Stopped-flow enzyme assays on a chip using a microfabricated mixer.

This paper describes a microfabricated enzyme assay system including a micromixer that can be used to perform stopped-flow reactions. Samples and reagents were transported into the system by electroosmotic flow (EOF). Streams of reagents were merged and passed through the 100-pL micromixer in < 1 s. The objective of the work was to perform kinetically based enzyme assays in the stopped-flow mode using a system of roughly 6 nL volume. Beta-galactosidase (beta-Gal) was chosen as a model enzyme for these studies and was used to convert the substrate fluorescein mono-beta-D-galactopyranoside (FMG) into fluorescein. Results obtained with microfabricated systems using the micromixer compared well to those obtained with an external T mixing device. In contrast, assays performed in a microfabricated device by merging two streams and allowing mixing to occur by lateral diffusion did not compare well. Using the microfabricated mixer, Km and kcat values of 75 +/- 13 microM and 44 +/- 3 s(-1) were determined. These values compare well to those obtained with the conventional stopped-flow apparatus for which Km was determined to be 60 +/- 6 microM and kcat was 47 +/- 4 s(-1). Enzyme inhibition assays with phenylethyl-beta-D-thiogalactoside (PETG) were also comparable. It was concluded that kinetically based, stopped-flow enzyme assays can be performed in 60 s or less with a miniaturized system of roughly 6 nL liquid volume when mixing is assisted with the described device.