Isolation and characterization of porcine somatotropin containing a succinimide residue in place of aspartate129

Mass Spectrometry Based Mechanistic Insights into Formation of Tris Conjugates: Implications on Protein Biopharmaceutics

We present here extensive mass spectrometric studies on the formation of a Tris conjugate with a therapeutic monoclonal antibody. The results not only demonstrate the reactive nature of the Tris molecule but also the sequence and reaction conditions that trigger this reactivity. The results corroborate the fact that proteins are, in general, prone to conjugation and/or adduct formation reactions and any modification due to this essentially leads to formation of impurities in a protein sample. Further, the results demonstrate that the conjugation reaction happens via a succinimide intermediate and has sequence specificity. Additionally, the data presented in this study also shows that the Tris formation is produced in-solution and is not an in-source phenomenon. We believe that the facts given here will open further avenues on exploration of Tris as a conjugating agent as well as ensure that the use of Tris or any ionic buffer in the process of producing a biopharmaceutical drug is monitored closely for the presence of such conjugate formation. Graphical Abstract ᅟ.

Unexpected functional implication of a stable succinimide in the structural stability of Methanocaldococcus jannaschii glutaminase

Protein ageing is often mediated by the formation of succinimide intermediates. These short-lived intermediates derive from asparaginyl deamidation and aspartyl dehydration and are rapidly converted into β-aspartyl or D-aspartyl residues. Here we report the presence of a highly stable succinimide intermediate in the glutaminase subunit of GMP synthetase from the hyperthermophile Methanocaldoccocus jannaschii. By comparing the biophysical properties of the wild-type protein and of several mutants, we show that the presence of succinimide increases the structural stability of the glutaminase subunit. The protein bearing this modification in fact remains folded at 100 °C and in 8 M guanidinium chloride. Mutation of the residue following the reactive asparagine provides insight into the factors that contribute to the hydrolytic stability of the succinimide. Our findings suggest that sequences that stabilize succinimides from hydrolysis may be evolutionarily selected to confer extreme thermal stability.

Succinimide is a post-translational modification susceptible to rapid hydrolysis and generally associated with protein destabilisation. Here, the authors use mass spectroscopy to identify a stable succinimide intermediate that is responsible for the high thermostability of a thermophilic enzyme.

Acetic acid can catalyze succinimide formation from aspartic acid residues by a concerted bond reorganization mechanism: a computational study

Succinimide formation from aspartic acid (Asp) residues is a concern in the formulation of protein drugs. Based on density functional theory calculations using Ace-Asp-Nme (Ace = acetyl, Nme = NHMe) as a model compound, we propose the possibility that acetic acid (AA), which is often used in protein drug formulation for mildly acidic buffer solutions, catalyzes the succinimide formation from Asp residues by acting as a proton-transfer mediator. The proposed mechanism comprises two steps: cyclization (intramolecular addition) to form a gem-diol tetrahedral intermediate and dehydration of the intermediate. Both steps are catalyzed by an AA molecule, and the first step was predicted to be rate-determining. The cyclization results from a bond formation between the amide nitrogen on the C-terminal side and the side-chain carboxyl carbon, which is part of an extensive bond reorganization (formation and breaking of single bonds and the interchange of single and double bonds) occurring concertedly in a cyclic structure formed by the amide NH bond, the AA molecule and the side-chain C=O group and involving a double proton transfer. The second step also involves an AA-mediated bond reorganization. Carboxylic acids other than AA are also expected to catalyze the succinimide formation by a similar mechanism.

Detection and quantitation of succinimide in intact protein via hydrazine trapping and chemical derivatization

The formation of aspartyl succinimide is a common post-translational modification of protein pharmaceuticals under acidic conditions. We present a method to detect and quantitate succinimide in intact protein via hydrazine trapping and chemical derivatization. Succinimide, which is labile under typical analytical conditions, is first trapped with hydrazine to form stable hydrazide and can be directly analyzed by mass spectrometry. The resulting aspartyl hydrazide can be selectively derivatized by various tags, such as fluorescent rhodamine sulfonyl chloride that absorbs strongly in the visible region (570 nm). Our tagging strategy allows the labeled protein to be analyzed by orthogonal methods, including HPLC-UV-Vis, liquid chromatography mass spectrometry (LC-MS), and SDS-PAGE coupled with fluorescence imaging. A unique advantage of our method is that variants containing succinimide, after derivatization, can be readily resolved via either affinity enrichment or chromatographic separation. This allows further investigation of individual factors in a complex protein mixture that affect succinimide formation. Some additional advantages are imparted by fluorescence labeling including the facile detection of the intact protein without proteolytic digestion to peptides; and high sensitivity, for example, without optimization, 0.41% succinimide was readily detected. As such, our method should be useful for rapid screening, optimization of formulation conditions, and related processes relevant to protein pharmaceuticals.

Mass spectrometric distinction of in-source and in-solution pyroglutamate and succinimide in proteins: a case study on rhG-CSF

Formation of cyclic intermediates involving water or ammonia loss is a common occurrence in any reaction involving terminal amines or hydroxyl group containing species. Proteins that have both these functional groups in abundance are no exception, and presence of amino acids such as asparagine, glutamines, aspartic acids, and glutamic acids aid in formation of such intermediates. In the biopharma scenario, such intermediates lead to product- or process-related impurities that might be immunogenic. Mass spectroscopy is a powerful technique that is used to decipher the presence and physicochemical characteristics of such impurities. However, such intermediates can also form in situ during mass spectrometric analysis. We present here the detection of in-source and in-solution formation of succinimide and pyroglutamate in the protein granulocyte colony stimulating factor. We also propose an approach for quick differentiation of such in-situ species from the tangible impurities. We believe that this will not only reduce the time spent in unambiguous identification of succinimide- and/or pyroglutamate-related impurity in bio-pharmaceutics but also provide a platform for similar studies on other impurities that may form due to stabilized intermediates.

Accurate determination of succinimide degradation products using high fidelity trypsin digestion peptide map analysis

We report an efficient, high fidelity trypsin digestion method for peptide map analysis. This method minimizes artifacts caused by the sample preparation process, and we show its utility for the accurate determination of succinimide formation in a degraded monoclonal antibody product. A basic charge variant was detected by imaged capillary isoelectric focusing and was shown with reduced antigen binding and biological activity. Samples were reduced under denaturing conditions at pH 5.0, and digestion of the reduced protein with porcine trypsin was performed at pH 7.0 for 1 h. Following reversed phase high-performance liquid chromatography and online mass spectrometric analysis, succinimide formation was identified at Asp30 in the light chain. This result contrasts with the observation of only iso-Asp and Asp residues under conventional sample preparation conditions, which are therefore concluded to be artificially generated. The Asp30 residue is seen in the cocrystal structure model to participate in favorable charge interaction with an antigen molecule. Formation of succinimide and the resulting loss of negative charge are therefore hypothesized to be the degradation mechanism. After treatment of the degraded antibody sample to mildly alkaline pH conditions, we observed only Asp residue as the succinimide hydrolysis product and concurrent recovery of biological activity.

Mink growth hormone structural-functional relationships: effects of renaturing and storage conditions

Fourier-transform infrared spectroscopy, in vitro bioassay and enzyme-linked immunoassay were used to study the structural-functional relationships of recombinant mink growth hormone (mGH), refolded and stored under different conditions. Porcine GH (pGH) was synthesized and used as an example. These two hormones, when refolded and stored the same way, had the same secondary structures, biological and immunological efficacy, and biological potency. Only the immunological potency differed, mGH being significantly less potent than pGH. Renaturation pH and storing frozen or at 4 degrees C in 5% glycerol did not affect either the secondary structure or the activity. However, freeze-drying raised the content of buried alpha-helices and lowered that of solvated alpha-helices and of unordered structures. These conformational changes were associated with a reduction of immunological and biological potency of mGH and of immunological potency of pGH. These findings provide original information on the secondary structure of mGH, and show that conformational changes induced by lyophilization adversely affect its activity.

Structural investigation of a phosphorylation-catalyzed, isoaspartate-free, protein succinimide: crystallographic structure of post-succinimide His15Asp histidine-containing protein

Aspartates and asparagines can spontaneously cyclize with neighboring main-chain amides to form succinimides. These succinimides hydrolyze to a mixture of isoaspartate and aspartate products. Phosphorylation of aspartates is a common mechanism of protein regulation and increases the propensity for succinimide formation. Although typically regarded as a form of protein damage, we hypothesize succinimides could represent an effective mechanism of phosphoaspartate autophosphatase activity, provided hydrolysis is limited to aspartate products. We previously reported the serendipitous creation of a protein, His15Asp histidine-containing protein (HPr), which undergoes phosphorylation-catalyzed formation of a succinimide whose hydrolysis is seemingly exclusive for aspartate formation. Here, through the high-resolution structure of postsuccinimide His15Asp HPr, we confirm the absence of isoaspartate residues and propose mechanisms for phosphorylation-catalyzed succinimide formation and its directed hydrolysis to aspartate. His15Asp HPr represents the first characterized protein example of an isoaspartate-free succinimide and lends credence to the hypothesis that intramolecular cyclization could represent a physiological mechanism of autophosphatase activity. Furthermore, this indicates that current strategies for succinimide evaluation, based on isoaspartate detection, underestimate the frequencies of these reactions. This is considerably significant for evaluation of protein stability and integrity.

Hydrophobic interaction chromatography of soluble interleukin I receptor type II to reveal chemical degradations resulting in loss of potency

A hydrophobic interaction chromatography method was developed to analyze recombinant soluble Interleukin 1 receptor type II (sIL-1R type II) drug substance and assess the stability of the drug under accelerated degradation studies. HIC resolved the degraded molecules into three peaks. A combination of several analytical techniques, including cyanogen bromide cleavage, reversed-phase chromatography, mass spectrometry, and N-terminal sequencing, were used to identify the origins of these peaks. We found that accelerated degradation resulted from three different events, deamidation and isomerization at asparagine 317 (Asn317), C-terminal cleavage, and aggregation. The iso-aspartate 317 (iso-Asp317)-containing species were shown to elute in HIC peak I and the Asp317-containing species in HIC peak II, respectively. Deamidation-isomerization to iso-Asp317, but not deamidation to Asp317, resulted in altered retention time on HIC companied by loss of potency, presumably by introducing a significant conformational change. CNBr C-terminal analysis showed that the inactive HIC peak I consisted of sIL-1R type II with "large" C-terminal truncations of 13 or 14 amino acids, whereas the active HIC peak II contained C-terminally full length and "small" C-terminal clips of two amino acids. Molecular modeling indicates that the short loop D317-S320, in the third domain of IL-1R type II, has a crucial impact on the stability of the molecule.

Dimerization of a PACAP peptide analogue in DMSO via asparagine and aspartic acid residues

To optimize the stability of a peptide development candidate for the treatment of type II diabetes, formulation studies were initiated in organic solvents and compared to results obtained in aqueous solutions. Stability was assessed by reversed phase liquid chromatography (RPLC) and electrospray ionization mass spectrometry (ESI-MS). Previous studies had shown deamidation and hydrolysis to be the primary mechanisms of degradation in aqueous formulations. Surprisingly, the use of an organic solvent did not decrease the rate of degradation and, as presented here, produced degradation products including dimers. We propose here that deamidation can readily occur in polar anhydrous organic solvents such as DMSO and that the dimer forms through intermolecular nucleophilic attack of an amino acid side chain on a stabilized cyclic imide intermediate.

Analysis of protein phosphorylation in the regions of consecutive serine/threonine residues by negative ion electrospray collision-induced dissociation. Approach to pinpointing of phosphorylation sites

Pinpointing of phosphorylation sites by positive ion collision-induced dissociation (CID) in phosphopeptides containing consecutive Ser/Thr residues (Ser/Thr clusters) is frequently hampered by the lack of backbone cleavage between adjacent Ser/Thr or pSer/pThr sites. In this study, we demonstrate that in negative ion collision-induced dissociation phosphorylated and unmodified residues of Ser/Thr clusters exhibit a very selective behavior toward cleavage of their N-Calpha bonds. Ser/Thr clusters were defined as two and more consecutive serine or threonine residues in phosphopeptide sequences. Dissociation reactions at pSer are significantly more abundant than those of unmodified sites. Thr residues exhibit the same effect, but the cleavages occurring at pThr are generally less prominent than those at pSer. The correlation observed between the facility of the amine backbone bond dissociation of phosphopeptides and the presence of the phosphate group on the side chain residues of Ser and Thr is attributed to the different magnitudes of electron density on the Calpha atoms of the amino acid in phosphorylated and unmodified forms. The results of this study indicate that the intensity ratio of the fragments generated by N-Calpha bond cleavage within the phosphopeptide Ser/Thr clusters represents a reliable and general marker for pinpointing of phosphorylation sites. The presented data illustrate that negative ion electrospray CID is superior over the standard positive ion mode approach for the localization of protein phosphorylation inside Ser/Thr clusters.

Monoclonal antibodies as a probe for the unfolding of porcine growth hormone

Monoclonal antibodies (mAbs) were generated against pituitary porcine growth hormone (pGH). Ten mAbs were selected for their specificity and affinity for pGH. These mAbs were of the immunoglobulin G (IgG)(1) kappa subclass, with dissociation constants (K(d)) between 7.42 and 0.26 nM, and recognised seven non-overlapping epitopes. We measured whether the mAbs detected alterations of the pGH three-dimensional structure by comparing the antibody reactivity to native pGH and to pGH experimentally unfolded by heating at 50 degrees C, 75 degrees C and 100 degrees C or by reduction and S-carboxymethylation. The antibody-antigen interactions were studied with two enzyme-linked immunosorbent assays (ELISA), based either on a direct binding or inhibition format. The results show that: 1) one mAb, mAb D12, is a conformation-sensitive antibody that recognises an epitope present only in the native pGH. Because the intact three-dimensional structure is essential for the expression of biological activity, mAb D12 could be used to detect altered pGH molecules in biological samples (blood, pituitary extracts or material produced with recombinant technology), and for the one-step purification of biologically active pGH by immunoaffinity chromatography; 2) one mAb, mAb I4, binds to a linear epitope that is not significantly modified in the denatured hormone. This mAb was able to detect the hormone in assays where protein conformation is usually strongly altered, i.e. immunoblotting and immunohistochemistry; 3) the performances of the other eight mAbs differed significantly in the competitive and non-competitive ELISA.

Atomic resolution structure of a succinimide intermediate in E.coli CheY

Isomerization of aspartate to isoaspartate occurs spontaneously in proteins, causes changes in protein structures, and correlates positively with the aging processes of many organisms, including Alzheimer disease in humans. Aspartate isomerization proceeds through an unstable cyclic succinimide intermediate. There are few protein structure determinations that have characterized the intermediates and products of this isomerization reaction. Here we report the discovery of an unusually stabilized succinimide ring in the 1.1A structure of the Escherichia coli CheY protein, as determined from a crystal eight years old. The ring is formed by the side-chain of aspartate 75 and the backbone nitrogen of glycine 76 in an exposed loop of the molecule. Stabilization of the succinimide is through interaction of a sulfate ion oxygen atom with the imide nitrogen atom. Formation of the ring caused conformational changes in the loop, but did not alter the overall structure of the protein.

Effects of conformation on the chemical stability of pharmaceutically relevant polypeptides

Control of chemical instability in protein pharmaceuticals continues to be a critical issue in developing stable formulations. While the effects of pH, buffer composition, ionic strength and temperature remain the most effective methods for controlling hydrolysis and oxidation reactions, it appears that conformational control may also be important. Addition of excipients to maintain native structure and reduce the propensity of the protein to denature and/or aggregate is already a central theme in stabilizing proteins (Arakawa et al., 1993). The same additives have now been found to slow both deamidation and oxidation, whether in solution or in the solid state. What is emerging is an additional approach for producing protein pharmaceuticals that maintain native structure and activity during long-term storage.

Amino acid modifications in canine, equine and porcine pituitary growth hormones, identified by peptide-mass mapping

Modified amino acid residues in porcine, canine and equine growth hormones purified from pituitary glands were characterised by tryptic mapping and high-performance liquid chromatography with on-line coupled electrospray ionisation mass spectrometry (HPLC-ESI-MS) detection. Hormones from all three species showed the same changes. Conversion of Asp128 to iso-Asp128 was a component of native hormones, while deamidation of Asn12 and Asn98 to Asp and iso-Asp, oxidation of Met4, and cyclisation to the pyroglutamyl derivative of Gln139, probably occurred in vitro, during isolation, storage or hydrolysis. Porcine and canine hormones had indistinguishable protein fingerprints, confirming the assumption, based on their cDNA sequences, that their mature primary structures are identical.

Neighboring side chain effects on asparaginyl and aspartyl degradation: an ab initio study of the relationship between peptide conformation and backbone NH acidity

The rate of spontaneous degradations of asparagine and aspartyl residues occurring through succinimide intermediates is dependent upon the nature of the residue on the carboxyl side in peptides. For nonglycine residues, we show here that this effect can largely be attributed to the electrostatic/inductive effect of the side chain group on the equilibrium concentration of the anionic form of the peptide bond nitrogen atom that initiates the succinimide forming reaction. However, the rate of degradation of Asn-Gly and Asp-Gly containing peptides is about an order of magnitude greater than predicted solely using this explanation. To understand the nature of the glycine effect, ab initio calculations were performed on model compounds. These calculations indicate that there is little to no change in the stability of the transition state or the tetrahedral intermediate of succinimide formation with Asn-/Asp-Gly and Asn-/Asp-Ala derivatives. However, we have found that the acidity of the backbone peptide nitrogen NH is highly dependent upon the conformation of the molecule. Since glycine residues lack the beta-carbon common to all other protein amino acids, these residues can sample additional regions of conformational space where it is possible to further stabilize the backbone amide anion and thus increase the rate of degradation. These results provide the first rationale for the particular rate enhancement of degradation in peptidyl Asn-/Asp-Gly sequences. The results also can be applied to asparagine and aspartyl residues in proteins where the 3-dimensional structure provides additional constraints on conformation that can either increase or decrease the equilibrium concentration of the backbone amide anion and thus their rate of degradation via succinimide intermediates. Understanding this chemistry will assist attempts to minimize the deleterious effect of aging at the molecular level. The relationship between these results and proton exchange experiments is discussed in the Appendix.

Identification of multiple sources of charge heterogeneity in a recombinant antibody

Seven forms of a therapeutic recombinant antibody that binds to the her2/neu gene product were resolved by cation-exchange chromatography. Structural differences were assigned by peptide mapping and HIC after papain digestion. Deamidation of light chain asparagine 30 to aspartate in one or both light chains is responsible for two acidic forms. A low potency form is due to isomerization of heavy chain aspartate 102; the Asp102 succinimide is also present in a basic peak fraction. Forms with both Asn30 deamidation and Asp102 isomerization modifications were isolated. Deamidation of heavy chain Asn55 to isoaspartate was also detected. Isoelectric focusing in a polyacrylamide gel was used to verify the assignments. All modifications were found in complementarity determining regions.

Kinetic and thermodynamic control of the relative yield of the deamidation of asparagine and isomerization of aspartic acid residues

Selective deamidation of Asn67 of RNase A to beta-Asp67 and Asp67 residues at neutral pH initially produces greater amounts of the beta-Asp derivative. As the reaction proceeds the relative concentration of [Asp67]-RNase A increases and, at equilibrium, becomes predominant. Such a discrepancy between the kinetic and thermodynamic control on reaction products is discussed in light of information from X-ray three-dimensional analysis and the lower thermodynamic stability of the beta-Asp derivative relative to the parent enzyme.

Analysis of isoaspartate in peptides by electrospray tandem mass spectrometry

In view of the significance of Asn deamidation and Asp isomerization to isoAsp at certain sites for protein aging and turnover, it was desirable to challenge the extreme analytical power of electrospray tandem mass spectrometry (ESI-MS/MS) for the possibility of a site-specific detection of this posttranslational modification. For this purpose, synthetic L-Asp/L-isoAsp containing oligopeptide pairs were investigated by ESI-MS/MS and low-energy collision-induced dissociation (CID). Replacement of L-Asp by L-isoAsp resulted in the same kind of shifts for all 15 peptide pairs investigated: (1) the b/y intensity ratio of complementary b and y ions generated by cleavage of the (L-Asp/L-isoAsp)-X bond and of the X-(L-Asp/L-isoAsp) bond was decreased, and (2) the Asp immonium ion abundance at m/z 88 was also decreased. It is proposed that the isoAsp structure hampers the accepted mechanism of b-ion formation on both its N- and C-terminal side. The b/y ion intensity ratio and the relative immonium ion intensity vary considerably, depending on the peptide sequence, but the corresponding values are reproducible when recorded on the same instrument under identical instrumental settings. Thus, once the reference product ion spectra have been documented for a pair of synthetic peptides containing either L-Asp or L-isoAsp, these identify one or the other form. Characterization and relative quantification of L-Asp/L-isoAsp peptide mixtures are also possible as demonstrated for two sequences for which isoAsp formation has been described, namely myrG-D/isoD-AAAAK (deamidated peptide 1-7 of protein kinase A catalytic subunit) and VQ-D/isoD-GLR (deamidated peptide 41-46 of human procollagen alpha 1). Thus, the analytical procedures described may be helpful for the identification of suspected Asn deamidation and Asp isomerization sites in proteolytic digests of proteins.

Studies on the dephosphorylation of phosphotyrosine-containing peptides during post-source decay in matrix-assisted laser desorption/ionization

Phosphorylation of tyrosine residues in proteins is a common regulatory mechanism, although it accounts for less than 1% of the total O-phosphate content in proteins. Whereas aromatic phosphorylation sites can be identified by a number of different analytical techniques, sequence analysis of phosphotyrosine-containing proteins at the low picomole or even femtomole level is still a challenging task. This paper describes the post-source decay in matrix-assisted laser desorption/ionization mass spectrometry of phosphotyrosine-containing model peptides by comparing their fragmentation behavior with sequence-homologous unphosphorylated peptides. Whereas the parent ions showed significant losses of HPO3, all phosphorylated fragment ions of the b- and y-series displayed only minor dephosphorylated signals, which often were not detectable. Surprisingly, one of the studied phosphotyrosine-containing sequences displayed, in addition to the [M + H - 80]+ ion, a more abundant [M + H - 98]+ ion, which could be explained by elimination of phosphoric acid. This dephosphorylation pattern was very similar to the patterns obtained for phosphoserine- and phosphothreonine-containing peptides. Because the dephosphorylation pattern of the parent ion is often used to identify modified amino acids in peptides, we investigated possible dephosphorylation mechanisms in detail. Therefore, we substituted single trifunctional amino acid residues and incorporated deuterated phosphotyrosine residues. After excluding direct elimination of phosphoric acid from tyrosine, we could show that the obtained loss of H3PO4 depends on aspartic acid and arginine residues. Most likely the HPO3 group is transferred to aspartic acid followed by cleavage of phosphoric acid forming a succinimide. On the other hand, arginine appears to induce the H3PO4 loss by protonation of phosphotyrosine leaving a phenyl cation.

Isolation and identification of cyclic imide and deamidation products in heat stressed pramlintide injection drug product

This report summarizes the identification of six cyclic imide [Asu] and two deamidation products from a sample of pramlintide final drug product that had been stressed at 40 degrees C for 45 days. The pramlintide degradation products were isolated by cation exchange high-performance liquid chromatography (HPLC) followed by reversed-phase HPLC. The isolated components were characterized by mass spectrometry (MS), tandem MS (MS/MS) and when necessary, by enzymatic (thermolysin) digestion followed by liquid chromatography/mass spectrometry (LC/MS) and sequence analysis. The isolated products were identified as [Asu14]-pramlintide, [Asu21]-pramlintide, [Asu22]-pramlintide, [Asu35]-pramlintide, [1-21]-succinimide-pramlintide, and [1-22]-succinimide-pramlintide. Also identified were [Asp35]-pramlintide, the deamidation product of pramlintide at Asn35, and [Tyr37-OH]-pramlintide, the deamidation product of the pramlintide amidated C-terminal Tyr. Together these data support those presented earlier (C. Hekman et al., Isolation and identification of peptide degradation products of heat stressed pramlintide injection drug product. Pharm Res 1998;15:650-9) indicating that the primary mechanism of degradation for pramlintide in this pH 4.0 formulation is deamidation, with six of the eight possible deamidation sites observed to undergo deamidation. Gln-10 and Asn-31 are the only two residues subject to deamidation for which none is observed. The data indicate that the cyclic imide products account for approximately 20% of the total thermal degradation while the deamidation products account for 64%. The remaining degradation is due to peptide backbone hydrolysis.

The aspartyl replacement of the active site histidine in histidine-containing protein, HPr, of the Escherichia coli Phosphoenolpyruvate:Sugar phosphotransferase system can accept and donate a phosphoryl group. Spontaneous dephosphorylation of acyl-phosphate autocatalyzes an internal cyclization

The active site residue, His(15), in histidine-containing protein, HPr, can be replaced by aspartate and still act as a phosphoacceptor and phosphodonor with enzyme I and enzyme IIA(glucose), respectively. Other substitutions, including cysteine, glutamate, serine, threonine, and tyrosine, failed to show any activity. Enzyme I K(m) for His(15) --> Asp HPr is increased 10-fold and V(max) is decreased 1000-fold compared with wild type HPr. The phosphorylation of Asp(15) led to a spontaneous internal rearrangement involving the loss of the phosphoryl group and a water molecule, which was confirmed by mass spectrometry. The protein species formed had a higher pI than His(15) --> Asp HPr, which could arise from the formation of a succinimide or an isoimide. Hydrolysis of the isolated high pI form gave only aspartic acid at residue 15, and no isoaspartic acid was detected. This indicates that an isoimide rather than a succinimide is formed. In the absence of phosphorylation, no formation of the high pI form could be found, indicating that phosphorylation catalyzed the formation of the cyclization. The possible involvement of Asn(12) in an internal cyclization with Asp(15) was eliminated by the Asn(12) --> Ala mutation in His(15) --> AspHPr. Asn(12) substitutions of alanine, aspartate, serine, and threonine in wild type HPr indicated a general requirement for residues capable of forming a hydrogen bond with the Nepsilon(2) atom of His(15), but elimination of the hydrogen bond has only a 4-fold decrease in k(cat)/K(m).

Identification of Asp95 as the site of succinimide formation in recombinant human glial cell line-derived neurotrophic factor

Human glial cell line-derived neurotrophic factor is a single polypeptide of 134 amino acids and functions as a disulfide-linked dimer. Incubation of the protein in pH 5.0 and at 37 degreesC for 1 week showed that 5% of the material was converted to a form that eluted after the major protein peak on a cation-exchange column. The modified component gave an average molecular mass of 30367.0 u (theoretical = 30384.8 u). Within measurement error, this 17.8-u decrease in mass indicated the loss of a water molecule. This observation, together with the protein's behavior on cation-exchange chromatography and the mode of incubation used to generate the modification, was consistent with cyclic imide (succinimide) formation at an aspartyl residue. Hence, only a monomer of the dimeric protein was modified. The modified monomer was purified and subjected to peptic degradation. By a combination of N-terminal analysis and mass spectrometry, the region containing Asp95-Lys96 was identified to be modified. This was further confirmed by carboxypeptidase Y digestion of the modified peptide where the modified region was found to be resistant to further enzymatic degradation. Furthermore, incubation of the modified monomer in pH 8. 5 for 2 h yielded two peaks, in agreement with the succinimide model where the cyclic imide was hydrolyzed into a mixture of isoaspartate and aspartate. Tryptic mapping of the isoaspartyl-containing protein showed that Asp95 was refractory to Edman degradation, confirming it was in the isoaspartate form. Hence, the modification observed was due to succinimide formation at Asp95. This is the first report of succinimide formation at an Asp-Lys linkage.

Synthesis of stereoisomers and isoforms of a tryptic heptapeptide fragment of human growth hormone and analysis by reverse-phase HPLC and capillary electrophoresis

The amino acid sequence Asn-Gly has at pH 7 a tendency to induce deamidation of asparagine to aspartic acid via the formation of a cyclic imide. This imide opens up to yield Asp-Gly or the isoaspartic acid (isoAsp) form, isoAsp-Gly. Both isomers may be found in their L-form or D-form. Like Asn-Gly, the sequence Asp-Gly has a tendency for isomerization and racemization via the formation of a cyclic imide intermediate. When human growth hormone is digested with trypsin, one of the fragments is a heptapeptide (amino acid residues 128-134) containing the amino acid sequence Asp-Gly (amino acid residues 130 and 131). This heptapeptide, as well as stereoisomers and isoforms where L-Asp was replaced by D-Asp, L-isoAsp, D-isoAsp or the L-cyclic imide, respectively, has been synthesized and used as a standard to achieve separation of the five forms by capillary electrophoresis and by reverse-phase HPLC. Capillary electrophoresis analysis was performed in uncoated capillaries by the use of aspartic acid/cyclodextrin buffers at low pH. The elution order of the aspartic-acid-containing heptapeptides was D-Asp, L-Asp, L-isoAsp, D-isoAsp and L-cyclic imide. Reverse-phase HPLC analysis was performed on a C18 column by the use of a shallow acetonitrile gradient in trifluoroacetic acid/water. The elution order was D-isoasp, L-isoASp, L-Asp, D-Asp and L-cyclic imide. Human growth hormone samples were degraded by incubation at high temperature and analyzed for their potential content of isomerization and racemization products. Only L-forms of aspartic acid and isoaspartic acid of the heptapeptide fragment were found.

Aqueous stability of human epidermal growth factor 1-48

Human epidermal growth factor 1-48 (hEGF 1-48, Des(49-53)hEGF) is a single chain polypeptide (48 amino acids; 3 disulfide bonds; 5445 Da) possessing a broad spectrum of biologic activity including the stimulation of cell proliferation and tissue growth. In this study, three primary aqueous degradation products of hEGF 1-48 were isolated using isocratic, reverse phase/ion-pair HPLC. The degradation products were characterized using amino acid sequencing, electrospray ionization mass spectrometry, isoelectric focusing, and degradation kinetics. Results indicate that hEGF 1-48 degrades via oxidation (Met21), deamidation (Asn1), and succinimide formation (Asp11). The relative contribution of each degradation pathway to the overall stability of hEGF 1-48 changes as a function of solution pH and storage condition. Succinimide formation at Asp11 is favored at pH < 6 in which aspartic acid is present mostly in its protonated form. Deamidation of Asn1 is favored at pH > 6. The relative contribution of Met21 oxidation is increased with decreasing temperature, storage as a frozen solution (-20 degrees C), and exposure to fluorescent light.

Isolation of Escherichia coli synthesized recombinant eukaryotic proteins that contain epsilon-N-acetyllysine

Recombinant porcine (rpST) and bovine somatotropins (rbST) synthesized in Escherichia coli contain the amino acid, epsilon-N-acetyllysine. This amino acid was initially discovered in place of the normal lysine144 in a modified reversed-phase HPLC (RP-HPLC) species of rpST. Mass spectrometry and amino acid sequencing of a tryptic peptide isolated from this RP-HPLC purified protein were used to identify this altered residue as epsilon-N-acetyllysine. Ion-exchange chromatography was utilized to prepare low isoelectric point (pI) forms of rpST and rbST, which are enriched in epsilon-N-acetyllysine. Electrospray mass spectrometry demonstrated that the majority of the protein in these low pI fractions contained species 42 Da larger than normal. Immobilized pH gradient electrophoresis (IPG) of the ion-exchange purified low pI proteins was used to isolate several monoacetylated species of rpST and rbST. The location of the acetylated lysine in each IPG-purified protein was determined by tryptic peptide mapping and amino acid sequencing of the altered tryptic peptides. Amino acid analyses of enzymatic digests of rpST and rbST were also used to confirm the presence of epsilon-N-acetyllysine in these recombinant proteins. These data demonstrate that a significant portion of rpST and rbST produced in E. coli contain this unusual amino acid.

Major degradation products of basic fibroblast growth factor: detection of succinimide and iso-aspartate in place of aspartate

The degradation products of basic fibroblast growth factor (bFGF) were isolated by ion exchange HPLC (HP-IEC) and characterized. The predominant product at pH 5 was a succinimide in place of aspartate15 as determined by LC/MS, N-terminal sequencing, and susceptibility to degradation at pH > 6.5. The rate of appearance of the succinimidyl-bFGF at 22 degrees C was comparable to that reported for small peptides, consistent with a high flexibility predicted for asp15-gly. Tryptic mapping together with [3H]-methylation indicated that iso-aspartate was formed at the position of asp15. Size exclusion HPLC indicated the presence of intact and truncated dimers and trimers which associated through disulfide linkages. Two truncated monomer forms were found that co-eluted by HP-IEC; the cleavages were determined to be at asp28-pro and asp15-gly using LC/MS and N-terminal sequencing. These degradation products which occurred at sites that are away from receptor or heparin binding domains of bFGF remained bioactive in a cell proliferation assay.

Identification of succinimide sites in proteins by N-terminal sequence analysis after alkaline hydroxylamine cleavage

Under favorable conditions, Asp or Asn residues can undergo rearrangement to a succinimide (cyclic imide), which may also serve as an intermediate for deamidation and/or isoaspartate formation. Direct identification of such succinimides by peptide mapping is hampered by their lability at neutral and alkaline pH. We determined that incubation in 2 M hydroxylamine, 0.2 M Tris buffer, pH 9, for 2 h at 45 degrees C will specifically cleave on the C-terminal side of succinimides without cleavage at Asn-Gly bonds; yields are typically approximately 50%. N-terminal sequence analysis can then be used to identify an internal sequence generated by cleavage of the succinimide, hence identifying the succinimide site.