Monoamine oxidase in sympathetic nerves: a transmitter specific enzyme type

1. When rat brain or superior cervical ganglion monoamine oxidase was incubated with increasing concentrations of clorgyline, using tyramine as substrate, the inhibition of the enzyme could be represented by a pair of sigmoidal curves joined by a horizontal region where inhibition was constant. Tyramine appeared to be metabolized by two enzymes, one of which was highly sensitive to clorgyline, designated A, whereas the other enzyme, designated B, was less sensitive to clorgyline.2. The ratio of A/B activity for brain was 6/4 while in the ganglion it was 9/1.3. When the experiments were repeated using noradrenaline as the substrate, the inhibition of the enzyme followed a simple sigmoidal curve where deamination was inhibited by low concentrations of clorgyline as observed with enzyme A.4. We conclude that tyramine is deaminated by both A and B enzymes whereas noradrenaline is deaminated only by enzyme A, the enzyme which is most active in the ganglion. Our observations are consistent with the hypothesis that a specific intraneuronal monoamine oxidase plays an important role in the catabolism of noradrenaline in sympathetic nerves.