The molecular evolution of pituitary hormones

Synteny and phylogenetic analysis of paralogous thyrostimulin beta subunits (GpB5) in vertebrates

At some point early in the vertebrate lineage, two whole genome duplication events (1R, 2R) took place that allowed for the diversification and sub-/neo-functionalization of the glycoprotein hormones (GpHs). All jawed vertebrates possess the GpHs luteinizing hormone (LH), follicle stimulating hormone (FSH), and thyroid stimulating hormone (TSH), each of which are heterodimers with a common alpha subunit and unique beta subunits. In 2002, a novel glycoprotein hormone named thyrostimulin was described to have unique GpA2 and GpB5 subunits that were homologous to the vertebrate alpha and beta subunits. The presence of GpA2 and GpB5 in representative protostomes and deuterostomes indicates their ancestry in the GpH family. There are several reports of GpH subunit evolution, but none have included GpA2 and GpB5 for species in each major vertebrate class. Thus, we addressed the ancestry of two paralogous GpB5 subunits (GpB5a and GpB5b) that were previously only recognized in two teleost species. Our search for orthologous GpB5a and GpB5b sequences in representative vertebrates and phylogenetic analysis, in addition to the currently published evolutionary scenarios of the GpH family, supports that GpB5a and GpB5b are paralogs that arose from the first vertebrate whole genome duplication event (1R). Syntenic analysis supports lineage specific losses of GpB5a in chondrichthyes, basal actinopterygians, and tetrapods, and retention in coelacanth and teleosts. Additionally, we were unable to identify GpA2 transcripts from tilapia mRNA, suggesting that this species does not produce heterodimeric thyrostimulin. While the conserved or even species-specific functional role of thyrostimulin or its individual subunits are still unknown in vertebrates, the analyses presented here provide context for future studies on the functional divergence of the GpH family.

Molecular evolution of the neurohypophysial hormone precursors in mammals: Comparative genomics reveals novel mammalian oxytocin and vasopressin analogues

Among vertebrates the neurohypophysial hormones show considerable variation. However, in eutherian mammals they have been considered rather conserved, with arginine vasopressin (AVP) and oxytocin (OT) in all species except pig and some relatives, where lysine vasopressin replaces AVP. The availability of genomic data for a wide range of mammals makes it possible to assess whether these peptides and their precursors may be more variable in Eutheria than previously suspected. A survey of these data confirms that AVP and OT occur in most eutherians, but with exceptions. In a New-World monkey (marmoset, Callithrix jacchus) and in tree shrew (Tupaia belangeri), Pro(8)OT replaces OT, confirming a recent report for these species. In armadillo (Dasypus novemcinctus) Leu(3)OT replaces OT, while in tenrec (Echinops telfairi) Thr(4)AVP replaces AVP. In these two species there is also evidence for additional genes/pseudogenes, encoding much-modified forms of AVP, but in most other eutherian species there is no evidence for additional neurohypophysial hormone genes. Evolutionary analysis shows that sequences of eutherian neurohypophysial hormone precursors are generally strongly conserved, particularly those regions encoding active peptide and neurophysin. The close association between OT and VP genes has led to frequent gene conversion of sequences encoding neurophysins. A monotreme, platypus (Ornithorhynchus anatinus) has genes for OT and AVP, organized tail-to-tail as in eutherians, but in marsupials 3-4 genes are present for neurohypophysial hormones, organized tail-to-head as in lower vertebrates.

The effects of dose and method of administration of biosynthetic bovine somatotropin on live-weight gain, carcass composition and wool growth in young lambs

Exogenous bovine pituitary somatotropin (GH) can influence markedly body composition in fattening lambs. However, neither the effects of biosynthetic somatotropin nor the effects of dose and method of administration have been reported. Fifty Dorset-cross lambs (female and castrated male) were given concentrate ad libitum and treated between 10 and 22 weeks of age with biosynthetic bovine somatotropin either dissolved in buffer and injected subcutaneously (s.c.) in proportion to body weight (0·025, 0·1 or 0·25 mg/kg per day), dissolved in buffer and continuously infused s.c. (0·1 mg/kg per day) or suspended in olive oil and injected s.c. (0·1 mg/kg per day), and compared with 10 untreated control lambs. Somatotropin had little effect on live-weight gain (controls = 228 g/day; final live weight 37 kg), food intake and food conversion efficiency, and only marginally increased the weight of muscle and bone dissected from the shoulder joint. The weights of the major fat depots in the abdominal cavity and of fat dissected from the shoulder joint were linearly related to dose of somatotropin (P < 0·001). Lambs given the highest dose had less visceral fat (1·18 v. 2·84 kg; P < 0·001) and proportionately less fat (285 v. 374 g/kg; P < 0·001) and more muscle (542 v. 447 g/kg) and bone (172 v. 149 g/kg; P < 0·001) in the shoulder joint than control lambs. There was a positive curvilinear relationship (P < 0·01) between clean wool growth (mid-side patch sample) and dose; lambs injected daily with 0·1 mg somatotropin per kg grew one-third more wool than control lambs. Method of administration affected plasma somatotropin profiles but had no significant influence on any of the responses measured. The anabolic actions of somatotropin may have been limited in this experiment by a low sodium concentration in the diet. It is concluded that the lipolytic/anti-lipogenic effect of somatotropin can occur in the absence of conditions conducive to the stimulation of muscle protein deposition and that the response is dose dependent but not influenced by method of administration of the hormone.

Lipids and lipid-transporting proteins in Chrysemys picta: role of gonadal steroids and growth hormone in intact and hypophysectomized turtles

In the freshwater turtle, the homeostatic control of plasma lipids and lipid-transporting proteins may be coordinately regulated by ovarian steroids and pituitary hormones such as growth hormone (GH). In order to elucidate the role of these hormones in the regulation of vitellogenesis and ovarian growth, we have investigated lipid metabolic changes in normal male and female turtles, and in hypophysectomized females with and without GH injections, in response to combinations of exogenously administered gonadal steroids (estradiol (E2), progesterone (P), and testosterone (T)). Determinations of total plasma triglycerides, cholesterol, vitellogenin, and apoA-I were performed. We have demonstrated that E2 alone and in combination with P significantly increased plasma apoA-I and triglyceride levels in both intact female and male turtles. Testosterone administered alone to males had no effect on any of the parameters measured. In hypophysectomized females, plasma apoA-I, vitellogenin, and triglyceride levels were all significantly elevated in animals which received GH and E2, compared to controls (sham and hypox) and those which received GH alone.

Fish gonadotropin and thyrotropin receptors: the evolution of glycoprotein hormone receptors in vertebrates

We have cloned and characterized, for the first time in fish, two different gonadotropin receptors (GTHR) and a single thyrotropin receptor (TSHR) from amago salmon (Oncorhynchus rhodurus) and Nile tilapia (Oreochromis niloticus). Phylogenetic analyses and intron/exon structure suggest that the two GTHRs in fish are comparable to tetrapod follicle stimulating hormone and luteinizing hormone receptors. Temporal and spatial expression patterns, examined by Northern blot analysis and in situ hybridization, paralleled those seen in mammals and birds. Consequently, genetic and functional divergence of two GTHRs and TSHR probably occurred before the teleost and tetrapod split.

cDNA cloning of the beta subunit of teleost thyrotropin

cDNA clones encoding the beta subunit of thyrotropin (thyroid-stimulating hormone; TSH) were isolated from a cDNA library made from the pituitaries of immature rainbow trout and sequenced. The precursor of rainbow trout TSH beta consists of 147 aa, which can be cleaved into a signal peptide (20 aa) and a mature protein (127 aa) containing one potential N-glycosylation site and 12 cysteine residues. The protein showed highest homology with human TSH beta (51%) and lesser homology with human follitropin (42%), human lutropin (32%), and salmon gonadotropin (31-33%) beta subunits. The identification of TSH in addition to two gonadotropins (gonadotropins I and II) in the teleost fish suggests that the divergence of three kinds of glycoprotein hormones from an ancestral molecule took place earlier than the time of divergence of teleosts from the main line of evolution leading to tetrapods. Northern blot analysis showed that the expression of the rainbow trout TSH beta gene is specific to the pituitary gland and is significantly higher in immature fish than in mature fish, suggesting that TSH plays some role in the biological processes of immature fish.

Recombinant bovine and porcine somatotropin: safety and benefits of these biotechnologies

This article reviews the literature about the safety and benefits of two recombinantly derived proteins, bovine somatotropin (bST) and porcine somatotropin (pST), that likely will be used in animal agriculture in the future. When administered to dairy cows, bST increases milk production per cow approximately 15% to 20% and improves productive efficiency approximately 10%. Administration of pST to growing pigs reduces carcass fat content by as much as 70% to 80% and improves productive efficiency 15% to 35%. Because meat is a major source of total fat and saturated fatty acids in the diets of human beings, pST will allow consumers to include leaner, more nutrient-dense pork in their diets and still meet current dietary guidelines. Although these biotechnologies have not yet received regulatory approval from the Food and Drug Administration (FDA) for commercial use, information published by the FDA, the National Institutes of Health, the US Congress Office of Technology Assessment, and the American Academy of Pediatrics, as well as an extensive body of scientific evidence, indicate that these products are safe for the consumer. Nonetheless, it is important that consumers understand the benefits and safety of these biotechnologies. Dietitians can play an important role in providing information to consumers about the safety and benefits of bST and pST.

Bovine somatotropin: review of an emerging animal technology

One of the first potential biotechnology products for animal production is bST. Research in the technology of bST has involved scientists and support from federal agencies, universities, and private industry. As a consequence of this extensive cooperation, more than 1000 bST studies have been conducted, which involved over 20,000 dairy cows, and results have been confirmed by scientists throughout the world. This quantity of published research is unprecedented for a new technology and greater than most dairy technologies in use. In contrast to steroids, bST is a protein hormone. Milk yield and persistency responses to bST have been observed for all dairy breeds examined. Quality of management is the major factor affecting magnitude of milk response to bST. The mechanism of action of bST involves a series of orchestrated changes in the metabolism of body tissues so that more nutrients can be used for milk synthesis. It is these coordinated changes that allow the animal to achieve an increased milk yield while remaining normal and healthy. Bioenergetic studies demonstrated that bST-supplemented animals are not stressed. Similarly, there are no adverse health effects from bST even under poor management conditions. Composition of milk (fat, protein, lactose, cholesterol, minerals, and vitamins) is not substantially altered when bST is used and does not differ in manufacturing characteristics. Public perception is of paramount importance if bST or any new technology is to be effectively implemented. New technology must be understood and perceived as safe and beneficial both by farmers, who would utilize it, and consumers, who would purchase the dairy products. With bST use, a unit of milk is produced with less feed and protein supplement and with a reduction in animal excreta (manure, urine, and methane). Nationally, the use of bST simply reinforces, but does not fundamentally change, dairy industry trends of increased milk yield per cow, reduced number of cows, and declining dairy farm numbers. For individual farms, bST technology is size-neutral. However, poorly managed farms where animals are stressed, underfed, or sick are at an economic disadvantage because they will achieve negligible milk response to bST.

Blockade of the antigen-antibody reaction using benzil condensation with the guanidyl residue of arginine

Benzil blockade of the guanidyl group of arginine was tried on sections of paraffin-embedded tissue fixed in two different fixatives, in an attempt to evaluate the relevance of this amino acid to the reaction of several proteins with their corresponding antibodies. The two fixatives were 10% formaldehyde, and Bouin's fluid without acetic acid. Both polyclonal and monoclonal antibodies against proteins or peptides (lysozyme, adrenocorticotropic hormone, growth hormone, placental lactogen, and prolactin) were used on human biopsies or material from autopsies. The blockade was effective when monoclonal antibodies were used, whereas no effect or only a small decrease of the intensity of the reaction was observed with polyclonal antibodies. The least definitive result was obtained with prolactin, where a complete blockade was never achieved with monoclonal antibodies. Calcitonin, a peptide that does not contain arginine, was used as a control not susceptible to benzil blockade; no blockade of immunostaining was observed.

Treatment of growth hormone deficiency

According to the results reported in the literature and from our own experience, the following recommendations for the treatment of children with GHD can be given: In order to start GH replacement therapy in early childhood the diagnosis of GHD should be made as early as possible. The growth hormone dose during prepubertal age should not fall short of 12 IU/m2 per week. During spontaneous or induced puberty, the dose needs to be increased, possibly by a factor of two. Daily subcutaneous injections appear most suitable. Treatment with growth hormone releasing factors in cases with hypothalamic GHD, although a promising alternative to the treatment with hGH (Thorner et al, 1985), must be considered experimental at this point. Thyroxine replacement at a daily dose of 75-100 micrograms/m2 should be given in cases of secondary hypothyroidism. Glucocorticoid replacement, if required, should be given at low doses (e.g. hydrocortisone 10 (to 15) mg/m2 per day in divided doses). In cases with additional gonadotropin deficiency, sex steroids (or anabolic steroids) should be given with frequent monitoring of bone maturity not before the age of 13 in girls or 15 years in boys. In boys depot testosterone starting at low doses (e.g. 50-100 mg/month i.m.) will induce a puberty-like increment in height velocity. Since the effect of oestrogens--even in low doses--on growth is uncertain, their administration before achievement of near-normal adult height should be avoided. With the advancement of diagnostic techniques and with the experience in treatment accumulated over the past 25 years, patients with GHD need no longer become dwarfs.

Monoclonal antibodies to bovine somatotropin: immunoadsorbent reagents for mammalian somatotropins

Twenty-nine stable hybridoma cell lines secreting monoclonal antibodies to bovine somatotropin (bST) have been produced and characterized. Five of the monoclonal antibodies bind porcine and human somatotropins as well as bST. One of these antibodies was used as a reagent in immunoadsorbent chromatography of recombinant bST or pituitary bST from cell extracts. Following chromatography, the bST preparations retained activity in a rabbit liver radioreceptor assay and in a radioimmunoassay. The immunoadsorbent reagent bound human and porcine somatotropins as well as bovine.

Contrasted, a steel allele in the mouse with intermediate effects

The steel allele, contrasted (Slcon), arose in a neutron irradiation experiment.Slconis fully penetrant and heterozygotes can be recognized at or soon after birth by darkly pigmented external genitalia in both sexes, while the adult coat tends to be a little lighter than normal. Homozygotes also have dark genitalia and a markedly diluted coat. Both eumelanin and phaeomelanin are affected, with reduced numbers of cortical and medullary pigment granules in the hairs. Contrasted also affects the haematopoietic system, causing slight macrocytic anaemia in the homozygote.Slconhomozygous males are fertile but testes weigh on average 20% less than in their heterozygous litter-mates. Homozygous females are usually sterile although if mated early (4½−6 weeks) they occasionally have a single litter. Ovarian sections showed a gradual degeneration of oocytes in Graafian follicles so that most had gone by 2 months. Similarly, vaginal smears indicated that after about three normal cycles homozygous females lapsed into a state of persistent dioestrus; injections with gonadotrophins did not prolong their period of fertility or cause a resumption of their oestrous cycles. The effects on fertility, pigmentation and haematology of contrasted when combined with other steel alleles are also described.

Ultrastructural changes in the adenohypophysis during the ovarian cycle of the viviparous teleost Poecilia latipinna. II. The thyrotrophic cells and the thyroid gland

Electron microscopical observations were made on the thyrotrophic (TSH) cells during the monthly cycle of oocyte development and pregnancy in female sailfin mollies. Parallel light microscopic studies were made on the thyroid gland. By morphological criteria, TSH secretion and thyroid activity run in parallel, and the findings indicate that TSH secretion and thyroid hormone output are higher during vitellogenic oocyte growth than during earlier oocyte development and during pregnancy. Measurements of TSH cell area and nuclear area and of thyroid epithelial cell height confirm this pattern. The results are discussed in the light of what is known of thyroid hormone involvement in teleostean ovarian physiology.

Production and characterization of monoclonal antibodies against bovine luteinizing hormone

Five mouse hybridoma cell lines producing monoclonal antibody against bovine luteinizing hormone (LH) have been established and the respective antibodies characterized by radioimmunoassay, immunofluorescence and immunoelectrophoresis. All antibodies belong to the IgG class and bind to staphylococcus protein A. Intraspecies cross-reactivity studies revealed no reaction with bovine follicle stimulating hormone (FSH). However, all antibodies showed partial cross-reaction with bovine thyroid stimulating hormone (TSH) suggesting a close conformational similarity between bovine LH and TSH. Studies on interspecies cross-reactivity (rat and human) showed that three of these five antibodies strongly react with rat LH but not at all with either rat FSH or rat TSH thus representing monospecific reagents for investigations concerning LH in this species. One of these three antibodies also strongly binds to human LH and to the same extent to human chorionic gonadotropin (CG) but not to human FSH or TSH. It was concluded that at least three different epitopes on the bovine LH molecule are recognized and that they are located on the beta-chain of the hormone.

Purification and primary structure of two neuroactive peptides that cause bag cell afterdischarge and egg-laying in Aplysia

Two neuroactive peptides, A and B, have been isolated from the atrial gland in the reproductive tract of Aplysia. Each of the two peptides is able to induce egg-laying behavior in recipient animals. In vitro recordings from the abdominal ganglion show that both peptides also trigger longlasting discharges in the bag cell neurons at concentrations around 0.1 muM. The peptides were purified by a combination of ammonium sulfate precipitation, agarose gel filtration, and cation exchange chromatography. Each peptide has 34 amino acid residues. Microsequencing together with carboxypeptidase Y degradation and analysis of tryptic peptides revealed the following sequence for peptide A: H-Ala-Val-Lys-Leu-Ser-Ser-Asp-Gly-Asn-Tyr-Pro-Phe-Asp-Leu-Ser-Lys-Glu-Asp-Gly -Ala-Gln-Pro-Tyr-Phe-Met-Thr-Pro-Arg-Leu-Arg-Phe-Tyr-Pro-Ile. Peptide B differs from A in only four positions. The first nine residues of B are: Ala-Val-Lys-Ser-Ser-Ser-Tyr-Glu-Lys-, whereas residues 10-34 of B are identical to those of A. The calculated M(r) of A is 3924 and that of B is 4032. The pI of peptide A as determined by isoelectric focusing in polyacrylamide gels is 7.9-8.1 and that of peptide B is 9.0-9.2. It is estimated that each atrial gland contains at least 150 mug of peptide A and 50 mug of B. Neither peptide resembles the egg-laying hormone isolated from bag cell neurons. It is postulated that the atrial gland peptides are released during copulation, and then by interacting with neuronal receptors in the head ganglia and pleuroabdominal connectives they cause the bag cells to afterdischarge, thereby releasing egg-laying hormone.

Endocrine control of energy metabolism in the cow: the effect on milk yield and levels of some blood constituents of injecting growth hormone and growth hormone fragments

1. Circulating concentrations of some hormones and metabolites and nitrogen balance were measured in lactating beef and dairy cows given daily injections of growth hormone (GH) and were compared to values before injection. Changes in milk yield and composition were recorded in these cows and in additional cows injected with GH fragments. 2. GH, but not GH fragments, raised milk yield while milk composition did not change. GH injection caused a large, rapid increase in the level of the hormone in blood and slower, smaller increases in the concentrations of insulin, prolactin and thyroxine. Blood metabolite levels were unaltered except for a rise in glucose concentration in beef cows during GH injection and a fall in 3-hydroxybutyrate concentration in the same cows after the injections ceased. An increase in N output in milk was partly balanced by a reduction in urinary N so that N retention did not change significantly. 3. GH caused a reduction in food intake in the beef cows. 4. The results are discussed in relation to control of partition of nutrients in the lactating cow, where GH is considered to play an important role. It was concluded that this role may be in increasing the supply of energy metabolites for milk synthesis, rather than a direct effect on the activity of the mammary gland.

Tryptophylglycine dipeptide in ACTH/MSH cells of the human hypophysis: its identification and studies on its antinociceptive effects in mice

The ACTH/MSH cells of the pars distalis and pars intermedia of the mammalian hypophysis contain peptides with amino-terminal tryptophan which exhibit a strong fluorescence after treatment with modified formaldehyde vapour methods and with glyoxylic acid in the tissue sections from freeze-dried specimens. By homogenization of the hypophyses in ethanolic glyoxylic acid and subsequent heating the peptides can be converted to highly fluorescent beta-carboline derivatives; these can then be extracted with glacial acetic acid, separated by silica gel thin-layer chromatography and identified in UV light. Amino-terminal tryptophyl peptide from adult human hypophysis extracted and treated in this way gave the structure L-tryptophylglycine after acid hydrolysis. This structure was subsequently confirmed by producing a fluorescent derivative from authentic L-tryptophylglycine using the same reaction conditions as for the tissue homogenate. This derivative moved in the same way in thin-layer chromatography as fluorecent amino-terminal tryptophyl peptide extracted from human hypophysis. Thereafter a study was made of the antinociceptive effects of authentic L-tryptophylglycine administered subcutaneously in mice both alone and together with morphine. L-tryptophylglycine had no antinociceptive effects alone and neither did it change morphine antinociception. Also it had no apparent effects on the behaviour of mice. Thus, ACTH/MSH cells contain a dipeptide whose physiological function differs from the effects of ACTH, MSH and endorphins.

Quantitative fluorescence histochemistry of combined formaldehyde-chloral-induced fluorescence of amino-terminal tryptophyl-peptide in model experiments and in the pars intermedia of the rat hypophysis

The relationship between the intensity of combined formaldehyde-chloral vapour-induced fluorescence and the concentration of amino-terminal tryptophyl-peptide in model experiments was found to be non-linear. At a certain concentration the intensity began to increase more slowly than the concentration, and when the concentration further increased the intensity even began to decrease. Based on the studies previously reported and on the above findings it seems that fluorescence induced by combined formaldehyde-chloral vapour, glyoxylic acid vapour and possibly also other combined formaldehyde and carbonyl compounds in the hypophyseal cells containing amino-terminal tryptophyl-peptides is quenched in normal conditions due to the high local concentration. Thus, small to moderate changes in the amounts of amino-terminal tryptophyl-peptides cannot be observed by measuring the fluorescence intensity. In tissue experiments the intensity of combined formaldehyde-chloral vapour-induced fluorescence in the rat pars intermedia was measured after reserpine treatment, which decreases the number of hormone storage granules as demonstrated electron microscopically. The fluorescence intensity measurements were combined with an estimation of the amounts of amino-terminal tryptophyl-peptides extracted from hypophyses and separated in thin-layer chromatography, and subsequently demonstrated by combined formaldehyde-chloral vapour and a protein stain (amido black). Reserpine treatment decreased the fluorescence intensity in the pars intermedia and in thin-layer chromatography, and the staining of the fluorescent band with amido black was also decreased. Amino-terminal tryptophyl-peptides appeared to be depleted from the pars intermedia cells together with endorphins and other hormones of the ACTH/MSH cells containing tryptophan.

Effect of conformation of hCG-beta on generation of hCG-specific antibody

Most antisera generated to isolated highly purified beta subunits of human glycoprotein hormones are not sufficiently sensitive to detect physiologic blood levels of the native hormone. In the dissociated state, beta subunits assume a conformation different from that in the native hormone. Since antisera to alpha subunits have essentially no cross-reactivity between species, highly purified hCG-beta was combined with bTSH-alpha. That hybrid served as immunogen to assess whether sensitive, specific hCG antisera would more likely result than using hCG-beta alone. Of five animals immunized, three developed sufficiently sensitive and specific antisera. The results of these studies strongly suggests that human glycoprotein beta subunits combined with non-human alpha subunit are more likely to yield specific, sensitive antisera than when either isolated beta subunit or the native human glycoprotein hormone, containing common alpha determinants, serves as immunogen.

In vitro activation of glycoprotein hormones. Hybridization of subunits from thyrotropin, lutropin and human choriogonadotropin

In vitro assembly of thyrotropin alpha and beta subunits led to an increase in content of alpha helix and beta sheet very similar to that found for gonadotropins. This association-dependent active folding involved the burying of three tyrosine residues tentatively assigned to Tyr alpha 41, Tyr beta 37 and Tyr beta 59 and common to all studied glycoprotein hormones. In vitro hybridizations between alpha and beta subunits of various hormones (thyrotropin, lutropin and choriogonadotropin) from different species (ovine, bovine and human) triggered the same molecular events as assembly of homologous subunits: the burying of three tyrosine residues and the increase of periodic structure of the folding. These changes are slow, time-dependent processes. Rates and yields of hybrid formation measured by sedimentation analysis and difference spectroscopy of tyrosines are identical, within experimental error, with the rates and yields measured by the recovery of the biological activity either the stimulation of chick thyroids for thyrotropin-beta hybrids or binding to porcine testis receptors for gonadotropin-beta hybrids. Whatever the origin of the alpha subunit, the thyrotropin-beta hybrids were not able to bind to testis receptors although active on chick thyroids. Rates and yields of hybrid formation essentially depended on the origin of the beta subunit. All the hybrids could be dissociated at acid pH with rates similar to those of native hormone. The extension to thyrotropin and various hybrids of the structural features of the in vitro assembly already recognized for gonadotropins strengthens the hypothesis that one deals with a basic activation process which also occurs in vivo after the synthesis of the subunits.