Microfluidic assays of acetylcholinesterase inhibitors

A microfabricated device for flow injection analysis and electrophoretic separation of acetylcholinesterase (AChE) inhibitors is described. Solutions of inhibitor, enzyme, substrate, and derivitizing agent were mixed within the channels of the microchip using computer-controlled electrokinetic transport. AChE-catalyzed hydrolysis of acetylthiocholine to thiocholine was measured in an onchip reaction of thiocholine with coumarinylphenylmaleimide, and the resulting thioether was detected by laser-induced fluorescence. Inhibitors reduced the fluorescence signal and produced a negative peak diagnostic for the type of inhibition. A Gaussian peak was observed for competitive inhibitors, whereas a broad negative peak was observed for irreversible inhibitors. From a microchip assay for tacrine, an inhibition constant, K-i, of 1.5 +/- 0.2 nM was derived, which compared well with a standard cuvette assay, A now injection assay of two irreversible inhibitors, carbofuran and eserine, was performed. With a Ei-min stopped-flow reaction time, a detection limit of 10 nM carbofuran was obtained. As a potential multiplex screening device, a mixture of four cationic inhibitors, tacrine, edrophonium, and tetramethyl- and tetraethylammonium chloride, was separated and detected within 70 s.