with an expanded genetic code

We describe a detailed protocol for incorporating non-natural amino acids, 3-iodo-L-tyrosine (IY) and p-benzoyl-L-phenylalanine (pBpa), into proteins in response to the amber codon (the UAG stop codon) in mammalian cells. These amino acids, IY and pBpa, are applicable for structure determination and the analysis of a network of protein-protein interactions, respectively. This method involves (i) the mutagenesis of the gene encoding the protein of interest to create an amber codon at the desired site, (ii) the expression in mammalian cells of the bacterial pair of an amber suppressor tRNA and an aminoacyl-tRNA synthetase specific to IY or pBpa and (iii) the supplementation of the growth medium with these amino acids. The amber mutant gene, together with these bacterial tRNA and synthetase genes, is introduced into mammalian cells. Culturing these cells for 16-40 h allows the expression of the full-length product from the mutant gene, which contains the non-natural amino acid at the introduced amber position. This method is implemented using the conventional tools for molecular biology and treating cultured mammalian cells. This protocol takes 5-6 d for plasmid construction and 3-4 d for incorporating the non-natural amino acids into proteins.

Thyroid hormone transport by the human monocarboxylate transporter 8 and its rate-limiting role in intracellular metabolism

Cellular entry of thyroid hormone is mediated by plasma membrane transporters. We have identified rat monocarboxylate transporter 8 (MCT8) as an active and specific thyroid hormone transporter. The MCT8 gene is located on the X-chromosome. The physiological relevance of MCT8 has been demonstrated by the identification of hemizygous mutations in this gene in males with severe psychomotor retardation and elevated serum T(3) levels. We have characterized human (h) MCT8 by analysis of iodothyronine uptake and metabolism in cell lines transiently transfected with hMCT8 cDNA alone or together with cDNA coding for iodothyronine deiodinase D1, D2, or D3. MCT8 mRNA was detected by RT-PCR in a number of human cell lines as well as in COS1 cells but was low to undetectable in other cell lines, including JEG3 cells. MCT8 protein was not detected in nontransfected cell lines tested by immunoblotting using a polyclonal C-terminal hMCT8 antibody but was detectable in transfected cells at the expected size (61 kDa). Transfection of COS1 and JEG3 cells with hMCT8 cDNA resulted in 2- to 3-fold increases in uptake of T(3) and T(4) but little or no increase in rT(3) or 3,3'-diiodothyronine (3,3'-T(2)) uptake. MCT8 expression produced large increases in T(4) metabolism by cotransfected D2 or D3, T(3) metabolism by D3, rT(3) metabolism by D1 or D2, and 3,3'-T(2) metabolism by D3. Affinity labeling of hMCT8 protein was observed after incubation of intact transfected cells with N-bromoacetyl-[(125)I]T(3). hMCT8 also facilitated affinity labeling of cotransfected D1 by bromoacetyl-T(3). Our findings indicate that hMCT8 mediates plasma membrane transport of iodothyronines, thus increasing their intracellular availability.

Structural basis of nonnatural amino acid recognition by an engineered aminoacyl-tRNA synthetase for genetic code expansion

The genetic code in a eukaryotic system has been expanded by the engineering of Escherichia coli tyrosyl-tRNA synthetase (TyrRS) with the Y37V and Q195C mutations (37V195C), which specifically recognize 3-iodo-L-tyrosine rather than L-tyrosine. In the present study, we determined the 3-iodo-L-tyrosine- and L-tyrosine-bound structures of the 37V195C mutant of the E. coli TyrRS catalytic domain at 2.0-A resolution. The gamma-methyl group of Val-37 and the sulfur atom of Cys-195 make van der Waals contacts with the iodine atom of 3-iodo-L-tyrosine. The Val-37 and Cys-195 side chains are rigidly fixed by the neighboring residues forming the hydrophobic core of the TyrRS. The major roles of the two mutations are different for the 3-iodo-L-tyrosine-selective recognition in the first step of the aminoacylation reaction (the amino acid activation step): the Y37V mutation eliminates the fatal steric repulsion with the iodine atom, and the Q195C mutation reduces the L-tyrosine misrecognition. The structure of the 37V195C mutant TyrRS complexed with an L-tyrosyladenylate analogue was also solved, indicating that the 3-iodo-L-tyrosine and L-tyrosine side chains are similarly discriminated in the second step (the aminoacyl transfer step). These results demonstrate that the amino acid-binding pocket on the 37V195C mutant is optimized for specific 3-iodo-L-tyrosine recognition.

SPECT and PET amino acid tracer influx via system L (h4F2hc-hLAT1) and its transstimulation

System L amino acid transport is increased in various types of cancer. The tracer 123I-2-iodotyrosine (2IT), which is accumulated via system L, could thus serve to allow visualization of cancer in vivo. Here, we studied the transport of 125I-2IT by h4F2hc-hLAT1, the major transporter subserving system L in growing cells, using the Xenopus laevis oocyte expression system. We compared the apparent affinity of 125I-2IT with that of tyrosine, tested the influence of intracellular methionine availability on the influx rate of this substrate, and then compared the transport of 2IT with that of the other tracers-iodo-alpha-methyltyrosine (IMT), fluoroethyltyrosine (FET), and 2-fluorotyrosine (2FT)-by measuring their transstimulating effect on phenylalanine efflux.

METHODS

Transport experiments were performed with Xenopus laevis oocytes expressing h4F2hc-hLAT1 (the functional transporter) and oocytes expressing only h4F2hc (negative control). The values obtained for the functional transporter were corrected for endogenous background transport by subtracting the values for the negative controls.

RESULTS

The apparent affinity for 125I-2IT and 3H-tyrosine was 29.3 +/- 9.3 micromol/L and 21.2 +/- 4.2 micromol/L, respectively. The influx rate of 125I-2IT was, similarly to that of 3H-phenylalanine, transstimulated by a factor of > or =3 when the oocytes were preinjected with methionine or phenylalanine. The proportion of preinjected 3H-phenylalanine that effluxed within 90 s in the presence of an extracellular 2IT concentration of 0.1 mmol/L was 4.1% +/- 0.5%, compared with 3.3% +/- 0.4% for extracellular IMT, 1.3% +/- 0.3% for FET, 9.3% +/- 0.8% for 2FT, and 9.1% +/- 0.5% for phenylalanine.

CONCLUSION

2IT has a high affinity for h4F2hc-hLAT1, comparable to that of natural tyrosine, and its influx rate is transstimulated by intracellular amino acids. The 2IT influx rate is comparable to that of IMT but lower than that of phenylalanine. In contrast to FET, which is only poorly transported, 2FT displays a high influx rate equal to that of phenylalanine.

An ascidian homolog of vertebrate iodothyronine deiodinases

In all classes of vertebrates, the deiodination of the prohormone T(4) to T(3) represents an essential activation step in thyroid hormone action. The possible presence of iodothyronine deiodinase activity in protochordates has been demonstrated in vivo. Recent molecular cloning of the genomes and transcripts of several ascidian species allows further investigation into thyroid-related processes in ascidians. A cDNA clone from Halocynthia roretzi (hrDx) was found to have significant homology (30% amino acid identity) with the iodothyronine deiodinase gene sequences from vertebrates, including the presence of an in-frame UGA codon that might encode a selenocysteine (SeC) in the active site. Because it was not certain that the 3' untranslated region (UTR) contained a SeC insertion sequence (SECIS) element essential for SeC incorporation, a chimeric expression vector of the hrDx coding sequence and the rat deiodinase SECIS element was produced, as well as an expression vector containing the intact hrDx cDNA. COS, CHO, and HEK cells were transfected with these vectors, and deiodinase activity was measured in cell homogenates. Outer-ring deiodinase activity was detected using both T(4) and reverse T(3) as substrates, and activity was enhanced by the presence of the reductive cofactor dithiothreitol. The enzyme activity was optimal during incubation between 20 and 30 C (pH 6-7) and was strongly inhibited by gold-thioglucose. The Halocynthia deiodinase appears to be a high Michaelis-Menten constant (K(m)) enzyme (K(m) reverse T(3), 2 microM; and K(m) T(4), 4 microM). Deiodinase activity was completely lost upon the substitution of the SeC residue in the putative catalytic center by either cysteine or alanine. Transfection of the full-length hrDx cDNA produced deiodinase activity confirming the presence of a SECIS element in the 3'UTR, as revealed by the SECISearch program. In conclusion, our results show, for the first time, the existence of an ascidian iodothyronine outer-ring deiodinase. This raises the hypothesis that, in protochordates, the prohormone T(4) is activated by enzymatic outer-ring deiodination to T(3).

Ontogenic and adult whole body distribution of aminopeptidase N in rat investigated by in vitro autoradiography

Aminopeptidase N (APN), which is widely distributed in mammalian tissues, is able to cleave numerous regulatory peptides. The selective inhibitor of APN, [(125)I] RB129, has been used to study the distribution of this exopeptidase during rat prenatal development and adult life by in vitro whole-body autoradiography. In the central nervous system, APN shows a weak labeling compared to the major part of the non-nervous tissues in the embryo and in the adult. APN is progressively expressed in kidney, intestine, heart, lung, sensory organs, eye, and thymus. In organs such as the liver, the cartilages and the bones, altered levels of APN expression are observed during the development, or in the embryo compared to the adult, suggesting a role of APN during the liver haematopoiesis and bone growth. At this time, all the physiological functions of APN are still incompletely known, however its developmental pattern of expression strongly suggests a function of modulation of this enzyme during the development, next in physiological and/or pathological situations in adult. In this way, APN could represent a new therapeutic target in pathological processes, such as tumoral proliferation and/or angiogenesis associated with cancer development, where an increase in the level of this enzyme has been observed.

Site-specific incorporation of an unnatural amino acid into proteins in mammalian cells

A suppressor tRNA(Tyr) and mutant tyrosyl-tRNA synthetase (TyrRS) pair was developed to incorporate 3-iodo-L-tyrosine into proteins in mammalian cells. First, the Escherichia coli suppressor tRNA(Tyr) gene was mutated, at three positions in the D arm, to generate the internal promoter for expression. However, this tRNA, together with the cognate TyrRS, failed to exhibit suppressor activity in mammalian cells. Then, we found that amber suppression can occur with the heterologous pair of E.coli TyrRS and Bacillus stearothermophilus suppressor tRNA(Tyr), which naturally contains the promoter sequence. Furthermore, the efficiency of this suppression was significantly improved when the suppressor tRNA was expressed from a gene cluster, in which the tRNA gene was tandemly repeated nine times in the same direction. For incorporation of 3-iodo-L-tyrosine, its specific E.coli TyrRS variant, TyrRS(V37C195), which we recently created, was expressed in mammalian cells, together with the B.stearothermophilus suppressor tRNA(Tyr), while 3-iodo-L-tyrosine was supplied in the growth medium. 3-Iodo-L-tyrosine was thus incorporated into the proteins at amber positions, with an occupancy of >95%. Finally, we demonstrated conditional 3-iodo-L-tyrosine incorporation, regulated by inducible expression of the TyrRS(V37C195) gene from a tetracycline-regulated promoter.

An engineered Escherichia coli tyrosyl-tRNA synthetase for site-specific incorporation of an unnatural amino acid into proteins in eukaryotic translation and its application in a wheat germ cell-free system

Tyrosyl-tRNA synthetase (TyrRS) from Escherichia coli was engineered to preferentially recognize 3-iodo-L-tyrosine rather than L-tyrosine for the site-specific incorporation of 3-iodo-L-tyrosine into proteins in eukaryotic translation systems. The wild-type TyrRS does not recognize 3-iodo-L-tyrosine, because of the bulky iodine substitution. On the basis of the reported crystal structure of Bacillus stearothermophilus TyrRS, three residues, Y37, Q179, and Q195, in the L-tyrosine-binding site were chosen for mutagenesis. Thirty-four single amino acid replacements and 16 of their combinations were screened by in vitro biochemical assays. A combination of the Y37V and Q195C mutations changed the amino acid specificity in such a way that the variant TyrRS activates 3-iodo-L-tyrosine 10-fold more efficiently than L-tyrosine. This engineered enzyme, TyrRS(V37C195), was tested for use in the wheat germ cell-free translation system, which has recently been significantly improved, and is now as productive as conventional recombinant systems. During the translation in the wheat germ system, an E. coli suppressor tRNA(Tyr) was not aminoacylated by the wheat germ enzymes, but was aminoacylated by the E. coli TyrRS(V37C195) variant with 3-iodo-l-tyrosine. After the use of the 3-iodotyrosyl-tRNA in translation, the resultant uncharged tRNA could be aminoacylated again in the system. A mass spectrometric analysis of the produced protein revealed that more than 95% of the amino acids incorporated for an amber codon were iodotyrosine, whose concentration was only twice that of L-tyrosine in the translation. Therefore, the variant enzyme, 3-iodo-L-tyrosine, and the suppressor tRNA can serve as an additional set orthogonal to the 20 endogenous sets in eukaryotic in vitro translation systems.

Identification of a new class of inhibitors of the voltage-gated potassium channel, Kv1.3, with immunosuppressant properties

The voltage-gated potassium channel, K(v)1.3, is a novel target for development of immunosuppressants. Using a functional (86)Rb(+) efflux assay, a new class of high-affinity K(v)1.3 inhibitors has been identified. The initial active in this series, 4-phenyl-4-[3-(2-methoxyphenyl)-3-oxo-2-azaprop-1-yl]cyclohexanone (PAC), which is representative of a disubstituted cyclohexyl (DSC) template, displays a K(i) of ca. 300 nM and a Hill coefficient near 2 in the flux assay and in voltage clamp recordings of K(v)1.3 channels in human T-lymphocytes. PAC displays excellent specificity as it only blocks members of the K(v)1 family of potassium channels but does not affect many other types of ion channels, receptors, or enzyme systems. Block of K(v)1.3 by DSC analogues occurs with a well-defined structure-activity relationship. Substitution at the C-1 ketone of PAC generates trans (down) and cis (up) isomer pairs. Whereas many DSC derivatives do not display selectivity in their interaction with different K(v)1.x channels, trans DSC derivatives distinguish between K(v)1.x channels based on their rates of C-type inactivation. DSC analogues reversibly inhibit the Ca(2+)-dependent pathway of T cell activation in in vitro assays. Together, these data suggest that DSC derivatives represent a new class of immunosuppressant agents and that specific interactions of trans DSC analogues with channel conformations related to C-type inactivation may permit development of selective K(v)1.3 channel inhibitors useful for the safe treatment of autoimmune diseases.

First discrete autoradiographic distribution of aminopeptidase N in various structures of rat brain and spinal cord using the selective iodinated inhibitor [125I]RB 129

The selective and potent aminopeptidase N inhibitor [125I]RB 129 has been used for the radioautographic localization of this enzyme in rat brain, spinal cord and intestine. Brain microvessels and intestine brush-border cells were shown to present a high concentration of aminopeptidase N. Moreover, a labeling of various brain structures was observed. A very high level of binding occurred in the meninges, choroid plexus, pineal gland, paraventricular nucleus and pituitary gland. Moderate to high labeling was also observed in the cortex, caudate-putamen, subthalamic nucleus, central periaqueductal gray, thalamus, as well as in the dorsal and ventral horn of the spinal cord, which are known to contain a high concentration of enkephalins, opioid receptors and neutral endopeptidase. This co-localization confirms the physiological implication of aminopeptidase N in the inactivation of enkephalins accounting for the requirement of dual inhibition of neutral endopeptidase and aminopeptidase N to observe highly significant morphine-like effects induced by the protected endogenous opioid peptides. Aminopeptidase N was also visualized in moderate to high levels in other brain structures such as the hippocampus, nucleus accumbens, substantia nigra, hypothalamus (dorsomedial and ventromedial nuclei), raphe nucleus, pontine nucleus, inferior olive, and in high concentration in the granular layer of cerebellum. In summary, aminopeptidase N has been visualized for the first time in numerous brain areas using the selective inhibitor [125I]RB 129. This iodinated probe could allow the ex vivo and in vivo localization of aminopeptidase N in various tissues to be investigated and may also be used to evaluate quantitative changes in aminopeptidase N expression in pathological situations. Aminopeptidase N, which preferably removes NH2-terminal neutral amino acids from peptides, has probably a host of substrates. Nevertheless, a certain in vivo selectivity could be achieved by the presence of the enzyme in structures where the peptide effector and its receptors are also co-localized.

Helix packing in the lactose permease of Escherichia coli: distances between site-directed nitroxides and a lanthanide

By exploiting substrate protection of Cys148 in lactose permease, a methanethiosulfonate nitroxide spin-label was directed specifically to one of two Cys residues in a double-Cys mutant, followed by labeling of Cys148 with a thiol-reactive chelator that binds Gd(III) quantitatively. Distances between bound Gd(III) and the nitroxide spin-label were then studied by electron paramagnetic resonance. The results demonstrate that the Gd(III)-induced relaxation effects on nitroxides at positions 228, 226 (helix VII), and 275 (helix VIII) agree qualitatively with results obtained by studying spin-spin interactions [Wu, J., Voss, J., et al. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 10123-10127]. Thus, a nitroxide attached to position 228 (helix VII) is closest to the lanthanide at position 148 (helix V), a nitroxide at position 275 (helix VIII) is further away, and the distance between positions 226 (helix VII) and 148 is too long to measure. However, the Gd(III)-spin-label distances are significantly longer than those estimated from nitroxide-nitroxide interactions between the same pairs due to the nature of the chelator. Although the results provide strong confirmation for the contention that helix V lies close to both helices VII and VIII in the tertiary structure of lactose permease, other methods for binding rare earth metals are discussed which do not involve the use of bulky chelators with long linkers.

Binding properties of a highly potent and selective iodinated aminopeptidase N inhibitor appropriate for radioautography

Aminopeptidase N (APN) is a zinc metallopeptidase involved in the inactivation of biologically active peptides. The knowledge of its precise distribution is crucial to investigate its physiological role. This requires the use of appropriate probes such as the recently developed highly potent and selective radiolabeled APN inhibitor 2(S)-benzyl-3-[hydroxy(1'(R)-aminoethyl)phosphinyl]propanoyl-L-3-[ (12 5)I]iodotyrosine ([(125)I]RB 129). Its binding properties were investigated using rat brain homogenates (K(d)=3.4 nM) or APN expressed in COS-7 cells (K(d)=0.9 nM). The specific binding was 95% at [K(d)], and preliminary autoradiography in intestine is promising. The decreased affinity of [(125)I]RB 129 (=10(-6) M) for the E(350)D APN mutant, supports the critical role of E(350) in the amino-exopeptidase action of APN.

Brain uptake of iodinated L-meta-tyrosine, a metabolically stable amino acid derivative

The possibility of using L-meta-tyrosine (L-mTyr) with high metabolic stability and amino acid transport affinity was evaluated. mTyr was first separated into D- and L-isomers with high-performance liquid chromatography and both were labelled with non-carrier-mediated 125I. Biodistribution and pharmacological studies of radioiodinated mTyr in mice and rats were then performed. 125I-L-mTyr showed greater accumulation in the brain and the pancreas. It accumulated in the brain stereospecifically in the in vivo studies and by the L-tyrosine competitive energy dependent transport system in the in vitro studies. It was resistant to deiodination, appeared to have no retention mechanism and was rapidly excreted. 123I-L-mTyr has the potential of an amino acid transport marker, especially in the brain and the pancreas.

Influence of Gz and Gi2 transducer proteins in the affinity of opioid agonists to mu receptors

The affinity displayed by different opioids to mu receptors (ORs) was determined in mouse brain membranes incubated with antibodies directed to Galpha subunits of the guanine nucleotide-binding proteins Gi2 and Gz. Assays were conducted with 10 pm 125I-Tyr27-beta-endorphin in the presence of 300 nm N, N-diallyl-Tyr-(alpha-aminoisobutyric acid)2-Phe-Leu-OH (ICI-174 864), which prevented the binding of the iodinated neuropeptide to delta-ORs. Gpp(NH)p or the preincubation of mouse brain membranes with IgGs to Gi2alpha or Gzalpha subunits, promoted reductions in the affinity exhibited by the labelled probe. The potencies of beta-endorphin, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and [D-Pen2,5]enkephalin (DPDPE) were reduced after impairing the coupling of mu-ORs to Gi2 or Gz proteins. Morphine showed a loss of affinity towards the mu-OR after preincubation of membranes with IgGs to Gzalpha subunits. However, it retained its potency after treatment with the anti-Gi2alpha IgGs. Conversely, [D-Ala2, D-Leu5]enkephalin (DADLE) and [D-Ser2, Leu5] enkephalin-Thr6 (DSLET) showed decreased affinity to mu-ORs after treatment with anti-Gi2alpha IgGs, with no noticeable change following the use of IgGs to Gzalpha subunits. The affinity exhibited by the opioid antagonists naloxone, naltrexone, naloxonazine and [Cys2,Tyr3,Orn5, Pen7 amide]somatostatin analogue (CTOP) remained unchanged after either treatment. Therefore, the affinity exhibited by opioid agonists of mu-ORs, but not antagonists, depends on the nature of the G-protein coupled to these receptors.

Mechanism for the anti-thyroid action of minocycline

Administration of minocycline (MN), a tetracycline antibiotic, produces a black pigment in the thyroids of humans and several species of experimental animals and antithyroid effects in rodents. We have previously shown that these effects appear to be related to interactions of MN with thyroid peroxidase (TPO), the key enzyme in thyroid hormone synthesis. In the present study, the mechanisms for inhibition of TPO-catalyzed iodination and coupling reactions by MN were investigated. MN was stable in the presence of TPO and H2O2, but adding iodide or a phenolic cosubstrate caused rapid conversion to several products. TPO-dependent product formation, characterized by on-line LC-APCI/MS and 1H-NMR, involved oxidative elimination to form the corresponding benzoquinone with subsequent dehydrogenation at the aliphatic 4-(dimethylamino) group. Addition of thiol-containing polymers (bovine serum albumin or thiol-agarose chromatographic beads) had a minimal effect on MN oxidation by TPO, but substantially reduced product formation and produced concomitant losses in free thiols. Covalent bonding through a thioether linkage of a reactive intermediate, the benzoquinone iminium ion, was inferred from these findings. Iodide- and phenolic cosubstrate-dependent oxidation of tetracycline to demethylated and dehydrogenated products was also observed, although at a slower rate than MN. The products and kinetics observed with MN were consistent with oxidation of MN by either the enzymatic iodinating species formed by reaction of TPO compound I with iodide or phenoxyl radicals/cations generated by TPO-mediated oxidation of a phenolic cosubstrate. The proposed reaction mechanism is consistent with alternate substrate inhibition of TPO-catalyzed iodination of tyrosyl residues in thyroglobulin (Tg) by MN, as previously reported. Furthermore, the observed phenoxyl radical-mediated oxidation of MN is consistent with its previously reported potent inhibition of the coupling of hormonogenic iodotyrosine residues in Tg in the reaction that forms thyroid hormones. The proposed reaction mechanism also implicates a reactive benzoquinone iminium ion intermediate that could be important in toxicity of MN.

The properties of beta-galactosidases (Escherichia coli) with halogenated tyrosines

An Escherichia coli tyrosine auxotroph (MR1) with an inducible lacZ was generated by mutagenesis. Of several tyrosine derivatives tested, only m-fluorotyrosine supported the growth of this mutant and allowed synthesis of active beta-galactosidase. The pH profiles of the beta-galactosidase that was obtained when this mutant was grown on m-fluorotyrosine (81.5% of the tyrosine was replaced by m-fluorotyrosine) indicated that a tyrosine may be acting as a general acid-base catalyst and that it (or another tyrosine with the same pKa) may be involved in substrate binding. Inactivation of normal beta-galactosidase by treatment with lactoperoxidase in the presence of I- did not affect affinity-column binding, but incubation of this iodinated beta-galactosidase with chymotrypsin caused a rapid degradation of a portion of the treated enzyme equal to the portion of the activity that was lost. A study with 125I- showed that the rapid degradation was mainly confined to iodinated molecules of enzyme. These studies indicate that iodination of beta-galactosidase does not affect binding ability, but causes the enzyme to lose catalytic activity and become susceptible to chymotryptic action. Chloroperoxidase also caused rapid inactivation of normal beta-galactosidase in the presence of Br- or I-, but there was a lag followed by a slow inactivation in the presence of Cl-.

Kinetics and mechanism of the peroxidase-catalyzed iodination of tyrosine

The kinetics of iodination of tyrosine by hydrogen peroxide and iodide, catalyzed by both horseradish peroxidase (HRP) and lactoperoxidase (LPO), were studied. The initial rates of formation of both molecular I2 and monoiodotyrosine (MIT) were measured with stopped flow techniques. The following reactions occur in both systems. Enzymatic: FeIII + H2O2-->Fev = O + H2O; Fev = O + I(-)-->FeIII-O-I-; FeIII-O-I- + H(+)-->FeIII + HOI; FeIII-O-I- + I- + H(+)-->FeIII + I2 + HO-. Iodine equilibria: I2 + I-<-->I3-; I2 + H2O<-->HOI + I- + H+. Nonenzymatic iodination, one or both of the following: Tyr + HOI-->MIT + H2O; Tyr + I2-->MIT + I- + H+, where FeIII is native peroxidase, Fev = O is compound I and Tyr is tyrosine. The big difference in the two systems is that the following reaction also occurs with LPO: FeIII-O-I- + Tyr-->MIT + FeIII + HO-, which is the dominant mechanism of iodination for the mammalian enzyme. The overall rate of formation of MIT is about 10 times faster for LPO compared to HRP under comparable conditions. A small decrease in rate occurs when D-tyrosine is substituted for L-tyrosine in the LPO reaction. Thus LPO has a tyrosine binding site near the heme. A kinetically controlled maximum is observed in I3- concentration. Once equilibrium is established, I2 is the dominant form of inorganic iodine in solution. However, hypoiodous acid may be the inorganic iodination reagent.

Examination of antithyroid effects of smoking products in cultured thyroid follicles: only thiocyanate is a potent antithyroid agent

We studied the antithyroid action of cigarette smoking products (nicotine, cotinine, and thiocyanate) in the physiological culture system of porcine thyroid follicles. Iodide uptake, iodine organification, de novo thyroid hormone formation, and iodide efflux were measured in the presence of 0-200 mumol/l nicotine, cotinine, or potassium thiocyanate. Nicotine and cotinine did not inhibit iodide transport or thyroid hormone formation. Thiocyanate concentrations equivalent to serum levels of smokers showed three independent antithyroid actions: (i) inhibition of iodide transport, (ii) inhibition of iodine organification, and (iii) increased iodide efflux. Inhibition of iodide transport by thiocyanate was competitive with iodide and independent of TSH concentration. Thiocyanate did not inhibit TSH mediated cAMP production or Na+K+ ATPase activity, a sodium pump for iodide transport. When 50 mumol/l thiocyanate was added 2 h after incubation with iodide or when 1 mumol/l thiocyanate was added from the beginning of incubation, iodine organification was inhibited without changing iodide transport. De novo thyroid hormone formation was clearly inhibited by 50 mumol/l thiocyanate. Thiocyanate increased iodide efflux although the degrees of iodide efflux by 10 mumol/l and 100 mumol/l thiocyanate did not differ significantly. In summary, thiocyanate, a product of smoking, has three independent antithyroid activities. The data of iodide transport kinetics suggest that thiocyanate can be an antithyroid agent particularly in iodine deficiency.

Expression of thyroglobulin gene in maternal and fetal thyroid in rats

The relationship between the changes in thyroglobulin (Tg) mRNA and Tg proteins during thyroid development in the fetus and in maternal thyroid glands during gestation and lactation is studied. While the appearance of Tg mRNA (fetal day 15) showed good temporal correlation with that of 12S Tg, no 19S Tg could be detected until 3 days later. The 12S Tg was the predominant protein on days 18 and 19 of gestation in the fetus, while 19S Tg was the predominant protein on fetal days 21-22 and during the postnatal period in the offspring; by the 20th postnatal day, the 19S Tg content per gland was 4 times the amount of 12S (155 vs. 37 micrograms/gland; P less than 0.001). The 19S iodine content in the fetus was the same as that in 12S up to the 21st day of gestation, except for lower values on day 18. From fetal day 22 and through the postnatal period, the iodine content in 19S was 1.6-5.9 times greater than that in 12S. Therefore, the ratio of atoms of iodine per mol Tg during the experimental period changed from 0.75 to 19.5 for 19S and from 0.72 to 7.2 for 12S. The levels of all of the iodoamino acids were low on fetal days 17-19, after which they increased at different rates for each protein. The greatest increase in monoiodotyrosine and T3 corresponded to 12S, while diiodotyrosine and especially T4 showed a greater increase in 19S than in 12S Tg; 20 days after birth, the T4 content in 19S was about 3 times greater than that in 12S Tg. The soluble thyroid proteins from pregnant, lactating, and nonpregnant female controls contained a main protein, 19S, and a smaller amount of 27S. Both 19S Tg and 19S iodine contents were already lower than those in nonpregnant rats at 14 days of pregnancy, and the levels continued to decrease during the experimental period. In contrast, the 27S Tg and 27S iodine levels remained constant and similar to nonpregnant values. Surprisingly, a decrease in the level of Tg mRNA was observed during pregnancy and lactation. We have no explanation for the dramatic decrease in Tg mRNA during the last days of pregnancy. Further studies should help to elucidate the mechanism responsible for the changes in Tg gene expression in the thyroids of pregnant and lactating rats.

Site-specific incorporation of nonnatural residues during in vitro protein biosynthesis with semisynthetic aminoacyl-tRNAs

A method is presented for the incorporation of nonnatural amino acids into proteins during in vitro cell-free translation. A combination of chemical synthesis and run-off transcription was employed to prepare a semisynthetic, nonhypermodified tRNA(Gly) nonsense suppressor acylated with L-3-[125I]iodotyrosine. The presence of this synthetic tRNA during in vitro translation of mRNA containing a nonsense suppression site (e.g., a UAG termination codon) results in the incorporation of the nonnatural amino acid L-3-iodotyrosine into the polypeptide exclusively at the position corresponding to that site. Incorporation of the nonnatural amino acid L-3-[125I]iodotyrosine into the model polypeptide was assessed by quantitative and unambiguous determination of suppression efficiency, read-through, and site specificity of incorporation. Minor modifications of the method employed in this initial experiment also allow the rapid analysis of unlabeled acylated tRNA analogues. Under optimum conditions, the unlabeled amino acid L-3-iodotyrosine was found to be incorporated with a suppression efficiency of 65%. Other nonnatural residues, including N-methylphenylalanine, D-phenylalanine, and phenyllactic acid, were tested in the assay under these same conditions. Suppression efficiencies for this series ranged from 0 to 72% depending on the structure of the residue incorporated. Several other aspects of this methodology, such as tRNA structure and context effects, are briefly discussed.

Colocalization of neurotensin receptors and of the neurotensin-degrading enzyme endopeptidase 24-16 in primary cultures of neurons

This paper compares the localization of neurotensin receptors and of endopeptidase 24-16, a peptidase likely involved in the inactivation of neurotensin in primary cultures of neurons. Neurotensin binding sites were radiolabeled with 125I-Tyr3-neurotensin, whereas endopeptidase 24-16 was stained by immunohistochemical techniques using a monospecific polyclonal antibody. Endopeptidase 24-16 is present in 80-85% of the nondifferentiated neurons. The proportion of immunoreactive neurons decreased during maturation to reach 35-40% after 4-8 d of culture. By contrast, neurotensin receptors were not detectable in nondifferentiated cells and appear during maturation. Specific 125I-Tyr3-neurotensin labeling is maximal after 4 d of culture and is located on about 10% of differentiated neurons. Double-labeling experiments show that about 90% of cortical, hypothalamic, and mesencephalic neurons bearing the neurotensin receptor also contained endopeptidase 24-16, supporting the hypothesis that one of the functions of endopeptidase 24-16 is the physiological inactivation of neurotensin. However, the presence of endopeptidase 24-16 on numerous neurons that do not contain neurotensin receptors also suggests that the enzyme could be involved in the degradation and/or maturation of other neuropeptides.

Characterization of lysosomal monoiodotyrosine transport in rat thyroid cells. Evidence for transport by system h

Lysosomal transport of monoiodotyrosine was characterized in countertransport experiments using rat FRTL-5 thyroid cell lysosomes. Monoiodotyrosine carrier activity was temperature-dependent (Ea = 11.65 kcal/mol) and had a pH optimum of 7.5. Carrier activity was minimally inhibited by KCl and NaCl, but unaffected by the presence of other ions or ATP. Monoiodotyrosine transport was unaffected by the presence of carbonyl cyanide m-chlorophenylhydrazone, nigericin, or ammonium chloride, indicating that a proton or K+ gradient is not necessary for monoiodotyrosine transport across the lysosomal membrane. Monoiodotyrosine countertransport showed a 6-fold increase in lysosomes from FRTL-5 cells grown in medium containing thyrotropin by comparison to cells grown without this hormone. Thyrotropin responsiveness raised the possibility that monoiodotyrosine was transported by system h, the only known lysosomal carrier whose activity is enhanced by thyrotropin. Consistent with this, monoiodotyrosine-loaded lysosomes exhibited countertransport of [3H]tyrosine, [3H]phenylalanine, and [3H]leucine, three system h ligands, but not [3H]cystine, a nonsystem h ligand. Unlabeled tyrosine, phenylalanine, and leucine, but not cystine or proline, inhibited [125I]monoiodotyrosine countertransport, and leucine inhibition of [3H]tyrosine countertransport and [125I]monoiodotyrosine countertransport yielded virtually identical KI values, 3.5 and 3.2 microM, respectively. Competition studies with monoiodotyrosine analogues showed that system h recognizes a broad range of ligands with an alpha-amino acid configuration at one end and a hydrophobic region at the other. Ring-substituted halogens, regardless of mass or ring position, but not amino, nitro, hydroxy, or methoxy groups, enhanced carrier recognition of system h analogues. It appears that a single system effects the transport of iodinated (e.g. monoiodotyrosine) and noniodinated (e.g. tyrosine) thyroglobulin catabolites into the cytosol for salvage and reutilization by FRTL-5 thyroid cells.

Rapid receptor-mediated catabolism of 125I-atrial natriuretic factor by vascular endothelial cells

The binding, internalization and degradation of 200 pM monoiodinated human atrial natriuretic factor-(99-126) (125I-hANF) by cultured bovine aortic endothelial cells (BAECs) were studied at 37 degrees C. 125I-hANF was rapidly cleared from the extracellular medium (t1/2 approximately 10 min), whereas preincubation of the cells in the presence of 20 mM-NH4Cl or 0.2 mM-chloroquine resulted in a significant inhibition of this process. The BAECs rapidly produce three major degradation products of 125I-hANF, namely [125I]iodotyrosine 126 (125I-Y), Arg125-[125I]iodotyrosine126 (125I-RY) and Phe124-Arg125-[125I]iodotyrosine126(125I-FRY), which were detected in the extracellular medium. NH4Cl and chloroquine acted to inhibit the generation of 125I-Y and 125I-RY, but not that of 125I-FRY. Furthermore, excess unlabelled hANF (300 nM) completely blocked the rapid production of 125I-Y and 125I-RY in the first 5 min, but only partially (49%) inhibited the generation of 125I-FRY. Thus, in contrast with our previous findings with cultured smooth-muscle cells [Johnson, Arik & Foster (1989) J. Biol. Chem. 264, 11637-11642], BAECs bind, internalize and rapidly degrade 125I-hANF, resulting in the release of 125I-Y and 125I-RY into the extracellular medium. Similarly to smooth-muscle cells, the BAECs generate 125I-FRY from 125I-hANF via an extracellular proteolytic event. The rapidity of the receptor-mediated process and its sensitivity to NH4Cl and chloroquine suggest that the 125I-hANF is proteolytically processed in the endosomes of BAECs and that its receptors cycle between the cell surface and intracellular stores.

Location of dehydroalanine residues in the amino acid sequence of bovine thyroglobulin. Identification of "donor" tyrosine sites for hormonogenesis in thyroglobulin

Thyroid hormonogenesis in thyroglobulin results in the conversion of an "acceptor" iodotyrosine to a hormone residue and a "donor" iodotyrosine to a dehydroalanine residue. Altogether five acceptor sites have been located as hormone residues in thyroglobulin of different animal species. To search for donor sites, we treated bovine thyroglobulin with 4-aminothiophenol to specifically modify dehydroalanine residues to S-(4-aminophenyl)cysteine (APC) residues, according to the principle of dehydroalanine determination developed by us (Kondo, T., Kondo, Y., and Ui, N. (1988) Mol. Cell. Endocr. 57, 101-106). After digesting thyroglobulin with lysyl endopeptidase, APC-containing peptides were separated from other peptides by trapping them on immobilized naphthylethylenediamine and from each other by size-exclusion and reverse-phase high performance liquid chromatography (HPLC). The HPLC patterns showed about 10 APC-containing peptides. Among them, four different peptides were purified by repeated reverse-phase HPLC. The results of partial sequencing of the four peptides by manual Edman degradation disclosed that Tyr5, Tyr926, Tyr1375, and Tyr986 or Tyr1008 are available for hormonogenesis as donor sites. These results strongly suggest that only specific tyrosine residues behave as donors.

Expression of tyrosine hydroxylase in cultured brain cells: stimulation with an extractable pituitary cytotropic factor

Expression of tyrosine hydroxylase (TH) in cultured cells of the ventral hypothalamus-midbrain of fetal rats has been investigated. TH mRNA and TH were quantified by an S1 nuclease protection assay and an immunoblot assay, respectively. Dihydroxyphenylalanine (DOPA) and dopamine secretion were evaluated using their rates of accumulation in the culture medium. The rate of accumulation of DOPA was 2-3 times that of dopamine. Inhibitors of TH activity caused a dose-dependent reduction in DOPA secretion. During an 11-week culture of dissociated cells, TH mRNA increased from 1.6 to 2.8 attomole/well between the first and fourth week of culture, remained steady to the ninth week, and then declined. TH increased from 12 to 105 fmol/well between the first and seventh week and then declined. DOPA secretion increased until the sixth week and then remained steady to the tenth week. An extract of rat pituitaries stimulated DOPA secretion by the cultures in a dose-dependent manner. This activity, attributed to a cytotropic factor (CTF), was inactivated by heating for 10 min in a boiling water bath, but was unaffected by trypsin digestion. Incubation with CTF for 24, 48, 72, and 96 h resulted in a day by day increase in the secretion of DOPA. After 96 h of culture with CTF, the amount per well of TH mRNA, but not TH, was significantly (P less than 0.01) greater than the control value. Pituitary CTF is probably not PRL, since rat PRL did not appreciably affect DOPA secretion or the amount of TH mRNA or TH in the cells. Withdrawal of CTF from CTF-stimulated cells resulted in a marked reduction in DOPA secretion as well as a decrease in TH mRNA. These results support the hypothesis that the pituitary gland contains a cytotropic factor that stimulates TH expression in fetal brain cells of the hypothalamus-midbrain.

Reaction of alpha-tubulin with iodotyrosines catalyzed by tubulin:tyrosine ligase: carboxy-terminal labeling of tubulin with [125I]monoiodotyrosine

We have studied the capacity of different iodinated derivatives of phenylalanine and tyrosine to inhibit the incorporation of [3H]tyrosine into tubulin catalyzed by tubulin:tyrosine ligase. In contrast to thyronine and its iodinated derivatives, iodotyrosines were efficient inhibitors. That they also functioned as substrates of the enzyme was shown by the effective incorporation of [125I]mono- and diiodotyrosine into tubulin. The label was shown to be located at the carboxy terminus. Labeling by this method conserves the polymerization capacity of tubulin in contrast with classical radioiodination methods involving oxidation.

Improved assay method for activity of thyroid peroxidase-catalysed coupling of iodotyrosine residues of thyroglobulin utilizing h.p.l.c. for analysis of iodothyronines

The coupling of iodotyrosine residues of thyroglobulin (Tg) catalysed by thyroid peroxidase (TPO) has scarcely been studied with respect to the TPO of abnormal human thyroid glands. The present paper proposes a rapid and convenient assay method applicable for determining the coupling activity of a sample of less than 500 mg from each patient's thyroid. The main characteristics of the method are as follows: (i) mitochondrial/microsomal fractions of thyroid glands were treated with sodium cholate plus trypsin, and the supernatants obtained by ultracentrifugation were directly used for the assay of coupling and peroxidase activity of TPO; (ii) the formation of iodotyrosine residues catalysed by TPO was performed by using chemically iodinated Graves'-disease Tg containing 41 iodine atoms per molecule and with a high iodotyrosine and a low iodothyronine content; (iii) newly synthesized iodothyronine residues (thyroxine, 3,5,3'-tri-iodothyronine, and 3,3',5'-tri-iodothyronine) were analysed by h.p.l.c. after hydrolysis of Tg with proteinases and extraction of iodothyronines with ethyl acetate.

Carrier-mediated transport of monoiodotyrosine out of thyroid cell lysosomes

Monoiodotyrosine (MIT) crosses the lysosomal membrane of rat FRTL-5 thyroid cells by a carrier-mediated process. In egress studies, MIT lost from inside lysosomes was quantitatively recovered outside lysosomes as MIT, indicating that the compound was transported intact across the lysosomal membrane. In uptake studies, [125I]MIT entry required intact lysosomes and exhibited saturation kinetics. The apparent Km for MIT was approximately 1.5 microM and the Vmax was approximately 0.24 pmol/unit hexosaminidase/min. Countertransport of MIT was demonstrated, with an initial velocity of [125I]MIT uptake which reached a maximum at high intralysosomal MIT loading. Nonradioactive MIT and diiodotyrosine competed to approximately equivalent extents with [125I]MIT for uptake in countertransport experiments. The existence of a lysosomal MIT carrier in thyroid cells may explain how this product of thyroglobulin catabolism is transported to the cytosol for iodine salvage and reutilization.

In vitro studies of the thyroglobulin degradation pathway: endocytosis and delivery of thyroglobulin to lysosomes, release of thyroglobulin cleavage products--iodotyrosines and iodothyronines

Iodinated thyroglobulin stored in the thyroid follicular lumen is subjected to an internalization process and thought to be transferred into the lysosomal compartment for proteolytic cleavage and thyroid hormone release. In the present study, we have designed in vitro models to study: 1) the transfer of endocytosed thyroglobulin into lysosomes, and 2) the intracellular fate of free thyroid hormones and iodinated precursors generated by intralysosomal proteolysis of thyroglobulin. Open follicles prepared from pig thyroid tissue by collagenase treatment were used to probe the delivery of exogenous thyroglobulin to lysosomes via the differentiated apical cell membrane. Open follicles were incubated with pure [125I]thyroglobulin with or without unlabeled thyroglobulin in the presence or in the absence of chloroquine. Subcellular fractionation on a Percoll gradient showed that [125I]thyroglobulin was internalized and present in low (for the major part) and high density thyroid vesicles. In chloroquine-treated open follicles, we observed the appearance of a definite fraction of [125I]thyroglobulin in a lysosome subpopulation having the expected properties of phagolysosomes or secondary lysosomes. In contrast, in control open follicles, the amount of [125I]thyroglobulin or degradation products found in high density vesicles was lower and associated with the bulk of lysosomes, i.e., primary lysosomes. The content in thyroglobulin and degradation products of lysosomes at steady-state was analyzed by Western blot using polyclonal anti-pig thyroglobulin antibodies. Under reducing conditions, immunoreactive thyroglobulin species correspond to polypeptides with molecular weights ranging from 130,000 to less than 20,000. The presence of free thyroid hormones and iodotyrosines inside lysosomes and their intracellular fate was studied in dispersed thyroid cells labeled with [125I]iodide. Neo-iodinated [125I]thyroglobulin gave rise to free [125I]T4 which was secreted into the medium. In addition to released [125I]T4, a fraction of free [125I]T4 was identified inside the cells. Lysosomes isolated from dispersed thyroid cells did not contain significant amounts of free [125I]T4. The free intracellular [125I]T4 fraction seems to represent an intermediate 'hormonal pool' between thyroglobulin-bound T4 and secreted T4. Evidence for such a precursor-product relationship was obtained from pulse-chase experiments.

IN CONCLUSION

1) open thyroid follicles have the ability to internalize thyroglobulin by a mechanism of limited capacity and to address the endocytosed ligand to lysosomes.(ABSTRACT TRUNCATED AT 400 WORDS)

Preferential formation of triiodothyronine residues in newly synthesized [14C]tyrosine-labeled thyroglobulin molecules in follicles reconstructed in a suspension culture of hog thyroid cells

Early processes of thyroid hormone (T4 and T3) synthesis in thyroglobulin molecules were studied using follicles reconstructed in a primary culture of hog thyroid cells under the influence of TSH. When the reconstructed follicles were incubated with 14C-tyrosine, thyroglobulin containing the labeled tyrosine was newly synthesized and in the presence of iodide, some of the labeled tyrosine residues were iodinated and coupled to produce labeled iodothyronines, T4 and T3. Coupling efficiency, especially the efficiency of T3 production, was much higher than that obtained from the average iodoamino acid composition of mature thyroglobulin from the gland, indicating a preferential iodination of hormonogenic tyrosines and synthesis of T3. The total production of T3 was higher than T4 under the present conditions. However, free labeled T4 released into the medium was more than T3 after 16 h incubation of the labeled follicles with non-labeled tyrosine, suggesting the preferential liberation of T4 from the labeled peptide and/or release from the cells.

Monoiodotyrosine formation during thyroglobulin processing in Golgi vesicles

A golgi-enriched subfraction was obtained from porcine thyroid glands by differential centrifugation. When incubated in a suitable medium, these vesicles were able to concentrate iodide from the medium and bind it to protein. The iodination process was inhibited by methylmercapto-imidazole and was increased by the addition of an H2O2 generating system to the medium. Analysis of the protein content of the vesicles revealed the presence of 18 and 12-13 S thyroglobulin molecules, lacking mannose residues, and containing only monoiodotyrosine. It is concluded that in vitro, iodination can begin before exocytosis, in the smooth-surfaced vesicles derived from the golgi apparatus, as soon as N-acetylglucosamine is incorporated onto the pre-thyroglobulin molecule.

Why do total-body decay curves of iodine-labeled proteins begin with a delay?

The initial delay that occurs in total-body radiation curves reaching their single-exponential slopes was analyzed from 106 experiments involving several mammalian species (guinea pig, mouse, rabbit, and rat) and plasma proteins (alpha 1-acid glycoprotein, antithrombin III, fibrinogen, immunoglobulin G, and transferrin) in 14 different combinations. The time interval (Td) between injection and the intercept of the slope with the full-dose value was adopted as a measure of curve nonideality. The overall mean Td was 6.6 h, but individual values showed a significant correlation to protein half-lives, whereby proteins of unequal metabolic properties exhibited different mean Td values. Targeting protein to the liver abolished delay. Choice of the isotope (125I or 131I) and size of the labeled protein had no influence on the magnitude of delay. Whole-body radiation curves of animals that received [125I]iodotyrosines, Na131I, or 131I-polyvinylpyrrolidone exhibited no initial delays. These results do not support the earlier notion that delay is caused by a redistribution of the labeled protein in the body to radiometrically more favorable sites. However, they are compatible with the assumption that delayed passage of a protein dose through the extracellular matrix and/or retarded transfer of proteolytic products from extravascular catabolic sites to plasma may be responsible for the phenomenon.

Receptor-mediated degradation of insulin in isolated rat adipocytes. Formation of a degradation product slightly smaller than insulin

More than 90% of the radioactivity associated with isolated rat adipocytes incubated with [TyrA14-125I]monoiodoinsulin represented at steady state iodoinsulin possessing full binding affinity. In contrast, about half of the radioactivity dissociating from the cells was [125I]monoiodotyrosine. The other half was of a molecular size similar to that of iodoinsulin as judged from gel-filtration chromatography. However, the descending limb of the 'insulin' peak (i.e., the smaller molecules) possessed a reduced binding activity compared with native iodoinsulin, material from the ascending limb, or a similar fraction isolated from dissociation medium from IM-9 lymphocytes, a cell type devoid of receptor-mediated insulin degradation. The cells, thus, release an intermediary degradation product.

[mono[125I]iodo-Tyr10,MetO17]-vasoactive intestinal polypeptide. Preparation, characterization, and use for radioimmunoassay and receptor binding

Vasoactive intestinal polypeptide (VIP) was labeled with sodium [125I]iodide using the chloramine-T method and subsequently purified by reverse-phase high performance liquid chromatography. Three main 125I-labeled peaks designated A, B, and C resulted from the radioiodination and purification procedures. They were characterized by electrophoresis of tryptic fragments; Edman degradation (for Peaks A and C); enzymatic digestion to amino acids by leucine aminopeptidase, carboxypeptidase Y and Pronase; and treatment with cyanogen bromide. Peak A corresponds to VIP monoiodinated on Tyr10 and with the Met17 residue oxidized to methionine sulfoxide. This [mono[125I]iodo-Tyr10,MetO17]VIP displays the following characteristics. 1) It constitutes quantitatively the major product of the iodination procedure (62.5%); 2) it is well resolved from other labeled and unlabeled products; 3) it is stable (2 months at -20 degrees C); 4) it possesses a high specific activity (2050 Ci/mmol); 5) it maintains the biological activity of native VIP; and 6) it binds to antibody and membrane recognition sites in a specific, saturable, and reversible manner. Reduction of [mono[125I]iodo-Tyr10, Met-O17]VIP to [mono[125I]iodo-Tyr10]VIP does not improve the performance of the tracer in a radioimmunoassay. The method described in this article is simple and rapid and yields a molecular form of 125I-labeled VIP that has been fully characterized and is suitable for use in biological studies.

Endocrine control of extrathyroidal peroxidases and iodide metabolism

The role of the thyroid and adrenal glands on iodide transport and peroxidase-catalyzed formation of iodotyrosines in extrathyroidal tissues such as stomach and submaxillary glands has been investigated. Thyroidectomy stimulates iodide concentration and iodotyrosine formation in stomach, sensitive to the administration of thyroxine but having no effect on the peroxidase activity. In contrast, although thyroidectomy stimulates the submaxillary peroxidase which is reversed on treatment with thyroxine, it has no effect on iodide concentration and organification in the submaxillary gland. Gastric peroxidase activity is specifically stimulated by adrenalectomy and is inhibited by glucocorticoids which also inhibit iodotyrosine formation in stomach.

Lysosomal digestion of thyroglobulin: role of cathepsin D and thiol proteases

Purified hog thyroid lysosomes, prepared by a procedure previously developed in this laboratory, were used to study lysosomal digestion of [131I]thyroglobulin [131I]Tg). The lysosomal proteases were solubilized with 0.1% Triton X-100. Rates of proteolytic digestion, measured by the release of ethanol-ammonium acetate-extractable 131I, were greatly stimulated by thiol reagents. The pH optimum was also affected by the presence of thiols. In the absence of a thiol reagent, a broad pH optimum was observed, ranging from 3.5-4.5. However, in the presence of 1 mM mercaptoethanol, the maximum rate of digestion occurred at pH 5.0, very close to reported values for the internal pH of lysosomes. Pepstatin, an inhibitor of cathepsin D, markedly inhibited lysosomal digestion of [131I]Tg at concentrations as low as 0.01 micrograms/ml. Its inhibitory effect was greater at pH 3.5 (pH optimum of cathepsin D) than at pH 5.0. Leupeptin, an inhibitor of thiol proteases, was not as potent as pepstatin, but it was significantly inhibitory at a concentration of 1 microgram/ml. In contrast to pepstatin, leupeptin displayed a greater inhibitory effect at pH 5.0 than at pH 3.5. The pH optimum of hog thiol proteases has been reported to range from 5.5-6.5. The effects of the two inhibitors were additive at pH 5.0. We conclude from these results that both cathepsin D and thiol proteases play a role in lysosomal digestion of Tg. Cathepsin D appears to be quantitatively more important than thiol protease in the initial phase of the digestion. The stimulatory effect of thiols on lysosomal digestion of [131I]Tg probably involves two separate effects: 1) stimulation of thiol proteases, and 2) reduction of S-S bonds in Tg, making the protein more susceptible to attack by proteolytic enzymes. Poorly iodinated [131I]Tg was more rapidly hydrolyzed than well iodinated [131I]Tg, based on the release of ethanol-ammonium acetate-extractable 131I. However, there was little or no difference in the rate of total peptide bond cleavage between poorly iodinated and well iodinated Tg. These results suggest that the first sites of iodination of Tg are preferentially attacked by lysosomal proteases. Long term (24-h) digestion of [131I]Tg with solubilized thyroid lysosomes at pH 5.0 in the presence of thiol compounds was just as effective as digestion with pronase at pH 8.0 in liberating free 131I-labeled iodothyronines and 131I-labeled iodotyrosines. Thus, thyroid lysosomes contain the full complement of proteases and peptidases required for cleaving free iodoamino acids from Tg.

Spectral characteristics and catalytic properties of thyroid peroxidase-H2O2 compounds in the iodination and coupling reactions

Hog thyroid peroxidase (TPO) was highly purified in order to study the spectral properties and catalytic specificities of its H2O2 compounds in iodothyronine biosynthesis. Purified TPO exhibited a Soret spectrum with an absorption maximum at 410 nm and had an A410/A280 value of 0.55. Protein iodination was only catalyzed under conditions which allowed formation of the transient TPO compound I (Fe(IV)-pi o+). On addition of an equimolar amount of H2O2, TPO formed a stable compound with an absorption maximum at 417 nm. This compound efficiently catalyzed the coupling reaction, but was unable to iodinate proteins. It catalyzed the formation of 1 mol iodothyronines/mol TPO, and therefore retained two oxidizing equivalents per molecule. It is proposed that this compound constitutes a second form of compound I whose structure might be Fe(IV)-Ro, analogous to that of cytochrome c peroxidase compound I. In the presence of an excess of H2O2, it formed TPO-compound III with an absorption maximum at 420 nm. TPO-compound III catalyzed neither the iodination nor the coupling reaction.