A one-step fluorometric method for the continuous measurement of monoamine oxidase activity

We have developed a one-step fluorometric method for the measurement of monoamine oxidase (MAO) activity in 96-well microplates with sensitivity 10-fold higher than the conventional spectrophotometric assay method. This assay is based on the detection of H2O2 in a horseradish peroxidase-coupled reaction using N-acetyl-3, 7-dihydroxyphenoxazine (Amplex Red), a highly sensitive and stable probe for H2O2. With a single sampling, this assay is useful for performing both end-point and continuous measurements of MAO activity. Using a commercially available enzyme, our assay allows the detection of MAO B activity as low as 1.2 x 10(-5) U/ml. When applied to crude tissue homogenates, we have been able to selectively detect both MAO A and MAO B from cow brain tissue with protein content as low as 200 microgram per sample. The potential applications of this assay include the measurement of MAO activity in normal and diseased tissues, blood samples, and other biological fluids and the screening of drugs for the treatment of MAO-mediated diseases.