The substrate- and inhibitor-related characteristics of monoamine oxidase (MAO) were studied with mitochondria of chick brain, liver, kidney and heart. The kinetic constants for MAO in these organs were determined, using 5-hydroxytryptamine (5-HT), tyramine and β-phenylethylamine (PEA) as substrates. For all the substrates, the V max values were highest in kidney, followed in decreasing order by brain, liver and heart. For tyramine and PEA, the K m values were lowest in liver, but for 5-HT it was lowest in heart. Inhibition experiments with clorgyline and deprenyl were carried out on mitochondria of the four organs with the three substrates at their K m concentrations. From the plateaus observed of inhibition by clorgyline, it was concluded that 5-HT was oxidized by both types of MAO in mitochondria of all the organs; PEA was fairly specific for type B MAO in brain, liver and kidney, but non-specific in heart. In heart mitochondria, appreciable amounts of the activities toward tyramine and PEA were due to an amine oxidase distinct from mitochondrial MAO; 5-HT, however, was oxidized exclusively by mitochondrial MAO in this organ. The above atypical characteristics in substrate specificity found in chick tissues support the idea that the type A and type B concept cannot be applied uncritically to all tissues from all species.
