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 α ‐ and β ‐cells. Adherent α TC1 clone 9 and β TC6 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.