On-Tissue Chemical Oxidation Followed by Derivatization for Mass Spectrometry Imaging Enables Visualization of Primary and Secondary Hydroxyl-Containing Metabolites in Biological Tissues

On-tissue chemical derivatization combined with mass
spectrometry
imaging (MSI) can effectively visualize low-abundance and poorly ionizable
molecules in biological tissues. Owing to the lack of an effective
chemical reaction environment on the tissue surface, the development
of direct one-step derivatization reactions is challenging. Herein,
we present a two-step reaction involving on-tissue chemical oxidation
followed by derivatization combined with airflow-assisted desorption
electrospray ionization-MSI, enabling the visualization of primary
and secondary hydroxyl-containing metabolites (PSHMs) within the tissue
sections. This method indirectly achieved on-tissue derivatization
by combining two reactions. Hydroxyl was converted to carbonyl using
chemical oxidants, and subsequently, carbonyl was derived using Girard’s
P reagent. Using this methodology, 169 PSHMs, including hydroxy fatty
acids (OH–FAs), fatty alcohols (FOHs), and sterol lipids, were
detected and imaged in the tissues of rat brain, kidney, and liver.
Moreover, we found that the abundant PSHMs, fatty aldehydes, and oxo
fatty acids were significantly dysregulated in the liver and kidney
tissues of type 2 diabetic rats; in particular, OH–FAs and
FOHs were remarkably up-regulated in the diabetic rat liver tissues.
The aberrations of these oxidative metabolites provide insights into
the understanding of the molecular pathological mechanism of diabetes.
This study demonstrates a novel, two-step reaction strategy for on-tissue
derivatization with the analysis of previously inaccessible molecules
using MSI.