Iodination of ovine luteinizing hormone and its subunits

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.

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

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

Enhancement of prolactin (PRL)-stimulated mitogenesis of Nb2 rat lymphoma cell cultures by insulin-like growth factor I (IGF-I)

IGF-I stimulated the mitogenesis of Nb2 cells in the presence of suboptimal mitogenic concentrations of prolactin (PRL, 0.01 or 0.1 ng/ml). In the presence of 1 ng/ml or 10 ng/ml, concentrations of PRL that produced a maximal or near maximal mitogenic response in Nb2 cells, the addition of insulin-like growth factor-I (IGF-I) at 5 or 10 ng/ml did not further enhance mitogenesis. This effect was selective for IGF-I since IGF-II, epidermal growth factor (EGF), insulin, transforming growth factor-beta (TGF-beta) or platelet-derived growth factor (PDGF) did not stimulate mitogenesis in the presence or absence of PRL. That the ability of IGF-I to stimulate mitogenesis of these cells required PRL was suggested by the ability of PRL antiserum to block the IGF-I effect. Monoclonal antibody to IGF-I reduced the mitogenic response to one identical to PRL alone. In the presence of a suboptimal mitogenic concentration of PRL, IGF-I was an equally effective comitogen when added at 0 time or at 6 h after PRL stimulation, suggestive of an effect of IGF-I in late G1 phase of the cell cycle. Effects of IGF-I on ornithine decarboxylase, a G1 enzymatic marker, were compatible with this interpretation. In order to be assured that IGF-I exerted its action through a receptor-mediated process, we evaluated the presence of IGF-I receptors on Nb2 cells. Receptors were present which bound IGF-I greater than IGF-II much greater than insulin. The IC50 was 35 nM for IGF-I, 280 nM for IGF-II and 875 nM for insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Human recombinant insulin-like growth factor I. II. Binding characterization and radioreceptor assay development using Balb/c 3T3 mouse embryo fibroblasts

The binding of human recombinant insulin-like growth factor I (IGF-I) to BALB/c 3T3 mouse embryo fibroblasts has been characterized, resulting in the development of a radioreceptor assay. Binding of radioiodinated IGF-I (125I-IGF-I) to washed monolayers of BALB/c 3T3 cells was specific, time dependent, and stable, being maximal after a 10-h incubation at 15 degrees C with no loss of bound ligand or cells through 25 h. Scatchard analysis identified a class of high affinity binding sites with Kd = 59.6 pM and an estimated 1.57 X 10(5) receptors/cell. Half-maximal displacement of bound 125I-IGF-I occurred with 15 to 20 ng/ml unlabeled IGF-I competitor. Insulin-like growth factor II and insulin were far less effective competitors, providing half-maximal displacement at concentrations of 130 to 170 ng/ml and 2 to 3 micrograms/ml, respectively. Epidermal growth factor, transforming growth factor type alpha, and acidic and basic fibroblast growth factors did not compete for 125I-IGF-I binding at 1 microgram/ml. Cells fixed with glutaraldehyde before ligand binding did remain attached to culture dishes more tightly; however such pretreatment destroyed approximately 70% of ligand binding. Crosslinking data indicated that 125I-IGF-I binds specifically to a 330-kDalton receptor as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. This receptor dissociated into 130-kDalton subunits when analyzed in the presence of dithiothreitol.

Preparation and characterization of bovine growth hormones produced in recombinant Escherichia coli

Two analogues of bovine growth hormone (BGH) have been produced in Escherichia coli by recombinant DNA techniques. In analogue Delta-1, the N-terminal alanine residue of the full-length bovine sequence is replaced by methionine. In analogue Delta-9, which is expressed at much higher levels than is Delta-1, the full-length bovine sequence is truncated at the N-terminus by eight residues and there is a serine-for-glycine substitution in the first position of the truncated protein. Both analogues, which were characterized by isoelectric focusing (i.e.f.), polyacrylamide-gel electrophoresis in the presence of SDS (SDS/PAGE), amino acid analysis and N-terminal amino acid sequence determination using combined g.l.c.-m.s., are compared with BGH isolated from pituitaries. In contrast with pituitary-derived BGH, the recombinant-derived proteins are homogeneous on SDS/PAGE and on i.e.f. In a radioimmunoassay, a radioreceptor assay and a bioassay in vivo (rat tibia), Delta-9 BGH showed very similar characteristics to the pituitary-derived hormone. Similar results have also been obtained with the Delta-1 analogue.

Pattern of immunoreactivity after reverse-phase high performance liquid chromatography of ovine lutropin at different pH

Unlabelled and radioiodinated ovine lutropin (oLH) was fractionated using reverse-phase high pressure liquid chromatography, employing a 5 micron Spherisorb C18 column. Following chromatography of unlabelled oLH at pH 3.3, 4.3 and 5.3, all eluted fractions were tested by radioimmunoassay of oLH, oLH alpha and oLH beta. Regardless of pH, the material corresponding to the first peak contained only alpha-subunit immunoreactivity. With increasing pH, the oLH alpha, oLH and oLH beta immunoreactivity separated into two groups of fractions. The retention times of the radioiodinated alpha- and beta-subunits of oLH corresponded to those of the first (descending part) and last peaks from unlabelled oLH, respectively. Following chromatography of radioiodinated oLH at pH 3.3, 5.3 and 6.5, all eluted fractions were analyzed using antiserum against either alpha- or beta-subunits of oLH. At pH 3.3 two radioactive peaks were detected: the first corresponded to the fraction with maximum binding to alpha-subunit antiserum, and the second corresponded to the fraction with maximum binding to beta-subunit antiserum. At pH 5.3 and 6.5, maximum radioactivity occurred in conjunction with maximum binding to both antisera. A free alpha-subunit in the radioiodinated oLH was even detected after chromatography at pH 6.5.

High-performance liquid chromatography of amino acids, peptides and proteins. LVII. Analysis of radioiodinated thyrotropin polypeptides by reversed-phase high-performance liquid chromatography

The chromatographic behaviour of bovine and human thyrotropin, radiolabelled stoichiometrically with lactoperoxidase, on octadecylsilicas and other reversed-phase n-alkylsilicas has been investigated. As part of this investigation the effects of a variety of elution systems on resolution and recovery have been examined. Analysis of the tryptic peptides of radioiodinated bovine thyrotropin (bTSH) preparations by reversed-phase high-performance liquid chromatographic mapping methods resulted in the separation of more than fourteen major radioactive peptide components. The data indicate that bTSH is radioiodinated with unequal incorporation into both the alpha- and beta-subunits. Further, assessment of the level of microheterogeneity of radiolabelled bTSH preparations can be achieved with these reversed-phase techniques.