Complete sequencing of the recombinant granulocyte-colony stimulating factor (filgrastim) and detection of biotinylation by mass spectrometry

Granulocyte-colony stimulating factor stimulates production and antibacterial function of neutrophiles. Therapy using the recombinant protein drug represents a major step forward in oncology. The protein has not been, however, completely sequenced at the protein level and this formed the rationale of the current study. Recombinant G-CSF (filgrastim) was run on two-dimensional gel electrophoresis (2DE), the protein was in-gel digested with trypsin and chymotrypsin, and peptides were analysed on Nano-ESI-LC-MS/MS (high performance ion trap, HCT). Bioinformatic tools used were Mascot v2.2 and Modiro(TM) v1.1 softwares. A single spot was detected on 2DE and peptides resulting from in-gel digestion were unambiguously identified by the MS/MS approach leading to complete sequencing when both searching engines were applied. N-terminal methionine loss, N-terminal methionine oxidation and amidination were observed. Both softwares identified modifications. Complete sequencing by a non-sophisticated and rapid gel-based mass spectrometry approach confirmed the primary structure predicted from nucleic acid sequences. A chemical modification of glutamine 26 with the interim name PentylamineBiotin (Unimod accession number #800) compatible with biotinylation with 5-(biotinamido) pentylamine by the producer was detected by both softwares. Although there is some evidence that biotinylated G-CSF analogues are active, it remains open whether this modification may be responsible for the side effects observed or lead to changes of antigenicity.