Isomeric discrimination and quantification of thyroid hormones, T3 and rT3, by the single ratio kinetic method using electrospray ionization mass spectrometry

Simultaneous Identification of Major Thyroid Hormones by a Nickel Immobilized Biological Nanopore

Thyroid hormones (THs) are a variety of iodine-containing hormones that demonstrate critical physiological impacts on cellular activities. The assessment of thyroid function and the diagnosis of thyroid disorders require accurate measurement of TH levels. However, largely due to their structural similarities, the simultaneous discrimination of different THs is challenging. Nanopores, single-molecule sensors with a high resolution, are suitable for this task. In this paper, a hetero-octameric Mycobacterium smegmatis porin A (MspA) nanopore containing a single nickel ion immobilized to the pore constriction has enabled simultaneous identification of five representative THs including l-thyroxine (T4), 3,3',5-triiodo-l-thyronine (T3), 3,3',5'-triiodo-l-thyronine (rT3), 3,5-diiodo-l-thyronine (3,5-T2) and 3,3'-diiodo-l-thyronine (3,3'-T2). To automate event classification and avoid human bias, a machine learning algorithm was also developed, reporting an accuracy of 99.0%. This sensing strategy is also applied in the analysis of TH in a real human serum environment, suggesting its potential use in a clinical diagnosis.

Mass spectrometry in the diagnosis of thyroid disease and in the study of thyroid hormone metabolism

The importance of thyroid hormones in the regulation of development, growth, and energy metabolism is well known. Over the last decades, mass spectrometry has been extensively used to investigate thyroid hormone metabolism and to discover and characterize new molecules involved in thyroid hormones production, such as thyrotropin-releasing hormone. In the earlier period, the quantification methods, usually based on gas chromatography-mass spectrometry, were complicated and time consuming. They were mainly focused on basic research, and were not suitable for clinical diagnostics on a routine basis. The development of the modern mass spectrometers, mainly coupled to liquid chromatography, enabled simpler sample preparation procedures, and the accurate quantification of thyroid hormones, of their precursors, and of their metabolites in biological fluids, tissues, and cells became feasible. Nowadays, molecules of physiological and pathological interest can be assayed also for diagnostic purposes on a routine basis, and mass spectrometry is slowly entering the clinical laboratory. This review takes stock of the advancements in the field of thyroid metabolism that were carried out with mass spectrometry, with special focus on the use of this technique for the quantification of molecules involved in thyroid diseases.

Molecular Basis for the Remarkably Different Gas-Phase Behavior of Deprotonated Thyroid Hormones Triiodothyronine (T3) and Reverse Triiodothyronine (rT3): A Clue for Their Discrimination?

Thyroid hormones are biologically active small molecules responsible for growth and development regulation, basal metabolic rate, and lipid and carbohydrate metabolism. Liquid chromatography mass spectrometry (LC–MS) can be used to quantify thyroid hormones blood level with high speed and selectivity, aiming to improve the diagnosis and treatment of the severe pathological conditions in which they are implicated, i.e., hypo- and hyperthyroidism. In this work, the gas-phase behavior of the isomeric thyroid hormones triiodothyronine (T3) and reverse triiodothyronine (rT3) in their deprotonated form was studied at a molecular level using MS-based techniques. Previously reported collision-induced dissociation experiments yielded distinct spectra despite the high structural similarity of the two compounds, suggesting different charge sites to be responsible. Infrared multiple photon dissociation spectroscopy on [T3-H]⁻ and [rT3-H]⁻ was performed, and the results were interpreted using DFT and MP2 calculations, assessing the prevalence of T3 in the carboxylate form and rT3 as a phenolate isomer. The different deprotonation sites of the two isomers were also found to drive their ion-mobility behavior. In fact, [T3-H]⁻ and [rT3-H]⁻ were successfully separated. Drift times were correlated with collisional cross section values of 209 and 215 Ų for [T3-H]⁻ and [rT3-H]⁻, respectively. Calculations suggested the charge site to be the main parameter involved in the different mobilities of the two anions. Finally, bare [T3-H]⁻ and [rT3-H]⁻ were made to react with neutral acetylacetone and trifluoroacetic acid, confirming rT3 to be more acidic than T3 in agreement with the calculated gas-phase acidities of T3 and rT3 equal to 1345 and 1326 kJ mol^–1, respectively.

Clinical and laboratory aspects of 3,3',5'-triiodothyronine (reverse T3)

Reverse T3 (3,3',5'-triiodothyronine or rT3) is the third most abundant iodothyronine circulating in human blood and is produced by the inner ring deiodination of the pro-hormone thyroxine (T4). Unlike the more abundant and active metabolite T3, the measurement of serum rT3 is yet to find a routine clinical application. As rT3 binds weakly to the T3 thyroid nuclear hormone receptors, it is thought to represent an inactive end-product of thyroid hormone metabolism, diverting T4 away from T3 production. The analysis of serum rT3 has, up until recently, been measured by competitive radioimmunoassay, but these methods have been superseded by mass-spectrometric methods which are less susceptible to interference from other more abundant iodothyronines. Serum rT3 concentration is increased as part of the non-thyroidal illness syndrome, and by administration of common medications such as amiodarone which inhibit the metabolism of rT3. Serum rT3 concentration is also affected by genetic conditions that affect the iodothyronine deiodinases, as well as thyroid transporters and transport proteins. Analysis of rT3 can provide a useful diagnostic fingerprint for these conditions. rT3 has been shown to bind extra-nuclear iodothyronine receptors with a potential role in cell proliferation; however, the clinical relevance of these findings awaits further study.

Direct differentiation of stereoisomers of ezetimibe/ambrisentan/atorvastatin and their mechanism study by electrospray ionization quadrupole time-of-flight mass spectrometry

A mass spectrometric method is introduced for rapid and accurate chiral quantification by examining a trimeric metal complex into which a chiral reference is incorporated with the analyte. Several metal ions (Cu^II , Ni^II , Mg^II , Mn^II , Co^II , and Zn^II ) were selected as the central metal ion, and chiral drugs ezetimibe (EZM) and ambrisentan (AMB) were used as the reference to each other for isomeric differentiation by using electrospray ionization quadrupole time-of-flight mass spectrometry. Doubly charged trimeric cluster ions instead of the singly charged clusters were applied in this study. Kinetic method (KM) and chiral recognition (CR) method were used for construction of a calibration curve for chiral quantitation. The results from the two methods were found to be complementary to each other, which improved quantitative analysis of stereoisomers for EZM. Furthermore, we have successfully used S-AMB as reference for the chiral differentiation of enantiomeric atorvastatin (ATO), which is frequently combined with EZM as a codrug. Experimental results showed that the binary mixture of EZM and ATO enantiomers can be determined simultaneously without prior separation steps. The direct measurement of chiral purity within 5% was demonstrated. This mass spectrometric method represents an effective alternative to commonly used chromatographic techniques as means of chiral purity determination and is of potential use in rapid screening experiments.

Correlation between Serum Levels of 3,3',5'-Triiodothyronine and Thyroid Hormones Measured by Liquid Chromatography-Tandem Mass Spectrometry and Immunoassay

Objective

For measuring serum 3,3′,5′-triiodothyronine (rT3) levels, radioimmunoassay (RIA) has traditionally been used owing to the lack of other reliable methods; however, it has recently become difficult to perform. Meanwhile, liquid chromatography-tandem mass spectrometry (LC-MS/MS) has recently been attracting attention as a novel alternative method in clinical chemistry. To the best of our knowledge, there are no studies to date comparing results of the quantification of human serum rT3 between LC-MS/MS and RIA. We therefore examined the feasibility of LC-MS/MS as a novel alternative method for measuring serum rT3, thyroxine (T4), and 3,5,3′-triiodothyronine (T3) levels.

Methods

Assay validation was performed by LC-MS/MS using quality control samples of rT3, T4, and T3 at 4 various concentrations which were prepared from reference compounds. Serum samples of 50 outpatients in our department were quantified both by LC-MS/MS and conventional immunoassay for rT3, T4, and T3. Correlation coefficients between the 2 measurement methods were statistically analyzed respectively.

Results

Matrix effects were not observed with our method. Intra-day and inter-day precisions were less than 10.8% and 9.6% for each analyte at each quality control level, respectively. Intra-day and inter-day accuracies were between 96.2% and 110%, and between 98.3% and 108.6%, respectively. The lower limit of quantification was 0.05 ng/mL. Strong correlations were observed between the 2 measurement methods (correlation coefficient, T4: 0.976, p < 0.001; T3: 0.912, p < 0.001; rT3: 0.928, p < 0.001).

Conclusions

Our LC-MS/MS system requires no manual cleanup operation, and the process after application of a sample is fully automated; furthermore, it was found to be highly sensitive, and superior in both precision and accuracy. The correlation between the 2 methods over a wide range of concentrations was strong. LC-MS/MS is therefore expected to become a useful tool for clinical diagnosis and research.

Enantiomeric differentiation of β-amino alcohols under electrospray ionization mass spectrometric conditions

The enantiomeric differentiation of a series of chiral β-amino alcohols (A) is attempted, for the first time, by applying the kinetic method using L-proline, L-tryptophan, 4-iodo-L-phenylalanine or 3, 5-diiodo-L-tyrosine as the chiral references (Ref) and Cu(2+) or Ni(2+) ion (M) as the central metal ion. The trimeric diastereomeric adduct ions, [M+(Ref)2+A-H](+), formed under electrospray ionization conditions, are subjected for collision-induced dissociation (CID) experiments. The products ions, formed by the loss of either a reference or an analyte, detected in the CID spectra are evaluated for the enantiomeric differentiation. All the references showed enantiomeric differentiation and the R(chiral) values are better for the aromatic alcohols than for aliphatic alcohols. Notably, the R(chiral) values of the aliphatic amino alcohols enhanced when Ni(2+) is used as the central metal ion. The experimental results are well supported by computational studies carried out on the diastereomeric dimeric complexes. The computational data of amino alcohols is correlated with that of amino acids to understand the structural interaction of amino alcohols with reference molecule and central metal ion and their role on the stabilization of the dimeric complexes. Application of flow injection MS/MS method is also demonstrated for the enantiomeric differentiation of the amino alcohols.

Isomeric distinction of small oligosaccharides: a bottom-up approach using the kinetic method

Isomeric distinction of di- and tri-saccharides could be efficiently achieved by using data previously obtained while performing experiments aimed at discriminating monosaccharides using trimeric ion dissociation with data analysis by the kinetic method. This study shows that effects observed for lower homologues when one of the partners is changed in the metal/reference system (typically a transition metal divalent cation associated to amino acids) can be extrapolated to upper homologues, at least for the tested analyte series. Systems allowing galactose, glucose, and fructose distinction were used as starting conditions to resolve cellobiose, lactose, maltose, and saccharose disaccharides. When a unique dissociation reaction was observed from the trimeric clusters, a new reference was selected based on its propensity to favor the analyte or the reference release, as revealed from monosaccharide experiments, depending on the desired effect. The same approach could be implemented from data obtained for disaccharides to select efficient metal/reference systems to distinguish cellotriose, isomaltotriose, maltotriose, and panose trisaccharides. As a result, method optimization is greatly improved due to an enhanced rationalization of the search for discriminant systems. While 40 systems had to be tested for monosaccharides, by screening five transition metals and eight amino acids, the proposed approach allowed efficient metal/reference systems to be found for disaccharides after testing 18 combinations; then, only four systems had to be scrutinized to achieve trisaccharide distinction. Accurate quantitative analyses could be performed in binary mixtures using three-point calibration curves to correct for competition effects between analytes for the formation of the trimeric clusters.