Colorimetric assay of membrane-bound enzyme based on lipid bilayer inhibition of ion transport.

Membrane-bound enzymes (MBEs), which make up a very high proportion of intracellular enzymes, catalyze a variety of activities that are currently analyzed by various techniques after purification. However, due to their amphipathic character, the purification of MBEs is difficult. Therefore, the most productive approach represents analysis of MBEs in the cellular membrane. In this study, using membrane-bound α-glucosidase (α-Glu) as an example, we have developed a colorimetric assay for MBEs based on the inhibitory effect of lipid bilayer on ion transport. The enzyme substrate could mediate the self-assembly of phospholipid PEG derivative around magnetic nanospheres that were modified with boronic acid. The formation of lipid bilayer could inhibit the leaking of iron ions under acidic conditions. However, the product of the catalytic reaction had no capability for self-assembly of the lipid bilayer, leading to the release of iron ions from the magnetic nanospheres under acidic pH. The colorimetric assay for MBEs could not only analyze the activity of membrane-bound α-Glu located on Caco-2 cells but could also evaluate the α-Glu inhibitors in cell medium. The simple, economic, and efficient method proposed here offers a potential application for high-throughput testing of α-Glu and its inhibitors. Our study also outlines a strategy for exploring the colorimetric method to detect the activities of MBEs .