The effects of synthetic alpha-subunit peptides on thyrotropin interaction with its receptor

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Binding of immunoglobulin G from patients with autoimmune thyroid disease to rat sodium-iodide symporter peptides: evidence for the iodide transporter as an autoantigen

The recent cloning of the rat sodium-iodide symporter (rNIS) from FRTL-5 cells makes possible studies of the role of this thyroid-specific protein as an antigen in autoimmune diseases of the thyroid (AITD). We generated 21 synthetic peptides replicating the entire sequence of the extramembranous domains (ExMD) of rNIS. Each was synthesized by automated chemistry, purified by high-pressure liquid chromatography (HPLC), and characterized by mass spectroscopy. Immunoglobulins were purified using protein A from serum of 27 patients with Graves' disease (GD), 27 patients with autoimmune hypothyroidism (HT), and 20 normal controls. Binding of IgG from patients and controls to each of the rNIS peptides was measured by enzyme-linked immunosorbent assay (ELISA). Binding of patient IgG significantly greater than control was observed with six peptides: peptide 262-280 (representing ExMD 8 between transmembrane [TM] domains VII and VIII), peptide 437-444 (ExMD 11), peptides 468-487, 483-602, and 498-517 from ExMD 12, and peptides 560-579 from the proximal portion of the carboxyl terminus (ExMD 13). 63% of GD patients and 26% of HT patients immunoglobulin G (IgG) bound peptide 498-517 compared to zero controls. Similarly, 59% of GD were positive against peptide 468-487 versus zero controls. Peptide 262-280 bound IgG from 44% of GD patients, 15% of HT patients, and none of the controls. The remaining peptides showed little or no binding of patient IgG. These data indicate that patients with GD and HT possess antibodies that recognize rNIS significantly greater than do normal individuals, suggesting that the iodide transporter represents an important autoantigen in AITD. They further suggest that the incidence of the antibodies is higher in GD than HT, and that the antigenic epitopes involve ExMD 8, 11, 12, and 13.

Chimeric proteins can exceed the sum of their parts: implications for evolution and protein design

Chimeric analogs derived from pairs of homologous proteins routinely exhibit activities found in one or both parents. We describe chimeras of two glycoprotein hormones, human chorionic gonadotropin (hCG) and human follitropin (hFSH), that exhibit activity unique to a third family member, human thyrotropin (hTSH). The results show that biological activity can be separated from hormone-specific amino acid residues. This is consistent with a model for the evolution of homologous ligand-receptor pairs involving gene duplication and the creation of inhibitory determinants that restrict binding. Disruption of these determinants can unmask activities characteristic of other members of a protein family. Combining portions of two ligands to create analogs with properties of a third family member can facilitate identifying key determinants of protein-protein interaction and may be a useful strategy for creating novel therapeutics. In the case of the glycoprotein hormones, this showed that two different hormone regions (i.e., the seat-belt and the intersubunit groove) appear to limit inappropriate contacts with receptors for other members of this family. These observations also have important caveats for chimera-based protein design because an unexpected gain of function may limit the therapeutic usefulness of some chimeras.

Structure-function studies of human TSH: new advances in design of glycoprotein hormone analogs

Recent progress in structure-function studies of glycoprotein hormones has provided new insights into the molecular mechanisms of action of these hormones and has further supported the concept that physiological modulation of assembly, bioactivity, and clearance of these hormones is dependent on specific structural components. This review emphasizes current advances in the structure-function relationships of human TSH, which have contributed to further elucidation of common and hormone specific features within the glycoprotein hormones family. Novel strategies are now being applied to investigate the role of individual structural elements. The principks discovered in such studies are essential to understand the physiological regulation of hormone bioactivity and allow for the rational design of novel analogs with potential therapeutic applications.

Secondary structure prediction of beta-subunits of the gonadotropin-thyrotropin family from its aligned sequences using environment-dependent amino-acid substitution tables and conformational propensities

The secondary structures of beta-subunits of the glycoprotein hormone family, LH (luteinizing hormone), CG (chorionic gonadotropin), FSH (follicle stimulating hormone), TSH (thyroid stimulating hormone), and GTH I/GTH II (two types of fish gonadotropins), are predicted by comparing an amino-acid substitution pattern at equivalent sites in their aligned sequences with environment-dependent amino-acid substitution tables and conformational propensities calculated from other protein families whose three-dimensional structures are known. According to the prediction results, together with other structural information obtained from experiments, the following points come up as important structural features of the beta-subunits of this family; The regions assigned to regular secondary structures (one alpha-helix and three beta-strands) are considered to constitute a core of the beta-subunits. They involve interaction sites with carbohydrate and alpha-subunit. Out of the six disulfide bonds formed in the beta-subunit, four are located together on one side of the core, and the other two on the opposite side. The two regions assumed to be a receptor binding region from experiments (therefore, species-specific regions) are predicted as loops located on the same side of the beta-subunit in this study. Some of the predicted loops are rich in proline residues. While the positions of proline residues are conserved in the family generally, there are hormone- or species-specific ones in the loop that is assumed to take part in receptor binding. The possible importance of proline residues in hormone or species specificity is discussed. (After submitting the manuscript the X-ray crystal structure of human CG was published. In order to evaluate the prediction, the original manuscript is kept intact and a comparison has been made between the prediction results and the crystal structure in an appendix).

Identification of epitopes and affinity purification of thyroid stimulating auto-antibodies using synthetic human TSH receptor peptides

We prepared a series of overlapping peptides (29 in total, 20 amino acids each) containing the sequence of the entire extracellular domain of the human TSH receptor. Three peptides (181-200, 376-394, and EC3 (629-639)) bound IgG from patients with Graves' disease in an enzyme linked immunoassay. Peptide 181-200 bound IgG from 9 of 10, EC3 from 8 of 10, and 376-394 from 6 of 10 patients respectively, compared to 0 of 9 controls. We affinity purified TSHr auto-antibodies from four Graves' patients using the three above noted peptides bound to epoxy-activated sepharose. Thyroid stimulating activity was enriched in the bound fraction from at least two of the three peptide affinity columns in each of the four patients, although the pattern of affinity enrichment differed between patients. One patient was found to possess a combination of stimulatory and inhibitory TSHr antibodies and, after affinity purification, the anti-376-394 and anti-EC3 fractions were enriched in stimulatory activity, suggesting that those regions of the receptor were epitopes for stimulatory antibodies. However, affinity purification against peptide 181-200 produced an IgG preparation that was not stimulatory, but was a potent thyroid inhibitor. Thus, we have not only partially purified TSHr auto-antibodies, but also successfully separated stimulatory and inhibitory antibodies from a single patient using combination TSHr peptide affinity.

Disruption of the first extracellular loop of thyrotropin receptor prevents ligand binding

In order to understand the function of the first extracellular loop of the human thyrotropin receptor (hTSHR), each of two peptides of nine amino acids was inserted into the first extracellular loop of hTSHR. hTSHR cDNA was subcloned into the eukaryotic expression vector, pRc/CMV (hTSHR/pRc/CMV). B-hTSHR/pRc/CMV, a mutant hTSHR cDNA which encodes a hydrophilic peptide insert (AGTTRRVAI) and C-hTSHR/pRc/CMV which encodes a hydrophobic peptide insert (ATVLVVPMI) between +486 Ileu and +487 Asp of hTSHR were transfected into Chinese hamster ovary cells to generate the B-1 and C-6 cell lines, respectively. Neither thyrotropin (TSH) nor thyroid stimulating antibody (TSAb) stimulated cAMP production by B-1 or C-6 cells. An 125I-TSH binding assay showed that neither cell line bound TSH. Our data demonstrated that these mutations impaired both TSH binding and cAMP production. This evidence suggests that the first extracellular loop of hTSHR may have a crucial role in the TSH- and TSAb-dependent signal transduction.

A receptor binding site identified in the region 81-95 of the beta-subunit of human luteinizing hormone (LH) and chorionic gonadotropin (hCG)

Two series of overlapping peptides comprising the entire sequences of the beta-subunits of human lutropin (LH) and choriogonadotropin (hCG) were prepared by a comprehensive synthetic strategy in order to identify all linear regions of the subunit that may participate in binding of the hormone to its receptor. Each series of peptides (15 residues in length) spanned the entire amino acid sequences of the two beta-subunits. The peptides were tested for their ability to inhibit the binding of 125I-labeled hCG or LH to rat ovarian membranes and for their ability to inhibit hCG-stimulated progesterone production in a Leydig cell bioassay. The most potent inhibitor of LH/hCG binding was a peptide containing the sequence beta 81-95, a receptor binding site of the LH/hCG beta subunit not previously described. The concentration at which LH/hCG binding was inhibited at 50% (IC50) was 20 microM and 30 microM for hCGbeta 81-95 and LH beta 81-95, respectively. These peptides also inhibited the stimulation of progesterone production by hCG in Leydig cell bioassays. In order to determine important residues that inhibit binding within this region, a third set of peptides was synthesized in which each residue of hCG beta 81-95 was sequentially replaced with the residue L-alanine. Five residues (Leu-86, Cys-88, Cys-90, Arg-94, and Arg-95) were critical for maximal inhibition of hCG binding by CG beta 81-95. In addition to site beta 81-95, other sites that inhibited hCG/LH binding but with significantly lower potencies included hCG beta 1-15, LH beta 41-55, and LH beta 91-105.(ABSTRACT TRUNCATED AT 250 WORDS)

Topography of equine chorionic gonadotropin epitopes relative to the luteinizing hormone and follicle-stimulating hormone receptor interaction sites

In order to localize the epitopes of equine chorionic gonadotropin (eCG) involved in interaction with luteinizing hormone (LH) and follicle-stimulating hormone (FSH) receptors, we used 14 monoclonal anti-eCG antibodies (mAbs). Different effects of these mAbs on the bioactivities of eCG were observed in in vitro bioassays, but the effects of each mAb on the two bioactivities were similar for all but four mAbs. All mAbs were found to inhibit the binding of eCG to LH receptors except 3A3 mAb, in radioreceptor assay. Six mAbs, which were strong inhibitors of eCG binding to LH receptors and of both bioactivities, recognized the same area on the alpha subunit of eCG. All others, except 3A3, recognized epitopes close to the former, and close to each other. 3A3 mAb had a hyperstimulatory effect on FSH bioactivity, and was the only mAb that did not inhibit binding. It appeared to recognize a different epitopic area. These observations suggest that there is a main antigenic area on eCG, which corresponds to the interaction site of eCG with both receptors. It mostly involves the alpha subunit and to a lesser extent the beta subunit.

Identification of assembled epitopes on the alpha-subunit of human follicle stimulating hormone

In order to characterize further the antigenic structure of human follitropin (hFSH), BALB/c mice were immunized with hFSH and anti-hFSH monoclonal antibodies (mAbs) were generated. The hFSH subunit specificity of the mAbs was assessed by a solid-phase enzyme-linked immunosorbent assay (ELISA) and a solution-phase radioimmunoassay (RIA), each using hFSH, hFSH alpha, and hFSH beta. Five mAbs bound hFSH and hFSH alpha in the ELISA and the RIA. In addition, some mAbs recognized hFSH beta, albeit to a much lower degree, as demonstrated by displacement of [125I]hFSH binding to the mAbs by hFSH beta, in the solution-phase RIAs. Next, synthetic peptides corresponding to the hFSH alpha-subunit sequence were used to identify sequences specific to the epitopes of each of the five mAbs. Using this epitope mapping strategy, two assembled epitopes were identified. mAbs 3A and 4B distinguish one discontinuous epitope comprised minimally of sequences alpha-16-21 and alpha-66-92, whereas mAbs 5F and 2E distinguish a second discontinuous epitope comprised minimally of sequences alpha-40-50 and alpha-66-72.

Effect of 80 kDa protein of porcine follicular fluid on gonadotropin stimulated progesterone production in rat granulosa cells in vitro

We reported the presence of a 80 kDa polypeptide in porcine follicular fluid that inhibited the binding of 125I-radiolabelled hFSH as well as hCG to the rat ovarian gonadotropin receptors. In the present study, the biological activity of the receptor binding inhibitor is determined using an in vitro bioassay procedure. Granulosa cells isolated from PMSG primed immature rat ovaries respond to exogenously added gonadotropins in terms of progesterone production. Addition of fractions containing the gonadotropin receptor binding inhibitory activity inhibited progesterone production stimulated by the gonadotropins in a dose-dependent fashion. The receptor binding inhibitory activity was also capable of inhibiting progesterone production stimulated by PMSG, which has both FSH- and LH-like activities in rats. In contrast, progesterone production stimulated by dbcAMP was not inhibited by the receptor binding inhibitor. This result indicates that the site of action of the inhibitor is proximal to the formation of the cAMP. The above observations point out to a possible role for this factor in modulating gonadotropin activity at the ovarian level.

Progress and approaches in mapping the surfaces of human follicle-stimulating hormone: comparison with the other human pituitary glycoprotein hormones

The pituitary glycoprotein hormones, including human follicle-stimulating hormone (hFSH), are involved in the physiologic functions of receptor binding, in subunit association during assembly and processing, and in untoward effects such as antibody formation during clinical intervention. These activities derive from specific macromolecular associations; a major research trend has been to map the surfaces of these glycoprotein hormones (hFSH, hLH, hCG, and hTSH) into discrete areas responsible for each activity by using a variety of biochemical approaches. Each surface domain or determinant of the hormone is likely to comprise discontinuous amino acid sequences, from one or both subunits, assembled into a precise, unique, macromolecular surface. The paradigm of antigen-antibody interaction may help to explain how the surfaces are assembled, how the common alpha-subunit combines with the unique beta-subunit of each, and how the receptor interacts with heterodimeric hormone.

Asialoagalacto-human chorionic gonadotropin, a carbohydrate-modified variant of human chorionic gonadotropin, antagonizes the stimulatory actions of bovine thyroid-stimulating hormone on thyroid function and HLA-DR expression in human thyroid in vitro and in vivo

The concept of using thyroid-stimulating hormone (TSH) receptor antagonists in the management of Graves' disease is intriguing. Therefore, we investigated a TSH receptor antagonist derived from human chorionic gonadotropin (hCG) with respect to TSH receptor binding, adenylate cyclase activity, thyroid hormone release, and HLA class II antigen expression in vitro and in an in vivo model. A variant of hCG, asialoagalacto-hCG, like asialo-hCG and unlike hCG itself, inhibited both 125I-bTSH binding and cAMP response to bTSH in human thyroid membranes. However, like intact or deglycosylated hCG and unlike asialo-hCG, asialoagalacto-hCG displayed a limited affinity for hepatic asialoglycoprotein receptors, a likely marker for its in vivo turnover rate. It proved capable of inhibiting bTSH-stimulated thyroid hormone release in human thyroid slices as well as in the nude mouse bearing human thyroid transplants. It also prevented bTSH induced hypertrophy of transplanted thyrocytes. Further, HLA-DR expression induced by bTSH in the presence of gamma-interferon on human thyrocytes was inhibited. In conclusion, we present evidence that asialogalacto-hCG antagonizes bTSH actions on thyroid function and HLA-DR expression in human thyroid in vitro and, more importantly, in an in vivo model. Hence, the hCG variant described here or similar agents should warrant further exploration in the study and treatment of Graves' disease.

Further characterization of the receptor-binding region of the thyroid-stimulating hormone alpha subunit: a comprehensive synthetic peptide study of the alpha-subunit 26-46 sequence

Previously, using a synthetic peptide strategy, we determined that the region of the common glycoprotein hormone alpha subunit between residues 26 and 46 is a site of interaction of the hormone with the thyroid membrane-bound receptor for thyroid-stimulating hormone (TSH). We have undertaken to identify further the specific residues within this 21-amino acid span that are critical in hormone receptor binding. We synthesized three nested sets of peptide, two in which we systematically truncated the amino-terminal region of the sequence and another in which we truncated the carboxyl-terminal region, and we synthesized a fourth nested set in which we systematically substituted alanine for the native residues from the region of highest activity. Each peptide was tested in a TSH radioreceptor assay for its ability to inhibit binding of 125I-labeled bovine TSH to porcine thyroid membranes. Removal either by truncation or alanine substitution, of several specific residues resulted in a significant reduction in the ability of the sequence to interact with receptor; these residues included Cys31, Cys32, Phe33, Arg35, Arg42, Lys44, and Lys45, suggesting that they are crucial for binding activity. Loss of activity also occurred with substitution for Gly30 and Ser34, but the reduction was less pronounced. Amino-terminal truncation of the sequence through Arg35 (leaving the alpha-subunit peptide 36-46) resulted in greater than 98% loss of activity of the sequence. We conclude that two distinct receptor binding regions lie within the alpha-subunit 26-46 sequence. The first lies between residues Gly30 and Arg35 and includes Cys31, Cys32, and Phe33 as important constituents, and the second region lies between residues Arg42 and Lys45 and includes Lys44 as an important residue and Ser43 as a less important component.

Carbohydrate-dependent epitope mapping of human thyrotropin

To probe possible effects of carbohydrate chains in the conformation of pituitary glycoprotein hormones, two radiolabeled derivatives of human thyroid-stimulating hormone (hTSH), either partially deglycosylated in the beta-subunit or fully deglycosylated in both the alpha- and beta-subunits, were compared to the native hormone for binding to monoclonal as well as polyclonal antibodies. Monoclonal antibodies were screened for their ability to bind the intact hormone (anti-hTSH), hTSH and its free alpha-subunit (anti-alpha) or its free beta-subunit (anti-beta). A panel of 14 monoclonal antibodies directed against at least eight out of the 12 epitopes known to be present in the hormone was tested in solid-phase assays for their capacity to bind intact and deglycosylated forms of hTSH. All of them displayed identical recognition of native and partially deglycosylated 125I-hTSH. In contrast, binding of fully deglycosylated 125I-hTSH to anti-hTSH and anti-beta antibodies was dramatically lost while that of anti-alpha was preserved. This clearly indicates that most of the epitopes specific for subunit association as well as those present on the beta-subunit are glycosylation dependent. No alteration was found in antibody recognition following deglycosylation of free individual subunits, indicating that the carbohydrate effect can only occur in the combined dimer. Using polyclonal antisera raised against the International Reference Preparations, we found that the deglycosylated hormone could be bound by the anti-beta antiserum although at a much lower dilution than the native antigen, suggesting the presence of at least one glycosylation-independent epitope in the beta-subunit. Competitive binding assays revealed that deglycosylated hTSH is 5 times less immunoreactive toward the anti-beta compared to the anti-alpha antiserum. The current data thus demonstrate the presence of the glycosylation-independent epitopes in the alpha-subunit of hTSH and the localization of most of the glycosylation-dependent domains in the beta-subunit.

Subunit interaction of human chorionic gonadotropin (hCG) with rat ovarian luteinizing hormone (LH)/CG receptor

The subunit interaction of hCG with its rat ovarian LH/CG receptor was studied by cross-linking the solubilized receptor-hormone complex with glutaraldehyde (GA), disuccinimidyl suberate (DSS) or dithiobis(succinimidyl propionate) (DSP) and analyzing the complexes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography. The hormone was labeled either in its alpha-subunit (125I-hCG) or in its beta-subunit (3H-hCG) or the label (3H) was introduced into the receptor molecule instead of the hormone. All of the labeling procedures led to the detection of only the receptor-(alpha,beta)hCG and receptor-(alpha)hCG complexes on the autoradiograms. The sizes of these complexes were 137,000 and 106,000, respectively, under reducing conditions. These results suggest that the receptor binds one hormone molecule, and that hCG interacts with the receptor mainly through its alpha-subunit. In addition, polyclonal antibodies directed against the LH/CG receptor and the alpha- and beta-subunits of hCG were used to detect the non-reduced receptor-(alpha,beta)hCG complex in immunoblotting. As antibodies directed against both the alpha-subunit and the beta-subunit were able to detect the Mr 130,000 complex, it is conceivable that both of the subunits are at least partially exposed on the receptor-hormone complex. 125I-hCG was also cross-linked to the membrane-bound receptor. The membrane-bound complex had an Mr of 144,000 under reducing conditions, i.e. approximately 7000 higher than that of the solubilized complex (Mr 137,000). This may indicate that the membrane-bound receptor is covalently modified or differs in conformation from the solubilized receptor.

The role of carbohydrate in human choriogonadotropin (hCG) action. Effects of N-linked carbohydrate chains from hCG and other glycoproteins on hormonal activity

Deglycosylation of gonadotropins and thyrotropin results in a major loss of hormonal bioactivity, while not impairing receptor-binding activity. However, a direct role of the glycan moieties in hormonal signal transduction has not been demonstrated. The addition of carbohydrate chains together with the deglycosylated hormone does not restore the hormonal activity. In contrast, glycopeptides were found to inhibit human choriogonadotropin (hCG)-stimulated adenylyl cyclase activity and hCG binding to its receptor. An inhibition of hCG-stimulated adenylyl cyclase activity but not hCG binding to receptor by glycopeptides specifically from hCG, has previously been reported as a lectin-like membrane component has been implicated in hCG action. In the present study we have shown that glycopeptides and oligosaccharides prepared from hCG, transferrin, fetuin, alpha 1-acid glycoprotein and ovalbumin inhibit the binding of hCG to its receptor. The inhibition was also observed with a highly purified preparation of the receptor, thus suggesting a lack of involvement of other lectin-like membrane components as previously proposed. We suggest that a lectin-like interaction with the hormone, if any, involves the receptor itself. Adenylyl cyclase activity stimulated by hCG, isoproterenol or forskolin was inhibited by oligosaccharides, indicating a non-specific interaction. Our results suggest that Asn-linked oligosaccharide chains from various glycoproteins perturb hCG-receptor interactions through a putative carbohydrate binding site on the receptor.

Peptide mapping of intersubunit and receptor interactions of human choriogonadotropin

Seven peptides covering the entire sequence of human choriogonadotropin (hCG) alpha-subunit, eight peptides covering the hCG beta-subunit sequence and two peptides, one of human beta-lutropin and one of beta-thyrotropin were synthesized. We checked their ability to prevent reassociation between hCG alpha- and beta-subunits and between hCG and its receptor. Only the alpha 1-22, alpha 59-92 and beta 1-16 peptides inhibited the reassociation between the alpha- and beta-subunits of hCG with an ED50 of respectively 2 mM, 2 mM and 4 mM. Using porcine Leydig cells in primary culture, we showed that alpha 33-59, alpha 41-59 and beta 1-16 peptides decreased both the specific binding to the cell surface and the internalization of [125I]hCG and [125I]porcine LH with ED50 of 0.3, 0.1 and 0.5 mM, respectively. From these results, the following minimal area may be assigned, (i) to the alpha-beta interaction: alpha 5-16, alpha 52-72 (or alpha 59-70) and beta 8-16, and (ii) to the hormone-receptor association: alpha 41-45 and beta 8-16.

[Free alpha-subunit glycoprotein hormones: physiological and pathological data]

The glycoprotein hormone alpha-subunit is secreted as a free molecule as well as a molecule combined to a glycoprotein hormone beta-subunit. In human subjects, plasma levels of the free alpha-subunit were measured by means of a specific radioimmunoassay. Plasma concentrations were high during the neonatal period, then decreased to a nadir at the age of 6 years. A significant pubertal increase occurred in both sexes, more pronounced in girls. In female subjects mean levels (+/- SEM) were 0.21 +/- 0.05 before puberty and 0.51 +/- 0.03 ng 1 degrees IRP-hCG alpha/ml in follicular phase. During menstrual cycle, a typical preovulatory surge was seen simultaneous with the LH surge. During aging, plasma levels increased slowly in males, abruptly in menopausal females. The pituitary reserve as assessed by LH-RH stimulation test (100 micrograms i.v./m2) exhibited a significant pubertal maturation in boys and girls. Chronic administration of LH-RH agonist induced a marked increase of alpha-subunit levels, whereas LH levels were deeply suppressed. LH-RH injections in children treated for precocious puberty with a LH-RH agonist induced a significant release of alpha-subunit despite an almost complete abolition of LH release. In conclusion, from a quantitative point of view, the glyco-protein hormone alpha-subunit is a major secretory product of the pituitary. It seems that there is a specific regulation of its secretion, resembling but not identical to LH secretion regulation. Whether or not it plays a biological role remains uncertain.