The primary structure of ovine interstitial cell-stimulating hormone. I. The -subunit

The effects of denaturant and temperature on the circular dichroic spectrum of ovine lutropin-alpha

Circular dichroic (CD) spectra of the glycoprotein hormone subunit, lutropin-alpha, have been obtained under denaturing conditions (6 M guanidinium chloride, GdmCl) with the disulfides intact, reduced, and reduced and S-carboxymethylated. Above 260 nm, these spectra are similar (although not identical), showing that the disulfide chromophores do not contribute appreciably to the near ultraviolet CD spectrum under denaturing conditions. However, these spectra differ considerably from that of native lutropin-alpha over this spectral region demonstrating that the tertiary structure of lutropin-alpha has a major influence on the near ultraviolet CD spectrum. The magnitude of the 232.5 nm positive CD extremum of lutropin-alpha was found to increase with increasing GdmCl concentration and with decreasing temperature. Between 5 degrees C and 45 degrees C, the ellipticity [theta] at 232.5 nm changed in a linear fashion with temperature; at other wavelengths the thermal changes in [theta] were quite small. [theta] at 232.5 nm increased by about 80% over a 3 M GdmCl range; in contrast, [theta] at 216 nm (indicative of beta-structure) exhibited maximal changes over a 0.5 M GdmCl range. These results indicate some conformational flexibility in lutropin-alpha. The positive 232.5 nm CD extremum has been assigned to one or more tyrosyls which are influenced by an intact disulfide. The latter was demonstrated by showing that reduced and S-carboxymethylated lutropin-alpha failed to exhibit the positive extremum.

The primary structure of ovine interstitial cell stimulating hormone. IV: Disulfide bridges of the beta subunit

The disulfide linkage of the 12 half-cystine residues in the beta subunit of ovine interstitial cell stimulating hormone has been investigated by enzymic and partial acid hydrolysis of the intact molecule. Results indicate that the disulfide bridges are formed by residues 9-38, 23-72, 26-110, 34-90, 57-88, and 93-100.

Follicle-stimulating hormone in the brushtail possum (Trichosurus vulpecula): purification, characterization, and radioimmunoassay

Follicle-stimulating hormone (FSH) was purified from brushtail possum (Trichosurus vulpecula) pituitary glands by using the following purification techniques: fractional ammonium sulfate precipitation, triazinyl-dye affinity chromatography, hydrophobic interaction chromatography, and gel filtration. A yield of 18 micrograms of FSH per gram of pituitary, with a recovery of 12%, was obtained from 1400 glands (20.3 g wet weight). The purified FSH activity per gram of protein was 1320 times more potent than the initial pituitary homogenate. Contamination with possum luteinizing hormone (LH) was < 0.02%. The amino acid composition of possum FSH was similar to that of ovine FSH. Amino-terminal sequencing for 11 cycles indicated that the alpha subunit has the same sequence as ovine FSH except for residue 7, where the possum FSH alpha subunit contains isoleucine compared to the ovine subunit which contains threonine. The beta subunit has two substitutions in the first 11 residues and does not contain the N terminal serine that is found in ovine FSH. Amino acid sequencing did not detect any contaminating proteins. Possum FSH bound possum and bovine testicular receptors with similar affinities. It was also able to stimulate in vitro cAMP production by Chinese hamster ovary cells which express recombinant FSH receptors. In the receptor assays and the bioassay possum FSH has about 21% of the potency of ovine FSH (USDA-oFSH-19-SIAFP-RP2). An RIA was developed for possum FSH using 125I-possum FSH and an antiserum raised against human FSH. The RIA has a sensitivity of 0.3 ng/ml, a 50% displacement of 2.7 ng/ml, and a cross reactivity of 0.05% against possum LH. Plasma FSH levels in male possums (10.4 +/- 2.4 ng/ml, n = 4) were higher (P < 0.05) than levels in females (1.0 +/- 0.1 ng/ml, n = 4). Five days after gonadectomizing these possums the plasma FSH levels increased (P < 0.05) to 27.1 +/- 0.2 ng/ml in the males and to 6.6 +/- 2.0 ng/ml in the females. In summary, we have purified and partially characterized possum FSH. We have also set up an RIA for the hormone and shown that males have higher levels than females and that plasma FSH increases after gonadectomy.

Molecular structures of glycoprotein hormones and functions of their carbohydrate components

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Cloning and sequence analysis of the common alpha-subunit complementary deoxyribonucleic acid of turkey pituitary glycoprotein hormones

Two cDNA clones of nearly full length that encode the turkey pituitary common alpha-subunit glycoprotein hormone have been isolated from a pituitary cDNA library and their nucleotide sequences have been determined. The longer alpha-subunit clone was 777 bp in length. It contained 88 bp of the 5'-untranslated region (UTR), an open reading frame of 360 bp (that encodes the turkey alpha-subunit 24 amino acid leader polypeptide fragment and 96 amino acid apoprotein), and a 312-bp 3'-UTR followed by a 17-bp poly A tract. When the nucleotide sequence of the turkey alpha-subunit was compared with the sequence of the chicken alpha-subunit clone, the coding region was greater than 98% homologous, but was only 69 to 76% homologous when compared with mammalian alpha-subunit sequences. Northern blot analysis showed an approximate 800 bp processed transcript that hybridized to the labeled turkey alpha-subunit cDNA. There was a greater than fourfold up-regulation of the steady-state levels of turkey alpha-subunit transcription when intact cultured pituitaries were treated with chicken gonadotropin-releasing hormone I.

Primary structure of two mRNAs encoding putative salmon alpha-subunits of pituitary glycoprotein hormone

1. By random sequence analyses, we isolated from the cDNA library of salmon pituitary glands two clones, the deduced amino acid sequences corresponding to the C-terminal region of which are almost the same as those of the alpha subunits of mammalian glycoprotein hormones. 2. Comparison of the nucleotide sequences and deduced amino acid sequences from these two clones with those of mammalian species revealed that the two newly-isolated cDNAs corresponded to mRNAs encoding the putative salmon pre-alpha subunit of glycoprotein hormones. 3. Homology in the nucleotide sequences of these two clones suggested that corresponding mRNAs may be encoded by separate genes which probably evolved from a common ancestral gene.

Primary structure of sperm whale luteinizing hormone

Luteinizing hormone was extracted from sperm whale pituitaries and separated into alpha- and beta-subunits. These subunits were cleaved with cyanogen bromide, and digested with trypsin and chymotrypsin. The fragments obtained were separated and purified by gel filtration on Sephadex and by ion exchange chromatography, reversed phase chromatography and chromatoelectrophoresis. The amino acid sequence of peptides obtained was studied by dansyl-Edman's method and Edman's modification of Chang et al. The study made it possible to establish the complete amino acid sequence of sperm whale LH alpha- and beta-subunits.

Pituitary gonadotropic hormone from a chondrostean fish, starred sturgeon (Acipenser stellatus Pall.) III. Polymorphism

Four biologically active fractions of gonadotropic hormone (aci-GTH-A, -B, -C, -D) were isolated and purified from acetonized pituitaries of the starred sturgeon (Acipenser stellatus Pall.). Their separation was achieved by DEAE-cellulose chromatography. Disc-electrophoresis and especially isoelectric focusing in polyacrylamide gel showed that each fraction contained several components. Not less than 15 different components as a whole with isoelectric points ranging from 4.5 to 7.0 could be counted in four aci-GTH preparations. All these components were active in toad oocyte maturation test. Only two of four preparations (aci-GTH-A and -D) were practically free of common components. All aci-GTH preparations were shown to be homogeneous and identical by molecular weight, sedimentation coefficient, sialic acid content, and some immunological properties. N-terminal amino acid analysis revealed tyrosine and leucine in all aci-GTH preparations, with the only exception of aci-GTH-D that contained an additional polypeptide with N-terminal glycine. No differences in the spectra of aci-GTH isoforms were found when pituitary extract, newly purified or 3 years older hormone preparations were submitted to isoelectric focusing.

Nucleotide sequence of the mRNA encoding the pre-alpha-subunit of mouse thyrotropin

We have constructed and cloned in bacteria recombinant DNA molecules containing DNA sequences coding for the precursor of the alpha subunit of thyrotropin (pre-TSH-alpha). Double-stranded DNA complementary to total poly(A)+RNA derived from a mouse pituitary thyrotropic tumor was prepared enzymatically, inserted into the Pst I site of the plasmid pBR322 by using poly(dC).poly(dG) homopolymeric extensions, and cloned in Escherichia coli chi 1776. Cloned cDNAs encoding pre-TSH-alpha were identified by their hybridization to pre-TSH-alpha mRNA as determined by cell-free translations of hybrid-selected and hybrid-arrested RNA. The nucleotide sequences of two cDNAs (510 and 480 base pairs) were determined with chemical methods and corresponded to much of the region coding for the alpha subunit and the 3' untranslated region of pre-TSH-alpha mRNA. The sequence of the 5' end of the mRNA was determined from cDNA synthesized by using total mRNA as template and a restriction enzyme DNA fragment as primer. Together these sequences represented greater than 90% of the coding and noncoding regions of full-length pre-TSH-alpha mRNA, which was determined to be 800 bases long. The amino acid sequence of the pre-TSH-alpha deduced from the nucleotide sequence showed a NH2-terminal leader sequence of 24 amino acids followed by the 96-amino-acid sequence of the apoprotein of TSH-alpha. There is greater than 90% homology in the amino acid sequences among the murine, ruminant, and porcine alpha subunits and 75-80% homology among the murine, equine, and human alpha subunits. Several regions of the sequence remain absolutely conserved among all species, suggesting that these particular regions are essential for the biological function of the subunit. The successful cloning of the alpha subunit of TSH will permit further studies of the organization of the genes coding for the glycoprotein hormone subunits and the regulation of their expression.

Primary structure of the ovine pituitary follitropin alpha-subunit

All the tryptic peptides of reduced and aminoethylated alpha-subunit of ovine pituitary follitropin were isolated. From their their composition and partial sequence analysis of the N- and C-termini of the oxidized protein and reliance on homology with the sequence of lutropin alpha-subunit, an entire structure for the follitropin alpha-subunit has been proposed. The structure of the alpha-subunits from the two ovine hormones are identical.

Sugar residues on proteins

Glycoproteins have become increasingly important in the structure and function of many different mammalian systems; for example, membrane glycoproteins and glycoprotein hormones. It is, therefore, important to understand their chemistry, which would include an understanding of both the carbohydrate and protein parts of the molecule. Since the chemical characterization of the protein moiety has been extensively examined and the techniques for its characterization are well worked out, only the carbohydrate portion of glycoproteins will be reviewed in this article. The chemical nature of the carbohydrate moiety of glycoproteins will be examined. First, the types of monosaccharides present in animal systems, especially those in the mammalian systems, will be described. Next, various types of simple and complex carbohydrate chains will be discussed to establish the diversity, size, and number of chains present in the carbohydrate units in different glycoproteins. Then, the type of linkages of the carbohydrate to the protein will be examined to determine if the primary sequence of protein is important in determining the size and type of carbohydrate chains present in glycoproteins. Finally, the current methods of structural elucidation such as monosaccharide sequence, intersugar bonds, and anomeric linkages in the carbohydrate moiety of glycoproteins will be reviewed. These methods include the techniques of periodate oxidation, methylation, partial acid hydrolysis, and specific glycosidase digestion of glycoproteins, as well as the latest techniques using micromethods of carbohydrate quantitation and characterization involving gas chromatography and mass spectrometry. The function of the carbohydrate in glycoproteins will also be considered. First, hormone glycoproteins will be discussed in their relationship to the immunological and biological function of the glycoprotein when the carbohydrate is sequentially removed. Next, the function of the carbohydrate in the turnover of glycoproteins will be discussed. These topics will be considered in order to develop an understanding of a specific function(s) of the carbohydrate in glycoproteins.

Secondary structure prediction of anterior pituitary hormones. Lack of correlation between predicted values and circular dichroism data

The secondary structures of human somatotropin, human choriomammotropin, ovine and porcine prolactin, human, ovine and porcine beta-lipotropin, human and ovine lutropin, human thyrotropin, human corticotropin, alpha-melanotropin and human beta-melanotropin have been predicted by the method of Chou & Fasman. Predicted contents of alpha-helix and beta-sheet do not correspond well with values estimated from circular dichroism spectra.

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.

The evolution of gonadotropins: some molecular data concerning a non-mammalian pituitary gonadotropin, the hormone from a teleost fish (Cyprinus carpio L.)

The amino acid and sugar compositions as well as long N-terminal sequences and the C-terminal amino acids of the two subunits of carp gonadotropin, SU I and SU II, were determined. An important homology was demonstrated between SU I and alpha-subunits and between SU II and beta-subunits of mammalian gonadotropins. Moreover SU II was more closely related to the beta-subunit of LH than to the beta-subunit of FSH.

Studies of disulfide bond location in ovine lutropin beta subunit

By combinations of selective chemical cleavage (cyanogen bromide), selective enzymatic cleavage (trypsin, thermolysin), and random cleavage (partial acid hydrolysis) a series of disulfide-containing peptides have been isolated from ovine lutropin beta subunit. These peptides suggest six disulfide linkages between half-cystine residues in positions 23-72, 26-110, 93-100, 34-88, 9-90, and 38-57. The latter pair was placed by elimination of other possibilities. The first three pairs are in agreement with a report by Chung, D., Sairam, M. R. and Li, C. H. (1975) Int. J. Peptide Protein Res. 7, 487-493; the pair 93-100 has also been detected by Reeve, J. R., Cheng, K. W. and Pierce, J. G. (1975) Biochem. Biophys. Res. Commun. 67, 149-155, using partial reduction and alkylation. In an attempt to improve the efficiency of enzymatic attack, a preliminary partial reduction as per Reeve et al. [16] was done. In this instance a peptide suggesting an additional disulfide linkage between half-cystines 23-26 was obtained as well as peptides consistent with the 23-72 and 26-110 placements. This was interpreted as an artifactual opening and recombining during partial reduction-reoxidation to produce the 23-26 linkage. The placement of three disulfide bonds (34-88, 9-90, and 38-57) is in disagreement with the pairings Chung et al. [15] suggest for these six half-cystine residues. Six reasons for uncertainty in the placement of disulfide bonds are discussed. It is concluded the definitive placement of the disputed three disulfide bonds in ovine lutropin beta subunit remains an open question.

On the optical activity of ionized tyrosyl residues in ovine lutropin

The effect of alkali on the circular dichroic (CD) spectra of ovine lutropin and its subunits has been studied. Mild alkaline pH induces the appearance of a new optically active band in the 250-nm region of the spectra of lutropin without any detectable alteration in the secondary structure of the protein. This change is reversible and can be correlated with ionization of 2--3 exposed tyrosyl residues in the intact hormone. In a previous report from this laboratory it was concluded that the three exposed tyrosyl residues are located in the alpha subunit, in positions 21, 92 and 93 [Burleigh, B.D., Liu, W.-K, and Ward, D.M. (1976) J. Biol. Chem. 251, 308--315]. Nitration of these residues lowers the pH at which the intensity of the 250-nm band is maximal. The importance of the tyrosyl residues of lutropin alpha (as opposed to those of lutropin beta) is also supported by the similarity of the effect of alkali on the CD spectra of lutropin and lutropin alpha. Further evidence for this involvement was also obtained by a comparison of the alkali-induced changes of refolded lutropin (alpha + beta recombinant) and the product obtained by recombination of des-(92--96)-lutropin alpha (obtained from carboxypeptidase treatment of the alpha-subunit) and lutropin beta. The results indicate that removal of tyrosines alpha 92 and alpha 93 results in a decrease of the intensity of the 235-nm band of ovine lutropin (at pH7.5) as well as that of the 250-nm band observed under alkaline conditions. It is therefore concluded that the 250-nm band observed in alkaline solutions of lutropin arises (at least partially) from the red shift produced in the short-wavelength optically active band of tyrosines alpha 21, alpha 92, and alpha 93 upon ionization.

Carboxymethylation of methionine residues in bovine pituitary luteinizing hormone and its subunits. Location of specifically modified methionine residues

Bovine lutropin (luteinizing hormone) was carboxymethylated at pH3.0 for 12 h at 37 degrees C with iodoacetic acid for specific modification of methionine residues. To facilitate the location of preferentially modified methionine residues, iodoE114C]acetic acid was added as tracer. The alpha and beta subunits of bovine lutropin were carboxymethylated with a 2- or 5-fold molar excess of iodoacetic acid either in the presence or absence of their counterpart subunits. The modified subunits were separated and isolated by counter-current distribution followed by gel filtration on Sephadex G-100. To locate the modified methiones, the isolated alpha or beta chain was reduced. S-carboxymethylated and subjected to tryptic hydrolysis. The tryptic peptides were fractionated by gel filtration on Bio-Gel P-10. From analyses of the purified 14C-labelled tryptic peptides, it was observed that methionine-8 and -33 in bovine lutropin alpha chain and methionine-52 in the beta chain were preferentially modified. Similar results were obtained when isolated alpha and beta subunits were individually carboxymethylated in the absence of their counterpart subunit under identical conditions. The fact that a recombinant of native human lutropin alpha chain, in which a valine residue is present in the position corresponding to methionine-8 of bovine lutropin alpha chain, and carboxymethylated bovine lutropin beta chain regenerated a substantial amount of receptor-site-binding activity indicated that methionine-8 in bovine alpha chain was biologically not essential. These studies showed clearly that both methionine-33 in the alpha chain and methionine-52 in the beta subunit were involved for optimum binding between bovine lutropin and its receptors for expression of hormonal activity.

Preparation of lutropin with acetyl or acetimidinyl substituents on the amino groups of the beta-subunit

The free amino groups in ovine lutropin beta subunit were acylated with acetic anhydride and methyl acetimidinate-HCl to produce the corresponding acetyl and acetimidinyl ovine lutropin beta derivative. These two derivatives recombined with ovine lutropin alpha as well as native ovine lutropin beta, but produced lutropin derivatives which were 33-50% less active than the ovine lutropin alpha + ovine lutropin beta in biological assays.

Guanidination of ovine luteinizing hormone and effects on activity

The free amino groups in oLH, oLHalpha and oLHbeta were guanidinated by O-methylisourea. The epsilon-NH2 groups of lysine residues reacted bo substitute these positions in the sequence with the more basic homoarginine residue. The alpha-NH2 groups did not react under the conditions used. Guanidinated oLH or the products of guanidinated oLHalpha + native oLHbeta or guanidinated oLHalpha + guanidinated oLHbeta were inactive in two bioassay systems. Native oLHalpha + guanidinated oLHbeta, however, showed potencies of 39% to 55% of that observed with the native subunit recombinant or native oLH. Possible structural implications for hormone-receptor site interactions are discussed.