Difference in monoamine oxidase activity measured by either liquid ion exchange or ion exchange resin chromatography in rat and cat brain

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Cat and rat brain monoamine oxidase (MAO) activity was measured with a radioisotopic procedure and two extraction methods. Results indicated an underestimation of MAO activity when liquid ion exchange chromatography (LIEC) was used instead of an ion exchange chromatographic method (IEC) to separate the different products of the deaminated tyramine, phenylethylamine, or serotonin. MAO produced aldehydic products which may be found in the incubation medium and may be extracted with the substrate in the chloroform phase by the LIEC method. In cat brain, the resulting underestimation of the MAO activity was prevented by the addition of nicotinamide adenine dinucleotide (10 −3 m) in the incubation medium or by allowing a 2-h period between the end of incubation and the LIEC extraction procedure. In the rat brain, the same result was obtained by the addition of an equimolar mixture of nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate in reduced form (NAD-NADPH, 10 −3 m). Using the IEC method, the NAD decreased only the deamination of tyramine and serotonin in rat brain. This study suggests that the use of an IEC method to evaluate MAO activity is more accurate for the estimation of the enzymatic activity.