Quantitative analysis capability of DIUTHAME mass spectrometry verified by acetylcholinesterase enzyme-catalyzed reaction assays

Matrix-assisted laser desorption/ionization (MALDI) is advantageous for mass spectrometry applications where throughput is important. However, quantitative analysis is essentially problematic for MALDI-MS whose results depend on the intrinsically stochastic microcrystalline state of the matrix. High-throughput screening (HTS) of drug candidates is a typical example that requires high throughput. The application of MALDI-MS to HTS, which is quantitative analysis, imposes restrictions on designing an experimental system. Surface-assisted laser desorption/ionization (SALDI) methods, which do not depend on matrix crystal formation, are expected to be applied to quantitative analyses such as HTS. A recently developed one type of SALDI methods, desorption ionization using through hole alumina membrane (DIUTHAME), possesses a distinct feature that the surface microstructure effective for ionization is formed by through holes. In this study, the quantitative analysis capability of DIUTHAME was verified by applying DIUTHAME to enzyme-catalyzed reaction measurements, which are also used for HTS. Quantitative DIUTHAME-MS was conducted on various conditions of acetylcholinesterase-catalyzed reaction solutions containing cow milk as a substitute of biological media. Even for the enzyme-catalyzed reaction solutions containing complex additives that make the quantitative analysis extremely difficult, DIUTHAME based on the through hole structure enables quantitative measurements of the analytes by applying the reaction solutions to the back side of the laser exposed surface. In comparison with those obtained by MALDI-MS, the results obtained by DIUTHAME-MS showed less variability of data and delivered a better linearity of the Lineweaver-Burk plots and a more reasonable value of the Michaelis constant. Accordingly, it was demonstrated that DIUTHAME-MS possesses the quantitative analysis capability much better than that of MALDI-MS.

Matrix-Free High-Resolution Atmospheric-Pressure SALDI Mass Spectrometry Imaging of Biological Samples Using Nanostructured DIUTHAME Membranes

Applications of mass spectrometry imaging (MSI), especially matrix-assisted laser desorption/ionization (MALDI) in the life sciences are becoming increasingly focused on single cell analysis. With the latest instrumental developments, pixel sizes in the micrometer range can be obtained, leading to challenges in matrix application, where imperfections or inhomogeneities in the matrix layer can lead to misinterpretation of MS images. Thereby, the application of premanufactured, homogeneous ionization-assisting devices is a promising approach. Tissue sections were investigated using a matrix-free imaging technique (Desorption Ionization Using Through-Hole Alumina Membrane, DIUTHAME) based on premanufactured nanostructured membranes to be deposited on top of a tissue section, in comparison to the spray-coating of an organic matrix in a MALDI MSI approach. Atmospheric pressure MALDI MSI ion sources were coupled to orbital trapping mass spectrometers. MS signals obtained by the different ionization techniques were annotated using accurate-mass-based database research. Compared to MALDI MSI, DIUTHAME MS images captivated with higher signal homogeneities, higher contrast and reduced background signals, while signal intensities were reduced by about one order of magnitude, independent of analyte class. DIUTHAME membranes, being applicable only on tissue sections thicker than 50 µm, were successfully used for mammal, insect and plant tissue with a high lateral resolution down to 5 µm.

Feasibility of Acetylcholinesterase Reaction Assay Monitoring in DIUTHAME-MS

DIUTHAME (desorption ionization using through hole alumina membrane) is a novel matrix-free laser desorption/ionization method that enables highly reproducible acquisition of mass spectra. This study aims to evaluate the applicability of DIUTHAME to the acetylcholinesterase reaction assay (AChE assay) commonly used in high-throughput screening (HTS) in the drug discovery process. The commercially available 9-ch DIUTHAME chip was applied to a series of AChE assays prepared with different reaction times. Numerous positive-mode TOF mass spectra were acquired from the raster-scanned sample spots of the AChE assays to analyze the progress of the enzyme reaction and to perform mass spectrometry imaging of the sample spots. The reaction kinetics plots obtained by DIUTHAME were found to reflect the time course of reaction progression as much as those obtained by MALDI and were found to have less error variation than MALDI. DIUTHAME allowed the already irradiated sample spots to be reused to reproduce the reaction kinetics plots by the second measurement conducted a week after, whereas MALDI was not able to properly reproduce the kinetics plots by remeasuring the irradiated sample spots. In DIUTHAME, which does not require a matrix, the experimental procedure for measuring AChE assay becomes extremely simple. DIUTHAME is potentially able to provide more precise AChE reaction kinetics plots than what have been accomplished by MALDI.

Simple Pretreatment for the Analysis of Additives and Polymers by Surface-Assisted Laser Desorption/Ionization Mass Spectrometry Using a Through-Hole Alumina Membrane as a Functional Substrate

The analysis of additives and polymers was performed by desorption ionization using through-hole alumina membrane (DIUTHAME) as a functional substrate for both sample pretreatment and surface-assisted laser desorption/ionization (SALDI) mass spectrometry. Using the unique absorbing/filtering capabilities of DIUTHAME and investigating the solubility of analytes/bulk materials in some solvents, three pretreatment techniques were demonstrated with (1) the selective removal of hydrophilic poly(ethylene oxide) (PEO)-based components from a "PEO-monostearate" sample, (2) the on-chip filtration of solubilized decabromodiphenylether (DBDE) from a solution of polystyrene that had been preliminarily precipitated, and (3) the on-chip extraction of antioxidants (Irganox 1010, Irgafos 168, and dimyristyl 3,3'-thiodipropionate) from a suspension of polypropylene powder or from the powder itself. The extracted analytes were further mass-analyzed using a spiral high-resolution time-of-flight analyzer to assess their elemental composition or molecular distribution.