Mass spectrometrical analysis of recombinant human growth hormone (Genotropin®) reveals amino acid substitutions in 2% of the expressed protein

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Abstract

Background

The structural integrity of recombinant proteins is of critical importance to their application as clinical treatments. Recombinant growth hormone preparations have been examined by several methodologies. In this study recombinant human growth hormone (rhGH; Genotropin ^®), expressed in E. coli K12, was structurally analyzed by two-dimensional gel electrophoresis and MALDI-TOF-TOF, LC-MS and LC-MS/ MS sequencing of the resolved peptides.

Results

Electrospray LC-MS analysis revealed one major protein with an average molecular mass of 22126.8 Da and some additional minor components. Electrospray LC-MS/MS evaluation of the enzymatically digested Genotropin ^®sample resulted in the identification of amino acid substitutions at the residues M ₁₄, M ₁₂₅, and M ₁₇₀; di-methylation of K ₇₀(or exchange to arginine); deamidation of N ₁₄₉, and N ₁₅₂, and oxidation of M ₁₄₀, M ₁₂₅and M ₁₇₀. Peak area comparison of the modified and parental peptides indicates that these changes were present in ~2% of the recombinant preparation.

Conclusion

Modifications of the recombinant human growth hormone may lead to structural or conformational changes, modification of antigenicity and development of antibody formation in treated subjects. Amino acid exchanges may be caused by differences between human and E. coli codon usage and/or unknown copy editing mechanisms. While deamidation and oxidation can be assigned to processing events, the mechanism for possible di-methylation of K ₇₀remains unclear.

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Most cited references 44

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A rapid method for the identification of known proteins separated by two-dimensional gel electrophoresis is described in which molecular masses of peptide fragments are used to search a protein sequence database. The peptides are generated by in situ reduction, alkylation, and tryptic digestion of proteins electroblotted from two-dimensional gels. Masses are determined at the subpicomole level by matrix-assisted laser desorption/ionization mass spectrometry of the unfractionated digest. A computer program has been developed that searches the protein sequence database for multiple peptides of individual proteins that match the measured masses. To ensure that the most recent database updates are included, a theoretical digest of the entire database is generated each time the program is executed. This method facilitates simultaneous processing of a large number of two-dimensional gel spots. The method was applied to a two-dimensional gel of a crude Escherichia coli extract that was electroblotted onto poly(vinylidene difluoride) membrane. Ten randomly chosen spots were analyzed. With as few as three peptide masses, each protein was uniquely identified from over 91,000 protein sequences. All identifications were verified by concurrent N-terminal sequencing of identical spots from a second blot. One of the spots contained an N-terminally blocked protein that required enzymatic cleavage, peptide separation, and Edman degradation for confirmation of its identity.

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Comparison of a structural and a functional epitope.

Adam Wells, Ross B. Cunningham (1993)

A comprehensive analysis of the energetic importance of the 31 side-chains buried at the interface between human growth hormone (hGH) and the extracellular binding domain of its receptor (hGHbp) has been carried out to assess the roles of contact side-chains in modulating the affinity and kinetics of binding. Each side-chain in hGH was converted to alanine, and the kinetics and affinity were measured using a biosensor device. This detects binding of the mutated hormones to the immobilized hGHbp by changes induced in refractive index. The data generated on the biosensor match affinities obtained by radio-immune assay in solution. The study shows that only one-quarter of the side-chains buried at the interface can account for the majority of the binding energy. These residues cluster near the center of the structural epitope. The role of these side-chains is predominantly to slow dissociation because most of the effect of the alanine substitutions is to increase the off-rate, not to slow the on-rate. The hormone associates about 10,000 times slower than expected from random diffusion but 1000 times faster than may be expected if one imposes strict orientation restraints for a productive collision. Electrostatic interactions partly modulate association because mutations at Arg residues most affect association and together contribute a factor of about 20 to the on-rate. The data suggest that the hormone and receptor associate by diffusion and electrostatics to form an ensemble of weak collisional complexes. From these a bound complex is produced that is stabilized by only a small proportion of the contacts. We suggest that solvation energies and/or side-chains interactions within the free hormone or receptor may be so favorable that little energy is gained at most side-chains upon binding. The fact that the functional binding epitope is much smaller than the structural epitope suggests it may be possible to design smaller hormone mimics.

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Author and article information

Journal

Journal ID (nlm-ta): Proteome Sci

Title: Proteome Science

Publisher: BioMed Central (London )

ISSN (Electronic): 1477-5956

Publication date Collection: 2005

Publication date (Electronic): 11 February 2005

Volume: 3

Page: 1

Affiliations

[1 ]Department of Pediatrics, Medical University of Vienna, Vienna, Austria

[2 ]Mass Spectrometry Unit, University of Vienna, Austria

Article

Publisher ID: 1477-5956-3-1

DOI: 10.1186/1477-5956-3-1

PMC ID: 549540

PubMed ID: 15707495

SO-VID: e872c322-0c52-4bfe-8819-33e44331e2f7

License:

This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.