Restoring Carboxylates on Highly Modified Alginates Improves Gelation, Tissue Retention and Systemic Capture

Alginate hydrogels are gaining traction for use in drug delivery, regenerative medicine, and as tissue engineered scaffolds due to their physiological gelation conditions, high tissue biocompatibility, and wide chemical versatility. Traditionally, alginate is decorated at the carboxyl group to carry drug payloads, peptides, or proteins. While low degrees of substitution do not cause noticeable mechanical changes, high degrees of substitution can cause significant losses to alginate properties including complete loss of calcium cross-linking. While most modifications used to decorate alginate deplete the carboxyl groups, we propose that alginate modifications that replenish the carboxyl groups could overcome the loss in gel integrity and mechanics. In this report, we demonstrate that restoring carboxyl groups during functionalization maintains calcium cross-links as well as hydrogel shear-thinning and self-healing properties. In addition, we demonstrate that alginate hydrogels modified to a high degree with azide modifications that restore the carboxyl groups have improved tissue retention at intramuscular injection sites and capture blood-circulating cyclooctynes better than alginate hydrogels modified with azide modifications that deplete the carboxyl groups. Taken together, alginate modifications that restore carboxyl groups could significantly improve alginate hydrogel mechanics for clinical applications. STATEMENT OF SIGNIFICANCE: Chemical modification of hydrogels provides a powerful tool to regulate cellular adhesion, immune response, and biocompatibility with local tissues. Alginate, due to its biocompatibility and easy chemical modification, is being explored for tissue engineering and drug delivery. Unfortunately, modifying alginate to a high degree of substitution consumes carboxyl group, which are necessary for ionic gelation, leading to poor hydrogel crosslinking. We introduce alginate modifications that restore the alginate's carboxyl groups. We demonstrate that modifications that reintroduce carboxyl groups restore gelation and improve gel mechanics and tissue retention. In addition to contributing to a basic science understanding of hydrogel properties, we anticipate our approach will be useful to create tissue engineered scaffolds and drug delivery platforms.

Examining ubiquitinated peptide enrichment efficiency through an epitope labeled protein

Ubiquitination is a dynamic process that is responsible for regulation of cellular responses to stimuli in a number of biological systems. Previous efforts to study this post-translational modification have focused on protein enrichment; however, recent research utilizes the presence of the di-glycine (Gly-Gly) remnants following trypsin digestion to immuno-enrich ubiquitinated peptides. Monoclonal antibodies developed to the cleaved ubiquitin modification epitope, (tert-butoxycarbonyl) glycylglycine (Boc-Gly-Gly-NHS)(1), are used to identify the Gly-Gly signature. Here, we have successfully generated the Boc-Gly-Gly-NHS modification and showed that when conjugated to a lysine containing protein, such as lysozyme, it can be applied as a standard protein to examine ubiquitinated peptide enrichment within a complex background.

Structure and structure-function relationships in glycoprotein hormones

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

Penetrating ulcer of the thoracic aorta: what is it? How do we recognize it? How do we manage it?

BACKGROUND

Although classic type A and B aortic dissections have been well described, less is known about the natural history of penetrating atherosclerotic ulcers of the thoracic aorta. This study differentiates penetrating ulcer from aortic dissection, determines the clinical features and natural history of these ulcers, and establishes appropriate correlates regarding optimal treatment.

METHODS

A retrospective review of patient records and imaging studies was conducted with 198 patients with initial diagnoses of aortic dissection (86 type A, 112 type B) at our institution from 1985 to 1997.

RESULTS

Of the 198 patients, 15 (7.6%) were found to have a penetrating aortic ulcer on re-review of computed tomographic scans, magnetic resonance images, angiograms, echocardiograms, intraoperative findings, or pathology reports. Two ulcers (13.3%) were located in the ascending aorta; the other 13 (86.7%) were in the descending aorta. In comparison with those with type A or B aortic dissection, patients with penetrating ulcer were older (mean age 76.6 years, p = 0.018); had larger aortic diameters (mean diameter 6.5 cm); had ulcers primarily in the descending aorta (13 of 15 patients, 86.7%); and more often had ulcers associated with a prior diagnosed or managed AAA (6 of 15 patients, 40.0%; p = 0.0001). Risk for aortic rupture was higher among patients with penetrating ulcers (40.0%) than patients with type A (7.0%) or type B (3.6%) aortic dissection (p = 0.0001).

CONCLUSIONS

Accurate recognition and differentiation of penetrating ulcers from classic aortic dissection at initial presentation is critical for optimal treatment of these patients. For penetrating ulcer, the prognosis may be more serious than with classic type A or B aortic dissection. Surgical management is advocated for penetrating ulcers in the ascending aorta and for penetrating ulcers in the descending aorta that exhibit early clinical or radiologic signs of deterioration.

Cesarean scar pregnancy: imaging and treatment with conservative surgery

BACKGROUND

Pregnancy in a cesarean scar represents a rare type of secondary abdominal pregnancy. Early diagnosis can be challenging and optimal treatment is unknown.

CASE

A 21-year-old woman presented for an abortion at 8 weeks' gestation. A cesarean delivery had been performed 5 months earlier. Suspecting a cervical pregnancy, her physician referred her to us, and an 8-week cesarean scar gestation was diagnosed and then confirmed by serial sonograms, cystoscopy, and magnetic resonance imaging. The patient elected pregnancy termination, which was accomplished by hysterotomy with uterine preservation followed by intramuscular methotrexate.

CONCLUSION

We report a case of cesarean scar pregnancy treated surgically with uterine preservation. This approach should be considered when cesarean scar ectopic pregnancy is diagnosed.

The beta subunits of glycoprotein hormones. Formation of three-dimensional structure during cell-free biosynthesis of lutropin-beta

The folding of the hormone-specific (beta) subunit of the glycoprotein hormone bovine lutropin was studied after the translation and processing of bovine pituitary mRNA in a system derived from Krebs ascites tumor cells. Of the three forms of beta subunit recognized in this system, only the subunit which had both its prepeptide removed and an oligosaccharide moiety attached formed a tertiary structure which could be immunoprecipitated by an antiserum specific to isolated (folded) lutropin-beta. This glycosylated subunit also combined with added alpha subunit to form the dimeric alpha-beta complex. The results of the translation and processing experiments parallel those of previous experiments in which alpha subunit folding was examined. In contrast to alpha subunit, however, the difficulty of demonstrating correct refolding of beta subunit after reduction and reoxidation of its disulfide bonds strongly suggests that the formation of its correct tertiary structure not only requires carbohydrate attached to the peptide chain but also must occur during formation of the nascent beta chain.

TSH crosslinks to the TSH receptor through the beta subunit

TSH binds specifically to the TSH receptor present on the surface of thyroid follicular cells and consequently activates adenylate cyclase. It was not known, however, which of the two subunits of the TSH bound to the receptor. By covalently crosslinking TSH to its receptor and characterizing the product with antisera specific for either the alpha or beta subunit of TSH, we have shown that the TSH beta subunit but not the TSH alpha subunit crosslinks to the TSH receptor.

Glycosylation of ovine prolactin during cell-free biosynthesis

Recently, a glycosylated form of ovine PRL (oPRL) was isolated from a crude pituitary preparation. As glycosylation of PRL was unexpected and because the composition of the oligosaccharide-containing peptide indicated the carbohydrate portion to be extensively degraded, studies of the glycosylation of oPRL during cell-free biosynthesis were initiated. Two glycosylated forms of oPRL can be recognized when biosynthesis occurs in ovine pituitary microsomes. Both forms are converted to mature PRL by digestion with endoglycosidase H and, thus, appear to contain only asparagine-linked, high mannose-type carbohydrate moieties. In contrast, immunoprecipitates from bovine pituitary microsomes consist of the expected (and nonglycosylated) pre-PRL and PRL. The results are consistent with the absence of a sequence segment in the bovine hormone which permits glycosylation (Asn-X-Ser- or Thr-) and the presence of the segment Asn31-Leu-Ser- in the ovine hormone. The occurrence of two glycosylated forms of oPRL is not understood; it may result from an additional site in oPRL capable of glycosylation.

The common alpha subunit of bovine glycoprotein hormones: limited formation of native structure by the totally nonglycosylated polypeptide chain

The folding of the bovine glycoprotein hormone alpha subunit, synthesized in bacteria following insertion of the nucleotide sequence coding for this polypeptide, has been studied to determine the effect that a complete lack of carbohydrate has on this process. The bacterially derived alpha polypeptide (bac-alpha), extracted from E. coli in the presence of reductant and denaturant, had an estimated 0.2% native structure as determined by a conformationally sensitive radioimmunoassay. Upon reduction of disulfide bonds and reoxidation in air, the amount of native structure increased about 18-fold. Approximately 2% of the refolded bac-alpha preparation combines with the beta subunit of human chorionic gonadotropin (hCG beta) to form a complex that binds to the gonadotropin receptor and elicits a biological response. Since the correct folding (by immunological criteria) of bac-alpha (ca 3%) is significantly greater than expected from a random formation of disulfide bonds (0.1%), it appears that correct folding of alpha subunit can occur in the complete absence of carbohydrate, though in very low yield. Native bovine lutropin alpha subunit (LH alpha) and chemically deglycosylated LH alpha (which retains two asparagine-linked N-acetyl glucosamine residues per alpha oligosaccharide) were subjected to the same reduction/reoxidation regimen as the bacterially produced alpha subunit. As has been reported previously [Giudice LC, Pierce, JG, J Biol Chem 251: 6392, 1976] intact LH alpha fully regained its native structure. The partially deglycosylated LH alpha also refolds to a native-like structure in high yield as assessed by immunological assays and by its ability to combine with HCG beta to form a biologically active complex. The data show that carbohydrate, while not obligatory for correct folding, greatly facilitates the formation of functional alpha subunit.

Tyrosine residues of bovine thyrotropin. Accessibility to iodination in the intact hormone and isolated subunits

The susceptibility of the tyrosines of bovine thyrotropin (bTSH) and of its isolated alpha and beta subunits to lactoperoxidase catalyzed iodination was examined as a probe of the reactivity of these residues. In isolated TSH alpha, tyrosines at positions 21, 41, 92 and 93 were labeled with radioactive iodine to a nearly equivalent extent while residue 30 did not incorporate iodine. In intact TSH tyrosine 41, as well as 30, was not modified showing that, upon association with beta subunit, residue 41 becomes substantially less reactive. The other three tyrosines of the alpha subunit retained reactivity; residue 21 incorporated relatively more label than residues 92 and 93. The pattern of reactivity of the TSH alpha tyrosines in intact TSH parallels previous studies on the alpha subunits of lutropin (LH) and human choriogonadotropin (hCG). Thus the different primary sequences of the beta subunits do not influence environments around tyrosines at positions which are spaced throughout the alpha subunit structure. Most of the 11 tyrosine residues of TSH beta incorporate significant amounts of iodine. In intact TSH, beta tyrosines 25, 37, and 119 are most readily iodinated and tyrosines 14, 25 and 37 are the most reactive in the isolated subunit. Of particular interest in intact TSH is that tyrosine 37 and to a lesser extent 61 are modified. These tyrosines are found in analogous and highly conserved domains in the beta subunits of all glycoprotein hormones whose sequences have been determined, but in LH and hCG they can only be iodinated in isolated beta subunits.

Rapid and easy separation of the subunits of bovine and human glycoprotein hormones by use of high performance liquid chromatography

A single method of reverse phase high performance liquid chromatography is used to separate the subunits of human and bovine glycoprotein hormones. This rapid and easy method is applicable for the separation and detection of subunits from as little as 10 micrograms hormone or the isolation of subunits from as much as 100 mg hormone. Separation is achieved by chromatography on a Vydac 218TP1010 column with a linear (60-min) gradient of 0.1 M sodium phosphate, pH 6.8, plus 1 mM sodium azide to a solvent containing 50% acetonitrile and 50% 0.1 M sodium phosphate, pH 6.8, plus 1 mM sodium azide. Although in some cases the interaction between the hydrophobic support and the hormone is sufficient for dissociation, preincubation of the hormone with guanidine hydrochloride ensures optimum dissociation and improves resolution of the subunits. The subunits isolated by high performance liquid chromatography are functional in that they will reassociate with their counterpart subunits.

Free alpha-like material from bovine pituitaries. Removal of its O-linked oligosaccharide permits combination with lutropin-beta

Further characterization of the free alpha subunit immunoreactive material, not combined with beta subunit in extracts of bovine pituitaries, shows that the only significant modifications, relative to alpha subunits themselves, are the oligosaccharide O-linked to threonine-43, and heterogeneity of the carboxyl terminus. Removal of the O-linked carbohydrate with a mixture of glycosidases from Streptococcus pneumoniae results in an alpha-like material capable of combining with lutropin beta subunit and, thus, the presence of the oligosaccharide is responsible for the inability of the free alpha-like material to combine with beta subunits. Amino acid compositions of tryptic peptides spanning the entire sequence indicate no change in amino acid sequence of the free alpha-like material as compared to lutropin alpha. Further, based on the similar behavior reverse phase high performance liquid chromatography of the tryptic peptides as compared to their lutropin alpha counterparts, it is concluded that no additional post-translational modifications are present. The N-linked oligosaccharides of the free alpha-like material most likely contain terminal O-sulfated N-acetylhexosamines (as do the asparagine-linked carbohydrates from the pituitary hormones) as indicated by the presence of 3 mol of sulfate/mol of free alpha-like material and the resistance of these oligosaccharides to enzymatic deglycosylation. The O-linked oligosaccharide does not contain sulfated residues.

Binding and degradation of doubly radioiodinated luteinizing hormone by mouse Leydig cells

Doubly radioiodinated LH (**LH) was used to examine the fate of both subunits during interaction with freshly isolated Leydig cells. Cells that had bound **LH and were then resuspended in **LH-free medium released radioactivity continuously from both subunits in acid-soluble form, but to only a limited extent in acid-precipitable form. With cells from BALB/c mice, the initial rate of release of acid-soluble radioactivity was substantially greater from alpha-subunit than from beta-subunit; this difference was not apparent with cells from Swiss-Webster mice. Appearance of acid-soluble radioactivity was inhibited by leupeptin; testosterone production was not affected. Cell-associated radioactivity declined when the resuspension medium contained unlabeled LH, but assumed a steady state when cells were incubated continuously in **LH. Thus, upon binding of LH to receptor, both subunits are internalized and degraded within the lysosome. Binding and degradation can proceed simultaneously, yet independently. LH degradation has no role in acute testosterone production.

Studies of the histidine residues of human and bovine glycoprotein hormones by nuclear magnetic resonance

Titration curves of the histidine residues in lutropin, thyrotropin, follitropin and chorionic gonadotropin have been assigned using imidazole C-2 proton nuclear magnetic resonance spectra and their estimated pK values determined. Spectra of reassociated hormone preparations, in which one or the other of their two subunits (alpha or beta) have had their accessible histidines exchanged with deuterium, permitted assignment of C-2 resonance to specific residues. Similar titration curves were found for residues which are conserved from one hormone to another. However, these conserved histidines do not have identical pK values, indicating that differences in the conformation or microenvironment around these residues occur in these hormones. Changes in some pK values also occur as a function of subunit association. The most dramatic change seen in all cases is the exposure to solvent of histidine alpha-83; in isolated alpha subunits this residue is unavailable for titration over a wide pH range. This change appears to be a general consequence of the association of the two subunits in any of these hormones. The data show that all histidines in the intact hormones are accessible to the environment, including those proposed to be in domains involved in subunit-subunit interaction.

Purification of an alternate form of the alpha subunit of the glycoprotein hormones from bovine pituitaries and identification of its O-linked oligosaccharide

Extracts of bovine anterior pituitary glands contain significant amounts of material with immunological properties similar to those of the common, alpha, subunit isolated from the pituitary glycoprotein hormones. Purification of this "free alpha-like" material and analysis show it to contain an additional site of glycosylation not present in the alpha subunit isolated from intact glycoprotein hormones. This additional oligosaccharide is O-linked to a threonine residue corresponding to threonine-43 of bovine lutropin-alpha. Carbohydrate analysis shows 1.7 mol of sialic acid, 0.8 mol of galactose and 0.9 mol of galactosamine/mol of oligosaccharide. A similar structure for the free alpha-like material as compared to bovine lutropin-alpha is evident from equal potency in an anti-lutropin-alpha radioimmunoassay, a similar amino acid composition and similar but not identical peptide maps. The free alpha-like material is distinct from lutropin-alpha in that the free alpha-material contains sialic acid and galactose, has a slightly higher apparent molecular weight, an increased negative charge, and will not reassociate with native lutropin-beta. Peptide maps of the tryptic peptides of the free alpha-like material show additional differences (other than the O-linked oligosaccharide) when compared to peptide maps of lutropin-alpha; thus additional modifications are probably present.

Enzymatic deglycosylation of the subunits of chorionic gonadotropin. Effects on formation of tertiary structure and biological activity

Both the O- and N-linked oligosaccharide moieties of the subunits of the placental glycoprotein hormone, human choriogonadotropin (hCG), are removed by treatment with a mixture of glycosidases produced by Streptococcus (Diplococcus) pneumoniae. The resulting deglycosylated subunits recombine with their native counterparts in good yield, and the reassociated hormones bind to gonadotropin receptors equally as well as the untreated hormone. Stimulation of steroidogenesis by the deglycosylated alpha-native beta recombinant, however, was markedly less than the stimulation by unmodified hCG both in terms of relative potency (0.10-0.15) and the maximal amount of steroid (40-50%) produced. The native alpha-deglycosylated beta recombinant produced a maximum level of steroid production of 80-90% that of control hCG although its relative potency had decreased approximately 4-fold. The data are in accord with results by others in which either hCG or lutropin was partially deglycosylated by treatment with anhydrous hydrofluoric acid. In addition, the effects of deglycosylation on the ability of each subunit to refold after reduction of their disulfide bonds was studied. Of particular interest is that, after deglycosylation, the beta subunit can correctly refold to a significant degree, in contrast to several unsuccessful attempts to demonstrate correct refolding of the unmodified beta subunit of either lutropin or hCG. Alpha subunit, as measured by a conformation sensitive radioimmunoassay, refolds with equal facility both before and after deglycosylation.

Doubly radioiodinated luteinizing hormone: preparation and characterization

Bovine [131I]Iodo-alpha LH-[125I]iodo-beta LH (**LH) has been prepared and shown to be physically and biologically equivalent to unmodified hormone. The beta-subunit was modified with 125I, purified by adsorption to Concanavalin A-Sepharose and elution with methylmannoside, added to alpha-subunit, and allowed to reassociate to intact hormone. Iodination with 131I was then carried out in the reassociation mixture and **LH was isolated by gel filtration. Both gel electrophoresis and rechromatography on Sephadex G-100 showed that both radiolabels comigrated with unmodified hormone. Sodium dodecyl sulfate gel electrophoresis showed that 131I was found in the alpha-subunit and 125I in the beta-subunit; this result is in agreement with studies by others which show that the tyrosines of the beta-subunit are nonreactive in intact hormone. In receptor-binding assays, both radiolabels were specifically displaced in a similar fashion by LH. Scatchard analysis showed high affinity binding (Ka approximately equal to 1.5 X 10(10) M-1) for both labels. Comparison of receptor-binding activity with steroidogenic activity showed that iodinated hormone molecules not only bound to receptor but also stimulated testosterone production. The demonstration that full biological activity is retained with iodination in both subunits shows that such doubly labeled LH can be used to monitor the disposition of both subunits simultaneously during interaction of the hormone with target cells.

Differential effects of alkylation of methionine residues on the activities of pituitary thyrotropin and lutropin

Methionine residues of the alpha and beta subunits of bovine lutropin (LH) and bovine thyrotropin (TSH) have been specifically alkylated with iodoacetic acid. The alpha subunit has been modified so that two of the four methionines are quantitatively alkylated (residues 8 and 33, in agreement with studies by Cheng, K.-W. (1976) Biochem. J. 159, 71-77). Reassociation of the modified alpha subunit with unmodified LH-beta or thyrotropin (TSH)-beta resulted in reconstituted hormones which differed markedly in their respective biological activities. The alpha-modified TSH was fully active in both radioligand receptor and in vivo assays, while the alpha-modified LH, because of lowered affinity for receptor, lost approximately 70% of its activity in its radioligand receptor assay. This observation is the first to show that modification of the alpha subunit leads to a differential loss of activity in one glycoprotein hormone versus another. Circular dichorism studies revealed no changes in conformation; thus, the data strongly support, for LH, a direct interaction of the common subunit with receptor. Methionine 32 in TSH-beta can be modified with retention of full activity under conditions where methionines 8, 9, and 58 are not modified. In contrast, previous work on the modification of lysine 42 in LH-beta which lies in an analogous domain implicates that residue in receptor interaction (e.g. Liu, W.-K., Yang, K.-P., Nakagawa, Y., and Ward, D. N. (1974) J. Biol. Chem. 249, 5544-5550; Sairam, M. R., and Li, C.-H., (1975) ARch. Biochem. Biophys. 167, 534-539). These results further emphasize the probable importance of this domain in hormone specificity.

Production and release of thyrotropin and its subunits by monolayer cultures containing bovine anterior pituitary cells

We have developed a dispersed cell monolayer system derived from bovine anterior pituitary glands. Fresh 1- to 6-week-old calf anterior pituitaries were mechanically and enzymatically dispersed and incubated with Dulbecco's Modified Minimal Essential Medium containing 10% hypothyroid goat serum. The media and cell extracts from confluent monolayers were analyzed for bovine TSH and free alpha, and TSH beta subunits by specific homologous RIAs. Basal levels of TSH, free alpha, and free TSH beta subunits in the media were 6.2 +/- 0.3, and 0.95 +/- 0.05 ng/10(6) cells . 24 h, respectively. Hence, an 8- to 10-fold excess of free alpha over free TSH beta subunits was released into the medium. Intracellular basal levels of TSH, free alpha, and free TSH beta subunits were 27.6 +/- 1.7, 10.7 +/- 0.2, and 2.6 +/- 0.3 ng/10(6) cells . 24 h, respectively, and indicated a 3- to 4-fold excess of free alpha over free TSH beta subunits within the cells. The total alpha-subunit to total beta-subunit ratio was 2:1. TRH stimulated release of TSH and its subunits in a dose-dependent fashion, with a half-maximal dose of 2 nM and a maximal response dose of 10 nM. Stimulation with 100 nM TRH increased the levels of TSH, free alpha, and free TSH beta subunits (450-900%, 180-200%, and 300-400%, respectively) in medium, with concomitant decreases within cells. Treatment with thyroid hormones decreased basal and blunted TRH-stimulated levels of TSH and its subunits in medium but had no effect on intracellular stores. However, large doses of T4 (25 nM) or T3 (1 nM) did not completely abolish the TRH (100 nM)-stimulated hormone response. TRH and thyroid hormones affect the release of TSH and TSH beta to a greater extent than they do the alpha-subunit. Finally, total alpha and TSH beta subunit production was increased with TRH stimulation and decreased with thyroid hormone exposure. Thus, an in vitro system to study the net production and secretion of TSH and its subunits in the normal pituitary thyrotrope has been established. (Endocrinology 108: 387, 1981)

Oligosaccharide moieties of glycoprotein hormones: bovine lutropin resists enzymatic deglycosylation because of terminal O-sulfated N-acetylhexosamines

The oligosaccharides of the bovine pituitary gonadotropin lutropin are N-linked to asparagine residues. These carbohydrates are unusual in that, although they contain the mannose, N-acetylglucosamine, and fucose typical of N-linked oligosaccharides, they also contain one residue of N-acetylgalactosamine but insignificant amounts of sialic acid or galactose. These oligosaccharides exhibit complete resistance to several exoglycosidases. This is in contrast to the ready release of peripheral sugars from human chorionic gonadotropin, a placenta hormone which has oligosaccharides of the complex type with terminal sialic acid and galactose residues. Stability of the lutropin hexosamines to periodate oxidation and reduction (Smith degradation) together with other data show that one residue of N-acetylgalactosamine and one of N-acetylglucosamine are peripheral to two periodate-sensitive mannose residues. The insensitivity to periodate of these two terminal amino sugars is found to result from a sulfate group covalently linked to each; sulfation of these hexosamines is also the most probable reason for the resistance to enzymatic deglycosylation. The alpha subunits of bovine thyrotropin and human pituitary lutropin also contain sulfate, in contrast to human chorionic gonadotropin. The results indicate that sulfating enzymes are present in the pituitary and that sulfation of peripheral sialic acids in the placental gonadotropin. The data lead to a partial structure for the oligosaccharides of bovine LH as follows: (formula see text).

Receptor-binding activity of highly purified bovine luteinizing hormone and thyrotropin, and their subunits

Highly purified preparations of bovine TSH (bTSH) and LH (bLH) and their subunits have been obtained by affinity chromatography using immobilized antibodies directed against counterpart subunits. The purified preparations were assessed for biological activity in radioligand-receptor assays for TSH and LH. After affinity purification against bLH beta, a TSH preparation whose initial potency in the LH assay had been 0.15% that of LH, failed to compete with [125I]LH in amounts up to 100 microgram. Thus, it appears that bTSH does not bind to LH receptors in the rat testis and that interaction of less purified TSH with gonadotropin receptors is attributable to LH contamination. In contrast, LH, whose initial potency in the TSH receptor assay was 0.6% that of TSH, retained a potency of 0.004% of TSH (equivalent to 3.6 mU/mg) after immunoadsorption by anti-bTSH beta. The retention of TSH receptor-binding activity by affinity-purified LH indicates that the LH molecule (like hCG) has a low intrinsic thyroid-stimulating activity. Affinity-purified LH subunits have little or no demonstrable affinity for the LH receptor in vitro. Affinity-purified TSH subunits and affinity-purified LH, however, exhibit very weak receptor-binding activity in the TSH radioligand receptor assay. An evaluation of the capacity of the immunoadsorbents to remove TSH from artificial mixtures suggests that the residual binding does not result entirely from contamination, and therefore, that alpha-subunits as well as LH have some intrinsic TSH-binding activity.

Proton nuclear magnetic resonance studies on bovine lutropin, its subunits, and on the alpha subunit of pregnant mare serum gonadotropin. Assignment of histidine resonances in the alpha subunit

The pK values of the 3 histidine residues in the common alpha subunits of bovine and equine glycoprotein hormones have been determined from titration curves generated from their C-2 proton nuclear magnetic resonances at different pH values. Assignment of resonances to specific histidines is based on a comparison between the two species, which have 1 histidine residue in different positions in their sequences, and of the bovine alpha subunit after removal of its histidine 94 by treatment with carboxypeptidases. In both species, those histidines closest to the COOH terminus titrate with near normal pK values of 6.2. The histidine residue found in the bovine subunit at position 87 titrates with an approximate pK value of 5.4. Histidine 83, adjacent to an oligosaccharide moiety in both species, does not titrate over a pH range of 4.0 to 8.0 and thus appears inaccessible to solvent. Similarly, in bovine lutropin-beta, 1 of 3 histidine residues does not titrate between pH 5.0 and 7.0. In the intact hormone, 2 "nontitratable" histidine residues are found. Changes in the characteristics of the signals, however, preclude unambiguous assignment of these two resonances to the nontitrating histidines in the isolated subunits. It appears that changes in the environment of at least some histidines occur when the subunits combine to yield intact hormone.

Purification and receptor binding properties of complexes between lutropin and monovalent antibodies against its alpha subunit

A complex between bovine lutropin (LH) and monovalent antibodies (Fab fragments) directed against its alpha subunit, which is common to the glycoprotein hormones, has been purified by gel filtration and chromatography on concanavalin A-Sepharose. The complex is heterogenous with respect to molecular size; 70--80% of the hormone is complexed with either two or three Fab fragments. The LH-Fab alpha complexes retain only about 13% receptor binding activity as compared to LH when measured in a radioligand receptor assay in which the radiolabeled ligand is human choriogonadotropin. (Use of the human hormone as labeled ligand permits direct measurement of competition between receptor and the bovine complex because the alpha portion of the human hormone does not cross react significantly with antibodies directed against bovine alpha subunits.) Complex formation does not lead to dissociation of the lutropin into its subunits, as shown with a homologous LH-beta immunoassay which distinguishes free beta subunit from intact LH. Complexing of LH with Fab-alpha fragments also causes little or no change in the affinity of the hormone's beta subunit for anti-LH-beta antibodies indicating that significant changes in beta subunit conformation did not occur. The data show that at least two well-separated antigenic regions on the alpha subunit are exposed to the surface in the intact hormone. They are also in agreement with the proposal that the loss of binding activity to receptor is due to steric effects rather than to changes in conformation or dissociation, and that there may be sites on the alpha subunit which interact directly with the receptor.

Sepharose-linked concanavalin A in the purification and characterization of glycoprotein hormones of the bovine pituitary

Affinity chromatography on concanavalin A-Sepharose is a time saving step in both large and small scale isolations of the bovine pituitary glycoprotein hormones. After ion-exchange chromatography, the final yield of purified lutropin is 40-50% of material in starting concentrates and of purified thyrotropin is approximately 20%. The final products have the same electrophoretic and immunological properties and amino acid compositions as previous preparations. Less than 3% of the immunoreactive lutropin, follitropin and thyrotropin are present as non-glycosylated forms in either crude pituitary extracts or concentrates. Thyrotropin and follitropin elute from the immobilized lectin as a single fraction, whereas lutropin separates into two glycosylated fractions. Gel filtration of both crude extracts and the glycoprotein fractions shows that less than 5% of the immunoreactivity of the hormones is present as material of apparently high molecular weight. Substantial alpha subunit immunoreactivity, however, is in three fractions (as found by others in human pituitary extracts) corresponding to "high molecular weight material" (7%), intact hormones (46%) and free subunit (47%).

Studies on the reduction and reoxidation of the disulfide bonds of the alpha and beta subunits of human choriogonadotropin

Reoxidation of the disulfide bonds of the alpha-subunit of human choriogonadotropin after their complete reduction yields a product which is indistinguishable from the native subunit in its electrophoretic pattern in polyacrylamide gel and in its ability to recombine with the beta subunit of bovine lutropin. The circular dichroism of reoxidized human choriogonadotropin-alpha is essentially identical to that of the native alpha-subunit, except for slightly more negative ellipticity in the region of 240 mm. Hybrid hormone preparations obtained by recombination of reoxidized or native human choriogonadotropin-alpha with native lutropin-beta exhibit identical electrophoretic patterns in polyacrylamide gels, elution profiles in gel filtration, receptor binding activities, and CD spectra. However, reoxidation of human choriogonadotropin-beta under the same conditions does not yield a product which resembles the native beta subunit in its electrophoretic pattern on gels, its CD spectrum or its ability to recombine with the alpha subunit.

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.

Separation of functional and non-functional beta subunits of thyrotropin preparations by polyacrylamide gel electrophoresis

Electrophoretic patterns of intact human and bovine TSH and bovine LH can be clearly distinguished from those of their subunits in 12% polyacrylamide gels, thus providing an easy method of examining subunit recombination. Two distinct components of both bovine and human TSH-beta subunits are observed, of which only one recombines with alpha subunits. Both beta-components cross-react with antisera to TSH and TSH-beta and have, within experimental error, identical amino acid compositions. Thus, the non-recombining component is a non-functional form of TSH-beta which has retained its immunological specificity, and the data explain why the recovery of biological activity during the recombination of TSH subunits is substantially less than with several other glycoprotein hormone preparations.

Studies on the disulfide bonds of glycoprotein hormones. Complete reduction and reoxidation of the disulfide bonds of the alpha subunit of bovine luteinizing hormone

Reoxidation of the disulfide bonds of the alpha subunit of bovine luteinizing hormone (LH) after their complete reduction both in the presence and absence of denaturing agent yields a product which is indistinguishable from the native subunit in its electrophoretic pattern on polyacrylamide gels and in its ability to recombine with the beta subunits of both luteinizing hormone and thyrotropin. The circular dichroism spectrum of the reoxidized alpha subunit is essentially identical to that of native alpha subunit except that its maximum at 233 nm is smaller than observed with native LHalpha. The intact hormone preparations obtained by recombination of reoxidized alpha subunit with native LH-beta exhibit electrophoretic patterns in polyacrylamide gels, elution profiles on gel filtration, binding activities to a membrane fraction from rat testes, and circular dichroism spectra identical to those of native LH and recombinants of native LH-alpha with the beta subunit. Recombinants of native or reoxidized LH-alpha with the beta subunit of thyrotropin are also indistinguishable in their electrophoretic patterns on polyacrylamide gels and in their in vivo activities of stimulating 32P uptake in thyroids of day-old chicks. While this study does not preclude that the alpha subunit may be biosynthesized as part of a larger precursor protein, the data demonstrate that sufficient information is present in the linear sequence of the alpha subunit to allow folding and formation of disulfide bonds to yield a functional alpha subunit.

Studies on the disulfide bonds of glycoprotein hormones. Course of reduction of bovine luteinizing hormone, bovine thyroid-stimulating hormone, and their subunits

The five disulfide bonds of isolated alpha subunits of luteinizing hormone (LH) and thyroid-stimulating hormone (TSH) are completely reduced at pH 8.5 in 15 min with no denaturant required and with only a slight excess of reducing agent. At pH 7.0, reduction is complete after 6 to 10 h. These results together with an earlier study concerning the positions of the two most readily reduced bonds (Cornell J.S., and Pierce, J.G. (1974) J. Biol. Chem. 249, 4166-4174) show that, in the isolated alpha subunit, all disulfides are readily accessible, although it is possible that a change in conformation, after rapid initial reduction of two disulfides, makes the remaining three more susceptible to reduction. No partially reduced and S-carboxymethylated intermediates were found at pH 7.0 other than those seen at pH 8.5, nor were additional intermediates found at pH 8.5 when reduction was initiated in the presence of alkylating agent. In contrast, reduction of the beta, hormone-specific, subunits of LH and TSH, while complete at pH 8.5 after 2 to 6 h, does not proceed to completion at pH 7.0 even after 24 h or upon addition of 6 M urea or large concentrations of reducing agent, and partially reduced intermediates useful in location of disulfide bridges can be trapped (e.g. Reeve, J.R., Cheng, K.-W., and Pierce, J.G. (1975) Biochem. Biophys. Res. Commun. 67, 149-155). Little or no reduction of the intact hormones is found at pH 7.0 in the absence of denaturing agents. This protection by the intact structure shows that the two most readily reduced disulfides of the alpha subunit and the single most readily reduced sidulfide of the beta subunits are either in regions of subunit-subunit contact or that these bonds become more reactive in the isolated subunits because of different influences by neighboring groups. At pH 8.5, intact LH is completely reduced after 6 h, but intact TSH is more resistant to reduction, which may reflect a higher affinity between subunits than exists in LH.

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.

The carboxylic acid groups of bovine luteinizing hormone. The effects of their modification on receptor site binding and subunit-subunit interaction

The modification of the carboxyl groups of the subunits of bovine luteinizing hormone to neutral derivatives by carbodiimide-mediated coupling with glycine methyl ester has been studied. The modified alpha subunit, which has 8 residues of glycine methyl ester incorporated, will no longer recombine with native beta (hormone-specific) subunit, but the modified beta subunit, with 6 to 7 glycine methyl esters incorporated, will recombine with native alpha to yield a partially active hormone. Derivatization of the intact hormone results in dissociation to subunits together with formation of a major side product which is covalently cross-linked. Significant cross-linked product was not obtained during modification of individual subunits, thus indicating an orientation between an activated carboxyl group(s) and a nucleophile(s) in the intact hormone which favors coupling. Separation of subunits from the derivatized, noncross-linked fraction by countercurrent distribution reveals a heterogeneous preparation of the modified alpha subunit which also will not recombine with either a native or modified beta subunit. The beta subunit from the modified intact hormone was indistinguishable from the modified isolated beta subunit in amino acid composition and in ability to recombine with native alpha subunit. The results are consonant with data from this and other laboratories in which various modifications of the alpha chain, the subunit common to the glycoproteins, more seriously affect recombination than similar modifications of the beta subunits. The number of carboxyl groups modified in each subunit is compatible with but not in total agreement with assignments of amides reported from sequence studies.

Secretion of alpha subunits of luteinizing hormone (LH) by the anterior pituitary

Free alpha subunit chains of the glycopeptide pituitary hormones have been found in the sera of normal subjects and postmenopausal women. To ascertain whether the alpha subunit of LH is directly secreted by the pituitary or formed as a result of degradation of intact LY in the periphery, alpha subunits and intact LH were measured by radioimmunoassay in human volunteers after LRF stimulation and purified LH infusion. In 4 subjects a loading dose of 90 IU, followed by the infusion of 22.5 IJ of purified human LH over 30 min, produced peak serum LH levels of 41 mIU/ml but no change in alpha subunit levels of 35 IU of purified human LH to an additional 4 subjects, produced peak LH levels of 8* mIU/ml, but again, no change in alpha subunits. In the same two groups of subjects 100 mug of LRF produced peak LH levels of 25 mIU/ml and 75 mIU/ml, respectively, with significant alpha subunit elevations at 20 min of 1.7 ng/ml and 2.7 ng/ml, respectively. In separate groups of men LRF was administered over a wide dose range of 1 to 3,000 mug and LH and the alpha subunit measured. A dose-response curve existed over the entire LRF dose range for blood LH; no minimum or maximum plateaus were observed over the range studied. However, the alpha chain response appeared to reach a maximal plateau at a dose of 100 mug of LRF. The results are compativle with the hypothesis that the alpha subunits appearing in the peripheral circulation in response to LRF are due to secretion by the anterior pituitary and not due to peripheral degradation of intact secreted LH.