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.