In situ sequencing of peptides from biological tissues and single cells using MALDI-PSD/CID analysis

The ability to directly sequence peptides from biological cells using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) with postsource decay (PSD) and collision-induced dissociation (CID) fragment ion mass analysis is explored. Three different sample preparation methods are described for sequencing peptides in tissue samples and in single neurons from the invertebrate model Aplysia californica. To characterize peptides from the atrial gland, MALDI-PSD/CID is applied directly to a tissue blot covered with the matrix alpha-cyano-4-hydroxycinnamic acid (CHCA). The resulting fragment ions combined with database searching confirm the structure of several novel peptides encoded by egg-laying hormone genes. Moreover, MS profiling of a single unidentified neuron detects peptides with molecular weights of myomodulins C and E; this assignment is confirmed using MALDI-PSD with the matrix 2,5-dihydroxybenzoic acid (DHB). DHB does not always provide adequate fragmentation for PSD experiments; therefore, a unique dual-matrix sampling method, employing both DHB and CHCA, is developed to directly sequence a decapeptide from a single cerebral ganglion B cell. Mass accuracy of fragment ions from cellular samples is typical for the instrument employed and is not deleteriously affected by the morphology and complexity of the samples.