The oxidation of adrenaline and noradrenaline by the two forms of monoamine oxidase from human and rat brain

The selective Monoamine Oxidase inhibitors clorgyline and (?)-deprenyl were used to study the distribution of monoamine oxidase-A and -B (MAO-A, MAO-B) activities towards (?)-noradrenaline and (+),(?)-adrenaline in homogenates from seven different regions of human brain. The activities towards 5-hydroxytryptamine and 2-phenethylamine, which are essentially specific substrates for the A- and B-forms, respectively, under the conditions used in this work, were also determined. Noradreanline and adrenaline were substrates for both forms of the Enzyme in all regions studied. The total MAO activity was found to be highest in the hypothalamus and lowest in the cerebellar cortex. Use of the selective MAO inhibitors clorgyline and (?)-deprenyl also showed adrenaline and noradrenaline to be substrates for both forms of the Enzyme in rat brain. In human cerebral cortex and rat brain the two forms were found to have similar K(m)-values and maximum velocities towards adrenaline. These values for the two forms were also found to be similar in human cerebral cortex when noradrenaline was used as the substrate. In contrast MAO-A showed a significantly lower K(m) and a higher maximum velocity towards noradrenaline in rat brain. These results suggest that the rat may not provide a close model of the human for studies on the effects of MAO inhibitors on brain noradrenaline metabolism.