Direct visualisation of peptide hormones in cultured pancreatic islet alpha- and beta-cells by intact-cell mass spectrometry

The application of intact-cell mass spectrometry (ICM) by matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry to achieve direct protein-profiling of bacterial species is now well established. However, this methodology has not to our knowledge been applied to the analysis of mammalian cells in routine culture. Here, we describe a novel application of ICM by which we have identified proteins in intact cells from two lines representative of pancreatic islet aand β-cells. Adherent αTC1 clone 9 and βTC6 F7 cells were harvested into phosphate-buffered saline (PBS) using enzyme-free dissociation buffer before 1 μL of cell suspension was spotted onto MALDI plates. Cells were overlaid with sinapinic acid then washed with pure water before application of a final coat of sinapinic acid. Data in the 2000-20 000 m/z range were acquired in linear mode on a Voyager DE-Pro mass spectrometer. The proteins which ionised were composed in large part of peptide hormones (e.g. insulin and glucagon) known to be packaged into the secretory granules of the β- and α-cells respectively. However, in addition to visualising the peptides expected to be associated with these cells, a mass consistent with oxyntomodulin was identified in the cultured α-cells, a finding not previously reported to our knowledge. In summary, this paper describes, for the first time, a rapid and direct method useful for identifying secretory products in intact endocrine cells. Copyright © 2007 John Wiley & Sons, Ltd.