A biological assay for calcitonin

Y12 nitration of human calcitonin (hCT): A promising strategy to produce non-aggregation bioactive hCT

Irreversible aggregation can extremely limit the bioavailability and therapeutic activity of peptide-based drugs. There is therefore an urgent demand of effective strategy to control peptide aggregation. Recently, we found that tyrosine nitration at certain sites of peptide can effectively inhibit its aggregation. This minor modification may be an ideal strategy to the rational design of peptide-based drugs with low aggregation propensity yet without loss of bioactivity. Human calcitonin (hCT) is such a peptide hormone known for its hypocalcaemic effect but has limited pharmaceutical potential due to a high tendency to aggregate. In this study, by using multiple techniques including Fluorescence, TEM, Nu-PAGE and CD, we demonstrated that Y12 nitration of hCT would significantly inhibit its self-assembles, and we also found that this modification would not only reduce the cytotoxicity induced by peptide aggregation, but also had little effect on its potency. This finding may provide a novel strategy for clinically application of hCT instead of sCT.

Heme prevents highly amyloidogenic human calcitonin (hCT) aggregation: A potential new strategy for the clinical reuse of hCT

Irreversible aggregation can extremely limit the bioavailability and therapeutic activity of peptide-based drugs. Thus, peptide fibrillation is an excellent challenge for biotechnological drug development. Human calcitonin (hCT) is such a peptide hormone known for its hypocalcaemic effect but has limited pharmaceutical potential due to a high tendency to aggregate. hCT is therefore not widely used preparation in clinical practice. Nonetheless, hCT seems to be still an ideal target for clinical therapy when fibrillation is effectively inhibited, because the alternatives of hCT can stimulate undesirable immune responses in patients and cause side effects. Interestingly, heme is an essential component for many livings and has been shown a strong inhibitory effect on some amyloidogenic peptides aggregation. Here we demonstrate that it may be a most suitable, safe, biocompatible small molecule inhibitor on hCT aggregation, and thereby improving its activity when guiding the drug peptide in clinical therapeutics. In this work, we found that heme was able to reversibly bind with hCT to form a heme-hCT complex with a moderate binding constant (9.17 × 10⁶ M^-1) and significantly suppress the aggregation of hCT probably accomplished by heme binding to it, blocking the β-sheet structure assembly which is essential in hCT fibril aggregation. Meanwhile, the heme-hCT complexes showed enhanced bioactivity compared to hCT itself after a 24 h incubation time in reducing blood calcium levels in mice. This study may develop a new strategy to reuse the wild-type hCT in clinical therapeutics.

Thyrocalcitonin: evidence for release in a spontaneous hypocalcemic disorder

Thyrocalcitonin content of thyroid gland extracts from normal postparturient cows was 3.9 times greater than in cows with postparturient paresis. The parafollicular cells in diseased cows were less numerous and appeared to have discharged their secretory products. An abrupt release of thyrocalcitonin near parturition may be related to the development of the hypocalcemia and hypophosphatemia in this disorder.

Calcitonin impairs the anabolic effect of PTH in young rats and stimulates expression of sclerostin by osteocytes

The therapeutic goal of increasing bone mass by co-treatment of parathyroid hormone (PTH) and an osteoclast inhibitor has been complicated by the undefined contribution of osteoclasts to the anabolic activity of PTH. To determine whether active osteoclasts are required at the time of PTH administration, we administered a low dose of the transient osteoclast inhibitor salmon calcitonin (sCT) to young rats receiving an anabolic PTH regimen. Co-administration of sCT significantly blunted the anabolic effect of PTH as measured by peripheral quantitative computer tomography (pQCT) and histomorphometry in the femur and tibia, respectively. To determine gene targets of sCT, we carried out quantitative real time PCR and microarray analysis of metaphyseal samples 1.5, 4 and 6.5h after administration of a single injection of PTH, sCT or PTH+sCT. Known targets of PTH action, IL-6, ephrinB2 and RANKL, were not modified by co-administration with sCT. Surprisingly, at all time points, we noted a significant upregulation of sclerostin mRNA by sCT treatment, as well as down-regulation of two other osteocyte gene products, MEPE and DMP1. Immunohistochemistry confirmed that sCT administration increased the percentage of osteocytes expressing sclerostin, suggesting a mechanism by which sCT reduced the anabolic effect of PTH. Neither mRNA for CT receptor (Calcr) nor labeled CT binding could be detected in sclerostin-enriched cells differentiated from primary calvarial osteoblasts. In contrast, osteocytes freshly isolated from calvariae expressed a high level of Calcr mRNA. Furthermore immunohistochemistry revealed co-localization of CT receptor (CTR) and sclerostin in some osteocytes in calvarial sections. Taken together these data indicate that co-treatment with sCT can blunt the anabolic effect of PTH and this may involve direct stimulation of sclerostin production by osteocytes. These data directly implicate calcitonin as a negative regulator of bone formation through a previously unsuspected mechanism.

Multicenter collaborative study to calibrate salmon calcitonin by bioassay and high-performance liquid chromatography: establishment of the third international standard

Salmon calcitonin (sCT) is widely used therapeutically in the treatment of patients with postmenopausal osteoporosis, Paget's disease, and some forms of hypercalcemia. Preparations of synthetic calcitonin peptides of high purity and reproducibility are now routinely produced and physicochemical methods, particularly reverse-phase high-performance liquid chromatography (RP-HPLC), are replacing the in vivo biological assay for monitoring and calibration. Although the bioassay is no longer required for routine batch control in Europe, calcitonin bioassays are still required in some countries and in the development of new products. Stocks of the Second International Standard (IS) for salmon calcitonin are now depleted and, to replace it with a new calibrant, an international collaborative study was organized in which the aims were to: determine the activity of the candidate sCT by in vivo bioassay in terms of the second IS; assess the stability of the preparation after accelerated thermal degradation; estimate the purity of the ampouled candidate preparation; and determine the sCT content in gravimetric units by HPLC. The HPLC data in terms of ampoule content were in good agreement giving an estimate of 23.1 (coefficient of variation [CV] 3.8%) microg per ampoule. The HPLC chromatograms revealed a small, but detectable, degree of heterogeneity, which possibly occurred during the formulating or ampouling procedures, resulting in a reduction in monocomponent content (purity) from 96% to 92%. The biological activity of the ampoule contents in international units (IU) was calculated from the mass value and the internationally agreed-upon figure of 6000 IU/per mg for the specific activity of salmon calcitonin. This gave a value of 138 IU per ampoule, which was in good agreement with the biological assay estimate (140 IU per ampoule). The preparation of sCT was subsequently adopted as the Third International Standard by the World Health Organization with an assigned content of 138 IU per ampoule.

Biological potency and radioimmunoassay of canine calcitonin

Calcitonin (CT) is a major calcitropic hormone. Because of low cross reactivity of canine CT (cCT) in radioimmunoassays (RIA) developed for other species, a homologous RIA is needed. Synthesis of cCT allowed study of its biologic potency using a rat bioassay and its plasma half-life in dogs. The availability of cCT also made possible the development of a homologous RIA for measurement of basal and stimulated plasma CT concentrations in dogs. The biologic potency of the synthesized cCT in rats is 24 IU/mg of peptide, which is low in comparison with the 4,000 IU/mg of the salmon CT standard. In the dog, an even lower potency of 4.4 IU/mg of cCT was found. Measurement of the disappearance of iv-injected radioiodinated or nonradioiodinated cCT revealed a short biologic half-life of less than 3 min, followed by a long half-life of 20 min. A polyclonal antiserum against synthetic cCT was raised in a goat. Using a final antiserum dilution of 1:12,000 and 125I-labeled synthetic cCT, the RIA had a detection limit of 6.5 ng/l. The antibody did not crossreact with standard human CT and had <0.1% cross reactivity with porcine CT. For measurement of plasma cCT concentrations, an extraction procedure was developed using ethanol. Dilutions of synthetic cCT and canine plasma extracts revealed parallelism over a wide range of concentrations. Size exclusion chromatography of canine plasma extracts on Biogel P-10 revealed a single cCT peak at the same position as [125I]-cCT, showing that there was little interference by other proteins or cCT prohormone. Basal plasma CT concentrations were 12-80 ng/l, and there was an 8- and 20-fold increase after calcium (1 and 2.5 mg/kg body weight) bolus infusion.

Rational design, conformational studies and bioactivity of highly potent conformationally constrained calcitonin analogues

Calcitonin is known for its hypocalcaemic effect and the inhibition of bone resorption, and is used therapeutically for the treatment of osteoporosis and Paget's disease. Our studies on the conformational features of human calcitonin (hCt) bioactivity have led to the conformationally constrained hCt analogue cyclo17,21-[Asp17, Lys21]hCt (1), which had a 5-10 times higher in vivo hypocalcaemic potency than hCt [Kapurniotu, A. & Taylor, J.W. (1995) J. Med. Chem. 38, 836-847]. We hypothesized that a stabilized, possibly type I beta turn/beta sheet conformation between residues 17 and 21 could play a crucial role in hCt bioactivity. Here, we designed, synthesized and studied the conformation and bioactivity of 19-member to 17-member ring-size analogues of 1 with the structure cyclo17,21-[Asp17,XX21]hCt with XX = Orn (2), Dab (3) and Dap (4), of the control peptide [Asp17,Orn21]hCt (5), and of the 19-member cyclo17,21-[Glu17,Dab21]hCt (6). Analyses of the far-UV CD spectra indicated increased type I beta turn and antiparallel beta sheet content in the bicyclic analogues compared with hCt. In the in vivo hypocalcaemic assay, cyclo17,21-[Asp17,Orn21]hCt (2) was found to have a 400-fold higher potency than hCt and was fourfold more potent than salmon calcitonin (sCt), which has been the most potent known Ct. Analogue 3 had a 30-fold higher potency than hCt, whereas the highly constrained analogue 4 was as potent as hCt. Bioactivity was not enhanced for the nonbridged compound [Asp17, Orn21]hCt (5), whereas cyclo17,21-[Glu17,Dab21]hCt (6) showed the same bioactivity as 1. This study identifies 2 as exhibiting the highest in vivo potency among currently known Cts, while it differs in only one amino acid residue from hCt, strongly suggesting that the introduced constraint may have served in 'freezing' hCt in a bioactive conformation. Our findings provide evidence for the first time that a beta turn/beta sheet conformation in region 17-21 of hCt and the topological features of the side chain of Asn17 are strongly associated with in vivo bioactivity, and offer a novel lead structure for a hCt-based drug for the treatment of osteoporosis and other bone-disorder-related diseases.

Production of biologically active salmon calcitonin in the milk of transgenic rabbits

Salmon calcitonin (sCT) is an example of one of the many bioactive peptides that require amidation of the carboxy terminus for full potency. We describe a method for the production of amidated sCT in the mammary gland of transgenic rabbits. Expression of a fusion protein comprising human alpha lactalbumin joined by an enterokinase cleavable linker to sCT was directed to the mammary gland under the control of the ovine beta lactoglobulin promoter. C-terminal amidation in vivo was achieved by extending the sCT by a single glycine residue that provides a substrate for endogenous amidating activity in the mammary gland. Full characterization of the released sCT demonstrated it to be equivalent to synthetic standard in terms of structure, purity, and potency.

Correlations between biological activities and conformational properties for human, salmon, eel, porcine calcitonins and Elcatonin elucidated by CD spectroscopy

Calcitonin (CT) inhibits osteoclastic bone resorption and induces calcium uptake from body fluids. A comparative study of the conformational behaviours of therapeutic calcitonins [salmon (s), eel (e), a synthetic eel calcitonin analogue (Elcatonin), porcine (p) and human (h) calcitonins] as a function of solvent polarity and temperature have been performed by circular dichroism spectroscopy. Elements of secondary structure were lacking in H2O but could be observed in 2,2,2-trifluoroethanol and sodium dodecyl sulphate. In particular, similar amounts of alpha-helical content (four alpha-helical turns) were estimated in trifluoroethanol despite the considerable differences in amino acid sequences. The relative ability to form an alpha helix, assessed by trifluoroethanol/H2O titration, was found to be Elcatonin > sCT > pCT > eCT > hCT. In Elcatonin, sCT, pCT and eCT the four alpha-helical turns were promoted almost completely in a single step, between 0 and 35% trifluoroethanol, unlike hCT where helical structure formation has been reported to involve two steps over the whole trifluoroethanol/H2O range [Arvinte, T. & Drake, A. F. (1993) J. Biol. Chem. 268, 6408-6414]. In SDS, which mimics the membrane environment, conformational differences (3-4 helical turns in Elcatonin, sCT, eCT versus one helical turn in pCT, hCT) were observed and correlate well with biological activity (Elcatonin = sCT = eCT > pCT = hCT). Low-temperature studies in a cryogenic solvent mixture showed the formation of high alpha-helix content (similar to that in trifluoroethanol) in Elcatonin, sCT, eCT and pCT, whilst a left-handed extended helix (3(1) helix) was formed in hCT. This is consistent with the hypothesis of 'linear' and 'helical' calcitonin receptors [Nakanuta, H., Orlowski, R. C. & Epand, R. M. (1990) Endocrinology 127, 163-169].

Sustained release of salmon calcitonin in vivo from lactide: glycolide copolymer depots

Studies were carried out to determine whether monolithic depot formulations, prepared using lactide:glycolide copolymers, could be used to administer salmon calcitonin (sCT) to rats in vivo. Formulations containing 2, 5, or 10% (w/w) sCT were administered subcutaneously to female Wistar strain rats. Release of sCT was determined by measurement of peptide in plasma using a specific radioimmunoassay and by measurement of residual sCT in the depots after recovery at postmortem. Plasma calcium concentrations and cumulative weight gain of the animals were used to measure pharmacological effects of the released sCT. Release of sCT from the depots was controlled by the copolymer and was sustained for periods up to 10 days. However, the release of sCT from the depots did not significantly alter plasma calcium concentrations, and effects on cumulative weight gain were small and transient. Peptide loading of the formulations was shown to modify sCT release. Maximal release of sCT from depots containing 10% peptide occurred over a 7 to 14-day period postadministration, with 5% sCT release occurred between days 11 and 14, and with 2% sCT, the period of maximal release was between days 11 and 18. Release of peptide from the depots was essentially complete by 21 days postadministration irrespective of the peptide loading. These data suggest that lactide:glycolide copolymer depots may have application for the convenient clinical administration of sCT in metabolic bone diseases.

Production of recombinant salmon calcitonin by in vitro amidation of an Escherichia coli produced precursor peptide

Salmon calcitonin (sCT) is a 32 amino acid peptide hormone that requires C-terminal amidation for full biological activity. We have produced salmon calcitonin by in vitro amidation of an E. coli produced precursor peptide. Glycine-extended sCT, the substrate for amidation, was produced in recombinant E. coli as part of a fusion with glutathione-S-transferase. The microbially produced soluble fusion protein was purified to near homogeneity by affinity chromatography. Following S-sulfonation of the fusion protein, the glycine-extended peptide was cleaved from the fusion by cyanogen bromide. The S-sulfonated peptide was recovered and enzymatically converted to the amidated peptide in a reaction with recombinant peptidylglycine alpha-amidating enzyme (alpha-AE) secreted from Chinese hamster ovary (CHO) cells. After reformation of the intramolecular disulfide bond, the sCT was purified with a step yield of 60%. The ease and speed of this recombinant process, as well as its potential for scale-up, make it adaptable to production demands for calcitonin, a proven useful agent for the treatment of post-menopausal osteoporosis. Moreover, the relaxed specificity of the recombinant alpha-AE for the penultimate amino acid which is amidated allows the basic process to be applied to the production of other amidated peptides.

Lack of effect of ipriflavone on osteoclast motility and bone resorption in in vitro and ex vivo studies

Using in vitro and ex vivo experimental procedures specifically designed to visualize pharmacological effects on parameters of bone resorption, studies were performed to elucidate whether ipriflavone's reported effect in osteoporosis is due to an effect on the motility and resorptive activity of osteoclasts, as has been shown to be the case with salmon calcitonin. Concentrations of ipriflavone used were higher by a factor of > 100 than peak blood levels measured in patients given standard therapeutic doses. Despite this, neither quantitative nor qualitative changes were observed in the motility of isolated rat osteoclasts or in their resorptive activity when incubated with bone slices. The conclusion is that ipriflavone does not possess antiosteoclastic and antiresorptive activity of the type documented for salmon calcitonin in the models employed and that further investigation of its mode of action is therefore necessary.

A new in vitro bioassay for human calcitonin: validation and comparison to the rat hypocalcemia bioassay

The human breast cancer cell line T 47 D expresses calcitonin (CT) receptors that are coupled to adenylate cyclase and which reveal a dose-dependent cyclic AMP response to CT. We used this model to establish an in vitro bioassay for synthetic human CT (hCT) preparations to overcome some of the obstacles of the standard rat hypocalcemia in vivo bioassay. The detection limit of the in vitro bioassay was 1 x 10(-10) M hCT (EC 50: 8.7 pM +/- 26%) compared to 7.3 x 10(-9) M (EC 50: 7.2 microM +/- 32%) for the in vivo bioassay. The relative potencies of test preparations revealed a good correlation (r = 0.89) and several hCT-related substances produced comparable results when tested by the two methods. The standard deviations of precision and accuracy, however, were significantly smaller (P less than 0.05) for the in vitro bioassay. According to these data the T 47 D in vitro bioassay is more sensitive, superior in precision and accuracy, and comparable in specificity to the rat hypocalcemia bioassay.

International Standards for salmon calcitonin, eel calcitonin, and the Asu1-7 analogue of eel calcitonin: calibration by international collaborative study

Regulatory specifications in most countries require that the potency of salmon calcitonin (sCT) clinical products be expressed in International Units (IU) defined by the World Health Organization (WHO) International Standard. The first ampouled standard was prepared in 1972 and has been distributed world-wide since then. A batch of ampoules to serve as the replacement standard is now required. Other piscine calcitonins, eel calcitonin (eCT) and an amino-suberic acid analogue of eCT (Asu1-7 eCT) are now clinical products in some countries and international standards are required for these peptides which are similar to, but not identical with, sCT. This paper describes the preparation of three new ampouled standards and their biological calibration by international collaborative study comprising 17 participants from 10 countries. Following the recommendations in the final report of the collaborative study, the 2nd International Standard (IS) for sCT, the 1st IS for eCT and the 1st IS for Asu1-7 eCT were recently established by WHO, each with an assigned potency in IU, and are now available for issue.

A new approach for calcitonin determination based on target cell responsiveness

We report the development and validation of three microbioassays for calcitonin based on calcitonin-induced inhibition of the activity of isolated osteoclasts. Having precisely quantified osteoclast motility, spreading and bone resorptive activity, we have applied stringent analytical procedures to define assay characteristics. We have found that the appropriately transformed responses significantly regress on log dose of the peptides. Furthermore, potency estimates obtained using calcitonins from three species (human, salmon and a synthetic analogue of eel calcitonin) have been found to be consistent with those obtained using conventional calcitonin bioassays. In addition, the assays are remarkably sensitive (detection limit 10(-15) M), highly specific and precise. We have determined plasma levels of bioactive calcitonin on samples from patients with medullary thyroid carcinoma; these are several-fold lower than those obtained using our routine calcitonin radioimmunoassay. Our study thus, forms the basis of an entirely new approach for the determination of 'biologically active' calcitonin, and we envisage that such target cell-specific assays could become useful microanalytical methods.

Biological activity of different calcitonins in men

Calcitonin has been isolated and its structure defined from several species, including man. Synthetic preparations of several calcitonins are available for clinical use. Of these, porcine (pCT), human (hCT), and salmon (sCT) have been synthetized according to their natural sequences, while eel calcitonin (cCT) is available as amino-suberic acid derivative (ASU-eCT). The different molecular configuration results in different biological potency and tolerability. Currently, the potency of calcitonin in man is evaluated by its capacity of lowering serum calcium and stimulating cAMP plasma levels after acute infusion. In normal subjects, cAMP stimulation seems to be a more sensitive test, since plasma calcium in normal subjects is poorly affected by an acute treatment with calcitonin. On the other hand, the side effects are usually assessed by clinical observation, on the basis of duration and intensity of the symptoms. Our experience, emerging from several studies devoted to comparing the biological activity and tolerability of different calcitonin preparations in humans, indicates that the hypocalcemic effect and the increase of plasma cAMP are produced by all peptides, according to the potency order sCT greater than hCT greater than ASU-eCT. For all peptides, the most constant side effect is flushing, and the frequency order of side effects is hCT greater than sCT = ASU-eCT.

Pathophysiological study of diarrhoea in a patient with medullary thyroid carcinoma. Evidence against a secretory mechanism and for the role of shortened colonic transit time

Intubation techniques and scintigraphic studies were used to determine the origin and mechanism of diarrhoea in a patient with medullary thyroid carcinoma, high plasma immunoreactive calcitonin and normal circulating serotonin, substance P and prostaglandins E2 and F2 alpha. Normal function of the small intestine was found for the following: (a) absorption tests; (b) water and electrolyte absorption in the proximal jejunum; (c) 24 hour flow rate and composition of fluid entering the colon and (d) gastric emptying rate and small intestinal progression of a normal meal. By contrast, colonic function was markedly impaired in three ways: (a) water absorption was decreased by half; (b) as the main excreted solutes were organic acids, a large electrolyte gap was recorded in faecal water, and (c) colonic transit time of the meal marker was very short, and was in agreement with the rapid transit of ingested radioopaque markers. These data strongly suggest that decreased absorption in the colon secondary to a motor disturbance is the main mechanism of diarrhoea in this case of medullary thyroid carcinoma, while calcitonin induced small intestinal fluid secretion suggested earlier is either non-existent, or only of minor importance.

Synthesis of an analog of human calcitonin gene related peptide, [Asu]-h-CGRP

The analog of h-CGRP, des-Ala-deamino-dicarba-h-CGRP, was synthesized by the combination of the conventional solution and the solid phase peptide synthesis methods. This analog showed stronger and longer-lasting hypocalcemic and hypophosphatemic activities than the natural hormone.

The effect of intravenous sodium fluoride and synthetic salmon calcitonin on plasma total calcium, inorganic phosphate, and ionic fluoride

Various doses of sodium fluoride (NaF), salmon calcitonin (CT) and NaF combined with CT were given intravenously to rats (236 +/- 2 g). Blood plasma samples were collected at various times up to 24 hours and measured for total calcium (adjusted for variation in plasma albumin), inorganic phosphate, ionic fluoride, urea, and creatinine. Following injection of NaF alone, significant hypocalcemia and hyperphosphatemia was observed. In contrast, CT injections resulted in hypocalcemia and hypophosphatemia. NaF and CT given in combination generally resulted in the hypocalcemia being equal to or greater than that calculated from the simple addition of their individual effects. Significant increases in both plasma creatinine and plasma urea were observed following treatment with NaF alone or with CT and NaF together, whereas CT alone had negligible effect. These results suggest that NaF does not mimic the effects of CT; rather that NaF and CT interact to modify their individual effects. The influence of NaF probably occurs via an effect on kidney function.

Calcitonin induces hypercalcemia in grey mullet and immature freshwater and sea-water adapted rainbow trout

1. Changes in plasma calcium levels, in response to salmon calcitonin injections, were studied in freshwater and sea-water adapted trout (Salmo gairdnerii) and in grey mullet (Chelon labrosus). 2. Low doses (0.1 ng sCT/100 g body weight) elicited hypercalcemia in the two species studied. 3. High doses (0.5 microgram) provoked hypocalcemia only in freshwater and sea-water adapted trout. 4. An hypercalcemic response appears as the primordial effects of CT injections, higher doses of CT leading to hypocalcemic effects.

Purification and characterization of calcitonin receptors in rat kidney membranes by covalent cross-linking techniques

We have characterized the binding parameters of renal receptors (Scatchard analysis revealed the presence of two binding sites: site I, Ka1 = 1.29 X 10(9) M-1, number of binding sites = 9.9 X 10(6)/micrograms protein; site II, Ka2 = 0.93 X 10(8) M-1, number of binding sites = 4.27 X 10(8)/micrograms protein) and studied the effect of solubilization. The high-affinity sites are preserved during affinity chromatography and the process results in a 6080-fold purification of those sites. The lower-affinity sites are also preserved but the overall purification factor is about 40% lower than that obtained using molecular sieving. The purification of the renal calcitonin receptor by molecular sieving (Sephacryl S-200) is accompanied by total loss of the high-affinity site; however, the low-affinity site is enriched over 1642-fold. Binding parameters were obtained for the purified fractions. Synthetic salmon calcitonin was also bound to renal membranes using the bifunctional reagent disuccinimidyl suberate and photo-affinity cross-linking using hydroxysuccinimidyl azidobenzonate reagent. Cross-linked receptor eluted in the same volume as solubilized membranes specifically binding salmon calcitonin (S-200 chromatography). Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis of purified fractions showed several protein bands with apparent molecular masses ranging from 18 000 Da to 100 000 Da in the presence or absence of a reducing agent (2-mercaptoethanol). Autoradiography of polyacrylamide gels of cross-linked calcitonin receptor showed only three protein bands specifically binding salmon calcitonin. Their molecular masses were 70 000 Da, 40 000 Da and 33 000 Da respectively. The 40 000-Da molecule represents a major band (47% total binding species). This suggests that these three proteins are the principal components of the calcitonin receptor and that S-S bonds are not involved in the assembly of the receptor subunits.

Quantitative cytochemical responses to exogenously administered calcitonins in rat kidney and bone cells

Time- and dose-dependent changes in intracellular enzyme activities in kidney and bone from rats injected with calcitonin have been assessed by quantitative cytochemistry. The doses of salmon calcitonin given were similar to those suggested in the Pharmacopoeial rat hypocalcaemia bioassay (1-50 mIU/50 g body weight). The highest doses produced 30% inhibition of alkaline phosphatase activity, maximal within 20 min after injection, in cells of renal proximal tubules and a stimulation of calcium-dependent adenosine triphosphatase activity in kidney cortical and outer medullary cells. Alkaline phosphatase activity in the periosteal bone cells was markedly inhibited at the lowest doses. When doses of human and porcine calcitonins were given which would be equipotent with that of salmon calcitonin in the rat hypocalcaemia bioassay, the effect of the non-mammalian peptide on renal alkaline phosphatase activity was relatively greater than that of the mammalian peptides. Oxidized human calcitonin did not inhibit renal alkaline phosphatase activity even when an amount equivalent to 10 times the highest dose of the unmodified peptide was injected.

Identity of calcitonin extracted from normal human thyroid glands with synthetic human calcitonin-(1-32)

Calcitonin in human thyroid glands obtained at autopsy from normal subjects were extracted with 2 M acetic acid. The extracts were additionally purified by adsorption to Sep-Pak C18 cartridges, and calcitonin was identified after gel filtration analysis, reverse-phase high-performance liquid chromatography (HPLC), thin-layer chromatography and isoelectric focusing. The purification steps were monitored by radioimmunoassay, and partially purified calcitonin was used for biological and physicochemical comparison with synthetic human calcitonin-(1-32) and its Met8-sulfoxide form. On gel filtration analysis a predominant peak coeluted with the synthetic hormone, and on HPLC two discrete peaks with the retention times of monomeric and dimeric human calcitonin were found. Thin-layer chromatography allowed the detection of two peaks with the Rf of human calcitonin-(1-32) and of its sulfoxide, respectively. The pI (7.9) of the predominant peaks of synthetic calcitonin were identical. Our findings provide strong evidence that the predominant forms of human calcitonin extracted from normal thyroid glands correspond to synthetic calcitonin-(1-32) and to dimeric calcitonin.

Hypercalcemia in hyperthyroidism. Role of age and goiter type

Hyperthyroidism is often associated with hypercalcemia which is provoked by osteoclastic activity of the thyroid hormones. These data show that hypercalcemia develops with increasing age and in the presence of a special type of hyperthyroid goiter. Total serum calcium, total protein, and albumin as well as different parameters of thyroid function, namely T3 RIA1, T4 test, ETR and TRH test were determined in a group of 147 patients. The ionized calcium level was estimated from total calcium and albumin. 211 measurements were performed. Hyperthyroidism existed in 92 cases. Total calcium was not significantly elevated in hyperthyroidism. Hyperthyroid patients under 61 years of age showed elevated ionized calcium levels in only 2.3% and patients over 60 years of age in 18.8% of cases. Elevated ionized serum calcium levels were observed in 43.8% of hyperthyroid patients with multinodular goiters. The linear correlation between ionized calcium levels and different parameters of thyroid function is much more pronounced in the older group and it was found to be highly significant. 7 of 9 hyperthyroid patients with elevated ionized calcium levels showed multinodular goiters, though no autonomous adenoma. In the hyperthyroid group of patients of over 60 years of age with multinodular goiters the incidence of hypercalcemia was 43.8%. Direct action of thyroid hormone on calcium turnover as well as increasing age and special goiter type seem to be responsible for disturbances in calcium metabolism. A possible calcitonin deficiency in the above mentioned conditions is discussed.

Evidence that protease inhibitors reduce the degradation of parathyroid hormone and calcitonin injected subcutaneously

1 Agents known to delay absorption from a subcutaneous site were tested in chicks for their ability to prolong the hypercalcaemic response to parathyroid hormone (PTH). 2 Polyvinylpyrrolidone was found to enhance the response but gelatine greatly reduced the 2 h hypercalcaemia. 3 The reduction by gelatine was reversed when the protease inhibitor aprotinin was added to the injection vehicle, and hypercalcaemia then persisted for more than 8 h. 4 Of other protease inhibitors studied, epsilon-aminocaproic acid was also found to enhance the hypercalcaemic response to subcutaneous PTH and its fragments but, unlike aprotinin, it was ineffective in the presence of gelatine. 5 By radioimmunoassay and bioassay respectively, it was confirmed that aprotinin raised circulating levels of PTH and also of another peptide hormone, calcitonin, injected subcutaneously. 6 Addition of calcium to the solutions injected subcutaneously abolished the hypercalcaemic response to PTH while injection of calcium and PTH simultaneously but at separate sites left the response unaltered. 7 The two protease inhibitors, epsilon-aminocaproic acid and aprotinin, each restored the response to subcutaneous PTH despite the presence of calcium at the injection site. 8 It was concluded that protease inhibitors injected subcutaneously with PTH and calcitonin in the chick reduced the rate of degradation of these hormones and that the proteases responsible for hormone degradation at the subcutaneous injection site may be released or activated by calcium ions.

Hormonal responses of bone in a continuous flow cultural system

Calvaria from five-day-old mice incubated in a continuous flow system (10 calvaria per culture chamber with 1 ml volume; flow rate 1ml/hr.). In 24-hour cultures, calvaria showed less resorption than in stationary culture. Short-term responses to PTH and calcitonin were similar to those observed in vivo.

Synthesis of eel-calcitonin and (asu1,7)-eel-calcitonin: contribution of the disulfide bond to the hormonal activity

Eel-calcitonin and its [asu1,7]-analog, deamino-dicarba-analog, were synthesized by the conventional solution method. The natural-type product showed 4300 MRC U/mg in the hypocalcemic potency which was comparable to that of the natural hormone isolated from eel. Hormonal activity of the Asu-analog was also as high as 3400 MRC U/mg.

[Structure-activity relationship of human calcitonin. III. Biological activity of synthetic analogues with shortened or terminally modified peptide chains (author's transl)]

Assays of 8 synthetic analogues of human calcitonin in rats showed that their hypocalcaemic activity was drastically reduced by deletion of the C-terminal amide group, chain-shortening or opening of the disulphide ring, but unaffected or enhanced by modification of the N-terminal amino group.

Analogues of human calcitonin. IV. Influence of leucine substitutions in positions 12, 16 and 19 on hypocalcaemic activity in the rat

The replacement of the three aromatic amino acids in positions 12, 16 and 19 of human calcitonin (HCT) by leucine residues, which occupy the corresponding positions, in ultimobranchial, e.g. salmon and eel, calcitonins, increased the hypocalcaemic potency of the peptide, as determined by bioassay on the rat, about 10-fold. The individual substitutions were not all equally augmentative: leucine (12) enhanced the activity of HCT about 4--5 times, but leucine (16) and (19) afforded no increase at all. Combination of leucine (12) with a deamino cysteine at the N-terminus yielded an analogue 10 times more potent than HCT. Another analogue containing valine in position 8 in place of methionine as well as the three leucine substituents in position 12, 16 and 19 proved more active than the tri-leucine analogue, but the additional introduction of tyrosine (22) nearly doubled the hypocalcaemic potency of the latter. The duration of the hypocalcaemic effects of the substituted peptides closely followed the changes in potency.