Monoamine oxidase activities in catfish (Parasilurus asotus) tissues.

The substrate- and inhibitor-related characteristics of monoamine oxidase (MAO) were studied for catfish brain and liver. The kinetic constants for MAO in both tissues were determined using 5-hydroxytryptamine (5-HT), tyramine and beta-phenylethylamine (PEA) as substrates. For both tissues, the Vmax values were highest with 5-HT and lowest with PEA. The Km value for the brain was highest with 5-HT, followed by tyramine and PEA; but for the liver its value was highest with PEA, followed by 5-HT and tyramine, although all values were in the same order of magnitude. The inhibition of MAO by clorgyline and deprenyl by use of 5-HT, tyramine and PEA as substrates showed that the MAO-A inhibitor clorgyline was more effective than the MAO-B inhibitor deprenyl for both catfish tissues; a single form was present since inhibition by clorgyline or deprenyl with 1000 microM PEA showed single phase sigmoid curves. It is concluded that catfish brain and liver contain a single form of MAO, relatively similar to mammalian MAO-A.