Hormonal responses during various phases of autoimmune adrenal failure: no evidence for 21-hydroxylase enzyme activity inhibition in vivo

Adrenal autoantibodies (ACA) are markers of adrenal cortex involvement in idiopathic Addison's disease. Recently the 21-hydroxylase (21-OH) enzyme has been discovered to be the major autoantigen of the ACA. A potential role of these antibodies in determining adrenal failure by inhibition of the 21-OH has been recently postulated. To test this hypothesis, cortisol and aldosterone (final products of adrenal steroid synthesis) and 17-hydroxyprogesterone (17-OH-progesterone) (as a marker of 21-OH impairment) have been investigated in baseline conditions and after ACTH (1-24) stimulation test in a group of 42 patients positive for both ACA and 21-OH autoantibodies. Patients were divided into five groups according to the stages (0-4) of adrenal failure. With progression toward overt Addison's disease, baseline 17-OH-progesterone, cortisol, and aldosterone remained almost unchanged but with impairment of their responses to ACTH (1-24) stimulation. The 17-OH-progesterone/cortisol ration remained normal both in basal conditions and after stimulation at stages 0-3. At stage 4 (overt Addison's disease), this ratio increased in baseline condition with no changes after ACTH (1-24), probably because of persistent 17-OH-progesterone gonadal production. In conclusion, there was a progressive and concomitant impairment of the synthesis of all steroids tested over various phases of adrenal failure. The pattern of response of the 17-OH-progesterone/cortisol ratio to ACTH stimulation in patients with 21-OH autoantibodies was not consistent with the autoantibodies inhibiting the 21-OH activity. This suggests that the inhibiting effect of 21-OH autoantibodies on 21-OH activity is not usually evident in vivo.

Autoantibodies to steroidogenic enzymes in autoimmune polyglandular syndrome, Addison's disease, and premature ovarian failure

Autoantibodies to steroidogenic enzymes, steroid 17 alpha-hydroxylase (17 alpha-OH), cytochrome P450 side-chain cleavage enzyme (P450scc), and steroid 21-hydroxylase (21-OH), were measured using specific and sensitive immunoprecipitation assays (IPAs) in patients with various forms of autoimmune adrenal disease. Autoantibodies to 17 alpha-OH were detected in 6 of 11 (55%) patients with autoimmune polyglandular syndrome (APS) type I, 8 of 24 (33%) patients with APS type II, 11 of 56 (20%) patients with adrenal cortex antibody (ACA; measured by immunofluorescence)-positive patients without Addison's disease, and only 3 of 64 (5%) patients with Addison's disease. Autoantibodies to P450scc were found at a prevalence similar to those to 17 alpha-OH: in 5 of 11 (45%) APS type I patients, 10 of 24 (42%) APS type II patients, 11 of 56 (20%) ACA-positive patients without Addison's disease, and only 6 of 64 (9%) patients of the Addison disease group. Autoantibodies to 21-OH were found in a majority of patients with APS type I (7 of 11;64%), APS type II (23 of 24; 96%), Addison's disease (41 of 64; 64%), and ACA-positive patients without Addison's disease (48 of 56; 86%). All sera that were positive for 17 alpha-OH or P450scc were also positive for 21-OH autoantibodies, except in 1 case. There was good agreement between the presence of ACA measured by immunofluorescence and 21-OH antibodies measured by IPA in all patient groups studied, and this indicates that 21-OH is a major autoantigen in adrenal autoimmune disease regardless of whether the disease presents as isolated Addison's disease or APS type I or type II. Autoantibodies to 17 alpha-OH and P450scc appeared to be the major components of the steroid-producing cell antibodies measured by immunofluorescence. No autoantibodies to 21-OH, 17 alpha-OH, or P450scc were detected in 17 sera from patients with premature ovarian failure without evidence of adrenal autoimmunity (as judged by immunofluorescence studies), except for 1 serum in which low levels of 17 alpha-OH antibodies were found. Overall, our studies indicate that 35S-labeled 17 alpha-OH, P450scc, and 21-OH can be used successfully in IPAs for their respective autoantibodies. Assays such as these may well be valuable in the immunological assessment of patients at risk for or suspected of adrenal autoimmunity.

Immunoprecipitation assay for autoantibodies to steroid 21-hydroxylase in autoimmune adrenal diseases

Adrenal autoantibodies characteristic of autoimmune Addison disease are directed towards steroid 21-hydroxylase (21-OH; EC 1.14.99.10). We describe a new assay to measure 21-OH autoantibodies (21-OH Abs), based on immunoprecipitation by the antibodies of 35S-labeled human 21-OH. Using this immunoprecipitation assay (IPA), we detected 21-OH Abs in 42 of 64 (66%) patients with Addison disease and in 14 of 19 (74%) patients with autoimmune polyendocrine syndromes type I and type II. No 21-OH Abs were detected by the IPA in any patients with Addison disease attributable to tuberculosis (n = 9) or adrenoleukodystrophy (n = 9) or in patients with autoimmune thyroid disease (n = 28), systemic lupus erythematosus (n = 10), myasthenia gravis (n = 10), rheumatoid arthritis (n = 10), or insulin-dependent diabetes mellitus (n = 12). None of the 26 sera from healthy normal blood donors was positive for 21-OH Abs by the assay. We found good agreement between 21-OH Abs measured by IPA and by Western blotting (r = 0.83, n = 123, P < 0.001). The inter- and intraassay CVs for IPA were well < 10% at high, medium, and low concentrations of 21-OH Abs. Overall, our studies indicate that the IPA provides a specific, sensitive, and convenient system for measuring 21-OH Abs.

Immunoprecipitation assay for autoantibodies to steroid 21-hydroxylase in autoimmune adrenal diseases

Adrenal autoantibodies characteristic of autoimmune Addison disease are directed towards steroid 21-hydroxylase (21-OH; EC 1.14.99.10). We describe a new assay to measure 21-OH autoantibodies (21-OH Abs), based on immunoprecipitation by the antibodies of 35S-labeled human 21-OH. Using this immunoprecipitation assay (IPA), we detected 21-OH Abs in 42 of 64 (66%) patients with Addison disease and in 14 of 19 (74%) patients with autoimmune polyendocrine syndromes type I and type II. No 21-OH Abs were detected by the IPA in any patients with Addison disease attributable to tuberculosis (n = 9) or adrenoleukodystrophy (n = 9) or in patients with autoimmune thyroid disease (n = 28), systemic lupus erythematosus (n = 10), myasthenia gravis (n = 10), rheumatoid arthritis (n = 10), or insulin-dependent diabetes mellitus (n = 12). None of the 26 sera from healthy normal blood donors was positive for 21-OH Abs by the assay. We found good agreement between 21-OH Abs measured by IPA and by Western blotting (r = 0.83, n = 123, P &lt; 0.001). The inter- and intraassay CVs for IPA were well &lt; 10% at high, medium, and low concentrations of 21-OH Abs. Overall, our studies indicate that the IPA provides a specific, sensitive, and convenient system for measuring 21-OH Abs.

Monoclonal thyroglobulin autoantibodies: variable region analysis and epitope recognition

A panel of human monoclonal thyroglobulin (Tg) autoantibodies (TgAAb) has been used to analyze autoantigenic determinants on human Tg and to investigate the relationship between variable (V) region gene sequences and epitope specificity. Two monoclonal TgAAb bound to the same (or closely related) epitope on Tg, and these were defined as type I TgAAb. Three other monoclonals bound to a different site and were defined as type II TgAAb. Inhibition studies with mixtures of type I and type II monoclonal TgAAb (Fab)2 preparations indicated that a mixture of the (Fab)2s almost completely inhibited (> 75%) labeled Tg binding to intact TgAAb in the sera of apparently healthy blood donors and patients with autoimmune thyroid disease (AITD). Type I TgAAb predominated in apparently healthy blood donors' sera, whereas type II TgAAb predominated in AITD sera. Analysis of V region gene sequences of the TgAAb indicated that a range of light chain and heavy chain genes from different gene families was used. Furthermore, the same germline genes that are used by TgAAb are also well represented in the genes coding for other self- and nonself-reactive antibodies. No homology in terms of light chain and heavy chain gene families, germline gene usage, or complementarity determining region sequences was observed in TgAAb directed to the same or closely related epitopes. Our studies show that TgAAb are directed to two major conformational epitopes on the Tg molecule and that the proportion of TgAAb directed to these epitopes in apparently healthy blood donors and that in patients with AITD appear to be different. TgAAb derived from different germline genes and with different complementarity determining region sequences can display similar epitope specificity, and this indicates that AAb directed to the same or a closely related epitope show considerable heterogeneity at the molecular level.

Autoimmune Addison's disease--evidence for a role of steroid 21-hydroxylase autoantibodies in adrenal insufficiency

Autoantibodies to steroid 21-hydroxylase (21-OH) are characteristic of adult onset Addison's disease and we have investigated the effects of these autoantibodies on recombinant human 21-OH enzyme activity. Antibody preparations from 11/11 Addison sera inhibited the ability of 21-OH to convert progesterone to deoxycorticosterone with 8 IgGs showing almost complete inhibition, 2 partial inhibition and 1 weak inhibition. Control IgGs from patients with autoimmune thyroid disease and normal blood donors had little or no effect on 21-OH activity. Our results suggest that 21-OH autoantibodies have the potential to contribute to adrenal failure in Addison's disease by inhibiting the 21-OH enzyme.

Naturally occurring mutations in human steroid 21-hydroxylase influence adrenal autoantibody binding

Human 21-hydroxylase (21-OH) genes containing various mutations, truncations, and deletions were expressed in yeast, and autoantibody binding was studied by Western blotting using patient sera and rabbit antibodies to 21-OH. 21-OH autoantibodies in 13 Addisonian sera showed a marked reduction in their ability to recognize 21-OH mutated at Pro453-->Ser (mean +/- SD, 31 +/- 9% of binding to wild type), whereas the effect on rabbit antibody binding was small (88 +/- 11% of binding to wild type; n = 7). Mutation at Arg339-->His had a less pronounced effect on autoantibody binding (85 +/- 11% of binding to wild type; n = 13) and caused a small enhancement of rabbit antibody binding (124 +/- 16% of binding to wild type; n = 7). These studies indicate that Pro453 has a key role in forming an autoantigenic epitope on 21-OH. It is important to note, however, that the Pro453 mutation caused only partial loss of autoantibody binding, i.e. all Addisonian sera studied still reacted with the mutated protein. This may indicate that each serum sample contains at least two different populations of 21-OH autoantibodies, only one of which recognizes a site dependent on Pro453. A series of more extensive modifications of the 21-OH sequence, including truncations (amino acids 460-494, 448-494, and 418-494) and deletions (amino acids 165-379, 142-240, and 142-280) indicated that most of the sequence of amino acids from 241-494 is important for autoantibody binding. The involvement of such an extensive region of the molecule suggests that the binding sites are generated by three-dimensional folding, with Pro453 having a critical role in forming at least one major autoantigenic epitope.

Cloning and characterisation of TPO autoantibodies using combinatorial phage display libraries

Thyroid lymphocyte RNA from a Hashimoto patient with high serum levels of autoantibodies to thyroid peroxidase (TPO) was used to construct a phage display antibody library in the phagemid vector pComb3. The library (100,000cfu) encoded IgG1 heavy chains together with kappa light chains. Selection of the phages displaying TPO antibody on TPO-coated ELISA plates yielded a phage population enriched for surface expression of TPO antibody Fabs. 3 different Fabs specific for TPO were subsequently isolated with affinities in the region of 10(9) molar-1. 2 of the Fabs recognised the same, or closely related, epitopes on TPO whereas the third Fab recognised a different epitope. These 2 epitopes were recognised by TPO autoantibodies in the serum of the lymphocyte donor and a series of 10 patient sera. Available sequence data showed that several non-self antibodies and non-thyroid autoantibodies use the same V kappa and VH germline genes as TPO autoantibodies. There appeared to be no clear relationship between gene sequence or gene family usage by TPO autoantibodies of the same or similar epitope specificity.

Autoimmune Addison's disease. Analysis of autoantibody binding sites on human steroid 21-hydroxylase

Human steroid 21-hydroxylase (21-OH) expressed in an in vitro translation system was found to react specifically with adrenal autoantibodies from patients with Addison's disease. The epitopes on 21-OH which reacted with autoantibodies were studied by incorporating a series of terminal and internal deletions into the 21-OH gene and analysing the expressed proteins by Western blotting. N-Terminal deletions up to amino acid 280 had no effect on autoantibody binding whereas a series of C-terminal deletions and truncations (amino acids 281-494) showed marked effects. Our results indicate that a central segment (281-379) and a C-terminal segment (380-494) of 21-OH interact to form at least one major autoantibody binding site.

Expression of human thyroid peroxidase in the yeasts Saccharomyces cerevisiae and Hansenula polymorpha

Saccharomyces cerevisiae and the methylotrophic yeast Hansenula polymorpha have been used to express both full-length and a large hydrophilic domain of human thyroid peroxidase (TPO). Expression of TPO in S. cerevisiae, using the natural signal sequence or the yeast alpha-mating factor (MF alpha) signal sequence, resulted in undetectable or very low levels of recombinant TPO production. However, TPO was expressed when the natural TPO leader sequence was replaced by the yeast STE2 signal sequence. This recombinant TPO reacted with both rabbit anti-human TPO polyclonal and mouse anti-human TPO monoclonal antibodies on Western blots. In the case of H. polymorpha, TPO expression was achieved when the natural TPO leader sequence was replaced by the MF alpha leader and the construct placed under the control of the methanol-regulated promoter from the methanol oxidase gene. The recombinant TPO produced in H. polymorpha reacted with both TPO polyclonal and TPO monoclonal antibodies. No TPO was produced when the signal sequence of SUC2 (invertase) or the TPO natural signal sequence was used to direct expression.

Is TPO detectable in the circulation?

Recent reports have suggested that thyroid peroxidase (TPO) can be detected in the circulation of normal subjects and of patients with Graves' disease and we have attempted to confirm and extend these observations. A TPO radioimmunoassay with a sensitivity of 1 ng/mL was used to measure TPO in the sera from 20 normal subjects and 21 patients with Graves' disease. In addition, TPO was measured in serum samples from six normal subjects before and after oral TRH. We were unable to detect TPO in 46 out of the 47 sera studied (normals and autoimmune thyroid disease). In the one remaining serum (from a normal subject), low levels of TPO were apparently detected, but we demonstrated that this result was due to assay interference from TPO autoantibodies. Overall our studies suggest that (1) thyroid peroxidase is not detectable in normal subjects nor in TPO autoantibody negative patients with Graves' disease; (2) endogenous TPO autoantibodies can interfere in the TPO radioimmunoassay leading to false positive results; and (3) an acute increase of TSH in normal subjects does not result in TPO release into the circulation.

Production and characterisation of a human monoclonal thyroid peroxidase autoantibody

A human-mouse hybridoma has been produced by fusion of Hashimoto thyroid lymphocytes with the mouse myeloma line X63-Ag8.653. The cloned hybridoma secreted 2.5 micrograms per 10(6) cells per day of an IgG kappa thyroid peroxidase (TPO) autoantibody (2G4) with high affinity (2.5 x 10(9) molar-1) and specificity for human TPO. 2G4 did not react with lactoperoxidase, horseradish peroxidase or human myeloperoxidase or with porcine TPO or with human thyroglobulin. Plastic tubes coated with 2G4 bound about 50% of 125I-labelled human TPO added and the binding was inhibited by IgGs prepared from 18/18 TPO autoantibody-positive sera. This indicated that all 18 sera contained autoantibodies which recognised the same (or closely related) epitope as 2G4. Plastic tubes coated with IgGs from different TPO autoantibody-positive patient sera also bound 125I-labelled TPO but inhibition by 2G4 in this system was not complete. This suggested that the sera contained at least 2 types of TPO autoantibodies, with only one type of autoantibody reactive with the same epitope as 2G4.

Steroid 21-hydroxylase is a major autoantigen involved in adult onset autoimmune Addison's disease

An adrenal-specific protein reacting with autoantibodies in the sera of patients with adult onset Addison's disease has been purified from human adrenal glands. The protein, mol.wt. 55K, has the biochemical characteristics of steroid 21-hydroxylase and reacts on Western blots with rabbit antibodies to recombinant 21-hydroxylase. Absorption of the native human 55K adrenal protein with human adrenal autoantibodies prevented the subsequent reaction of the 55K protein with rabbit antibodies to 21-hydroxylase in Western blot analysis. In addition, human adrenal autoantibodies reacted with recombinant 21-hydroxylase expressed in yeast. These data indicate that the adrenal specific enzyme steroid 21-hydroxylase is a major autoantigen involved in adult onset autoimmune Addison's disease.

Cloning of a human autoimmune response: preparation and sequencing of a human anti-thyroglobulin autoantibody using a combinatorial approach

Thyroid lymphocyte RNA from a Hashimoto patient exhibiting high titre serum IgG autoantibodies against thyroglobulin (Tg) has been used to construct a Fab library in phage lambda. Screening of this library with radioiodinated Tg has permitted the cloning of an anti-Tg antibody (MH52) with an affinity of 4.5 x 10(9) molar-1 as determined by inhibition ELISA. Sequence analysis showed MH52 to be an authentic antibody of the IgG1/K isotype with variable region genes from the VHI and VKIII families in combination with the JH3, DK4 and JK2 gene segments. The MH52 light chain gene showed high sequence homology (93%) with the germline gene used by several rheumatoid factors and some DNA autoantibodies. Greater divergence from the germline was observed in the case of the MH52 heavy chain gene which showed 86% homology with a germline heavy chain gene isolated from human liver. Overall the similarity between the genes coding for MH52 and the genes coding for some other autoantibodies of non-related specificity might suggest that similar regulatory processes control the formation of these different autoantibodies.

Analysis of carbohydrate residues on human thyroid peroxidase (TPO) and thyroglobulin (Tg) and effects of deglycosylation, reduction and unfolding on autoantibody binding

The contribution of carbohydrate residues and peptide chain conformation to autoantibody binding sites on human thyroid peroxidase (TPO) and thyroglobulin (Tg) has been investigated. In addition the nature of carbohydrate residues associated with human TPO has been studied. 125I-labelled human TPO and Tg were treated with the following glycosidases: EndoD, EndoH, neuraminidase, O-glycanase, neuraminidase followed by O-glycanase and PNGaseF. Thereafter binding to different sera containing TPO autoantibodies and Tg autoantibodies was assessed using solid phase protein A to separate antibody-bound and free labelled antigens. In addition, labelled Tg and TPO were treated with reducing agent (dithiothreitol) or sodium acetate buffer pH 7.5, 5.5 and 3.2 (followed by neutralisation with 2 M Tris pH 8.3) prior to antibody binding studies. Furthermore, the effect of deglycosylation and treatment with acid buffers on TPO enzyme activity was studied. The nature of carbohydrate residues associated with hTPO was analysed by assessment of the effects of different glycosidases on 125I-TPO mobility on SDS-PAGE followed by autoradiography and by the use of lectins. Deglycosylation of labelled Tg and TPO had no clear effect on Tg and TPO autoantibody binding. Reduction of labelled Tg and TPO resulted in almost complete loss of autoantibody binding with all sera studied. Furthermore, adjusting the pH of labelled TPO or Tg transiently to pH 5.5 lowered autoantibody binding in the case of all the sera and the effect was more marked at pH 3.2. TPO enzyme activity (guaiacol assay) of unlabelled TPO was decreased after treatment with EndoH but not with other glycosidases. The low pH buffers affected unlabelled TPO enzyme activity measured by iodide assay. Treatment of 125I-labelled TPO with EndoH, neuraminidase and PNGaseF caused marked changes in the double band pattern characteristic of TPO on analysis by SDS gel electrophoresis (TPO doublet). Analysis of changes in the mobility of the 2 bands of the doublet after treatment with different glycosidases and binding studies with lectins indicated that both high mannose and complex type sugar residues were associated with hTPO. The high mannose type residues were associated mostly with the lower band of the hTPO doublet whereas complex type residues were associated mostly with the upper band. Overall, our studies indicate that (1) the major autoantibody binding sites on hTPO and hTg are conformational, (2) sugar residues do not appear to be important in forming the autoantibody binding sites on hTPO and hTg, and (3) both high mannose type and complex type sugar residues are associated with hTPO.

Analysis of TSH receptors and microsomal antigen in different human thyroid tissue specimens

Affinity labelling with a 125I-labelled photoactive derivative of TSH (HSAB-TSH) was used to analyse TSH receptor size in the following specimens of human thyroid tissue: (1) cold nodules; (2) autonomous nodules; (3) papillary carcinoma; (4) medullary carcinoma; (5) metastasis of papillary carcinoma to lymph node; (6) anaplastic carcinoma, and (7) Graves' thyroid. In addition, a sample of histologically normal thyroid tissue surrounding specimens 1-4 was analysed in each case. Thyroid microsomes were also prepared from the tissue samples, solubilized using 1% deoxycholate and labelled with 125I. The preparations were immunoprecipitated using microsomal autoantibodies and protein A and analysed by SDS-PAGE and autoradiography. These studies indicated that no differences in the characteristics of the TSH receptor or of microsomal antigen were observed in the tissue samples 1-3 and 7. Neither protein was detected in tissue specimens 4-6.

Immunoprecipitation of human adrenal microsomal antigen

Human adrenal microsomes have been labelled with 125I and immunoprecipitated with sera from patients with Addison's disease. The immunoprecipitates were then analysed by SDS-PAGE and autoradiography. 13 of the 23 sera from the Addison patients studied contained antibodies which reacted with a 55 kDa adrenal microsomal protein. The same 13 sera were also positive for adrenal antibodies as judged by immunofluorescence. The 55 kDa protein was not immunoprecipitated from placenta or thyroid microsomes by Addison sera. Furthermore, patients with Graves' disease or rheumatoid arthritis did not immunoprecipitate the 55 kDa protein from adrenal microsomes. Our studies suggest therefore that Addison sera contain antibodies to a 55 kDa adrenal specific protein which may well be the antigen observed on immunofluorescence.

Autoantibodies to the thyrotropin receptor

This review considers recent developments in our understanding of the properties of TRAb, particularly measurement of the antibodies and their sites of action and synthesis. Two new assay methods have allowed considerable improvements in the sensitivity, specificity, precision, and ease of measuring TRAb. In particular: 1) receptor assays based on inhibition of receptor-purified labeled TSH binding to detergent-solubilized TSH receptors and 2) bioassays based on stimulation of cAMP release from monolayer cultures of isolated thyroid cells. Detailed studies with the two assays indicate that TSH receptor antibodies nearly always act as TSH agonists in patients with a history of Graves' hyperthyroidism. Studies in areas of dietary iodine sufficiency suggest that measurement of the antibodies at various stages in the course of treating Graves' disease can be of value in predicting the outcome of therapy. However, in areas of iodine deficiency, difficulties in the ability of patients' thyroid tissue to recover from the effects of antithyroid drugs may prevent the receptor antibodies from causing a relapse of thyrotoxicosis. Consequently, the predictive value of receptor antibody measurements would be expected to be lower in these geographical areas. Although patients with a history of Graves' hyperthyroidism nearly always have TRAb which act as TSH agonists, about 20% of patients with frank hypothyroidism due to autoimmune destruction of the thyroid have TRAb which act as TSH antagonists (blocking antibodies). There is some evidence that these blocking antibodies can cause hypothyroidism particularly in the neonate. With regard to the site of synthesis of TRAb, there is now direct evidence that they are synthesized by thyroid lymphocytes, particularly the lymphocytes in close proximity to thyroid follicular cells. This is consistent with the well established effects of antithyroid treatment (drugs, radioiodine, or surgery) on TRAb levels in addition to their effects on thyroid hormone synthesis. Recent studies using affinity labeling with 125I-labeled TSH have enabled elucidation of the structure of the TSH receptor. TSH receptors in human, porcine, and guinea pig thyroid tissue have a two-chain structure in which the TSH binding site is formed on the outside surface of the cell membrane by a water-soluble A subunit (Mr approximately 50 K). The A subunit is linked by a disulfide bridge and weak noncovalent bonds to the amphiphilic B subunit (Mr approximately 30 K). This subunit, which penetrates the lipid bilayer, probably forms the site for interaction of the receptor with the regulatory subunits of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Antibodies to membrane antigens in autoimmune thyroid disease

The possibility that sera from patients with autoimmune thyroid diseases contain autoantibodies to thyroid membrane proteins distinct from microsomal antigen and the TSH receptor has been investigated using (a) solid phase assay system based on human thyroid membranes and 125I-labelled protein A and (b) immunoprecipitation of detergent solubilized 125I-labelled thyroid membranes followed by gel electrophoresis and autoradiography. In the solid phase assay binding to membranes showed a highly significant correlation with binding to microsomes (r = 0.82; P less than 0.001; N = 82) indicating that the interaction between the serum and the membranes was due principally to microsomal antibody binding to microsomal antigen contaminating the membrane preparations. However, there were some discrepancies suggesting that an additional antigen-antibody system was involved. This possibility was then investigated using immunoprecipitation of 125I-labelled thyroid membranes. A labelled protein with mol wt 54 K was specifically immunoprecipitated (relative to normal pool serum) by 3 out of 4 sera from patients with Graves' disease who showed high binding to thyroid membranes. A further 4 sera from such patients with low membrane binding affinity failed to immunoprecipitate the 54 K protein. Sera from some patients with Hashimoto's disease and some patients with rheumatoid arthritis and one patient with Addison's disease also immunoprecipitated the 54 K protein from solubilized thyroid membranes. These studies suggested that antibodies interacting with the 54 K protein contributed to the discrepancies between thyroid membrane and microsome binding. However, the 54 K protein was also immunoprecipitated from detergent solubilized membranes prepared from human placenta, skeletal muscle and adrenal tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

The TSH receptor: structure and interaction with autoantibodies in thyroid disease

Studies of the TSH receptor using affinity labelling with photoactive derivatives of TSH and analysis by SDS-PAGE have shown that the receptor contains 2 subunits (A and B), linked by a disulphide bridge. Similar results are obtained with TSH receptors from human, porcine and guinea pig thyroid tissue and from guinea pig fat. Analysis of affinity labelled receptors under non-denaturing conditions suggest that subunits additional to the A and B subunits are not present. Hydrodynamic measurements indicate that the receptor A subunit has an approximately spherical structure (Stokes' radius 70 A) and when this interacts with TSH (an elongated structure with Stokes' radius 56A) a very elongated complex (Stokes' radius 104A) is formed. Isoelectric focusing studies of the TSH receptor A subunit, TSH and TSH receptor antibodies indicate that charge-charge interactions are of considerable importance in the binding of hormone and antibody to the receptor.

Photoaffinity labelling of the TSH receptor on FRTL5 cells

An investigation of the properties of TSH receptors on FRTL5 cells using affinity labelling with a 125I-labelled photoactive derivative of TSH is described. Our studies suggest that FRTL5 cells contain 2 principal types of cell surface TSH receptors. One form, probably a precursor, consists of a single polypeptide chain (Mr 120,000) with an intrachain loop of amino acids formed by a disulphide bridge. The other type of receptor consists of a water-soluble A chain (Mr 55,000) linked to an amphiphilic B chain (Mr 35,000) by a disulphide bridge. The 2 chain structure is probably derived from the single chain 120,000 protein by enzymatic cleavage of peptide sequences within the loop of amino acids formed by the intrachain disulphide bridge.

Assessment of the shape and molecular size of TSH-TSH receptor complexes

Photoaffinity labelling and analysis under denaturing conditions (SDS-PAGE) have shown that the porcine TSH receptor contains an A subunit (Mr = 47,000) which forms the binding site for TSH and a B subunit (Mr = 25,000) linked to the A subunit by a disulphide bridge. In order to assess the size and shape of the receptor under non-denaturing conditions we have solubilized photoaffinity-labelled porcine TSH receptors using the small micelle-sized detergent sodium deoxycholate and analysed the preparations by sucrose density gradient centrifugation and gel filtration. Under these conditions, the cross-linked TSH-TSH receptor complex showed an S20,w of 6.4 S and a frictional ratio f/f0 of 1.8. These values were consistent with those which might be expected from an elongated protein complex with a molecular weight of about 100,000 (the value obtained by SDS-PAGE). Analysis of another thyroid membrane protein, human thyroid microsomal antigen (Mr = 110,000 by SDS-PAGE) under the same conditions gave an S20,w of 6.0 S and f/f0 = 1.3, suggesting that this protein has a compact structure. The TSH receptor A subunit cross-linked to TSH (Mr = 70,000 by SDS-PAGE) gave an S20,w of 4.6 S and f/f0 = 1.8 and these values could be compared with those obtained for the A subunit alone (S20,w = 3.6 S; f/f0 = 1.4; Mr by SDS-PAGE = 47,000) and TSH alone (S20,w = 2.6 S; f/f0 = 1.6; Mr = 28,000.(ABSTRACT TRUNCATED AT 250 WORDS)