Mass Microscopy to Reveal Distinct Localization of Heme B (m/z 616) in Colon Cancer Liver Metastasis

Abstract

Imaging mass spectrometry (imaging MS) of tissue sections has become one of the main subjects in surface analysis. However, it is not easy to identify the molecular species present in imaged signals. We successfully performed imaging MS and tandem mass spectrometry (MSⁿ) on the tissue section using a matrix-assisted laser desorption/ionization quadrupole-ion-trap time-of-flight mass spectrometer. We called the method “mass microscopy,” which is a combination of high spatial resolution imaging MS and MSⁿ. This paper describes mass microscopy of a human liver section with metastatic colon cancer. The imaging result of m/z 616 showed low ion intensity in a cancerous region. To identify the molecule that is responsible for the generation of m/z 616, we performed MS² and MS³ on the liver section. Considering the m/z value and peak pattern in MS² and MS³ data, it was found that m/z 616 was generated from heme B. Heme B consists of an iron atom and porphyrin, and is known as a prosthetic group in hemoglobin, which is a protein in erythrocytes. Our results indicate the difference between the blood rich organ liver and ischemic metastatic colon cancer.