High-performance liquid chromatography of aldehydes and acids formed in monoamine oxidase-catalyzed reactions

A rapid, sensitive, and specific method for the determination of monoamine oxidase (MAO) activities toward different substrates is described. The assay is based on high-performance liquid chromatographic (HPLC) separation and electrochemical detection of the aldehyde or acid products. The aldehyde metabolic intermediates were observed to be quite stable in 0.1 N perchloric acid containing antioxidant and EDTA, and therefore can be used to measure the MAO activity of washed mitochondrial membrane and partially purified or purified MAO. Incomplete conversion of aldehyde to acid was observed when the amine substrates were incubated with the crude enzyme preparations. These aldehydes can be converted to corresponding acids by addition of yeast aldehyde dehydrogenase and beta-NAD and the acid can also be measured by HPLC-electrochemical detection. A deuterium isotope effect in the oxidation of p-[alpha,alpha-2H2]tyramine and [alpha,alpha-2H2]serotonin has been demonstrated by this method.