Inhibition Of Iodination Research Articles

On the use of human data in assessing effects on human health: the case of perchlorate

Public health standards often rely on animal data, despite the availability of more relevant human data. The recent case of health recommendations regarding safe levels of perchlorate in drinking water is used as a case study illustrating how existing human data can offer a reliable basis for inferences about effects on human health. The direct effect of perchlorate exposure is the inhibition of iodine uptake by the thyroid gland. Subsequent effects on thyroid hormone production potentially may lead to adverse neurological and cognitive development or thyroidal effects. Existing human data are sufficient to determine a level of perchlorate exposure that will protect against these effects. The published literature on humans and perchlorate exposure includes medical, occupational, epidemiological, ecological, and laboratory studies. A systematic review of that literature provides a solid, robust, and consistent assessment of human risk from perchlorate exposure. This yields, in regulatory or risk assessment terminology, exposure levels for a lowest observed adverse effect level (LOAEL), a lowest observed effect level (LOEL), and a no observed effect level (NOEL). In vitro studies of the cellular mechanism support the human findings. The therapeutic level for perchlorate is in the range of 100–1000 mg/d. At occupational exposures of up to 35–50 mg/d, no effect on thyroid hormones is seen. Iodine inhibition has been demonstrated at levels down to 1.4 mg/d. No effect has been observed in persons exposed at 0.5 mg/d or below. The data indicate a threshold of about 0.4 mg/d or 200 μg/L if in drinking water. Ecological and epidemiological studies of long-term perchlorate exposure at levels up to 100 μg/L find no effects. Rat data should not trump the totality of human data to assess human risks. More than 50 years of clinical, occupational, and epidemiological experience with the effects of perchlorate exposure to humans is available and sufficient to identify safe exposure levels. These human data provide the most relevant basis for establishing public health standards and obviate the need to extrapolate from animal models that may not accurately represent humans.

#57 Perchlorate dosage on thyroid – laboratory confirmation of occupational epidemiology findings

Abstract PURPOSE: Perchlorate is known to affect the thyroid by blocking the uptake of iodine at the sodium-iodide symporter (NIS) receptor site of the basolateral membrane of the thyroidal cell in a dose-response manner. A previous occupational study of employees at a plant where perchlorate was manufactured examined for the level at which iodine uptake inhibition in chronically-exposed workers was sufficient to cause a reduction in the free thyroxine index (FTI). The data analysis predicted a 10% inhibition at an exposure of about 45 mg/day and demonstrated no effect on FTI at occupational exposures of about 35, 11, 4, and 1 mg/day. METHODS: The occupational exposures were a mixture of inhalation, ingestion, and possibly dermal routes. The dosages had been modeled based on urine perchlorate levels across one work shift with airborne exposures measured. The laboratory studies were conducted using measured oral dosages in drinking water for a two-week period with determination of radio-active iodine uptake before, during, and following exposure period. Thyroid function tests and urine and serum concentrations of perchlorate and iodine were also determined. RESULTS: Laboratory studies demonstrated a significant 38% inhibition of iodine uptake at 10 mg/day and a non-significant 10% inhibition at 3 mg/day (Lawrence 2000, 2000b). Another laboratory conducted studies at doses of about 35, 7, 1.4, and 0.5 mg/day and found iodine inhibition rates of 67%, 45%, 16%, and 2% (not significant) (Greer et al., 2002). Neither free thyroxine index nor free thyroxine levels were affected at any laboratory exposure level. Log-linear extrapolation of the data indicated that an exposure level slightly below 0.5 mg/day perchlorate would be below the threshold level for iodine uptake inhibition in these populations. CONCLUSION: An occupational health study found no adverse effect on thyroid hormone (FTI) at exposures of about 1, 4, 11, and 35 mg/day perchlorate, a chemical known to inhibit iodine uptake. Measurement of iodine uptake inhibition at similar levels showed that the uptake inhibition threshold was just below 0.5 mg/day and that adverse effects on thyroid hormone (FTI) were not seen with exposures equivalent to 67% inhibition. Thus, a range of iodine uptake inhibition at which thyroxine level appears not to be adversely affected is indicated.

Activation of bovine neutrophils by recombinant bovine tumor necrosis factor-α

The in vitro effect of bovine recombinant tumor necrosis factor-α (rbTNF-α) on bovine neutrophil function and the possibility that rbTNF-α and recombinant bovine interferon-γ (rbIFN-γ) act synergistically were investigated. Treatment of neutrophils with rbTNF-α (0.05 μg/ml; approximately 50 U/ml) at 37°C for 2.5 h resulted in enhancement of antibody independent neutrophil-mediated cytotoxicity (AINC) and inhibition of random migration and chemotaxis. The same treatment resulted in a slight decrease in iodination and cytochrome C reduction, but did not affect Staphylococcus aureus ingestion, or antibody dependent cell-mediated cytotoxicity. Kinetic and inhibitor studies indicated that the action of rbTNF-α was rapid and was independent of protein and RNA synthesis by neutrophils. Evaluation of the synergistic activities of rbTNF-α and rbIFN-γ indicated that treatment of neutrophils with these two cytokines simultaneously resulted in additive enhancement of AINC and inhibition of random migration and chemotaxis. There was no additive effect of the two cytokines on inhibition of iodination or cytochrome C reduction.

Isolation of components of Brucella abortus responsible for inhibition of function in bovine neutrophils.

The effects of fractions of Brucella abortus strain 2308 on functions of bovine polymorphonuclear neutrophils (PMNs) were examined in vitro. Ingestion of Staphylococcus aureus and reduction of nitroblue tetrazolium dye by bovine PMNs were not inhibited by heat-killed B. abortus. The ability of PMNs to iodinate proteins was significantly inhibited by live or heat-killed B. abortus and supernatant from heat-killed cells but not by washed heat-killed cells. Two inhibitory components isolated from the supernatant by high-performance liquid chromatography were characterized as nucleotide-like substances with molecular weights of less than 1,000. Inhibition of iodination by these components was concentration dependent. These results indicate that one of the mechanisms by which B. abortus may escape intracellular killing by PMNs is through the production of low-molecular-weight components that inhibit the myeloperoxidase-hydrogen peroxide-halide antibacterial system of bovine PMNs.

Reversible and irreversible inhibition of thyroid peroxidase-catalyzed iodination by thioureylene drugs.

The mechanism of reversible and irreversible inhibition of thyroid peroxidase (TPO)-catalyzed iodination by thioureylene drugs was investigated using a model incubation system. The major observations may be summarized as follows. 1) TPO is inactivated by 1-methyl-2-mercaptoimidazole and propylthiouracil even in the presence of a relatively high concentration of iodide. The extent of this inactivation depends on the ratio of iodide to drug. 2) Spectral changes observed on oxidation of the drugs with the peroxidase-iodide system were very similar to those observed when the drugs were oxidized nonenzymatically with I3-. These findings support the view that oxidized iodine is an intermediate in TPO-catalyzed oxidation of the drugs. 3) Under conditions where TPO is largely inactivated, inhibition of iodination is complete and irreversible. Drug metabolism, on the other hand, occurs to a limited extent. 4) Under conditions where TPO is only partially inactivated, inhibition of iodination is transient (reversible). In this case, drug metabolism is extensive, and higher oxidation products (sulfate and sulfinic acid) are observed. Inhibition of iodination occurs only during the interval required to reduce the drug concentration to a low level. Thereafter, iodination may occur at a rate close to that observed in the absence of drug. Based on these and other observations, a scheme is presented to explain the mechanism of reversible and irreversible inhibition of iodination. In essence, the type of inhibition depends on the relative rates and extent of TPO inactivation and drug oxidation. These rates, in turn, depend primarily on the iodide to drug concentration ratio. A high ratio favors extensive drug oxidation and reversible inhibition. A low ratio favors TPO inactivation and irreversible inhibition.

Levamisole stimulation of neutrophil chemotaxis and chemokinesis by protection of both the leucoattractant and the cellular chemotactic response from inactivation by the peroxidase/hydrogen peroxide/halide system in vitro.

The effects of levamisole, at concentrations known to stimulate neutrophil motility, on neutrophil post-phagocytic metabolic activity were investigated. Levamisole at these concentrations caused inhibition of hexose monophosphate shunt (HMS) activity, superoxide production, hydrogen peroxide generation and myeloperoxidase (MPO), mediated iodination of ingested Candida albicans,. The inhibition of MPO-mediated iodination was not solely due to lack of H2O availability as a result of decreased HMS activity but also to a primary inhibition of iodination as shown in a cell-free system with horse-radish peroxidase (HRP) and added H2O2 and sodium iodide. Further experiments were designed to investigate possible relationships between stimulation and motility and levamisole-induced inhibition of superoxide generation and peroxidase-mediated iodination. These showed that the peroxidase/halide/H2O2 system caused inactivation of both the leucoattractant and neutrophil chemotactic responsiveness. However, concentrations of levamisole, which stimulate motility and inhibit superoxide production and peroxidase-mediated iodination, protected both the leucoattractant response and the ability of the cell to respond to the leucoattractant from inactivation by the HRP/H2O2/iodide system.

The mechanism of action of the thioureylene antithyroid drugs.

A model incubation system containing purified thyroid peroxidase (TPO) was used to study the mechanism of action of the thioureylene anti-thyroid drugs--propylthiouracil (PTU), methylmercapto imidazole (MMI) and carbimazole. Two general types of experiments were performed: a) measurement of the inhibitory effects of the drugs on TPO-catalyzed iodination and on TPO-catalyzed oxidation of guaiacol, and b) studies of the metabolism of PTU and MMI by the TPO model system. The major observations can be summarized as follows: 1) The thioureylene drugs are potent inhibitors of TPO-catalyzed iodination of protein and tyrosine. Their potency increases greatly as the concentration of I- decreases. 2) The thioureylene drugs are also potent inhibitors of TPO-catalyzed oxidation of guaiacol, a reaction that does not involve iodide. 3) MMI and PTU are readily oxidized in the model incubation system when iodide is present but not in the absence of iodide. The rate of oxidation increased as the iodide concentration was increased from 10 to 100 muM. 4) Oxidation of PTU and MMI by the model incubation system is inhibited by relatively slight increases in the concentration of PTU and MMI. These drugs are capable of inhibiting their own and each other's metabolism. 5) Inhibition of iodination is competitively antagonized by iodide at low drug concentrations, but not at higher drug concentrations. 6) Inhibition of iodination by MMI and PTU may be either reversible (low ratio of drug to iodide), or irreversible (higher ratio of drug to iodide). In reversible inhibition the iodination is inhibited for a period which may be as brief as 2 min or as long as 20 min, but thereafter, iodination begins, and there is escape from inhibition. During the lag-period there is extensive metabolism of the drug. In the case of irreversible inhibition of iodination is inhibited completely or almost completely for 60 min, and drug oxidation during this period is relatively low. 7) Irreversible inhibition may be transformed into reversible inhibition by increasing the concentration of TPO or the concentration of iodide. However, increasing the concentration of H2O2 or of tyrosine does not overcome irreversible inhibition. On the basis of these findings and of current views concerning the mechanism of enzymatic iodination, a scheme is proposed for the mechanism of inhibition by thioureylene drugs of TPO-catalyzed iodination of protein and tyrosine.

Biosynthesis of the 27 S thyroid iodoprotein

Biosynthesis of the 27 S thyroid iodoprotein was studied in rats in vivo, and evidence was obtained that 19 S, the major thyroid iodoprotein, is the immediate precursor of 27 S. In untreated rats, labeled 19 S is found within minutes after the administration of [ 3H]leucine, whereas the appearance of 27 S is preceded by a lag-phase of at least 1 hr. Inhibition of protein synthesis by cycloheximide administration resulted in a several fold increase in the proportion of [ 3H]leucine-labeled 27 S relative to 19 S. In contrast, inhibition of iodination with antithyroid agents prevented the formation of 27 S, but not of 19 S synthesis. Since formation of 27 S is dependent on iodination, but not on de novo protein synthesis, 27 S must be derived from the preexisting pool of 19 S in the follicular colloid. It is suggested that 27 S is formed by chemical alterations produced as a by-product of iodination of the tyrosyl residues, probably involving oxidative side-reactions. It appears that the quaternary structure of the 27 S iodoprotein is not encoded into its structural gene, but is a result of post-transcriptional events.

Function of peroxidase and NADPH cytochrome C reductase during the Wolff-Chaikoff effect.

The effects of varying doses of stable iodide on thyroid peroxidase function and NADPH-cytochrome c reductase have been studied in rats. Wolff-Chaikoff effect was induced in vivo by injection of 250 or 500 μg of stable iodide, and several enzymatic activities of particulate fractions collected from the same thyroids were measured in vitro. There was no difference between the enzymatic activity (peroxidase and NADPH-cytochrome c reductase) of blocked and unblocked thyroid. Blocked thyroids showed increased 131I-PBI formation with NADPH in the same manner as unblocked thyroids. Iodide concentrations 10 times higher than those calculated to be present in the blocked thyroid (3 × 10-5M- 1.2 × 10-3M) were used in the in vitro reconstruction of the Wolff-Chaikoff effect with particulate fraction. Inhibition of iodination was not observed in vitro, even when low levels of tyrosine (10-5M) were used in the model system. Kinetic studies of the iodination reaction suggest that it follows a random mechanism, and sho...

Iodine Inhibition in the Flash Photolysis of Methyl Iodide

Iodine Inhibition in the Flash Photolysis of Methyl Iodide