The stability and degradation pathway of recombinant human parathyroid hormone: deamidation of asparaginyl residue and peptide bond cleavage at aspartyl and asparaginyl residues

Tutorial review for peptide assays: An ounce of pre-analytics is worth a pound of cure

The recent increase in peptidomimetic-based medications and the growing interest in peptide hormones has brought new attention to the quantification of peptides for diagnostic purposes. Indeed, the circulating concentrations of peptide hormones in the blood provide a snapshot of the state of the body and could eventually lead to detecting a particular health condition. Although extremely useful, the quantification of such molecules, preferably by liquid chromatography coupled to mass spectrometry, might be quite tricky. First, peptides are subjected to hydrolysis, oxidation, and other post-translational modifications, and, most importantly, they are substrates of specific and nonspecific proteases in biological matrixes. All these events might continue after sampling, changing the peptide hormone concentrations. Second, because they include positively and negatively charged groups and hydrophilic and hydrophobic residues, they interact with their environment; these interactions might lead to a local change in the measured concentrations. A phenomenon such as nonspecific adsorption to lab glassware or materials has often a tremendous effect on the concentration and needs to be controlled with particular care. Finally, the circulating levels of peptides might be low (pico- or femtomolar range), increasing the impact of the aforementioned effects and inducing the need for highly sensitive instruments and well-optimized methods. Thus, despite the extreme diversity of these peptides and their matrixes, there is a common challenge for all the assays: the need to keep concentrations unchanged from sampling to analysis. While significant efforts are often placed on optimizing the analysis, few studies consider in depth the impact of pre-analytical steps on the results. By working through practical examples, this solution-oriented tutorial review addresses typical pre-analytical challenges encountered during the development of a peptide assay from the standpoint of a clinical laboratory. We provide tips and tricks to avoid pitfalls as well as strategies to guide all new developments. Our ultimate goal is to increase pre-analytical awareness to ensure that newly developed peptide assays produce robust and accurate results.

Discovery and Control of Succinimide Formation and Accumulation at Aspartic Acid Residues in The Complementarity-Determining Region of a Therapeutic Monoclonal Antibody

PURPOSE

Succinimide formation and isomerization alter the chemical and physical properties of aspartic acid residues in a protein. Modification of aspartic acid residues within complementarity-determining regions (CDRs) of therapeutic monoclonal antibodies (mAbs) can be particularly detrimental to the efficacy of the molecule. The goal of this study was to characterize the site of succinimide accumulation in the CDR of a therapeutic mAb and understand its effects on potency. Furthermore, we aimed to mitigate succinimide accumulation through changes in formulation.

METHODS

Accumulation of succinimide was identified through intact and reduced LC-MS mass measurements. A low pH peptide mapping method was used for relative quantitation and localization of succinimide formation in the CDR. Statistical modeling was used to correlate levels of succinimide with basic variants and potency measurements.

RESULTS

Succinimide accumulation in Formulation A was accelerated when stored at elevated temperatures. A strong correlation between succinimide accumulation in the CDR, an increase in basic charge variants, and a decrease in potency was observed. Statistical modeling suggest that a combination of ion exchange chromatography and potency measurements can be used to predict succinimide levels in a given sample. Reformulation of the mAb to Formulation B mitigates succinimide accumulation even after extended storage at elevated temperatures.

CONCLUSION

Succinimide formation in the CDR of a therapeutic mAb can have a strong negative impact on potency of the molecule. We demonstrate that thorough characterization of the molecule by LC-MS, ion exchange chromatography, and potency measurements can facilitate changes in formulation that mitigate succinimide formation and the corresponding detrimental changes in potency.

Designing Formulation Strategies for Enhanced Stability of Therapeutic Peptides in Aqueous Solutions: A Review

Over the past few decades, there has been a tremendous increase in the utilization of therapeutic peptides. Therapeutic peptides are usually administered via the parenteral route, requiring an aqueous formulation. Unfortunately, peptides are often unstable in aqueous solutions, affecting stability and bioactivity. Although a stable and dry formulation for reconstitution might be designed, from a pharmaco-economic and practical convenience point of view, a peptide formulation in an aqueous liquid form is preferred. Designing formulation strategies that optimize peptide stability may improve bioavailability and increase therapeutic efficacy. This literature review provides an overview of various degradation pathways and formulation strategies to stabilize therapeutic peptides in aqueous solutions. First, we introduce the major peptide stability issues in liquid formulations and the degradation mechanisms. Then, we present a variety of known strategies to inhibit or slow down peptide degradation. Overall, the most practical approaches to peptide stabilization are pH optimization and selecting the appropriate type of buffer. Other practical strategies to reduce peptide degradation rates in solution are the application of co-solvency, air exclusion, viscosity enhancement, PEGylation, and using polyol excipients.

Comparative Physicochemical and Biological Characterisation of the Similar Biological Medicinal Product Teriparatide and Its Reference Medicinal Product

Background

In January 2017, the European Commission approved Terrosa^® (company code RGB-10) as one of the first biosimilar medicinal products of teriparatide for the same indications as for the reference medicinal product Forsteo^® (Lilly France S.A.S.), which has been on the market in the European Union since 2003. The active pharmaceutical ingredient of the reference medicinal product is the biologically active 1-34 fragment of the endogenous human parathyroid hormone [PTH(1-34)]. It is one of the three bone anabolic agents used in the treatment of osteoporosis promoting bone formation and preventing fragility fractures.

Objective

The objective of this paper is to summarise the results of the comparative analysis of representative batches of both the RGB-10 drug product and the reference medicinal product performed by physicochemical and in vitro biological methods.

Methods

A series of state-of-the-art analytical methods were applied in a comparative head-to-head manner for testing the similarity in respect to purity, content, structure and potency.

Results

Based on the results of the comprehensive physicochemical and biological characterisation, RGB-10 proved to be highly similar to the reference medicinal product with respect to the critical quality attributes investigated.

Conclusion

The results of the quality comparability study demonstrated similarity of RGB-10 to the reference medicinal product, providing the scientific basis for conducting a specifically designed clinical programme, and supported registration of the Marketing Authorisation Application of RGB-10 in the EU.

A RAPID Method for Blood Processing to Increase the Yield of Plasma Peptide Levels in Human Blood

Research in the field of food intake regulation is gaining importance. This often includes the measurement of peptides regulating food intake. For the correct determination of a peptide's concentration, it should be stable during blood processing. However, this is not the case for several peptides which are quickly degraded by endogenous peptidases. Recently, we developed a blood processing method employing Reduced temperatures, Acidification, Protease inhibition, Isotopic exogenous controls and Dilution (RAPID) for the use in rats. Here, we have established this technique for the use in humans and investigated recovery, molecular form and circulating concentration of food intake regulatory hormones. The RAPID method significantly improved the recovery for (125)I-labeled somatostatin-28 (+39%), glucagon-like peptide-1 (+35%), acyl ghrelin and glucagon (+32%), insulin and kisspeptin (+29%), nesfatin-1 (+28%), leptin (+21%) and peptide YY3-36 (+19%) compared to standard processing (EDTA blood on ice, p <0.001). High performance liquid chromatography showed the elution of endogenous acyl ghrelin at the expected position after RAPID processing, while after standard processing 62% of acyl ghrelin were degraded resulting in an earlier peak likely representing desacyl ghrelin. After RAPID processing the acyl/desacyl ghrelin ratio in blood of normal weight subjects was 1:3 compared to 1:23 following standard processing (p = 0.03). Also endogenous kisspeptin levels were higher after RAPID compared to standard processing (+99%, p = 0.02). The RAPID blood processing method can be used in humans, yields higher peptide levels and allows for assessment of the correct molecular form.

Forced degradation studies of biopharmaceuticals: Selection of stress conditions

Stability studies under stress conditions or forced degradation studies play an important role in different phases of development and production of biopharmaceuticals and biological products. These studies are mostly applicable to selection of suitable candidates and formulation developments, comparability studies, elucidation of possible degradation pathways and identification of degradation products, as well as, development of stability indicating methods. Despite the integral part of these studies in biopharmaceutical industry, there is no well-established protocol for the selection of stress conditions, timing of stress testing and required extent of degradation. Therefore, due to the present gap in the stability studies guidelines, it is the responsibility of researchers working in academia and biopharmaceutical industry to set up forced degradation experiments that could fulfill all the expectations from the stability studies of biopharmaceuticals under stress conditions. Concerning the importance of the function of desired stress conditions in forced degradation studies, the present review aims to provide a practical summary of the applicable stress conditions in forced degradation studies of biopharmaceuticals according to the papers published in a time period of 1992-2015 giving detailed information about the experimental conditions utilized to induce required stresses.

Demonstrated solid-state stability of parathyroid hormone PTH(1-34) coated on a novel transdermal microprojection delivery system

PURPOSE

This study assessed conditions necessary for at least a 2-year, ambient temperature storage stability of the peptide parathyroid hormone 1-34, or PTH(1-34), coated on a novel transdermal microprojection delivery system, or ZP-PTH.

METHODS

Liquid coating characterization of high concentration PTH(1-34) formulations (>20% w/w) was assessed by viscosity and contact angle measurements along with RP-HPLC and SEC-HPLC. Solid-state coating morphology of PTH(1-34) on microprojection arrays was determined by SEM, and stability on storage was assessed after dissolution and testing with stability indicating assays. Internal vapor analysis was performed to detect and quantify volatile organics released by patch components into the headspace inside the final package.

RESULTS

Aggregation and oxidation were the primary degradation mechanisms for solid-state PTH(1-34) in this transdermal delivery system. Although these two degradation pathways can be retarded by appropriate stabilizers and use of foil pouch packaging (nitrogen purged and desiccant), the solid-state drug formulation's compatibility with patch components, particularly the plastic retainer ring, surprisingly dictated PTH(1-34) stability. Internal vapor analysis demonstrated that PTH(1-34) was particularly vulnerable to vapors such as moisture, oxygen, and outgassed formaldehyde, and each of these volatiles played a unique and significant role in PTH(1-34)'s degradation mechanism.

CONCLUSIONS

Identifying degradation mechanisms of volatile compounds on solid-state PTH(1-34) peptide stability allowed for the rationale for selection of final formulation, system components and packaging conditions. A >2-yr, ambient temperature storage stability was demonstrated for solid-state drug coated on a novel transdermal microprojection delivery system. This system was successfully tested in a Phase 2 clinical trial for the treatment of post-menopausal women with osteoporosis.

Combined application of high resolution and tandem mass spectrometers to characterize methionine oxidation in a parathyroid hormone formulation

Identification and monitoring of degradation products is a critical aspect of drug product stability programs. This process can present unique challenges when working with complex biopharmaceutical formulations that do not readily lend themselves to straightforward HPLC analysis. The therapeutic 34 amino acid parathyroid hormone fragment (PTH1-34) contains methionine (Met) residues at positions 8 and 18. Oxidation of these Met residues results in reduced biological activity and thus efficacy of the potential drug product. Here, we present an effective approach for the identification of PTH1-34 oxidation products in a drug product formulation in which the stability indicating method used non-MS compatible HPLC conditions to separate excipients, drug substance and degradation products. High resolution and tandem mass spectrometers were used in conjunction with cyanogen bromide (CNBr) mediated digestion to accurately identify the oxidation products observed in an alternative MS compatible HPLC method used for drug substance analysis. All anticipated CNBr digested peptide fragments, including both oxidized and nonoxidized peptide fragments, were positively identified using TOF MS without the need for additional enzymatic digestion. Once identified, the oxidation products generated were injected onto the original non-MS compatible HPLC drug product stability indicating method and the respective retention times were confirmed. This allowed the oxidative stability of different formulations to be effectively monitored during the solid state stability program and during variant selection.

Parathyroid hormone PTH(1-34) formulation that enables uniform coating on a novel transdermal microprojection delivery system

PURPOSE

Assess formulation parameters to enable >24-h continuous accurate and uniform coating of PTH(1-34) on a novel transdermal microprojection array delivery system.

METHODS

Surface activity and rheology of the liquid formulation was determined by contact angle measurement and cone-plate viscometry. The formulation's delivery performance was assessed in vivo using the hairless guinea pig model. Peptide gelation was investigated by rheological and viscoelastic behavior changes.

RESULTS

Accurate and uniform coating was achieved by formulating the liquid formulation to a preferred contact angle range of 30-60 degrees with a surfactant and by establishing a Newtonian fluid (defined as a fluid maintaining a constant viscosity with shear rate and time) with a viscosity of > or =20 cps via adjusting the peptide concentration and using an appropriate acidic counterion. A non-volatile acidic counterion was found critical to compensate for the loss of the volatile acetate counterion to maintain the peptide formulation's solubility upon rehydration in the skin. Finally, the 15.5% w/w PTH(1-34) concentration was found to be the most physically stable formulation (delayed gelation) in the roll-coating reservoir. With a properly designed coating reservoir for shear force reduction, the liquid formulation could last for more than 24 h without gelation.

CONCLUSIONS

The study successfully offered scientific rationales for developing an optimal liquid formulation for a novel titanium microprojection array coating process. The resultant formulation has an enduring physical stability (>24 h) in the coating reservoir and maintained good in vivo dissolution performance.

The RAPID method for blood processing yields new insight in plasma concentrations and molecular forms of circulating gut peptides

The correct identification of circulating molecular forms and measurement of peptide levels in blood entails that the endocrine peptide being studied is stable and recovered in good yields during blood processing. However, it is not clear whether this is achieved in studies using standard blood processing. Therefore, we compared peptide concentration and form of 12 (125)I-labeled peptides using the standard procedure (EDTA-blood on ice) and a new method employing Reduced temperatures, Acidification, Protease inhibition, Isotopic exogenous controls, and Dilution (RAPID). During standard processing there was at least 80% loss for calcitonin-gene-related peptide and cholecystokinin-58 (CCK-58) and more than 35% loss for amylin, insulin, peptide YY forms (PYY((1-36)) and PYY((3-36))), and somatostatin-28. In contrast, the RAPID method significantly improved the recovery for 11 of 12 peptides (P < 0.05) and eliminated the breakdown of endocrine peptides occurring after standard processing as reflected in radically changed molecular forms for CCK-58, gastrin-releasing peptide, somatostatin-28, and ghrelin. For endogenous ghrelin, this led to an acyl/total ghrelin ratio of 1:5 instead of 1:19 by the standard method. These results show that the RAPID method enables accurate assessment of circulating gut peptide concentrations and forms such as CCK-58, acylated ghrelin, and somatostatin-28. Therefore, the RAPID method represents an efficacious means to detect circulating variations in peptide concentrations and form relevant to the understanding of physiological function of endocrine peptides.

Methods for syntheses of N-methyl-DL-aspartic acid derivatives

A novel practical method for the synthesis of N-methyl-DL-aspartic acid 1 (NMA) and new syntheses for N-methyl-aspartic acid derivatives are described. NMA 1, the natural amino acid was synthesized by Michael addition of methylamine to dimethyl fumarate 5. Fumaric or maleic acid mono-ester and -amide were regioselectively transformed into beta-substituted aspartic acid derivatives. In the cases of maleamic 11a or fumaramic esters 11b, the alpha-amide derivative 13 was formed, but hydrolysis of the product provided N-methyl-DL-asparagine 9 via base catalyzed ring closure to DL-alpha-methylamino-succinimide 4, followed by selective ring opening. Efficient methods were developed for the preparation of NMA-alpha-amide 13 from unprotected NMA via sulphinamide anhydride 15 and aspartic anhydride 3 intermediate products. NMA diamide 16 was prepared from NMA dimethyl ester 6 and methylamino-succinimide 4 by ammonolysis. Temperature-dependent side reactions of methylamino-succinimide 4 led to diazocinone 18, resulted from self-condensation of methylamino-succinimide via nucleophyl ring opening and the subsequent ring-transformation.

Mammalian Hsp22 is a heat-inducible small heat-shock protein with chaperone-like activity

A newly identified 22 kDa protein that interacts with Hsp27 (heat-shock protein 27) was shown to possess the characteristic alpha-crystallin domain, hence named Hsp22, and categorized as a member of the sHsp (small Hsp) family. Independent studies from different laboratories reported the protein with different names such as Hsp22, H11 kinase, E2IG1 and HspB8. We have identified, on the basis of the nucleotide sequence analysis, putative heat-shock factor 1 binding sites upstream of the Hsp22 translation start site. We demonstrate that indeed Hsp22 is heat-inducible. We show, in vitro, chaperone-like activity of Hsp22 in preventing dithiothreitol-induced aggregation of insulin and thermal aggregation of citrate synthase. We have cloned rat Hsp22, overexpressed and purified the protein to homogeneity and studied its structural and functional aspects. We find that Hsp22 fragments on storage. MS analysis of fragments suggests that the fragmentation might be due to the presence of labile peptide bonds. We have established conditions to improve its stability. Far-UV CD indicates a randomly coiled structure for Hsp22. Quaternary structure analyses by glycerol density-gradient centrifugation and gel filtration chromatography show that Hsp22 exists as a monomer in vitro, unlike other members of the sHsp family. Hsp22 exhibits significantly exposed hydrophobic surfaces as reported by bis-8-anilinonaphthalene-l-sulphonic acid fluorescence. We find that the chaperone-like activity is temperature dependent. Thus Hsp22 appears to be a true member of the sHsp family, which exists as a monomer in vitro and exhibits chaperone-like activity.

Analysis of aspartyl peptide degradation products by high-performance liquid chromatography and high-performance liquid chromatography-mass spectrometry

A reversed-phase HPLC method for the analysis of degradation products of the model aspartyl tripeptides Phe-Asp-GlyNH2 and Gly-Asp-PheNH2 after incubation at pH 2 and 10 was developed. Most of the compounds could be separated with a gradient of acetonitrile in water containing 0.1% trifluoroacetic acid. Resolution of the isomeric pairs L-Phe-alpha-L-Asp-GlyNH2/L-Phe-beta-L-Asp-GlyNH2 and L-Phe-alpha-D-Asp-GlyOH/L-Phe-beta-D-Asp-GlyOH was achieved with a gradient of acetonitrile in phosphate buffer, pH 5.0. Under acidic conditions the major degradation pathway was cleavage of the peptide backbone amide bonds yielding dipeptides and amino acids, C-terminal deamidation as well as formation of succidinimyl peptides. At alkaline pH both deamidation of the C-terminal amide as well as isomerization and concomitant enantiomerization of Asp were observed. The peaks were identified both by reference substances and by online electrospray mass spectrometry. The results were compared to a previous developed capillary electrophoresis method. Diastereomeric pairs ofpeptides that could not be separated by capillary electrophoresis were resolved by HPLC while the separation of corresponding pairs of alpha- and beta-Asp peptides was not always achieved by HPLC in contrast to capillary electrophoresis illustrating that both techniques can be complimentary in peptide analysis.

Improved rotorod performance and hyperactivity in mice deficient in a protein repair methyltransferase

The protein L-isoaspartate (D-aspartate)-O-methyltransferase participates in the repair of age-induced protein damage by initiating the conversion of abnormal aspartyl residues within proteins to normal L-aspartyl residues. Previous studies have shown that mice deficient in the gene encoding this enzyme (Pcmt1-/-) accumulate damaged proteins, have altered levels of brain S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy), and suffer from epileptic seizures that result in death at an average age of about 42 days. In this study, we found that the behavior of Pcmt1-/- mice is abnormal in comparison to their wild-type (Pcmt1+/+) and heterozygous (Pcmt1+/-) littermates in two standard quantitative behavioral assays - the accelerating rotorod and the open-field test. On the accelerating rotorod, we found Pcmt1-/- mice actually perform significantly better than their heterozygous and wild-type littermates, a situation that has only been infrequently described in the literature and has not been described to date for epilepsy-prone mice. The Pcmt1-/- mice show, however, hyperactivity in the open-field test that becomes more pronounced with age, with a partial habituation with time in the chamber. Additionally, these mice demonstrate a strong thigmotaxic movement pattern. We present evidence that these phenotypes are not related to the alterations of the AdoMet/AdoHcy ratio in the brain and thus may be a function of the accumulation of damaged proteins. These results implicate a role for this enzyme in motor coordination and cerebellum development and suggest the importance of the function of the repair methyltransferase in hippocampal-dependent spatial learning.

The influence of protein structure on the products emerging from succinimide hydrolysis

Proteins are vulnerable to spontaneous, covalent modifications that may result in alterations to structure and function. Asparagines are particularly labile, able to undergo deamidation through the formation of a succinimide intermediate to produce either aspartate or isoaspartate residues. Although aspartates cannot undergo deamidation they can form a succinimide and result in the same products. Isoaspartyls are the principal product of succinimide hydrolysis, accounting for 65-85% of the emerging residues. The variability in the ratio of products emerging from succinimide hydrolysis suggests the ability of protein structure to influence succinimide outcome. In the H15D histidine-containing protein (HPr), phosphorylation of the active site aspartate catalyzes the formation of a cyclic intermediate. Resolution of this species is exclusively to aspartate residues, suggestive of either a succinimide with restrained hydrolysis, or an isoimide, from which aspartyl residues are the only possible product. Deletion of the C-terminal residue of this protein does not influence the ability for phosphorylation or ring formation, but it does allow for isoaspartyl formation, verifying a succinimide as the cyclic intermediate in H15D HPr. Isoaspartyl formation in H15D Delta85 is rationalized to occur as a consequence of elimination of steric restrictions imposed by the C terminus on the main-chain carbonyl of the succinimide, the required point of nucleophilic attack of a water molecule for isoaspartyl formation. This is the first reported demonstration of the influence of protein structure on the products emerging from succinimide hydrolysis.

Analysis of human parathyroid hormone (1-84) products. Separation of a major impurity in synthetic products by ion-pairing reversed-phase high-performance liquid chromatography

Human parathyroid hormone (1-84) is a naturally occurring polypeptide that acts as the major regulator of calcium ion homeostasis. It can be efficiently produced through both synthetic and biosynthetic routes and, as such, highly selective analytical methods are required for the detection of a wide range of impurities. Herein we report on the development of an ion-pairing reversed-phase HPLC method for the analysis of human parathyroid hormone and the separation of impurities including a major, unidentified impurity detected in synthetic preparations. This impurity could not be resolved using trifluoroacetic acid-based methods generally used for monitoring purity levels in commercial products. Separation conditions consisted of a gradient elution of 0.155 M sodium chloride containing 0.037 M sodium pentanesulfonate, pH 5.6, as mobile phase A and acetonitrile as mobile phase B. Separations were carried out on an octadecylsilyl silica column maintained at 50 degrees C. Both column temperature and pH of mobile phase A significantly affected the separation of the major impurity. The major impurity eluted after the main human parathyroid peak and was detected in the two commercial synthetic products analyzed. Several minor impurities eluting before and after the main peak were also detected. Purity levels measured by the developed HPLC method (method C) were similar to those previously measured by capillary electrophoresis. Analysis of purified recombinant human parathyroid hormone did not show the presence of this impurity. This method offers a significant advantage for the purity assessment of human parathyroid hormone.

The effect of electroporation on iontophoretic transdermal delivery of calcium regulating hormones

Electrically-assisted delivery by iontophoresis and/or electroporation was used in vitro to deliver the calcium regulating hormones, salmon calcitonin (sCT) and parathyroid hormone (1-34) (PTH) through human epidermis. Such delivery could be useful for chronic treatment of post-menopausal osteoporosis and other clinical indications as a superior alternative to parenteral delivery. sCT (50 microg/ml) or PTH (1-34) (100 microg/ml) formulation was prepared in citrate buffer (pH 4.0 or 5.0, respectively). Epidermis separated from human cadaver skin was used. Iontophoresis was applied using a constant current power source and electroporation with an exponential pulse generator. Silver/silver chloride electrodes were used. A combination of electroporation and iontophoresis resulted in higher transdermal permeation than either one technique alone. Electroporation also shortened the lag time of iontophoretic transdermal delivery of salmon calcitonin. Pulsing at lower voltages followed by iontophoresis did not result in increased transport (over iontophoresis alone), perhaps because the transdermal voltage was very low. The transdermal transport of salmon calcitonin by pulsing with 15 pulses (1 ppm) of 500 V (200 ms) followed by iontophoresis led to a quick input and high flux. The average transdermal voltage was only about 50 V for a 500 V study.

High-performance liquid chromatography-mass spectrometry of an osteocalcin derivative

High-performance liquid chromatography (HPLC) was combined on-line with electrospray ionization mass spectrometry (ESI-MS) for structural analysis of a synthetic osteocalcin derivative and its degradation products. Initial determination of amino acid sequence of the synthetic peptide was performed after tryptic degradation. Hydrolytic degradation of the osteocalcin derivative was studied under different pH conditions: pH 2, pH 7 and pH 10 at 60 degrees C up to 20 h. According the HPLC-ESI-MS results, the chemical stability was dependent on pH. Two major degradation products and a number of other fragments were obtained in acidic solution, whereas the osteocalcin molecule was rather stable in neutral and alkaline conditions.