Isolation and characterization of the subunits of bovine follitropin.

Highly purified bovine follitropin was dissociated into its alpha- and beta-subunits after treatment with 1 M-propionic acid. The dissociated subunits were fractionated by chromatography on DEAE-cellulose and further purified by gel filtration on Sephadex G-100. The isolated alpha- and beta-subunits were biologically inactive, but their recombinants regenerated 80% of the follitropin activity. The alpha-subunit of bovine follitropin recombined with the beta-subunits of bovine lutropin and thyrotropin to regenerate 70% of lutropin and 50% of thyrotropin activities respectively. The beta-subunit of bovine follitropin recombined with the alpha-subunit of either bovine lutropin or thyrotropin to regenerate about 75% of follitropin activity. Recombinations were monitored by specific radioligand-receptor assays and polyacrylamide-gel electrophoresis. The elution volumes of the alpha- and beta-subunits of bovine follitropin after gel filtration on Sephadex G-100 were almost identical. The amino acid composition of bovine follitropin-alpha was low in histidine, arginine, isoleucine and leucine, but relatively high in lysine, threonine and glutamic acid. The bovine follitropin-beta contained one methionine residue and low amounts of histidine and phenylalanine, but relatively high in aspartic acid, threonine and glutamic acid. The N-terminal residues of the alpha- and beta-subunits of bovine follitropin were identified to be phenylalanine and glycine respectively.     

Physicochemical and biological characterizations of pregnant mare serum gonadotropin and its subunits.

Pregnant mare serum gonadotropin and its subunits have been further characterized. Ultracentrifugation of the gonadotropin at pH 1.3 and 11.5 showed little evidence of dissociation compared to pH 8.2. Highly purified subunits are obtained by urea dissociation and ion-exchange chromatography followed by gel-filtration. Circular dichroism spectra of the gonadotropin and its subunits are much like those of ovine lutropin and its subunits in that there is little evidence for secondary structure and one or more tyrosine residues are inaccessible in the intact gonadotropin compared to the subunits. The alpha-subunit possesses almost 3 times as much total carbohydrate as the beta-subunit; the individual sugar composition of each was determined as well as the amino acid composition. The alpha-subunit begins with the sequence NH2-Phe-Pro (Gly or Pro) ... and terminates with isoleucine. The beta-subunit has the sequence NH2-Ser-Pro-Gly ...; no C-terminal residue is detectable by either carboxypeptidase or hydrazinolysis. Biological studies show the gonadotropin to be active in assays specific for both lutropin and follitropin. Precipitin test in agar with rabbit antiserum against the gonadotropin show that the beta subunit cross-reacts whereas the alpha subunit does not.     

Primary structure of the ovine pituitary follitropin beta-subunit.

The complete amino acids sequence of the ovine pituitary follitropin beta-subunit was established by studying the tryptic, chymotryptic and thermolytic peptides. The N-terminal sequence of the subunit was confirmed by subjecting the oxidated protein to Edman degradation in an automated sequenator. Automated Edman degradation of the reduced and alkylated (with iodo [14C]acetamide) beta-subunit indicated that most of the molecules used in the sequence studies had lost the N-terminal serine residue. This also confirmed the location of the first five half-cystine residues in the sequence. The proposed structure shows the presence of 111 amino acid residues with the two oligosaccharide moieties linked to asparagine residues located at positions 6 and 23. Heterogeneity occurs at both the termini of the polypeptide chain. Comparison of the sequence of beta-subunit of the ovine hormone with that proposed for human follitropin beta-subunit shows the absence of any deletions in the middle of the peptide chain. Of the 13 replacements, 11 residues can be explained on the basis of a single base change in the codon. The single tryptophan residue of the follitropin occupies an identical position in all the four species that have been studied. The region corresponding to residues 63-105 of the ovine beta-subunit is highly conserved in all the species.     

Amphibian lutropin and follitropin from the bullfrog Rana catesbeiana. Complete amino acid sequence of the alpha subunit.

The amino acid sequence of the alpha-subunit of the gonadotropins, lutropin and follitropin from bullfrog, Rana catesbeiana, has been determined. The alpha-subunit was identified in both hormones by the amino acid composition and ovulation activity of lutropin in the Xenopus ovary, by means of reconstituted hormones in various combinations. The amino acid sequences of two identical alpha-subunits from lutropin and follitropin were determined or deduced by different strategies. The alpha-subunit of those gonadotropins have 97 amino acid residues, the longest among the known alpha-subunits of gonadotropins, and one arginine insertion at position 29. Ten cysteine residues and two sugar-chain binding sites at Asn57 and Asn83 are completely conserved among the species. The molecular mass of this subunit is 11,026 Da not including the sugar chains. The bullfrog alpha-subunit has approximately 70% sequence identity with mammalian alpha-subunits.     

Studies on pituitary follitropin. II. Isolation and characterization of the subunits of the ovine hormone.

The α and β subunits of highly potent ovine follitropin have been isolated by dissociation in 8 m urea, pH 7.5, and chromatography on DEAE-Sephadex A25. The isolated subunits display microheterogeneity on polyacrylamide gel electrophoresis and have very low activity in follitropin-specific radioreceptor and radioimmunoassays. The tryptophan fluorescence spectra of native follitropin and the isolated β subunit are different. The recombinant of follitropin α + β subunit had the same activity as the native hormone in the radioimmunoassay, but its activity in the radioreceptor and in vivo bioassay was about 65% of the intact hormone. Substitution of the follitropin α by ovine lutropin α subunit (prepared by a method not involving urea) to form the recombinant restored full activity in all the three assays investigated. The formation of recombined hormone proceeds at a rapid rate and is almost complete by 6 h. The α and β subunits of ovine follitropin differ from each other in amino acid composition. No significant differences were apparent in their carbohydrate composition. The amino acid composition of the ovine follitropin α and lutropin α subunits are very similar. The oxidized α subunit has phenylalanine at its NH₂-terminus while aspartic acid is present at this position in the oxidized β subunit.     

Studies on pituitary follitropin. V. Isolation of the subunits of the human hormone and reaction of the amino groups with acylating agents.

The alpha and beta subunits of human follitropin were isolated in a high state of purity. The tryptophan fluorescence of the native hormone and the isolated beta subunit are different. The N-terminus of the alpha and beta subunits was identified as valine and aspartic acid respectively. While recombination of the isolated alpha and beta subunits restores the electrophoretic mobility of the intact hormone, its receptor binding activity cannot be fully regenerated. Substitution of the human follitropin alpha by an ovine lutropin alpha subunit, to form a recombinant with the follitropin beta subunit, generates a complex with 2-3 receptor binding activity of the native human follitropin and the same activity as ovine follitropin. Acylation of the intact hormone does not disrupt the quaternary structure but leads to complete inactivation. Acylation studies with the subunits suggests the crucial role of the epsilon-amino groups of the alpha subunit in determining biological activity.     

Studies on the mechanism of follitropin action at the cellular level

With a view toward understanding better the mechanism of action of follitropin, an attempt was made using granulosa cells obtained from intact immature estrogenized rats to study in short-term incubations the effect of highly purified ovine follitropin on the binding of the hormone to the cells and the associated aromatase response. A modified radioimmunoassay procedure has been used to monitor unlabeled physiologically fully active follitropin bound to the cell. A linear relationship between the actual amount of hormone bound to the cells and the estradiol produced in vitro has been established. The amount of ovine follitropin bound that can elicit a half-maximal response in estrogen production was calculated to be 400 pg. The number of follitropin binding sites per cell was 375 and the K_d of binding was 3.03 × 10^?10m. By the addition of ovine follitropin antiserum at different time points of a 4-h incubation period, a continual need for follitropin support for estradiol production has been established.     

Studies on pituitary follitropin. IV. A conformation specific radioimmunoassay for the ovine hormone

An antiserum to partially purified ovine follitropin (50 x NIH-FSH-S10) shows species specificity. It is conformation dependent and requires the proper recombination of the alpha and beta subunits for maximal reactivity. The isolated alpha subunit is essentially inactive and the hormone specific beta subunit is weakly reactive. The homologous radioimmunoassay is valuable for estimating native ovine follitropin in the presence of free subunits. It also provides a sensitive method to study association-dissociation and structure-function relationships.     

A single gonadotropin alpha-subunit gene in normal tissue and tumor-derived cell lines

The structure of gene sequences coding for the mRNA of human chorionic gonadotropin (hCG) alpha-subunit was investigated by Southern blot analysis of genomic DNAs using a cloned, full length cDNA probe. While four hormones, lutropin, follitropin, thyrotropin, and choriogonadotropin, have homologous alpha-subunits, only one gene that bears hCG-alpha sequences could be detected per haploid complement. The structure of this single gonadotropin alpha-subunit gene, which contains intervening sequences, is the same in DNA from first trimester and term placentae. However polymorphism was observed for the presence of a HindIII site and of an Eco RI site in the gene's 3' flanking sequences. The organization of hCG-alpha sequences in several trophoblastic and nontrophoblastic tumor cell lines, which produce hCG subunits, was also examined. In each, the same gene copy number and structure were seen as in normal tissue. Thus, the characteristics of ectopic alpha-subunit expression in these cells seem not to be determined by DNA rearrangements.     

Follitropin receptors in rat testis. Characterization with enzymatically 125I-labeled human follitropin.

The interaction between enzymatically radioiodinated human follitropin and the follitropin receptors in testis homogenate was investigated in immature and adult rats. The 125I-labeled human follitropin exhibited high binding activity with specific binding of up to 17% in the presence of an excess of testis homogenate. Approx. 50% of the bound hormone could be eluted at pH 5, and the receptor purified tracer exhibited a 3.6-fold increase in binding activity when compared with the original tracer preparation. Quantitative analysis of equilibrium binding data was performed with corrections for the measured specific activity and maximum binding activity of the tracer hormone. The equilibrium association constants (Ka) determined 24 degrees C were not significantly different in immature and adult rat testis, and the mean value for Ka was 3.9 . 10(9) M-1. At 37 degrees C, the Ka value obtained using immature rat testis was 1.3 . 10(10) M-1. The association of 125I-labeled human follitropin with immature rat testis homogenate was time and temperature dependent. In the presence of an excess of unlabeled hormone, 30--60% of the preformed hormone . receptor complex was dissociated after 24 h incubation. A specific and sensitive radioligand-receptor assay for follitropin was developed using immature rat testis homogenate. The minimum detectable dose of purified human follitropin was 0.6 ng, and human urinary and pituitary follitropin, ovine follitropin and pregnant mare serum gonadotropin reacted in the assay with equivalent slopes. The potencies of highly purified pregnent mare serum gonadotropin and highly purified human follitropin were similar in the radioligand-receptor assay, consistent with the follitropin bioactivity of the equine gonadotropin.     

Differential sorting of lutropin and the free alpha-subunit in cultured bovine pituitary cells.

The glycoprotein hormones lutropin (LH) and follitropin (FSH) are both synthesized by gonadotrophs in the anterior pituitary but are stored in separate secretory granules prior to secretion. Despite having highly homologous beta-subunits and alpha-subunits with the identical amino acid sequence, the Asn-linked oligosaccharides on LH terminate with SO4-GalNAc while those on FSH terminate with sialic acid-Gal. In addition to LH and FSH, gonadotrophs secrete uncombined (free) alpha-subunit which bears the same sulfated oligosaccharides as LH. We have examined the synthesis and secretion of LH and free alpha-subunit in primary cultures of bovine pituitary cells in order to determine if the sulfated oligosaccharides have any impact on sorting. Our results show that newly synthesized free alpha-subunit is secreted exclusively via the constitutive pathway with a t1/2 of 1.8 h and is never found in dense-core secretory granules. In contrast, LH dimer is secreted by both the constitutive and the regulated pathways. Constitutive secretion and arrival in a dense secretory granule both occur with t1/2 values of 1-1.5 h for newly synthesized LH. Sulfation occurs immediately prior to arrival of LH in the secretory granule and is followed by a period of 1-1.5 h before the LH-containing granules become sensitive to release by gonadotropin releasing hormone. As a result the t1/2 for LH secretion in the presence of gonadotropin releasing hormone is 3.5 h. Sulfation of the free alpha-subunit oligosaccharides is not, therefore, sufficient to direct the alpha-subunit to secretory granules, and the information required for directing LH to granules must reside either in the beta-subunit or the alpha beta-complex.     

Effect of diverse mammalian gonadotropins on estrogen and progesterone production by cultured rat granulosa cells

The ability of gonadotropins from six mammalian species to stimulate estrogen and progesterone production was investigated in granulosa cells of hypophysectomized estrogen-primed immature female rats. Granulosa cells were cultured for 2 days in the presence of delta 4-androstenedione (10(-7) M) with or without various gonadotropin preparations. Treatment with follitropin (follicle-stimulating hormone, FSH) from human, rat, ovine, porcine, equine, and bovine origins resulted in dose-dependent increases in steroidogenesis from negligible amounts to maximal levels of approximately 4-8 and 12-30 ng/10(5) cells for estrogen and progesterone, respectively. The ED50 values of the FSH preparations for stimulation of steroidogenesis were: human: 1-4 ng/ml; ovine: 2.5-30 ng/ml; rat: 1.6-4.0 ng/ml; porcine: 7.5-20 ng/ml; equine 2.5-6 ng/ml; and bovine greater than 100 ng/ml. Lutropin (luteinizing hormone, LH) from rat, ovine, bovine, and porcine origins, human chorionic gonadotropin (hCG), the alpha-subunit of human FSH and the beta-subunit of human LH were ineffective in stimulating steroidogenesis, indicating the specificity of the assay system for FSH. In a high concentration (600 ng/ml), the beta-subunit of human FSH-stimulated steroidogenesis to a small extent. Furthermore, pregnant mare serum gonadotropin and equine LH also caused a dose-dependent stimulation of estrogen and progesterone production, the half-maximal response values (ED50) being 1.8-4 and 7.5-10 ng/ml, respectively. This is consistent with previous in vivo and in vitro findings, showing the potent FSH activities of these hormones. Thus, the cultured rat granulosa cell system provides a sensitive assay for measuring FSH activities of gonadotropins from various mammalian species.     

Glycoprotein hormone assembly in the endoplasmic reticulum: I. The glycosylated end of human alpha-subunit loop 2 is threaded through a beta-subunit hole

Glycoprotein hormone heterodimers are stabilized by their unusual structures in which a glycosylated loop of the alpha-subunit straddles a hole in the beta-subunit. This hole is formed when a cysteine at the end of a beta-subunit strand known as the "seatbelt" becomes "latched" by a disulfide to a cysteine in the beta-subunit core. The heterodimer is stabilized in part by the difficulty of threading the glycosylated end of the alpha-subunit loop 2 through this hole, a phenomenon required for subunit dissociation. Subunit combination in vitro, which occurs by the reverse process, can be accelerated by removing the alpha-subunit oligosaccharide. In cells, heterodimer assembly was thought to occur primarily by a mechanism in which the seatbelt is wrapped around the alpha-subunit after the subunits dock. Here we show that this "wraparound" process can be used to assemble disulfide cross-linked human choriogonadotropin analogs that contain an additional alpha-subunit cysteine, but only if the normal beta-subunit latch site has been removed. Normally, the seatbelt is latched before the subunits dock and assembly is completed when the glycosylated end of alpha-subunit loop 2 is threaded beneath the seatbelt. The unexpected finding that most assembly of human choriogonadotropin, human follitropin, and human thyrotropin heterodimers occurs in this fashion, indicates that threading may be an important phenomenon during protein folding and macromolecule assembly in the endoplasmic reticulum. We suggest that the unusual structures of the glycoprotein hormones makes them useful for identifying factors that influence this process in living cells.     

Follitropin receptors in rat testis characterization with enzymatically 125I-labeled human follitropin

The interaction between enzymatically radioiodinated human follitropin and the follitropin receptors in testis homogenate was investigated in immature and adult rats. The ¹²⁵I-labeled human follitropin exhibited high binding activity, with specific binding of up to 17% in the presence of an excess of testis homogenate.Approx. 50% of the bound hormone could be eluted at pH 5, and the receptor purified tracer exhibited a 3.6-fold increase in binding activity when compared with the original tracer preparation. Quantitative analysis of equilibrium binding data was performed with corrections for the measured specific activity and maximum binding activity of the tracer hormone. The equilibrium association constants (K_a) determined at 24°C were not significantly different in immature and adult rat testis, and the mean value for K_a was 3.9 · 10⁹ M^?1. At 37°C, the K_a value obtained using immature rat testis was 1.3 · 10¹⁰ M^?1. The association of ¹²⁵I-labeled human follitropin with immature rat testis homogenate was time and temperature dependent. In the presence of an excess of unlabeled hormone, 30–60% of the preformed hormone · receptor complex was dissociated after 24 h incubation. A specific and sensitive radioligand-receptor assay for follitropin was developed using immature rat testis homogenate. The minimum detectable dose of purified human follitropin was 0.6 ng, and human urinary and pituitary follitropin, ovine follitropin and pregnant mare serum gonadotropin reacted in the assay with equivalent slopes. The potencies of highly purified pregnant mare serum gonadotropin and highly purified human follitropin were similar in the radioligand-receptor assay, consistent with the follitropin bioactivity of the equine gonadotropin.     

Isolation and characteristics of cyanogen bromide peptides of transducin alpha and beta subunits

The alpha- and beta-subunits of the GTP-binding protein (transducin) from cattle retina were cleaved with cyanogen bromide. 21 peptides covering 90-100% of the amino acid sequence of the alpha- and beta-subunits were isolated from the hydrolyzate. Cyanogen bromide peptides complete or partial amino acid sequence was determined, the results were compared with those by Numa and coworkers [1] and Lochrie et al. [2] at the primary structure of the transducin alpha-subunit deduced from the nucleotide sequence of the cDNA. The structure by Lochrie is shown to differ much from the true structure of the alpha-subunit; probably, the investigators isolated cDNA, corresponding to the gene for some GTP-binding protein homologous to transducin, but not to the gene for the transducin alpha-subunit. The Numa's structure also contains an error. The final primary structure of the transducin alpha-subunit is given. The protein polypeptide chain consists of 349 amino acid residues and has an acetylmethionine residue as the N-terminal residue.     

Nucleotide sequence of cloned complementary deoxyribonucleic acid for the alpha subunit of bovine pituitary glycoprotein hormones

Recombinant DNA plasmids containing sequences coding for the alpha subunit of the bovine pituitary glycoprotein hormones have been isolated. The nucleotide sequences of three different cDNA clones have been determined. The largest alpha-subunit cDNA clone was found to contain 713 bases including 77 nucleotides from the 5'-untranslated region, 72 nucleotides coding for a precursor segment, 288 nucleotides coding for the mature alpha subunit, and 276 nucleotides from the 3'-untranslated region of the mRNA followed by a poly(A) segment. This cDNA likely represents most of the bovine alpha-subunit mRNA sequence. Nucleotide sequences were obtained from the cDNA inserts of two other alpha-subunit clones, and several differences among the three cDNA sequences have been detected. These differences in nucleotide sequence may represent either individual variation in genomic sequence or cloning artifacts. Comparison of the bovine alpha-subunit cDNA sequence to the sequences of human, rat, and mouse alpha-subunit cDNAs reveals that the bovine sequence has greater than 70% homology with the other cDNAs. The cloned alpha-subunit cDNA should provide a useful probe for further studies of the structure and expression of this interesting gene.     

Further characterization of the ovine lutropin alpha and beta subunits prepared by the salt precipitation method.

The subunits of ovine lutropin prepared by acid dissociation and salt precipitation were characterized by end group analysis, tryptic peptide mapping, SDS gel electrophoresis and biological activity. No evidence of internal peptide cleavage was found in the alpha subunit. The subunits possessed low activity. The alpha and beta subunits recombined effectively to generate a complex that had full receptor binding activity and in vitro biological activity. The recombinants of subunits prepared by countercurrent distribution showed only 50% activity in both assays. The salt precipitation method alpha subunit could be completely reduced and reoxidized in the absence of denaturants. The reoxidized alpha subunit combines with the native beta subunit generating full activity. However, this recombined hormone tends to lose activity with time, suggesting that the reoxidation may not fully restore the native structur of the reduced alpha subunit. The native lutropin alpha subunit effectively combined with follitropin beta subunit generating complete follitropin activity.     

Efficient preparation of glycoprotein hormones lacking an alpha-subunit oligosaccharide

The oligosaccharide on alpha-subunit loop 2 (alpha 2) is needed for full glycoprotein hormone efficacy. Efforts to prepare glycoprotein hormone antagonists usually involve removing the alpha 2 oligosaccharide and are hampered by its requirement for efficient heterodimer secretion from mammalian cells. Here we show that hormones lacking this oligosaccharide can be produced by treating them at low pH to dissociate the heterodimer and permitting the subunits to re-associate in the presence of peptide N-glycosidase F (PNGase F). Re-assembly of human choriogonadotropin, human follitropin, and bovine lutropin occurred rapidly and efficiently following removal of the alpha 2 oligosaccharide by PNGase F. Consequently, virtually all heterodimers formed in the presence of this enzyme lacked this oligosaccharide. These findings support the notion that heterodimer assembly in vitro occurs by a threading mechanism that is impeded by the presence of the alpha 2 oligosaccharide. This procedure should facilitate the study of glycoprotein hormone structure and function.     

Analysis of the kinetics of ovine follitropin agonist-antagonist interactions with pig ovarian membranes

The binding of 125I-labeled ovine follitropin (oFSH) and 125I-labeled deglycosylated ovine follitropin (DG-oFSH) to porcine granulosa cell membranes was studied at equilibrium and nonequilibrium binding conditions and statistically analyzed. Saturation and competition binding experiments revealed homogeneity in the population of binding sites labeled with 125I-oFSH, having a pK estimation of approximately equal to 10. 125I-DG-oFSH similarly interacts with a single uniform class of receptors of equal affinity (pK approximately equal to 10) and binding capacity as oFSH. In contrast, displacement experiments using 125I-DG-oFSH as tracer and unlabeled oFSH as competing ligand demonstrate slope factors less than unity, suggesting apparent heterogeneity of sites not observed with 125I-DG-oFSH vs. DG-oFSH competition experiments. Under these conditions, it appears that FSH binds to two sites in near equal proportion but of unequal affinities. The total specific binding capacities of these sites equal those observed in 125I-DG-oFSH/unlabeled DG-oFSH competition experiments. Analysis of oFSH association kinetics at 37 degrees C by curve-fitting methods is best explained by a biexponential rate equation describing a fast and a slow association component that are equally distributed. DG-oFSH demonstrates a disproportionately greater amount of fast vs. slow binding component. The binding half-times for each component of oFSH and DG-oFSH are similar, i.e., minutes for the fast and hours for the slow t 1/2 times. At 37, 25, and 4 degrees C, DG-oFSH exhibits greater velocity of binding to the receptor than oFSH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Molecular cloning of a cDNA for alpha-subunit of rat liver electron transfer flavoprotein

Two cDNA clones for the alpha-subunit of rat liver electron transfer flavoprotein were isolated and their nucleotide sequences were determined. The longer cDNA contained a protein-coding region of 900 nucleotides and 3'-noncoding region of 335 nucleotides. The identity of the clone was confirmed by matching the amino acid sequence predicted from the cDNA with the sequence of one of the lysyl endopeptidase-digested peptides from the purified alpha-subunit. The molecular weight of the protein calculated from the protein-coding nucleotides was approx. 3,000 daltons smaller than that of the precursor, suggesting that the cDNA was not of full length. The derived amino acid composition fairly agreed with the chemically determined amino acid composition of the purified alpha-subunit, indicating that the protein-coding region contains most of the mature alpha-subunit.