A microassay for the determination of iodide and its application to the measurement of the iodination of proteins and the catalytic activities of iodo compounds

In vitro selection and characterization of RNA aptamers binding thyroxine hormone

RNA possesses the ability to bind a wide repertoire of small molecules. Some of these binding interactions have been shown to be of primary importance in molecular biology. For example, several classes of mRNA domains, collectively referred to as riboswitches, have been shown to serve as RNA genetic control elements that sense the concentrations of specific metabolites (i.e. acting as direct sensors of chemical compounds). However, to date no RNA species binding a hormone has been reported. Here, we report that the use of an appropriate SELEX (systematic evolution of ligands by exponential enrichment) strategy results in the isolation of thyroxine-specific aptamers. Further biochemical characterization of these aptamers, including mutational studies, the use of transcripts with site-specific modified nucleotides, nuclease and chemical probing, binding-shift assays and CD, demonstrated that these RNA structures included a G-rich motif, reminiscent of a guanine quadruplex structure, adjacent to a helical region. The presence of the thyroxine appeared to be essential for the formation of the structural motif's scaffold. Moreover, the binding is shown to be specific to thyroxine (T4) and tri-iodothyronine (T3), the active forms of the hormone, whereas other inactive derivatives, including thyronine (T0), do not support complex formation. These results suggest that this aptamer specifically binds to the iodine moieties of the thyroxine, a previously unreported ability for an RNA molecule.

Transcellular thiocyanate transport by human airway epithelia

Human airway mucosa synthesizes and secretes lactoperoxidase (LPO). As H(2)O(2) and thiocyanate (SCN(-)) are also present, a functional LPO antibacterial defence system exists in the airways. SCN(-) concentrations in several epithelial secretions are higher than in serum, although the mechanisms of transepithelial transport and accumulation in these secretions are unknown. To examine SCN(-) accumulation in secretions, human airway epithelial cells, re-differentiated at the air-liquid interface, were used in open-circuit conditions. [(14)C]SCN(-), in the basolateral medium, was transported across the epithelium and concentrated tenfold at the apical surface. Measurement of the transepithelial potential showed that the basolateral compartment was positive relative to the apical surface (13.7 +/- 1.8 mV) and therefore unfavourable for passive movement of SCN(-). Transport was dependent on basolateral [SCN(-)] and saturable (K(m,app) = 69 +/- 25 microM); was inhibited by increased apical [SCN(-)]; and was dependent on the presence of basolateral Na(+). Perchlorate (K(i,app) = 0.6 +/- 0.05 microM) and iodide (K(i,app) = 9 +/- 8 microM) in the basolateral medium reversibly inhibited transport, but furosemide did not. Iodide was also transported (K(m,app) = 111 +/- 69 microM). RT-PCR and immunohistochemistry confirmed expression of Na(+)-I(-) symporter (NIS) in the airways. SCN(-) transport was insensitive to apical disulphonic acid Cl(-) channel blockers, but sensitive to apical glibenclamide and arylaminobenzoates. Forskolin and dibutyryl cAMP increased transport. These data suggest SCN(-) transport may occur through basolateral NIS-mediated SCN(-) concentration inside cells, followed by release through an apical channel, perhaps cystic fibrosis transmembrane conductance regulator.

Dityrosine bridge formation and thyroid hormone synthesis are tightly linked and are both dependent on N-glycans

Formation of dityrosine bridges is a ubiquitous process mainly attributed to oxidative stress leading to protein degradation and cellular damages. Here we show that dityrosine formation is involved in a physiological process, thyroid hormone synthesis, and is strictly dependent on structural characteristics, namely N-glycans, presented by the protein acting as the prothyroid hormone. We used two isoforms of the N-terminal thyroid hormone forming domain (NTD) of human thyroglobulin: one without N-glycan (19 kDa isoform) and the other with high mannose type structures (25 kDa isoform). Both isoforms were able to form iodotyrosines after in vitro iodination. However, iodotyrosine coupling to form thyroxine did not occur with the unglycosylated 19 kDa NTD. In contrast, the 25 kDa isoform formed thyroxine. Strikingly, thyroxine synthesis was accompanied by dimerization of the 25 kDa isoform and formation of a dityrosine bridge; none of this was observed with the 19 kDa isoform. Taken as a whole, our results indicate that dimerization through dityrosine bridging accompanies and could have a role in thyroid hormone synthesis.

The optimisation of a novel iodide microassay and its application in an immunoassay for human antibody levels in serum

Radioactive iodine has been used as an antibody label in many immunoassays. The feasibility of using non-radioactive iodine as a label was investigated. A microassay for iodide based on the Sandell-Kolthoff reaction was optimised. It was used to detect iodine-labelled antibody and successfully applied to the detection of human IgG in serum samples. The performance of this assay system compared with enzyme-linked immunosorbent assay using HRP was evaluated.

Simplified measurement of protein-bound iodine with epithermal neutron activation analysis

A refined method for the measurement of protein-bound iodine (PBI) in blood serum was demonstrated by the use of epithermal neutron activation analysis (ENAA). PBI in ammonium sulfate-precipitated serum protein, after epithermal neutron activation, was determined by high resolution gamma-ray spectrometry. From our results, the PBI concentration was 67.8 +/- 2.2 ng/mL. Good agreement was obtained with published data, ranging from 40 to 80 ng/mL, which had been obtained using different analytical techniques. The validity of these techniques for PBI has been born out by a very good accuracy and simplicity without temperature dependence.

Kinetic determination of iodide in pharmaceutical and food samples

A kinetic method for the determination of iodide based on its inhibitory effect on the Pd(III) catalysed reaction between ethylenediaminetetraacetic acid (EDTA)-Co(III) and the hypophosphite ion is described. The reaction was followed spectrophotometrically by measuring the decrease in the absorbance at 540 nm. Under the optimum experimental conditions of 2.6 x 10(-3) mol dm(-3)Co(III)-EDTA, 0.4 mol dm(-3)H2PO2-, pH 3.2 (adjusted with Britton-Robinson buffer), 0.57 micrograms ml(-1) Pd(III) and 20 +/- 0.2 degrees C, iodide was determined in the range 2-28 ng ml(-1). The method was applied to the determination of iodide in pharmaceutical products, iodinated salts, cow's milk and infants' powdered milk.

Use and detection of nonradioactive iodine-labeled antibodies for immunoassay

Traditionally radioactively labeled iodine has been used as a reporter group for the detection of antibodies in immunoassay. We have recently developed a microassay system for the detection of very low concentrations of iodide which eliminates the need for the use of radiolabeled iodine (O'Kennedy, R. et al. 1989, Anal. Biochem. 179, 138-144). The successful application of this assay for the detection of mouse IgG is described. The performance of this system compared with enzyme immunoassay was evaluated.

Bifunctional antibodies: concept, production and applications

Immunoglobulins, or antibodies, are monospecific, bivalent antigen-binding molecules. Bifunctional antibodies are bispecific, with each arm binding to a different antigen, and may be produced by biological or chemical methods. Biological production involves the fusion of two monoclonal antibody-producing hybridomas or of an immunised spleen cell and a hybridoma. The resulting hybrid hybridomas (quadromas or triomas) secrete a mixture of parenteral monoclonal antibodies and bifunctional antibody. In chemical production, the parental monoclonal antibodies can be 'chopped up and reconstituted' to produce the bifunctional antibody only. Bifunctional antibodies have a variety of potential uses. They were originally proposed as an aid to cancer chemotherapy where one of the arms of the antibody would bind to a tumour marker and the other to a drug, toxin, or cytotoxic cell. Functional agents can thus be target directly onto tumour cells, accumulating with higher density, yet with reduced side effects for the patient. Further applications have been proposed involving enzyme immobilization and novel immunoassay techniques. This review describes developments that have taken place in bifunctional antibody technology to date.