On-Tissue Direct Analysis: MALDI MassSpectrometric Imaging for Peptides/Protein

Introduction

MALDI imaging refers to a technique in which mass spectrometric analysis is conducted directly on a biological tissue sample. The distribution of biomolecules (low-molecular weight metabolites, lipids, peptides and proteins, etc.) on the tissue are mapped as a two-dimensional image based on measurement site location information and mass spectral information. This makes it possible to visually grasp the localization of biomolecules of interest. The application of the MALDI imaging technique has previously been reported for biomolecules in various tissues, and there are numerous recent manuscripts citing spatial distribution of disease-specific biomarker candidate compounds. Thus, the MALDI imaging is an effective technique for understanding the spatial distribution of molecules, and not only is there expectation for this technique with respect to the search for disease-specific biomarkers, but for its application in drug kinetics as well. Here we present an example of MALDI imaging of peptides and proteins in which we used a cross- section of rat kidney tissue as the sample. First, the matrix was coated on a frozen tissue section of rat kidney which was placed on an electrically conductive glass slide. Generally, when conducting MALDI imaging, the matrix must be coated uniformly on the tissue sample to ensure highly reproducible mass spectral data acquisition. One of these coating techniques uses a spotter instrument to deposit micro volumes of matrix solution. Here we used a chemical inkjet printer (CHIP-1000), a micro volume dispensing instrument, to conduct repeat deposition of 300 pL of matrix solution (5 mg/mL sinapinic acid) at 200 μm intervals from spot center-to-center over the surface of the tissue section. Next, after drying the matrix-applied sample in a desiccator, the AXIMA Confidence was used to conduct linear-mode mass spectrometric analysis (positive mode) on all of the matrix spots.

October 28, 2009 GMT