Deiodination of the iodinated amino acids

A Role for P-Glycoprotein in Clearance of Alzheimer Amyloid β -Peptide from the Brain

Most data indicates that Alzheimer's disease involves an accumulation of amyloid β - peptide (Aβ) in the CNS and that sporadic cases arise from a deficiency in Aβ clearance. Considerable attention has been given to mechanisms by which Aβ might be transported between the brain and blood, and evidence suggests that p-glycoprotein, also known as the multi-drug resistance (MDR) protein (product of the ABCB1 gene), plays a role in Aβ transport across the blood-brain barrier (BBB). We tested this possibility through two approaches: First, wild-type and MDR1A-knockout mice were compared after intravenous injection of [(125)I]-labeled Aβ; after 60 min, homogenates of brain parenchyma were subjected to γ-counting of TCA-precipitable material, and histological sections of brain were subjected to autoradiography. Second, MDR1Aknockout mice were crossed with Tg2576 APP transgenic mice, a line that routinely accumulates Aβ in the brain; SDS and formic acid extracts of brain homogenates were assessed for Aβ levels by ELISA. Each of these approaches yielded data indicating that Aβ accumulates to a greater degree in mice lacking MDR1A. These findings confirm other reports linking p-glycoprotein to Aβ clearance across the BBB and have important implications for Alzheimer's disease genetics, pharmacology, and epidemiology.

Triazole Appending Agent (TAAG): A New Synthon for Preparing Iodine-Based Molecular Imaging and Radiotherapy Agents

A new prosthetic group referred to as the triazole appending agent (TAAG) was developed as a means to prepare targeted radioiodine-based molecular imaging and therapy agents. Tributyltin-TAAG and the fluorous analogue were synthesized in high yield using simple click chemistry and the products labeled in greater than 95% RCY with (123)I. A TAAG derivative of an inhibitor of prostate-specific membrane antigen was prepared and radiolabeled with (123)I in 85% yield where biodistribution studies in LNCap prostate cancer tumor models showed rapid clearance of the agent from nontarget tissues and tumor accumulation of 20% injected dose g(-1) at 1 h. The results presented demonstrate that the TAAG group promotes minimal nonspecific binding and that labeled conjugates can achieve high tumor uptake and exquisite target-to-nontarget ratios.

Life without thyroxine to 3,5,3'-triiodothyronine conversion: studies in mice devoid of the 5'-deiodinases

Considerable indirect evidence suggests that the type 2 deiodinase (D2) generates T(3) from T(4) for local use in specific tissues including pituitary, brown fat, and brain, whereas the type I deiodinase (D1) generates T(3) from T(4) in the thyroid and peripheral tissues primarily for export to plasma. From studies in deiodinase-deficient mice, the importance of the D2 for local T(3) generation has been confirmed. However, the phenotypes of these D1 knockout (KO) and D2KO mice are surprisingly mild and their serum T(3) level, general health, and reproductive capacity are unimpaired. To explore further the importance of 5'deiodination in thyroid hormone economy, we used a mouse devoid of both D1 and D2 activity. In general, the phenotype of the D1/D2KO mouse is the sum of the phenotypes of the D1KO and D2KO mice. It appears healthy and breeds well, and most surprisingly its serum T(3) level is normal. However, impairments in brain gene expression and possibly neurological function are somewhat greater than those seen in the D2KO mouse, and the serum rT(3) level is elevated 6-fold in the D1/D2KO mouse but only 2-fold in the D1KO mouse and not at all in the D2KO mouse. The data suggest that whereas D1 and D2 are not essential for the maintenance of the serum T(3) level, they do serve important roles in thyroid hormone homeostasis, the D2 being critical for local T(3) production and the D1 playing an important role in iodide conservation by serving as a scavenger enzyme in peripheral tissues and the thyroid.

Notes on the history of cellular uptake and deiodination of thyroid hormone

In this mini review on the history of the research devoted to thyroid hormone metabolism two pathways are discussed, i.e. uptake and subsequent deiodination in cells and tissues. In the 1950's the investigations of these processes were greatly stimulated when 131I became available for research purposes. The true nature of both mechanisms surfaced in the 1970's when it became apparent that transport of thyroid hormones into cells was a regulated carrier mediated process, while deiodination appeared to be catalyzed by different types of enzymes. Kinetic data indicated that these processes were of great importance in the ultimate regulation of thyroid hormone bio-availability.

The deiodinase family of selenoproteins

The realization some forty years ago that several iodothyronine compounds are present in the circulation suggested that deiodination occurs in various tissues. Subsequently, deiodination was indeed documented in in vivo studies. Later, using in vitro assay techniques, three deiodinase processes, termed types 1, 2 and 3, were defined that differed in terms of tissue distribution, reaction kinetics, efficiency of substrate utilization and sensitivity to inhibitors. Although purification of the deiodinase enzymes has continued to be problematic, recent molecular cloning studies have identified cDNAs for these three deiodinase isoforms from multiple species. These cDNAs have provided important insights into the structural characteristics of this family of enzymes. Foremost among the structural features has been the demonstration that all three deiodinase isoforms contain at their active site the uncommon amino acid selenocysteine which is of critical importance to their catalytic activity. The availability of cDNAs for these enzymes provides important reagents for pursuing additional studies aimed at defining their biochemical features and roles in thyroid hormone economy.

Kinetics of degradation of levothyroxine in aqueous solution and in solid state

The kinetics of the deiodination of levothyroxine in aqueous solution were studied over the pH range 1 to 12. Temperature dependence of the reaction was also studied. The log k - pH profile indicated that the kinetics of deiodination include proton attack on the anion and dianion in acidic solution and water attack on the anion and dianion in alkaline solution. A possible mechanism of the deiodination was discussed. The solid-state stability of levothyroxine sodium was studied at elevated temperatures; and the compound was found to undergo deamination on heating. The decomposition follows biphasic first-order kinetics, with the most rapid decomposition occurring at the beginning of heating.

Glomerular sieving of high molecular weight proteins in proteinuric rats

To characterize the permeability of the glomerular capillary wall to high molecular weight proteins in normal and proteinuric rats, we determined the glomerular sieving coefficients (GSC) of radioiodinated marker proteins of known size and charge by means of a paired label, tissue accumulation method previously validated in this laboratory. In one group of rats (Series A) the GSCs of 125I-anionic IgG (aIgG-molecular weight [mol wt] 150,000, pI 4.9) and 131I-neutral IgG (nIgG-pI 7.4 to 7.6) were measured simultaneously. In Series B, the GSC of a second anionic marker, 131I-human ceruloplasmin (Crp-mol wt 137,000, pI 4.9) was compared to that of 125I-nIgG. As in the previous report, the labeled proteins were not degraded or deiodinated during the 20 minute clearance period for GSC determination. Within Series A and B, three subgroups of rats were studied: control saline-infused rats, rats made acutely proteinuric by infusion of the polycation hexadimethrine (HDM), and rats with chronic doxorubicin (Adriamycin-Adria) nephrosis. In the control rats, GSCs for the anionic markers aIgG (Series A) or Crp (Series B) were significantly greater than that of nIgG (both series). These large proteins crossed the filtration barrier by a different pathway from that available to smaller neutral molecules the size of albumin, which in our previous study had a much higher GSC than a native, anionic albumin marker. In a third group of control rats only (Series C), the GSCs of native anionic bovine albumin (BSA) and nIgG were compared directly. The GSC of BSA (0.0029) was only slightly larger than the GSC of nIgG (0.0025), indicating that most of the native albumin crosses the glomerular capillary wall via a nonselective pathway similar to that available to nIgG. The results in the control groups are compatible with recently-described heteroporous models of glomerular size selectivity. In the HDM proteinuric rats of both series, the filtration of all three markers was significantly increased, although the GSCs of the anionic markers continued to exceed that of nIgG. The finding of increased nIgG filtration in HDM rats confirms our previous observation that HDM, although it binds to and neutralizes anionic sites in glomerular basement membrane, induces a structural change which leads to altered pore size distribution. Adria rats also had increased GSCs of all markers, although the increase was not as great as in HDM rats, and the GSCs of the anionic and neutral markers became equal.(ABSTRACT TRUNCATED AT 250 WORDS)

Glomerular sieving of anionic and neutral bovine albumins in proteinuric rats

To characterize the defect in glomerular permeability leading to albuminuria in rats made nephrotic acutely by infusion of hexadimethrine (HDM) or chronically by administration of Adriamycin (doxorubicin) (Adria), we developed and validated a tissue accumulation method for simultaneous determination of the glomerular sieving coefficients (GSC) of anionic 131I-labeled bovine albumin (BSA-pI 4.9) and 125I-labeled charge-modified neutral BSA (nBSA-pI 7.5 to 8.0). Total filtered marker was calculated by adding marker excreted in the urine to that filtered but reabsorbed by the tubules. The latter was determined by subtracting interstitial marker present in the left kidney, rendered non-filtering by ureteral ligation during mannitol diuresis, from the total marker accumulating within the right, filtering kidney. Experiments showed that markers circulated and were excreted intact and were neither degraded nor deiodinated during the period of the clearance studies. In control animals the GSC of nBSA (0.026 +/- 0.004) greatly exceeded that of BSA (0.0006 +/- 0.0002), demonstrating the normal charge dependence of permeability. Both proteinuric groups had marked increases in the GSC of BSA (HDM: 0.021 +/- 0.005; Adria: 0.025 +/- 0.004), which correlated with appearance of rat albumin in their urine. HDM rats also had a twofold increase in the GSC of nBSA (0.049 +/- 0.005), indicating alteration of the size dependence of permeability. The absolute increase of GSC of BSA and nBSA was similar, suggesting that albuminuria resulted from appearance of new "pores" in the glomerular filter that were not charge selective for proteins of the size of albumin. Thus, infusion of HDM, which binds to and neutralizes GBM anions, appears to produce albuminuria by inducing a structural change in the glomerular filter. Conversely, Adria rats had no significant increase in the GSC of nBSA (0.031 +/- 0.005), indicating no significant change in the size dependence of permeability for proteins of the size of albumin. In these animals, the GSC of the anionic BSA approached that of the neutral nBSA, indicating that Adriamycin induces albuminuria by markedly reducing the normal charge dependence of permeability.

Evidence for the rapid direct control both in vivo and in vitro of the efficiency of oxidative phosphorylation by 3,5,3'-tri-iodo-L-thyronine in rats

1. Examination of the distribution of L-tri-iodothyronine among rat liver tissue fractions after its intravenous injection into thyroidectomized rats focused attention on mitochondria at very short times after administration. By 15 min this fraction contained 18.5% of the tissue pool; however, the content had decreased sharply by 60 min and even further over the next 3 h. By contrast, the content in all other fractions was constant or increased over 4 h. About 60% of tissue hormone was bound to soluble protein. 2. Mitochondria isolated from thyroidectomized rats showed P/O ratios that were about 50% of those found in normal controls, with both succinate and pyruvate plus malate as substrates. There was no evidence of uncoupling; the respiratory-control ratio was about 6. 3. Mitochondria isolated 15 min after injection of tri-iodothyronine into thyroidectomized rats showed P/O ratios and respiratory-control ratios that were indistinguishable from those obtained in mitochondria from euthyroid animals. The oxidation rate was, however, not restored. 4. Incubation of homogenates of livers taken from thyroidectomized animals injected with L-tri-iodothyronine before isolation of the mitochondria restored the P/O ratio to normal; by contrast, direct addition of hormone to isolated mitochondria had no effect. The role of extramitochondrial factors in rapid tri-iodothyronine action is discussed. 5. Possible mechanisms by which tri-iodothyronine might rapidly alter phosphorylation efficiency are considered: it is concluded that control of adenine nucleotide translocase is unlikely to be involved. 6. The amounts of adenine nucleotides in liver were measured both after thyroidectomy and 15 min after intravenous tri-iodo-thyronine administration to thyroidectomized animals. The concentrations found are consistent with a decreased phosphorylation efficiency in thyroidectomized animals. Tri-iodothyronine injection resulted in very significant changes in the amounts of ATP, ADP and AMP, and in the [ATP]/[ADP] ratio, consonant with those expected from an increased efficiency of ADP phosphorylation. This suggests that the changes seen in isolated mitochondria may indeed reflect a rapid response of liver in vivo to tri-iodo-thyronine.

Phenolic and nonphenolic ring deiodinations of iodothyronines in cultured hepatocarcinoma cell homogenate from monkey

Iodothyronine monodeiodinase activities in homogenates of cultured monkey hepatocarcinoma cells were measured by the deiodination of [3.5-(125)I]-diiodo-L-thyronine or 3-[3',5'-(125)I]triiodo-L-thyronine (phenolic ring-labeled 'reverse' triiodothyronine). The assay system utilized a small ion-exchange column (AG50W-X4, O.9 X approximately 1 cm) to measure 125I-. Both deiodinases were destroyed by boiling for 1 min. Maximal nonphenolic ring deiodination was observed at pH 7.9 whereas maximal phenolic ring deiodination was at pH 6.3. Both reactions were enhanced strongly by dithiothreitol (0.1-5mM), and slightly by 5 mM beta-mercaptoethanol. Phenolic ring deiodination was strongly inhibited by 0.1 mM propylthiouracil. Nonphenolic ring deiodination was accelerated by EDTA (1.2 MM) and inhibited by Mg(2+) (5mM). Methylmercaptoimidazol and Mg(2+), Ca(2+) and Mn(2+) (0.1-1.0 mM) had little or no effect on either reaction, but Zn(2+) (0.1 mM) strongly inhibited both. Both reactions were inhibited by excess iodothyronine analogues at 10 mM to 10 micron M, and thyroxine was shown to be a competitive inhibitor in both cases. On the basis of relative affinities and inhibitory effects, it appears that the order of affinity for the phenolic ring deiodinase is 3,3',5'-triiodo-L-thyronine(rT3) greater than L-thyroxine(T4) greater than 3,5,3'-triiodo-L-thyronine(T3), whereas for the nonphenolic ring deiodinase the order is T3 greater than T4 greater than rT3. Diiodotyrosine did not affect their deiodination.

Effects of 3-nitro-L-tyrosine on thyroid function in the rat: an experimental model for the dehalogenase defect

The effects on thyroid function of an inhibitor of tyrosine dehalogenase, 3-nitro-L-tyrosine (MNT) have been investigated in rats. In preliminary studies, marked inhibition of iodotyrosine deiodination was demonstrated in rats drinking 8 mM MNT. A series of experiments was then performed in which rats received Remington low iodine diet and 8 mM MNT as drinking fluid. This regimen had the following effects, compared to the effects of a low iodine diet alone: (a) a decrease in serum protein-bound iodine, elevation of serum thyrotropin level, goiter, and growth inhibition all prevented or reversed by iodine supplements: (b) on initiation of MNT, a 2- to 3-fold increase in the rate of release of radioiodine from the thyroid and concomitant urinary excretion of large amounts of organic iodine: and (c) after 2 wk of MNT, a greatly increased rate of thyroidal uptake and release of (131)I, an increase in the ratio of monoiodotyrosine-(131)I to diiodotyrosine-(131)I in thyroid proteolysates and the appearance of labeled iodotyrosines in serum. Acute administration of MNT intraperitoneally to rats on either an iodine-deficient or iodine-sufficient diet did not inhibit thyroidal uptake of (131)I or alter the distribution of (131)I among thyroidal iodoamino acids. It is concluded that MNT is an effective inhibitor of iodotyrosine deiodination in vivo, without other important actions on thyroid function. Thus, MNT treatment affords a model for the human dehalogenase defect. By provoking iodotyrosine secretion and consequent urinary loss of iodine, MNT can exaggerate the effects of a low iodine intake, producing goitrous hypothyroidism despite a rapid rate of iodine turnover in the thyroid.

The extrathyroidal conversion rate of thyroxine to triiodothyronine in normal man

Eight normal subjects were administered tracer amounts of a (14)C-labeled thyroxine, L-[tyrosyl-(14)C] T(4), by multiple injections. Then serial blood samples were collected for isolation of the thyroxine, triiodothyronine, and tetraiodothyroacetic acid fractions by a combination of column and paper chromatographies. The chromatographic artifacts were corrected by adding to the sera a purified (3)H-labeled thyroxine, D,L-[alpha,beta-(3)H] T(4) immediately after the separation of sera from blood. 1-2% of the serum (14)C radioactivity was observed in the triiodothyronine fraction and 2-4% of the serum (14)C radioactivity was observed in the tetraiodothyroacetic acid fraction. Complete kinetic studies of thyroxine and triiodothyronine were compared in the same individual in four of the subjects. The extrathyroidal conversion rates of thyroxine to triiodothyronine were calculated from data obtained during both the injection and the postinjection periods as functions of the (14)C-labeled thyroxine and triiodothyronine remaining in the body at time t and their fractional turnover rates. The average daily rate of the extrathyroidal conversion of thyroxine to triiodothyronine was 4% of the extrathyroidal thyroxine pool or 33% of the total thyroxine production. The amount of triiodothyronine generated by this pathway (22 mug/day) was found to contribute 31% of the extrathyroidal triiodothyronine pool or 41% of the daily triiodothyronine production. This pathway is a major source of triiodothyronine production. The extrathyroidal conversions of thyroxine to triiodothyronine and tetraiodothyroacetic acid are major metabolic pathways of thyroxine in normal man.

A new method for the measurement of acute alterations in thyroxine deiodination rate in man

A newly devised dual labeled iodine isotopic method is described for the detection and quantitation of alterations in thyroxine (T(4)) deiodination rate in man. This method employs the principle of a constant (125)I infusion to serve as a reference source for the generation of (131)I derived from the deiodination of T(4)-(131)I. Measurement of these two iodide isotopes are made in serially timed urine collections and are expressed in terms of a ratio value. Using this technique, it was possible to measure accurately the effects of a single dose of 6-propylthiouracil (6-PTU) in producing inhibition of T(4) deiodination in euthyroid subjects. It was also possible to assess the time of onset, duration of action, and degree of inhibition produced by 6-PTU. Employing single doses of 6-PTU, ranging from 100 to 1000 mg, there was found to be a log dose relationship with a degree of inhibition observed in T(4) deiodination. In control studies T(4) deiodination rate was found to be constant for periods ranging up to 72 hr in normal ambulating subjects. The acute administration of many other agents was employed in an attempt to alter the T(4) deiodination rate. These included diphenylhydantoin, methimazole, triiodothyronine, thyroxine, thyroid stimulating hormone (TSH), adrenocorticotropin (ACTH), hydrocortisone, predinsolone, potassium iodide, epinephrine, and oxytocin. No detectable change in T(4) deiodination rate was observed with these agents in the dosage ranges employed in this study. The lack of any observable alteration in the T(4) deiodination rate in response to this array of drugs and hormones appears to indicate that the availability of T(4) to intracellular sites of deiodination and possibly action is well modulated to resist abrupt changes.

THE EFFECT OF PROPYLTHIOURACIL ON THE OXYGEN CONSUMPTION AND IODINE METABOLISM OF THE FROG, RANA PIPIENS, IN WINTER

A study was made of the physiological importance of the thyroid gland in the adult frog, Rana pipiens, in winter. Male frogs were chemically thyroidectomized by oral administration of propylthiouracil three times a week from December 21 to February 23. Oxygen consumption was measured weekly with volumetric respirometers at a temperature of 16 ± 0.1 °C.Treatment with propylthiouracil for 9 weeks did not significantly alter the metabolic rate, although thyroid radioiodine uptake and blood conversion ratio were immediately blocked. Secretion of thyroid hormone was shown to be reduced from August to December, followed by reactivation in late February. This reduction of activity in December, together with the failure of propylthiouracil to alter the metabolic rate or eliminate thyroid iodine stores after 9 weeks of treatment, suggests that the frog thyroid is of no calorigenic importance during early winter.