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Isotopic labelling of peptides in tissues enhances mass spectrometric profiling.

RATIONALE
There is a need in imaging mass spectrometry to use the acquired isotope distribution to unequivocally determine the identity of a peptide ion. A way of achieving unambiguous differentiation of ions from protonated peptides from other [M + H](+) ions in a tissue would be via the direct on-tissue incorporation of (18)O into peptides.


METHODS
Tissues were first digested with trypsin for 3  h at 37 °C in a humidified chamber. For the (18)O-labelling of digested peptides 1  μL of H(2)(18)O/50  mM ammonium acetate (at pH 6.75) was added to the array of tryptic spots and incubated at room temperature for 20  min. α-Cyano-4-hydroxycinnamic acid was used as a matrix modifier. The mass spectral analysis of tissue sections was carried out using a matrix-assisted laser desorption/ionisation tandem time-of-flight (MALDI-TOF-TOF) instrument.


RESULTS
On-tissue incorporation of (18)O into peptides cannot be carried out during the digestion step inside a humidified chamber. After tissue digestion for 3  h at 37 °C in an humidified chamber, (18)O labelling was carried out for 20  min at room temperature (no humidified chamber). No trypsin was needed to enhance the labelling.


CONCLUSIONS
For first time the feasibility of (18)O-labelling of peptides in situ for tissues has been demonstrated. The method decouples protein digestion from peptide labelling and is performed in sequential steps. Furthermore, we observed that (18)O incorporation produces characteristic isotopic peptide distributions, thus making facile distinguishing peptides from other tissue molecular components that ionise in the MALDI ion source.