Kinetics of thyroglobulin iodination and thyroid hormone synthesis catalyzed by peroxidases: the role of H2O2

Is Glucose-6-Phosphate Dehydrogenase Deficiency a Risk Factor for Autoimmune Thyroid Disease? A Retrospective Case-Control Study

BACKGROUND

The risk of developing thyroid disorders (TDs) in subjects with inherited glucose-6-phosphate dehydrogenase (G6PD) deficiency is unknown. The aim of this study was to explore the association between autoimmune (AITD) and G6PD deficiency in Northern Sardinia, in a population with a high frequency of these two conditions.

METHODS

In this retrospective single-center case-control study, demographic and clinical data were collected from patients examined in a tertiary referral Gastroenterology Section of a teaching hospital.

RESULTS

In 8894 subjects examined (64.7% females), 1218 patients were diagnosed with TDs; more specifically, 767 were diagnosed with AITD and 451 were not (non-AITD). Overall, G6PD deficiency was more prevalent in TD patients compared with patients without TD (controls) (16.7% vs. 11.2%; p < 0.0001). Multivariable logistic regression analysis (after adjusting for age, sex, excess weight and smoking habits), confirmed a higher risk of AITD among G6PD deficient patients with an odds ratio (OR) of 1.36 and 95% confidence interval (CI) of 1.11-1.6, female patients (OR 1.33, 95% CI 1.07-1.65) and overweight patients (OR 1.22, 95% CI 1.03-1.44).

CONCLUSIONS

The risk of AITD is increased in carriers of G6PD deficiency. A careful assessment of thyroid function is advisable in patients with inherited G6PD defects.

Catalyzed Oxidation of Carotenoids by Lactoperoxidase in the Presence of Ethanol

The discovery of the lactoperoxidase system as a biocatalyst in milk was a landmark finding. The activation of this system using hydrogen peroxide (H₂O₂) raised hopes for oxidation of various organic substrates. The involvement of lactoperoxidase system in the catalyzed-oxidation of carotenoids in the whey proteins, and the effect of various solvents on carotenoids' oxidation reaction rate has been studied. However, there is no evidence for this reaction without the addition of oxidizing agents, such as peroxides. Here, we reveal that carotenoids are oxidized through the addition of just ethanol in the presence of lactoperoxidase. The oxidation of carotenoids through this exquisite strategy is ∼360 times faster than harnessing the lactoperoxidase system in whey proteins via the addition of hydrogen peroxide. Bearing in mind that ethanol is not an oxidizing agent, this observation suggests a potential paradigm shift in our understanding of lactoperoxidase and catalyzed oxidation in biochemical systems.

Metabolomic profile in hyperthyroid patients before and after antithyroid drug treatment: Correlation with thyroid hormone and TSH concentration

Hyperthyroidism (HT) is characterized by an intense metabolic impact which affects the lipid, carbohydrate and amino acids metabolism, with increased resting energy expenditure and thermogenesis. Metabolomics is a new comprehensive technique that allows to capture an instant metabolic picture of an organism, reflecting peculiar molecular and pathophysiological states. The aim of the present prospective study was to identify a distinct metabolomic profile in HT patients using ¹H NMR spectroscopy before and after antithyroid drug treatment. This prospective study included 15 patients (10 female, 5 male) who were newly diagnosed hyperthyroidism. A nuclear magnetic resonance (¹H NMR) based analysis was performed on plasma samples from the same patients at diagnosis (HypT₀) and when they achieved euthyroidism (HypT₁). The case groups were compared with a control group of 26 healthy volunteers (C). Multivariate statistical analysis was performed with Partial Least Squares-Discriminant Analysis (PLS-DA). PLS-DA identified a distinct metabolic profile between C and untreated hyperthyroid patients (R²X 0.638, R²Y 0.932, Q² 0.783). Interestingly, a significant difference was also found between C and euthyroid patients after treatment (R²X 0.510, R²Y 0.838, Q² 0.607), while similar cluster emerged comparing HypT₀vs HypT₁ patients. This study shows that metabolomic profile is deeply influenced by hyperthyroidism and this alteration persists after normalization of thyrotropin (TSH) and free thyroid hormone (FT3, FT4) concentration. This suggests that TSH, FT3 and FT4 assays may not be insufficient to detect long lasting peripheral effects of the thyroid hormones action. Further studies are needed to clarify whether and to what extent the evaluation of metabolomics profile may provide relevant information in the clinical management of hyperthyroidism.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Metabolic pathways of tetraidothyronine and triidothyronine production by thyroid gland: a review of articles

Tetraidothyronine (T4) and Triiodothyronine (T3) are the two vital hormones in human metabolism produced by thyroid gland. The major pathways in thyroid hormone biosynthesis begin with iodine metabolism which occurs in three sequential steps: active iodide transport into thyroid followed by iodide oxidation and subsequent iodination of tyrosyl residues of thyroglobulin (Tg) to produce idotyrosines monoidotyrosine (MIT) and diiodothyrosine (DIT) on Tg. Oxidized iodine and tyrosyle residues which are an aromatic amino acids are integral part of T4 and T3. The thyroid iodine deficiency of either dietary, thyroid malfunction, or disorder of hypothalamus and pituitary to produce enough Thyroid Stimulating Hormone (TSH), eventually lead to hypothyroidism with sever side effects. Iodine oxidation is the initial step for thyroid hormone synthesis within thyroid, is mediated by thyroperoxidase enzyme (TPO), which itself is activated by TSH required for production of MIT and DIT. T4 and T3 are subsequently are synthesized on Tg following MIT and DIT coupling reaction. Thyroid hormones eventually produced and released into circulation through Tg pinocytosis from follicular space and subsequent lysozomal function, a process again stimulated by TSH. The production of T4 and T3 are highly regulated externally by a negative feed-back interrelation between serum T4, T3 and TSH and internally by the elevated iodine within thyroid gland. It is believed the extra iodine concentration within thyroid gland control thyroid hormones synthesis by inhibition of the TPO and hydrogen peroxide (H2O2) formation which is also an essential factor of iodine oxidation, via a complex mechanism. In healthy subjects the entire procedures of T4 and T3 synthesis re-start again following a drop in serum T4 and T3 concentration. On conditions of thyroid disorders, which caused by the distruption of either of above mechanisms, thyroid hormone deficiency and related clinical manifestations eventually begin to show themselves.

Structural and functional aspects of thyroid peroxidase

Thyroperoxidase (TPO) is the enzyme involved in thyroid hormone synthesis. Although many studies have been carried out on TPO since it was first identified as being the thyroid microsomal antigen involved in autoimmune thyroid disease, previous authors have focused more on the immunological than on the biochemical aspects of TPO during the last few years. Here, we review the latest contributions in the field of TPO research and provide a large reference list of original publications. Given this promising background, scientists and clinicians will certainly continue in the future to investigate the mechanisms whereby TPO contributes to hormone synthesis and constitutes an important autoantigen involved in autoimmune thyroid disease, and the circumstances under which the normal physiological function of this enzyme takes on a pathological role.

Spectral studies with lactoperoxidase and thyroid peroxidase: interconversions between native enzyme, compound II, and compound III

Spectral scans in both the visible (650-450 nm) and the Soret (450-380 nm) regions were recorded for the native enzyme, Compound II, and Compound III of lactoperoxidase and thyroid peroxidase. Compound II for each enzyme (1.7 microM) was prepared by adding a slight excess of H2O2 (6 microM), whereas Compound III was prepared by adding a large excess of H2O2 (200 microM). After these compounds had been formed it was observed that they were slowly reconverted to the native enzyme in the absence of exogenous donors. The pathway of Compound III back to the native enzyme involved Compound II as an intermediate. Reconversion of Compound III to native enzyme was accompanied by the disappearance of H2O2 and generation of O2, with approximately 1 mol of O2 formed for each 2 mol of H2O2 that disappeared. A scheme is proposed to explain these observations, involving intermediate formation of the ferrous enzyme. According to the scheme, Compound III participates in a reaction cycle that effectively converts H2O2 to O2. Iodide markedly affected the interconversions between native enzyme, Compound II, and Compound III for lactoperoxidase and thyroid peroxidase. A low concentration of iodide (4 microM) completely blocked the formation of Compound II when lactoperoxidase or thyroid peroxidase was treated with 6 microM H2O2. When the enzymes were treated with 200 microM H2O2, the same low concentration of iodide completely blocked the formation of Compound III and largely prevented the enzyme degradation that otherwise occurred in the absence of iodide. These effects of iodide are readily explained by (i) the two-electron oxidation of iodide to hypoiodite by Compound I, which bypasses Compound II as an intermediate, and (ii) the rapid oxidation of H2O2 to O2 by the hypoiodite formed in the reaction between Compound I and iodide.

The role of compound III in reversible and irreversible inactivation of lactoperoxidase

In the presence of iodide (I-, 10 mM) and hydrogen peroxide in a large excess (H2O2, 0.1-10 mM) catalytic amounts of lactoperoxidase (2 nM) are very rapidly irreversibly inactivated without forming compound III (cpd III). In contrast, in the absence of I- cpd III is formed and inactivation proceeds very slowly. Increasing the enzyme concentration up to the micromolar range significantly accelerates the rate of inactivation. The present data reveal that irreversible inactivation of the enzyme involves cleavage of the prosthetic group and liberation of heme iron. The rate of enzyme destruction is well correlated with the production of molecular oxygen (O2), which originates from the oxidation of excess H2O2. Since H2O2 and O2 per se do not affect the heme moiety of the peroxidase, we suggest that the damaging species may be a primary intermediate of the H2O2 oxidation, such as oxygen in its excited singlet state (1 delta gO2), superoxide radicals (O-.2), or consequently formed hydroxyl radicals (OH.).

Inactivation of peroxidase and glucose oxidase by H2O2 and iodide during in vitro thyroglobulin iodination

Thyroglobulin iodination and thyroxine synthesis in vitro require the presence of peroxidase, H2O2 and iodide. H2O2 is usually continuously generated by glucose oxidase (GO) and glucose. The aim of this study was to investigate whether the two enzymes could possibly be inactivated by a particular concentration of H2O2 or iodide present during incubation. The results revealed that both enzymes were indeed inactivated under two distinct conditions: Lactoperoxidase and thyroid peroxidase were inactivated by modest concentrations of H2O2 accumulating during incubation. Glucose oxidase was inactivated by an oxidized species of iodine or singlet oxygen produced in the catalytic cycle. The results may explain some hitherto unsolved discrepancies between different iodination procedures. Moreover they may have an impact on the regulation of in vivo thyroglobulin iodination and hormone synthesis.

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.

Mechanism of enzymatic and non-enzymatic tyrosine iodination. Inhibition by excess hydrogen peroxide and/or iodide

Non-enzymatic (I2-mediated) and lactoperoxidase-catalyzed iodination of tyrosine are inhibited by excess iodide (I-) and/or hydrogen peroxide (H2O2). This phenomenon is a consequence of the concentration-dependent dual role of I- and H2O2 in the iodinating system. I- and H2O2, in addition to their function as primary substrates of peroxidase, may act as alternative 'iodine acceptors' and therefore compete with tyrosine for the active iodinating agent, irrespective of whether this compound is an enzyme-associated iodinium cation (E X I delta +) or an equivalent oxidized iodine species (IOH, IC1, I2). The competitive reaction pathways resulting from excess I- and/or H2O2 in the iodination system are I2/I-3 generation and/or pseudo-catalatic degradation of H2O2, respectively. Our results also demonstrate that I2 (and alternative medium-dependent oxidized iodine species such as IOH and IC1) generated in the iodination system may play an important role as iodinating agent(s). They serve as a substitute for the enzyme-bound iodinium species (E X I delta +), if the prevailing I- concentration favours this pathway. The proposed mechanism of the various antagonistic and interactive reaction pathways is summarized in a scheme.

Thyroid hormone synthesis and thyroglobulin iodination related to the peroxidase localization of oxidizing equivalents: studies with cytochrome c peroxidase and horseradish peroxidase

Cytochrome c peroxidase (CcP) and horseradish peroxidase (HRP), when combined with a stoichiometric amount of H2O2, form stable compounds I which are known as FeIV Ro and FeIV o pi + structures, respectively. These compounds were assayed in the catalysis of thyroid hormone synthesis and the iodination reaction. As previously shown for the lactoperoxidase FeIV Ro compound, the CcP FeIV Ro compound was involved in the coupling and not in the iodination reactions. In contrast, the HRP FeIV o pi + compound catalyzed both iodination and hormone formation. The possible role of the different peroxidase-H2O2 compounds in the two sequential reactions, thyroglobulin iodination and thyroid hormone formation, is discussed.

Reduction of lactoperoxidase-H2O2 compounds by ferrocyanide: indirect evidence of an apoprotein site for one of the two oxidizing equivalents

The titration by ferrocyanide and the localization of the oxidizing equivalents of lactoperoxidase "compound II" were studied as a function of pH. It was demonstrated that 1) whatever the pH, the structure of lactoperoxidase "compound II" was compatible with a Fe IV R degree state, 2) at acidic pH, ferrocyanide preferentially reduced the oxidizing equivalent localized on the heme iron to give an Fe III R degree compound, 3) at pH 4.2 only the Fe III R degree form was obtained after reduction of lactoperoxidase "compound II" with one mole of ferrocyanide and whereas at pH greater than 4.2, a mixture of both Fe III R degree and Fe IV R forms was present, 4) lowering the pH from 7.2 to 4.0 induced a transition of Fe IV R state to Fe III R degree state, but increasing the pH from 4.0 to 7.2 did not permit the formation of Fe IV R compound from Fe III R degree compound.

Initiation of lipid peroxidation by a peroxidase/hydrogen peroxide/halide system

A lactoperoxidase/H2O2/halide system caused the initiation of linoleate peroxidation as indicated by diene conjugation. Coupled lipid peroxidation was accelerated by iodide, chloride and bromide ions at pH 4.0 and 6.2. No peroxidation occurred in the presence of H2O2 or lactoperoxidase alone. The rate of linoleate peroxidation by lactoperoxidase in the presence of chloride depended on the concentration of H2O2. Linoleate peroxidation by the enzymatic system was inhibited by high concentration of H2O2 by methionine, tryptophan and BHT. Oxygen was absorbed during peroxidation and the major products were the 13-hydroperoxides. The mechanisms of the initiation of lipid peroxidation by a peroxidase/H2O2/halide system are discussed.

Lipid deterioration: beta-carotene destruction and oxygen evolution in a system containing lactoperoxidase, hydrogen peroxide and halides

A model system containing lactoperoxidase/H2O2/halide decomposed beta-carotene in a reaction greatly affected by the concentration of H2O2. The optimal concentrations of H2O2 for activation of iodide and bromide were 2 mM and 10 microM, respectively. The oxidation of chloride by a lactoperoxidase, using beta-carotene destruction as a sensitive method to determine the activity of the enzyme, is reported herein. In the presence of optimal amounts of H2O2, the rate of beta-carotene destruction increases slowly until a critical concentration of the halides, followed by a rapid increase in the rate when halide concentrations were further increased. A lactoperoxidase/H2O2/iodide and/or bromide system generates oxygen in the presence of high H2O2 and halide concentrations. beta-Carotene inhibited the evolution of oxygen. A possible mechanism of beta-carotene destruction and triplet unexcited oxygen evolution by a lactoperoxidase/H2O2/halide system are proposed.