Abstract
The effects of storage, and treatment with heat and inhibitors on MAO in a rabbit liver homogenate with various substrates were examined. Tyramine, benzylamine, serotonin, β-phenylethylamine, and tryptamine were used as substrates and pargyline, harmaline, and tranylcypromine as inhibitors. Oxygen consumption during the reaction was measured in a Warburg apparatus.
The substrate specificity of the freshly prepared enzyme was higher than that of the frozen enzyme. With fresh enzyme, the reactions with tyramine, serotonin, and tryptamine increased linearly for 70 min, whereas those with benzylamine and β-phenylethylamine decreased after 30 min. Therefore, the activities of the fresh preparation and preparations after various treatments were compared, using 10 mM tyramine and benzylamine as substrates.
The results obtained were as follows:
1) When the homogenate was stood at 22°C, the MAO activities with both tyramine and benzylamine did not change for 24 hrs but then decreased to almost zero after 29 hrs.
2) When the homogenate was stored at 4°C, the decrease with time in the MAO activity with benzylamine was more extreme than that with tyramine.
3) When the homogenate was stored at -20°C, the activities with both substrates did not change for 4 weeks. Then both activities decreased rapidly, the decreases after 12 weeks being 25 % with tyramine and about 51 % with benzylamine.
4) Tranylcypromine and pargyline had similar inhibitory effects on a fresh homogenate and a prepration that had been kept in a refrigerator for one week with both substrates. In the fresh preparation harmaline inhibited the activity with tyramine more than that with benzylamine, but in the stored preparation it caused similar inhibition with the two substrates.
5) The activity with tyramine was more stable on heat treatment than the activity with benzylamine.
6) The activities with tyramine and benzylamine were inhibited similarly by trypsin.
From these results, it is suggested that there are multiple forms of MAO in rabbit liver differing in enzymological properties.
