Identification of serum proteins discriminating colorectal cancer patients and healthy controls using surface-enhanced laser desorption ionisation-time of flight mass spectrometry

Figure 4 Identification of apolipoprotein C-I. A: Parts of the MS spectra of the gel eluates of an apolipoprotein C-I control and the 6.6×10³-Da protein isolated from HC serum run on the same gel. B: Parts of the MS spectra of the in-gel digests of an apolipoprotein C-I control and the 6.6×10³-Da protein isolated from HC serum and the results of sequencing of these two peptides with tandem MS. C: Part of the MS spectrum of the eluate from the apolipoprotein C-I antibody. Apart from the expected peaks at 9.3×10³, apolipoprotein C-I precursor, and 6.6×10³, a 6.4×10³-Da peak is seen, which is a known fragment of apolipoprotein C-I missing two N-terminal amino acids. The mass at 7.8×10³ is unknown and does not correspond to any of the apolipoproteins with which antibody cross-reactivity can occur. It is possibly an intermediate splice form of the precursor protein.