Radioimmunological characterization of human thyrotropin and its subunits: applications for the measurement of human TSH

Structure and structure-function relationships in glycoprotein hormones

Relationships between the sequences of thyrotropin (thyroid-stimulating hormone, TSH), lutropin (luteinizing hormone, LH), human choriogonadotropin (chorionic gonadotropin, hCG) and follitropin (follicle-stimulating hormone, FSH) are now well established. Each beta-subunit contains six disulphide bonds and considerable homology is seen when all four linear sequences are aligned with half-cystine residues in juxtaposition. Major questions about the tertiary structures of the subunits and their interactions to form active hormone remain. Determination of the disulphide bridges in both alpha- and beta-subunits has not yielded to usual methods and conflicting data about the alpha-subunit have been reported. Partial reduction of the beta-subunits of LH and TSH with subsequent labelling of the cysteines formed has shown that a single bond is first reduced. This bond is between positions 93 and 100 in LH-beta and the corresponding positions 88-95 in TSH-beta. Thus, as would be expected from the fact that interhormone hybrids can be made with the common alpha-subunits, the chemical data, though still limited, indicate similar tertiary structures for the different beta-subunits. To investigate whether other useful intermediates can be obtained after partial reduction, we have studied reduction and derivative formation in various conditions. Intact LH is more resistant to reduction than either its alpha- or beta-subunit but no intermediates have been observed which are not present after partial reduction of individual subunits. Preliminary experiments on the reoxidation of fully reduced alpha-subunit show that the reoxidized material will recombine with native beta-subunits to yield biologically active TSH or LH. Studies from this and other laboratories on chemical modifications of several amino acid residues of glycoprotein hormones and their subunits are also summarized.

Differences in resting metabolic rate between paraplegic and able-bodied subjects are explained by differences in body composition

BACKGROUND

Little is known about the relation between body composition and energy metabolism in paraplegia.

OBJECTIVE

We investigated the relation between body composition and energy metabolism in healthy paraplegics as compared with able-bodied control subjects. We hypothesized that paraplegics would have lower fat-free mass (FFM), body cell mass (BCM), resting metabolic rate (RMR), and thermic effect of feeding (TEF).

DESIGN

This cross-sectional study included 34 control subjects and 28 paraplegics (mean age: 29.1 +/- 7.6 and 33.9 +/- 9.2 y, respectively) with body mass indexes (in kg/m(2)) of 23.5 +/- 1.8 and 24.3 +/- 6.0, respectively. We measured RMR and TEF with indirect calorimetry, total body water with deuterium dilution, and extracellular water with corrected bromide space. We calculated FFM (total body water/0.732) and BCM [(total body water - extracellular water)/0.732)].

RESULTS

FFM was higher in control subjects than in paraplegics (77.2 +/- 7.2% and 69.2 +/- 8.7%, respectively; P = 0.0002), as were BCM (47.4 +/- 6.7% and 35.9 +/- 8.1%, respectively; P < 0.0001) and RMR (7016 +/- 935 and 6159 +/- 954 kJ/d, respectively; P = 0.0007). FFM was the single best predictor of RMR in both groups (r(2) = 0.83 for control subjects and 0.70 for paraplegics, P < 0.0001 for both). RMR adjusted for FFM did not differ significantly between control subjects and paraplegics (6670 +/- 504 and 6588 +/- 501 kJ/d, respectively). TEF also did not differ significantly between control subjects and paraplegics (6.25 +/- 2.2% and 5.53 +/- 1.8% of energy intake, respectively).

CONCLUSIONS

FFM, BCM, and RMR, but not obligatory TEF, are lower in paraplegics than in control subjects. RMR does not differ between control and paraplegic subjects after adjustment for FFM, indicating similar metabolic activity in the fat-free compartment of the body.

Biological aspects of pituitary tumors induced by synthetic salmon calcitonin (TZ-CT) in Sprague-Dawley rats. Immunohistochemical and ultrastructural studies

Rat pituitary tumors induced by synthetic salmon calcitonin (TZ-CT) were studied by the indirect peroxidase-labeled antibody method, together with ultrastructure and serum hormone measurement. Immunohistochemically, TZ-CT-induced pituitary tumors showed staining for only rLH alpha subunit, and were negative for other peptide hormones including GH, PRL, alpha MSH and ACTH, and the beta subunit of glycoprotein hormones. Electron microscopic examination showed that the majority of tumor cells possessed numerous small secretory granules, 100 to 200 nm in diameter. The serum PRL concentrations of rats with TZ-CT-induced pituitary tumors were markedly elevated, but not beyond 130 ng/ml. From our data, TZ-CT-induced pituitary tumors are considered to be endocrinologically inactive and to produce alpha subunit. Furthermore, these tumors are thought to be potentially useful models of alpha subunit-producing pituitary tumors in humans. This is the first report to document the tumorigenesis of alpha subunit-producing pituitary tumors in rats after long-term treatment with calcitonin.

Evaluation of the human gonadotroph free alpha-subunit secretory pools by administration of gonadotropin hormone-releasing hormone into normal subjects at different phases of the ovarian cycle

The specificity of a monoclonal antibody RIA for the measurement of free alpha-subunit in plasma is presently documented. This RIA was used to explore the pituitary gonadotroph free alpha-subunit reserve in normal ovulating women stimulated with gonadotropin hormone-releasing hormone (GnRH). RIA specificity was established by means of competitive inhibition curves with various glycoprotein hormone preparations, and Sephadex G-100 exclusion chromatography of purified hFSH (1-2), hLH (LER 960) and pituitary extract. The 3.8% and 5.5% crossreactivities of hFSH 1-2 and hLH LER 960 were shown by exclusion chromatography to result in part from free alpha-subunit contamination in these purified preparations. Pituitary extract chromatography indicated hFSH and hLH cross-reactivity below 2.5% and 1.5%, respectively. Normal females were stimulated with GnRH throughout the cycle: 3 tests were performed on Day 7, 1 test on Day 13, 16, 17 and 22, respectively, and 2 tests on Day 24. GnRH stimulation consisted of an initial 100 micrograms bolus (time 0) followed at 2 h by a 12.5 micrograms/h constant infusion, and a second 100 micrograms bolus at 5 h. In all subjects, baseline free alpha-subunit values were below 2 ng/ml. Total free alpha-subunit secretion was markedly enhanced in subjects Day 13 and 16, in concert with total hLH and hFSH secretion. In the three subjects Day 7, free alpha-subunit was released only after the second GnRH bolus. In periovulatory subjects, free alpha-subunit secretion became apparent after the initial bolus and with constant GnRH infusion. In the three subjects Day 22 and 24, peak levels of free alpha-subunit were obtained after the second GnRH bolus.(ABSTRACT TRUNCATED AT 250 WORDS)

Radioimmunoassay of human thyrotropin: analytical and clinical developments

Recent modifications of the radioimmunoassay systems for TSH have greatly extended the clinical utility of the measurement of this hormone, so that its use is no longer limited to the diagnosis of primary hypothyroidism. The newer assays provide improved sensitivity and specificity, such that it is now possible to distinguish TSH levels that are within the normal range from those that are suppressed, e.g., in thyrotoxicosis. New vistas of clinical applications are being revealed as we improve our understanding of human thyroid physiology and pathophysiology. It is the purpose of this communication to summarize information about the improved TSH radioimmunoassay, to demonstrate the new observations available regarding TSH concentrations in various normal and diseased conditions, and finally, to illustrate the various ways in which the assay provides more accurate guidance in the clinical diagnosis and management of thyroid and nonthyroid disease.

Pure alpha subunit-secreting pituitary tumors

The authors describe six patients with pituitary macroadenomas hypersecreting only the alpha subunit of the glycoprotein hormones. These patients had been previously diagnosed as having "non-functioning chromophobe adenomas." All of the patients had visual field abnormalities and partial hypopituitarism. The elevated serum alpha concentrations showed a variable response to stimulation by thyrotropin-releasing hormone, and could not be suppressed by thyroid hormone administration. Immunological, gel chromatographic, and immunocytochemical studies documented that only the alpha subunit was present. Following pituitary surgery and radiotherapy, serum alpha levels decreased. These patients represent a new subset of functioning pituitary tumors. Determination of alpha subunit concentration is useful in managing some patients with pituitary tumors previously thought to have non-functioning chromophobe adenomas.

Plasma immunoreactive TSH: spurious elevation due to antibodies to bovine TSH which cross-react with human TSH

A patient with thyroid carcinoma treated by thyroidectomy who received multiple injections of bovine (bTSH) exhibited elevated and nonsuppressible levels of plasma "immunoreactive TSH." Antibodies to TSH of the IgG class which bound bTSH and human TSH (hTSH) were demonstrated using specific radioimmunoassay and radioimmunoelectrophoretic techniques. Antibodies were present for more than one year and binding of bTSH was greater than that of hTSH throughout this period. Characterization of the antibodies with respect to the binding of human and bovine glycoprotein hormones and subunits revealed two populations of antibodies, one of which bound both bTSH and hTSH and the other which bound only bTSH. Both antibodies appeared to be directed toward antigenic sites on the beta subunit of TSH as both hTSH-beta and bTSH-beta displaced the binding of intact TSH from antibodies whereas the alpha subunits were virtually unreactive. The binding studies suggest that the cross-reactivity of the antibody to hTSH occurred on the basis of common antigenic determinants on the beta subunits of the two species. Documentation of a true elevation of plasma hTSH was achieved by gel filtration of plasma in which a peak of immunoreactivity co-eluting with [125I]-hTSH could be shown. Evidence for suppression of TSH secretion by thyroxine administration in the presence of interfering antibody was obtained by demonstrating that the level of plasma "immunoreactive TSH" did not change in response to TRH administration.

Pure alpha-secreting pituitary adenomas

Isolated hypersecretion of the alpha subunit of the glycoprotein hormones occurred in two men with previously diagnosed "nonfunctioning chromophobe adenomas." The alpha hypersecretion was unresponsive to hypothalamic releasing hormone, thyroid hormone, and sex-steroid hormones. After trans-sphenoidal surgery and conventional pituitary irradiation, alpha secretion was decreased. Increased quantities of immunologically active and biologically inactive luteinizing hormone (LH) material were detected in serum and in tumor homogenate. Immunologic and gel-chromatographic studies determined that only the alpha subunit was present and that it was cross-reacting in the LH immunoassay. These studies suggest that the alpha subunit may be a useful marker of pituitary tumors, particularly in patients without clinical evidence of hormonal hypersecretion.

Metabolic clearance and secretion rates of subunits of human thyrotropin

Metabolic clearance rates (MCR) of the alpha and beta subunits of human thyrotropin (hTSH-alpha and hTSH-beta) were determined by a constant infusion to equilibrium method. In 15 normal individuals (six men, six premenopausal women, and three post-menopausal women), the mean MCR of hTSH-alpha (68 ml/min per m2) was significantly faster than that of hTSH-beta (48 ml/min per m2) was significantly faster than that of hTSH-beta (48 ml/min per m2); both were two to three times more rapid than the previously determined MCR of hTSH. In patients with primary hypothyroidism, MCR were significantly slower with a mean value of 55 ml/min per m2 for hTSH-alpha and 37 ml/min per m2 for hTSH-beta. However, MCR of subunits were not significantly faster than normal in hyperthyroid patients. Serum concentrations of alpha subunits and hTSH-beta were measured by radioimmunoassay, and secretion rates of alpha and hTSH-beta from the pituitary were calculated using hTSH-alpha and hTSH-beta MCR, respectively. In the normal individuals, alpha secretion rates averaged 91 mug/day per m2, greater than those previously determined for hTSH and human follicle-stimulating hormone. Alpha secretion rates were significantly elevated in the normal postmenopausal women (211 mug/day per m2) and in the premenopausal hypothyroid women (202 mug/day per m2); they were also elevated in the postmenopausal hypothyroid women (277 mug/day per m2). Alpha secretion rates were significantly decreased in the premenopausal hyperthyroid women (66 mug/day per m2). Usually, the secretion rates of hTSH-beta could not be calculated in normal individuals, and the rates in hyperthyroid patients could never be calculated because serum hTSH-beta was not detected. Six normals had detectable hTSH-beta secretion rates (17 mug/day per m2); hTSH-beta secretion rates were significantly increased in patients with primary hypothyroidism (28 mug/day per m2). Although we had previously demonstrated a 50-fold increase in hTSH secretion rates in primary hypothyroidism, there was only a 2-fold increase in alpha and hTSH-beta secretion rates. Thus, increased subunit synthesis appears to be utilized predominantly for production of complete hTSH.

Evaluation of the specificity of a luteinizing hormone beta and alpha subunit assay

A non-specific antihuman FSH antiserum which is able to bind 125I-labelled FSH, LH, LHbeta and LHalpha subunits, has been shown to contain at least three different antibody populations directed against different parts of the LH and FSH molecule. This antiserum has been used to develop a rather specific radio-immunoassay for determination of the common alpha subunit. Further, by gel filtration it is shown that the LH IRC2/69 preparation contains about 30% of intact LH and about 10% of alpha and LHbeta subunit, respectively. In contrast, the LH 68/40 preparation contains about 96% of intact LH and only small amounts of alpha and LHbeta subunit. Also, a considerable difference in the content of the alpha sub-unit was found between two different FSH preparations. The contamination of the preparations of the intact hormones is shown to have a marked influence on the cross-reactivity of the substances in an LHbeta and alpha subunit radioimmuno-assay.

Secretion of alpha and beta subunits of TSH by the anterior pituitary

We have attempted to determine whether blood concentrations of the alpha chains of TSH arise by direct pituitary secretion of alpha chain or by degradation of the intact hormone after secretion. Highly purified human TSH was administered by infusion to euthyroid volunteers; blood TSH concentrations were raised to 36 muu/ml. After TSH infusion, circulating blood alpha chain concentrations did not change. The same volunteers received an infusion of TRH which stimulated pituitary secretion of TSH, raising blood concentrations to 15 muu/ml in one series of subjects and to 23 muu/ml in a second series. After TRH, blood concentrations of alpha chain increased from undetectable to over 400 pg/ml. We conclude that alpha subunits of TSH are directly secreted by the pituitary gland and are not formed by degradation of intact TSH in the peripheral circulation.

Radioimmunoassay and the hormones of thyroid function

Radioimmunoassay (RIA) has provided the tools for wide-reaching investigations that have changed and continue to change many important concepts of thyroid physiology and pathosphysiology. The RIA vor human thyrotropin (TSH) was developed in 1965; development of the RIA for triiodothyronine (T3), thyroxine (T4), thyroxine-binding globulin (TBG), and, recently, thyrothropin-releasing hormone (TRH) and thyroglobulin (Tg) followed. The capacity to measure nanogram and picogram concentrations with relative ease and speed has permitted the demonstration of dynamic relationships of the intrathyroidal and circulating thyroid hormones to each other and to the pituitary and hypothalamic regulating hormones. Evidence for the presence of cross-influences between TRH and other hypothalamic regulating hormones on the secretion of pituitary hormones has accumulated. The impact of the new information on clinical practice is now becoming evident. There is new appreciation of the value of assaying serum T3 and TSH concentrations in the clinical management of patients with disturbed function of the thyroid, pituitary, or hypothalamus. The necessary components for RIA performance can be purchansed separately or in kit form from commercial sources. With appropriate quality-control procedures, precise, sensitive, and reliable data can be generated. Awareness of the specific technical problems relating to the RIA of these hormones is absolutely necessary to assure reliable results. The availability of kits or their components permits the performance of these studies in the community hospital and in reliable commercial-service laboratories.

Thyroid stimulators in health and disease

Thyroid function in health is largely controlled by thyroid-stimulating hormone (TSH) produced by the thyrotroph cells of the anterior pituitary. Alterations of intrathyroidal iodine content may also be involved in autoregulation by the thyroid, particularly in states of iodine deficiency and excess (Ingbar, 1972). During pregnancy placental thyroid-stimulating agents-human chorionic thyrotrophin (HCT) and human molar thyrotrophin (HMT)-are produced. In the common variety of hyperthyroidism, Graves' disease, thyroid function appears to be regulated by thyroid-stimulating immunoglobulins (TSI) (Smith & Hall, 1974a). Human molar thyrotrophin can be produced by trophoblastic tumours in both sexes and thyroid-stimulating factors have been reported to be released by malignant tumours of the gut and by bronchogenic carcinomas, although this is one of the rarest of the ectopic hormone syndromes.