Highly sensitive two-site immunoradiometric assay of parathyrin, and its clinical utility in evaluating patients with hypercalcemia

We have developed a highly sensitive, two-site immunoradiometric assay (IRMA) for human parathyrin (PTH) that is specific for the intact, secreted, biologically active 84-amino-acid peptide. This assay has several technical advantages: it does not detect even high concentrations of inactive carboxyl-terminal fragments, results are available within 24 h, and the detection limit for intact hormone is low (1 ng/L). The assay readily measures concentrations of PTH in all healthy subjects and distinguishes these values from low or undetectable PTH values observed in clinical situations in which PTH secretion is expected to be suppressed. We found complete separation of results from 37 patients with surgically proven hyperparathyroidism and those from 23 patients with hypercalcemia associated with malignancy, the latter having PTH values at or below the lower limits of normal for this assay. The sensitivity, specificity, and rapid turnaround time of this two-site IRMA should advance the laboratory evaluation of patients with disorders of calcium metabolism.

The glycoprotein hormones: recent studies of structure-function relationships

The structural features of the heterodimeric glycoprotein hormones (LH, FSH, TSH, and hCG) are briefly reviewed. Removal of carbohydrate chains does not reduce binding of the hormones to membrane receptors, but markedly reduces biological responses. The glycopeptides from the hormone do not reduce binding of native hormone to receptors but do reduce biological responses. Newer data concerned with replication of different regions of the peptide chains of these molecules using synthetic peptides are reviewed and presented. These studies indicate that two regions on the common alpha subunit are involved with receptor binding of the LH, hCG, and TSH molecules. These regions are alpha 26 to 46 and alpha 75-92. Two synthetic disulfide loop peptides from the hCG beta subunit beta 38-57 and beta 93-100 also block binding of hCG to its receptor. In addition, the beta 38-57 peptide stimulates testosterone production by Leydig cells. These data indicate that glycoprotein hormone binding to plasma membrane receptors involves a discontinuous site on the hormone that spans both the alpha and beta subunits, and that the alpha subunit sites are similar for several hormones.

Synthesis of a biologically active N-terminal tetratriacontapeptide of parathyroid hormone

Determination of the amino acid sequence of bovine parathyroid hormone has led to the synthesis of a tetratriacontapeptide corresponding to the amino-terminal 1-34 residues of the native molecule. The specific biological effects of this synthetic peptide on bone and kidney are qualitatively identical to those of the native hormone in classical bioassays in vivo and in several systems in vitro. Potency of the synthetic peptide equals or exceeds that of a biologically active fragment of comparable size isolated from the native hormone; the synthetic and natural peptides show complete immunological cross-reactivity. Thus, essential requirements for the physiological actions of the peptide on both skeletal and renal tissue are contained within the 34 amino-terminal amino acids. The potency of the synthetic peptide, relative to that of the native (84-amino acid) polypeptide, is greater in vitro than in vivo; this suggests that the carboxyl terminal two-thirds of the native hormone may protect the circulating polypeptide from rapid metabolic degradation.

Structure of the precursor to an enzyme mediating COOH-terminal amidation in peptide biosynthesis

Many bioactive peptides terminate with an amino acid alpha-amide at their COOH terminus. The enzyme responsible for this essential posttranslational modification is known as peptidyl-glycine alpha-amidating monooxygenase or PAM. We identified cDNAs encoding the enzyme by using antibodies to screen a bovine intermediate pituitary lambda gt11 expression library. Antibodies to a beta-galactosidase/PAM fusion protein removed PAM activity from bovine pituitary homogenates. The 108,207 dalton protein predicted by the complete cDNA is approximately twice the size of purified PAM. An NH2-terminal signal sequence and short propeptide precede the NH2 terminus of purified PAM. The sequences of several PAM cyanogen bromide peptides were localized in the NH2-terminal half of the predicted protein. The cDNA encodes an additional 430 amino acid intragranular domain followed by a putative membrane spanning domain and a hydrophilic cytoplasmic domain. The forms of PAM purified from bovine neurointermediate pituitary may be generated by endoproteolytic cleavage at a subset of the 10 pairs of basic amino acids in the precursor. High levels of PAM mRNA were found in bovine pituitary and cerebral cortex. In corticotropic tumor cells, levels of PAM mRNA and pro-ACTH/endorphin mRNA were regulated in parallel by glucocorticoids and CRF.

Intracellular and extracellular processing of human apolipoprotein A-I: secreted apolipoprotein A-I isoprotein 2 is a propeptide

We have recently proposed that the major secreted isoprotein form of human apolipoprotein A-I (designated apo A-I2) is modified extracellularly to become the predominant apo A-I form seen in plasma (designated apo A-I4). In the current report we demonstrate that the primary translation product of human apo A-I (designated apo A-I2p) has a 24-amino-acid NH2-terminal extension with a sequence of Met-Lys-Ala-Ala-Val-Leu-Thr-Leu-Ala-Val-Leu-Phe- Leu-Thr-Gly-Ser-Gln-Ala-Arg-His-Phe-Trp-Gln-Gln. The first 18 amino acids of this NH2-terminal extension are cleaved intracellularly by the signal peptidase, resulting in the formation of apo A-I2, which is the secreted form of apo A-I. Sequence analysis of apo A-I2 confirmed that it contains a hexapeptide extension at its NH2 terminus compared to apo A-I4. This observation demonstrates that apo A-I2 is a propeptide and that the apo A-I2 to apo A-I4 conversion involves the removal of the NH2-terminal hexapeptide of apo A-I2 by a protease in plasma, lymph, or both. Our findings indicate that apo A-I is synthesized as a prepropeptide, which undergoes intracellular and extracellular proteolysis to attain the major plasma apo A-I4 isoprotein form.

The extracellular amino-terminal region of the parathyroid hormone (PTH)/PTH-related peptide receptor determines the binding affinity for carboxyl-terminal fragments of PTH-(1-34)

The recombinant human PTH/PTH-related peptide (PTHrP) receptor, when transiently expressed in COS-7 cells, binds [Nle8,18,Tyr34] bovine PTH-(7-34)amide [PTH-(7-34)], human PTH-(10-34)amide [PTH-(10-34)], and bovine PTH-(15-34)amide [PTH-(15-34)] with at least 50-fold higher affinity than does the rat receptor homolog. In contrast, PTH-(1-34) binding affinities are similar for both receptor homologs. To map those areas of the PTH/PTHrP receptors that determine the binding specificity for carboxyl-terminal fragments of PTH-(1-34), we constructed chimeric rat/human PTH/PTHrP receptors. These bound PTH-(1-34) with normal affinity and, therefore, must have an overall conformation that resembles that of native receptors. Chimeras with the amino-terminal extracellular domain of the human PTH/PTHrP receptor have a considerably higher binding affinity for PTH-(7-34), PTH-(10-34), and PTH-(15-34) than do the reciprocal receptor constructs in which the amino-terminal region is from the rat PTH/PTHrP receptor. The opossum PTH/PTHrP receptor homolog also binds PTH-(7-34) with higher affinity than the rat receptor, and studies of rat/opossum chimeras confirm the importance of the amino-terminal extracellular domain in determining the PTH-(7-34) binding specificity. Mutant rat and human PTH/PTHrP receptors in which either residues 61-105 of the extracellular region or most of the intracellular tail were deleted have PTH-(7-34) binding characteristics indistinguishable from those of either wild-type receptor. These findings indicate that the amino-terminal extracellular region of the PTH/PTHrP receptor contains a domain(s) that largely determines the binding affinity of amino-terminally truncated PTH analogs. This region, therefore, is likely to constitute a site for ligand-receptor interaction.

Amino acid sequence of salmon ultimobranchial calcitonin

Salmon ultimobranchial calcitonin has been isolated and rendered pure, as demonstrated by several chemical criteria. Its amino acid sequence was determined by means of manual Edman degradation of the intact molecule and of several peptide subfragments. Results of automated degradation provided confirmation of the structure. The salmon molecule possesses, in common with other calcitonins, a 32-amino acid peptide chain terminating in prolinamide and containing half-cystine residues at positions 1 and 7. Although the sequence of the salmon hormone differs considerably from that of the porcine, bovine and human calcitonins, the four hormones are homologous in 9 of 32 positions. The much higher biological potency possessed by the salmon calcitonin makes it of particular interest for future structure function studies.

Peptidylglycine alpha-hydroxylating monooxygenase: active site residues, disulfide linkages, and a two-domain model of the catalytic core

Peptidylglycine alpha-hydroxylating monooxygenase (PHM) is a copper, ascorbate, and molecular oxygen dependent enzyme that catalyzes the first step leading to the C-terminal amidation of glycine-extended peptides. The catalytic core of PHM (PHMcc), refined to residues 42-356 of the PHM protein, was expressed at high levels in CHO (DG44) (dhfr-) cells. PHMcc has 10 cysteine residues involved in 5 disulfide linkages. Endoprotease Lys-C digestion of purified PHMcc under nonreducing conditions cleaved the protein at Lys219, indicating that the protein consists of separable N- and C-terminal domains with internal disulfide linkages, that are connected by an exposed linker region. Disulfide-linked peptides generated by sequential CNBr and pepsin treatment of radiolabeled PHMcc were separated by reverse phase HPLC and identified by Edman degradation. Three disulfide linkages occur in the N-terminal domain (Cys47-Cys186, Cys81-Cys126, and Cys114-Cys131), along with three of the His residues critical to catalytic activity (His107, His108, and His172). Two disulfide linkages (Cys227-Cys334 and Cys293-Cys315) occur in the C-terminal domain, along with the remaining two essential His residues (His242, His244) and Met314, thought to be essential in binding one of the two nonequivalent copper atoms. Substitution of Tyr79 or Tyr318 with Phe increased the Km of PHM for its peptidylglycine substrate without affecting the Vmax. Replacement of Glu313 with Asp increased the Km 8-fold and decreased the kcat 7-fold, again identifying this region of the C-terminal domain as critical to catalytic activity. Taking into account information on the copper ligands in PHM, we propose a two-domain model with a copper site in each domain that allows spatial proximity between previously described copper ligands and residues identified as catalytically important.

A receptor-binding region in human choriogonadotropin/lutropin beta subunit

Synthetic fragments have not been widely used thus far to evaluate structure-activity relations in the glycoprotein hormones. We prepared a series of peptides representing the intercysteine "loop" sequence (residues 38-57) in human choriogonadotropin (hCG) and lutropin (hLH) beta subunits, anticipating that it might be oriented toward the surface and accessible to receptors. The peptides were characterized chemically and tested for bioactivity by binding to rat ovarian membrane receptor and stimulation of Leydig cell testosterone production. The hCG beta-(38-57) and hLH beta-(38-57) peptides inhibited binding of 125I-labeled hCG half-maximally at 1.51 X 10(-4) and 2.03 X 10(-5) M, respectively, while other peptide hormones and fragments from elsewhere in the beta subunit were inactive. Both peptides stimulated testosterone production, with half-maximal responses at 3.55 X 10(-5) M (hCG) and 2.18 X 10(-5) M (hLH). By radioimmunoassay with an antibody to thyroglobulin-conjugated hCG beta-(38-57) peptide, native hCG and beta subunit were highly reactive, as were the reduced and carboxymethylated subunit and peptide. Helical-wheel projection predicted an amphipathic region in the N-terminal portion of the 38-57 sequence, and circular dichroic measurements showed an increase in ordered structure, especially alpha-helix, when the 38-57 peptides were transferred from an aqueous to a more lipophilic (90% trifluoroethanol) environment. These results indicate that the 38-57 region of beta subunit is exposed on the surface and constitutes a component in the receptor-binding domain for hCG and hLH. A region of amphipathic-helical structure in the 38-57 sequence may promote hormone-receptor interactions in a manner proposed for several other peptide hormones.

Determinants of [Arg2]PTH-(1-34) binding and signaling in the transmembrane region of the parathyroid hormone receptor

Previously, we reported that [Arg2]PTH-(1-34) bound to the rat osteosarcoma cell line, ROS 17/2.8, with 2-fold higher apparent affinity than it did to the opossum kidney cell line, OK, yet the analog was only a weak partial agonist for cAMP stimulation with ROS 17/2.8 cells, whereas it was a full cAMP agonist with OK cells. These results suggested that the rat and opossum PTH receptors differ in a region recognized by the hormone's amino-terminus. In this report we show that the cloned PTH receptors derived from ROS 17/2.8 and OK cells, expressed in COS-7 cells, also displayed altered responses to [Arg2]PTH-(1-34). Thus, [Arg2]PTH-(1-34) bound to the cloned rat PTH receptor with 7-fold higher affinity than it did to the cloned opossum PTH receptor, and in cAMP stimulation assays, it was a much weaker agonist with the rat receptor than it was with the opossum receptor. Studies with rat/opossum PTH receptor chimeras suggested that the membrane-spanning region of the receptor contributed to the different binding and signaling responses to [Arg2]PTH-(1-34). Point mutation analysis identified three sites in or near the extracellular ends of transmembrane domains V and VI, which specifically affected [Arg2]PTH-(1-34) binding and signaling.

The N-terminal amino-acid sequence of bovine proparathyroid hormone

Proparathyroid hormone (calcemic fraction-A) is a biosynthetic precursor of parathyroid hormone in bovine glands. Limited amounts of the isolated prohormone have been obtained for the purpose of initial structural studies. The results of automated sequence analysis on two separate preparations indicate that the N-terminal region of the prohormone consists of the sequence Lys-Ser-Val-Lys-Lys-Arg followed by a sequence exactly corresponding to residues 1 to 34 of bovine parathyroid hormone. Also observed was a minor sequence (about 12% of the total) in which the N-terminal lysine was absent. These data suggest that more than one species of prohormone may exist. Due to the small quantities of sample available, the analyses were restricted to the N-terminal portion of the prohormone only. However, because the amino-acid composition of the prohormone indicates it to be a molecule containing more than 100 amino acids, the possibility remains that additional residues occur at the C-terminus. Thus, the prohormone structure based on these data is believed to consist of the hexapeptide sequence above, followed by the known sequence of the 84 residues in parathyroid hormone, possibly followed by an additional sequence of 10-15 residues.

The amino-acid sequence of the amino-terminal 37 residues of human parathyroid hormone

The sequence of the amino-terminal 37 residues of human parathyroid hormone has been established. The hormone used in these studies was isolated in highly purified form from parathyroid adenomata and was subjected to automated degradation in a Beckman sequencer. A high-sensitivity sequencing procedure employing (35)S-labeled phenylisothiocyanate of high specific activity as the coupling agent was used. The sequence obtained differs from that of bovine parathyroid hormone in three of the first 37 positions, and from that of porcine parathyroid hormone in two positions. A single human-specific residue was found (asparagine 16). The sequence obtained differs at three positions (22, 28, and 30) from the structure for human parathyroid hormone reported recently by Brewer et al. [(1972) Proc. Nat. Acad. Sci. USA 69, 3585-3588] and synthesized by Andreatta et al. [(1973) Helv. Chim. Acta, 56, 470-473] We have carefully reviewed our data, reported here in detail, on the sequence positions in dispute. We must conclude, on the basis of all available data, that the structure that we propose is the correct structure. The objective resolution of these discrepancies in structural analysis through further chemical and immunochemical studies is important, since synthesis of human parathyroid hormone, in which there is widespread interest for physiological and clinical studies, must be based on the correct sequence of the human hormone if the peptide is to be genuinely useful.

Non-homologous sequences of parathyroid hormone and the parathyroid hormone related peptide bind to a common receptor on ROS 17/2.8 cells

We and others have recently shown that amino terminal sequences of parathyroid hormone (PTH) and parathyroid hormone related peptide (PTHrP), which share a 62% homology within the first 13 residues, bind to the same receptor on ROS 17/2.8 cells. The remaining PTHrP sequence is markedly different from PTH, suggesting that receptor binding may be dependent on the first 13 amino acids of either peptide. However, since the amino acid residues 14-34 have previously been recognized as an important binding domain for PTH, conformational similarity within this portion's secondary structure of both peptides could contribute to their capacity to bind to the same receptor. To test this hypothesis, we synthesized [Tyr36,Cys38]PTHrP-(14-38) and [Tyr34]bPTH(14-34)NH2, and studied binding of both peptides to the common PTH/PTHrP receptor on ROS 17/2.8 cells. Radioiodinated, HPLC-purified [Nle8,18, Tyr34]bPTH(1-34)NH2 (NlePTH) and [Tyr36]PTHrP-(1-36)NH2 were used to functionally define receptor binding requirements. [Tyr36,Cys38]PTHrP(14-38) and [Tyr34]bPTH(14-34)NH2 competed with 125I-NlePTH for binding sites on ROS 17/2.8 cells with apparent Kds of 10 microM and 50 microM respectively. Both peptides also competed with 125I-[Tyr36]PTHrP(1-36)NH2 with apparent Kds of 30 microM and 10 microM respectively. In the same assay system, NlePTH and [Tyr36,Cys38]PTHrP(1-38)inhibited binding of either radioiodinated ligand with apparent Kds of 0.3 and 1.0 nM. These studies indicate that although [Tyr34]bPTH(14-34)NH2 and [Tyr36,Cys38]PTHrP(14-38) share virtually no sequence homology, their secondary structures must be sufficiently similar to permit binding to a common PTH/PTHrP receptor.

Alpha-enolase is restricted to basal cells of stratified squamous epithelium

We have developed a monoclonal antibody against a 50-kDa protein that binds preferentially to basal cells in the limbus of rat, rabbit, and human corneas (J. D. Zieske, G. Bukusoglu, and M. A. Yankauckas, Invest. Ophthalmol. Visual Sci. 33, 143-152, 1992). Here we report on the purification and identification of the antigen. The 50-kDa antigen was purified from rabbit limbal and corneal epithelium using HPLC methodology including anion exchange (DEAE) followed by reverse-phase (C18) chromatography. The purified 50-kDa protein was then digested with endoproteinase Lys-C, and a reproducible profile comprising approximately 20 peptides was observed by reverse-phase HPLC of the digest. Sequence analysis of five peptides ranging in length from 4 to 20 residues revealed that the 50-kDa protein was alpha-enolase, a glycolytic enzyme. Overall, 57 amino acids were identified with a 95% sequence homology. Localization of alpha-enolase in rat epithelium by immunofluorescence microscopy demonstrated that simple epithelium contained low or undetectable levels of the enzyme. Stratified squamous epithelium, however, showed high levels of alpha-enolase, which was localized specifically to cells of the basal layer. Epidermal, corneal limbal, oral mucosal, vaginal, and laryngeal epithelium all showed cytoplasmic binding specific to the basal cells. These data indicate that the glycolytic enzyme alpha-enolase is preferentially localized in the basal cell layer of stratified squamous epithelium and suggest that glycolytic activity is concentrated in these cells. The localization pattern suggests that a major change in metabolism occurs as cells leave the mitotically active basal cell layer and migrate toward terminal differentiation in the suprabasal cell layers.

Alternating zinc fingers in the human male associated protein ZFY: 2D NMR structure of an even finger and implications for "jumping-linker" DNA recognition

ZFY, a sex-related Zn-finger protein encoded by the human Y chromosome, is distinguished from the general class of Zn-finger proteins by the presence of a two-finger repeat. Whereas odd-numbered domains and linkers fit a general consensus, even-numbered domains and linkers exhibit systematic differences. Because this alternation may have fundamental implications for the mechanism of protein-DNA recognition, we have undertaken biochemical and structural studies of fragments of ZFY. We describe here the solution structure of a representative nonconsensus (even-numbered) Zn finger based on 2D NMR studies of a 30-residue peptide. Structural modeling by distance geometry and simulated annealing (DG/SA) demonstrates that this peptide folds as a miniglobular domain containing a C-terminal beta--hairpin and N-terminal alpha-helix (beta beta alpha motif). These features are similar to (but not identical with) those previously described in consensus-type Zn fingers (derived from ADR1 and Xfin); the similarities suggest that even and odd ZFY domains bind DNA by a common mechanism. A model of the protein-DNA complex (designated the "jumping-linker" model) is presented and discussed in terms of the ZFY two-finger repeat. In this model every other linker is proposed to cross the minor groove by means of a putative finger/linker submotif HX4HX3-hydrophobic residue-X3. Analogous use of a hydrophobic residue in a linker that spans the minor groove has recently been described in crystallographic and 3D NMR studies of homeodomain-DNA complexes. The proposed model of ZFY is supported in part by the hydroxyl radical footprint of the TFIIIA-DNA complex [Churchill, M.E.A., Tullius, T.D., & Klug, A. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 5528-5532].

Follistatin-related protein (FSRP): a new member of the follistatin gene family

The identification and characterization of follistatin related protein (FSRP) suggests that the follistatin (FS) gene family may actually contain two sub-families. The first includes FS and FSRP by virtue of their high degree of structural homology and comparable activin-binding activity, while the second sub-family contains extracellular matrix proteins that possess one or more 10-cysteine FS domains, but do not bind activin or related TGF-beta family members. Characterization of FSRP indicates that it binds activin with similar affinity and selectivity as FS, but does not bind heparin. Furthermore, although FSRP inhibits activin-mediated gene transcription in heterologous assays, FSRP is much less active than FS in the rat pituitary bioassay. When overexpressed in transgenic mice, FSRP may lead to interruption of follicular development and fertility in females but appears to have only a modest effect on males. These results suggest that FSRP is a structural, but not necessarily a functional homologue of FS.