Isolation and characterization of a resistant core peptide of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF); confirmation of the GM-CSF amino acid sequence by mass spectrometry

Applications of LC/MS in structure identifications of small molecules and proteins in drug discovery

With advancements in ionization methods and instrumentation, liquid chromatography/mass spectrometry (LC/MS) has become a powerful technology for the characterization of small molecules and proteins. This article will illustrate the role of LC/MS analysis in drug discovery process. Examples will be given on high-throughput analysis, structural analysis of trace level impurities in drug substances, identification of metabolites, and characterization of therapeutic protein products for process improvement. Some unique MS techniques will also be discussed to demonstrate their effectiveness in facilitating structural identifications.

Mass spectrometric mapping of disulfide bonds in recombinant human interleukin-13

Interleukin 13 (IL-13), a member of the a-helical family of cytokines, has approximately 30% primary sequence homology with IL-4 and shares a common receptor component. The biologically active rhIL-13 is monomeric and non-glycosylated, and contains two disulfide bonds as determined by comparative electrospray mass spectrometric (MS) analysis of the protein before and after reduction with dithiothreitol-dithioerythritol. A trypsin-resistant core peptide of rhIL-13 was isolated and analyzed by plasma desorption (PD) MS, identifying a disulfide-linked core peptide. Subsequent digestion of this core peptide by pepsin, followed by PDMS analysis of the resulting cystine-containing peptic fragments, provided rapid determination of the existing disulfide bonds between cysteine residues 28-56 and 44-70. This disulfide arrangement is similar to that observed for the analogous four internal cysteine residues in hIL-4. The conservation of disulfide bond arrangements between hIL-13 and hIL-4, coupled with their alpha-helical structure and sequence homologies, confirms that IL-13 and IL-4 are structural homologues. It is also consistent with their reported similarities in biological function and receptor binding kinetics.

Expression, purification, and characterization of recombinant human interleukin-13 from NS-O cells

Interleukin-13 is a cytokine which is secreted by activated T lymphocytes and primarily impacts monocytes, macrophages, and B cells. A synthetic gene coding for human interleukin-13 has been prepared and cloned into expression vector pEE12. The construct was transfected into NS-O cells, which showed stable expression of the recombinant protein. A four-step purification procedure consisting of S-Sepharose, Q-Sepharose, hydroxyapatite, and Sephacryl-100 chromatographies yielded bioactive interleukin-13 of > 98% purity. The purified protein was structurally characterized. The extinction coefficient at 280 nm was determined to be 5678 M-1 cm-1. Amino acid sequencing confirmed that the N-terminus of the purified protein was intact. Electrospray mass spectrometric analysis, size-exclusion chromatography, and SDS-PAGE revealed that the biologically active protein is monomeric and unglycosylated. Mass spectrometry and a chemical assay for free sulfhydryls indicated that the four cysteine residues of interleukin-13 are involved in two intramolecular disulfide bonds. The circular dichroism spectrum confirms that interleukin-13 belongs to the alpha-helical family of cytokines. A biologically inactive covalent trimer also forms in the cell culture, but can be separated from the monomer by the hydroxyapatite and size-exclusion chromatographies. These data indicate that human interleukin-13 retains many structural similarities to human interleukin-4, from which it arose by a gene duplication event.