Iodination of thyroid-stimulating hormone for receptor-binding studies with human thyroid membranes: effects of specific activity and method of iodination

Receptor-purified, Bolton-Hunter radioiodinated, recombinant, human epidermal growth factor: an improved radioligand for receptor studies

We report an assessment of the applicability of the Bolton-Hunter method to the radioiodination of epidermal growth factor (EGF). Recombinant human EGF (hEGF) could be radioiodinated successfully by this method, whereas murine EGF could not. Bolton-Hunter 125I-labeled hEGF was compared with commercial 125I-labeled hEGF prepared by the chloramine-T radioiodination method. Neither radioligand was sufficiently pure for a detailed characterization of the purportedly heterogeneous pattern of binding of EGF to its receptors. A procedure based on receptor adsorption was thus developed for repurification of the Bolton-Hunter 125I-labeled hEGF. This provided a much purer radioligand suitable for detailed studies of receptor-binding heterogeneity.

Means for the assessment of radioligand quality and its importance in receptor-binding studies. Observations with radiolabelled formylmethionyl-leucyl-phenylalanine

Various methods for testing the quality of radioligands were applied to two different radiolabelled forms of formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). The purpose of the study was both to examine the value of these methods for assessing radioligand quality and to determine the suitability of these particular radioligands for studying the chemotactic formylpeptide receptors on the rabbit neutrophil. It is useful in this context to distinguish two different aspects of radioligand quality: these are purity and equivalence to the native ligand. The two methods described for measuring receptor-reactivity (or 'bindability'), by measuring binding to an increasing excess of receptors and by a re-incubation procedure, provide a reliable measure of purity that should readily be applicable to other radioligands. Equivalence to the native ligand is more difficult to establish, and any uncertainty about the specific radioactivity of the radioligand can pose serious problems with this assessment. Commercial preparations of both tritiated and 35S-labelled fMet-Leu-Phe were found to be inadequately pure for detailed receptor studies. Repurification by t.l.c., however, consistently yielded radioligand preparations of high purity and close equivalence to the native ligand. Other radioligands may often also require a suitable repurification step before use for detailed receptor studies; this is especially important whenever a complex receptor-binding pattern is envisaged.

Lipoprotein-binding proteins in the human platelet plasma membrane

The binding of homologous plasma lipoproteins to specific receptor proteins in the plasma membrane of human blood platelets was studied by ligand blotting techniques. HDL3, HDL2 and LDL showed saturable binding to three bands of 156, 130 and 115 kDa, respectively. This binding was not markedly affected by the presence or absence of Ca2+ nor by covalent modification of lysine and arginine residues of the apoprotein moieties. However, it can be almost completely reversed by the addition of heparin or suramin.

Heterogeneity of binding of chemotactic formyl peptides to their receptors on rabbit neutrophils: methodological and analytical considerations in the study of complex receptor-binding patterns

The pattern of binding of chemotactic formyl peptides to their receptors on intact rabbit neutrophils was investigated using three different radiolabeled formyl peptides. The purpose of the study was both to establish suitable procedures for the analysis of complex receptor-binding data and to evaluate the heterogeneity of binding of the chemotactic formyl peptides. Radiolabeled formylmethionyl-leucyl-phenylalanine was found to require repurification by thin-layer chromatography before it was of sufficient purity for use in these studies. All three radioligands were essentially stable during the binding assay. The 'LIGAND' computer program proved suitable for a detailed and rigorous analysis of the binding data. Each of the three formyl peptides showed significant heterogeneity in its binding pattern and all appeared to interact with the same receptor sites. The degree of heterogeneity, however, was greater with formylnorleucyl-leucyl-phenylalanine than with the other two formyl peptides. The heterogeneity was close to the theoretical limit of discernibility; thus individual binding studies with one formyl peptide did not always show statistical evidence of the heterogeneity. The study also highlighted the severe uncertainty in estimates of the proportions of each type of binding site when such data are analyzed on the basis of the 'two binding site' model.

Thyrotrophin receptors, tumour radioiodine concentration and thyroglobulin secretion in differentiated thyroid cancers

Tumour radioiodine concentration has been compared with serum thyroglobulin (Tg) and, in a few cases, with tumour complement of thyrotrophin receptors in patients with differentiated thyroid carcinoma. All tumours examined possessed TSH receptors. In most the complement was similar to that of normal thyroid tissue although all but one of the tumours had no detectable 131I concentration in vivo even with excess TSH stimulation. Elevated serum Tg (patient taking T4 in suppressive dose) was generally associated with tumours which had 131I concentrating function when stimulated by excess TSH. Some patients, however, had high serum Tg concentration but only low or indetectable tumour 131I uptake. We conclude that (a) measurement of tumour TSH receptor complement is unlikely to be useful in clinical management as tumours which do not significantly concentrate 131I in vivo may have a normal TSH receptor complement and (b) the capacity to secrete Tg is usually associated with 131I concentration but quantitatively the relationship varies considerably between tumours.

High-performance liquid chromatography of amino acids, peptides and proteins. LVII. Analysis of radioiodinated thyrotropin polypeptides by reversed-phase high-performance liquid chromatography

The chromatographic behaviour of bovine and human thyrotropin, radiolabelled stoichiometrically with lactoperoxidase, on octadecylsilicas and other reversed-phase n-alkylsilicas has been investigated. As part of this investigation the effects of a variety of elution systems on resolution and recovery have been examined. Analysis of the tryptic peptides of radioiodinated bovine thyrotropin (bTSH) preparations by reversed-phase high-performance liquid chromatographic mapping methods resulted in the separation of more than fourteen major radioactive peptide components. The data indicate that bTSH is radioiodinated with unequal incorporation into both the alpha- and beta-subunits. Further, assessment of the level of microheterogeneity of radiolabelled bTSH preparations can be achieved with these reversed-phase techniques.

Degradation of beta 2-microglobulin in infected urine by leukocyte elastase-like activity

Loss of beta 2-microglobulin in human urine at near neutral pH is described. beta 2-Microglobulin was found to be unstable at pH 7.4 in 15 out of 70 specimens with bacterial infection. Breakdown of this protein was shown to be most active between pH 6.5 and 8.0 but some proteolysis also occurred between pH 5.0 and 6.0. Evidence that proteolysis of beta 2-microglobulin near neutral pH is due to neutrophil elastase-like activity was obtained by the following methods: use of the model elastase substrate L-pyroglutamyl-L-prolyl-L-valine-p-nitroanilide and the specific inhibitor 2-trifluoroacetylaminobenzenesulphonyl fluoride; use of neutrophil granule extracts and determination of the size distribution of beta 2-microglobulin proteolytic fragments. If increased numbers of pus cells are present, it is suggested that screening for elastase-like proteolysis should be undertaken before interpreting a measurement of beta 2-microglobulin.

A comparative study of the binding of Graves' immunoglobulins by the patients own and other thyroid membranes

Thyroid membrane preparations from six patients with active Graves' disease were tested in an assay which detects the thyroid interactive immunoglobulins of Graves' disease by their inhibition of binding of [125I]-thyroid stimulating hormone (TSH). With all preparations inhibition of binding of 125I-TSH by excess TSH could be demonstrated (specific binding). The patients' own immunoglobulins were assayed against their own thyroid membranes and against each other's under exactly comparable conditions. Inhibition of binding by IgGs from the patients varied between membrane preparations: with one preparation 5/6 IgGs were inhibitory but with another none were effective. Of the six patients, their own IgG inhibited binding of 125I-TSH to their own thyroid membrane preparation in only four instances, and when interaction did occur this did not reliably predict that the membrane preparation would interact with IgGs from other patients with Graves' disease. The selection of a membrane preparation for this assay cannot be made solely on ability to specifically bind TSH but the measure of the specific interaction with a Graves' IgG of proven potency must also be considered. Moreover, because of the variability between different membrane preparations, sequential clinical studies on individual patients, of the changes in concentration of Graves' IgG, must be performed using the same selected thyroid membrane preparation. We infer from these observations that the membrane structure in the vicinity of the TSH binding site is an important determinant of the interaction of Graves' IgGs with the TSH receptor, and that the configuration of this area is variable between individuals of the same species. The distinction between 'human-specific' and 'non-species-specific' thyroid stimulating antibodies is therefore probably not valid. The observation that the patient's own IgG was not often the most potent IgG inhibitor of binding of TSH also suggests that the Graves' IgG binding site is not identical or restricted to the TSH binding site; alternative explanations are discussed.