Monoamine Oxidase and Catechol-O-Methyl Transferase Activity in Tetrahymena*

SYNOPSIS Tetrahymena pyriformis strain HSM was found to have monoamine oxidase (MAO) and a catechol-O-methyl transferase-like (COMT) activity. As in mammalian tissues, the MAO activity is predominantly localized in the mitochondrial pellet and COMT in the cytosol. The COMT-like activity was present in amounts comparable to several mouse tissues and was inhibited by tropolone. MAO activity was much lower than in any of the mouse tissues tested, and its activity varied greatly from preparation to preparation. The substrate preference of Tetrahymena MAO was tryptamine > serotonin > dopamine, and activity increased with increasing pH from pH 6.5 to pH 7.8, as does that of mouse liver MAO. The K_m of Tetrahymena MAO for tryptamine was 4 μM, an order of magnitude lower than that of mouse liver MAO. Sensitivity to inhibition by MAO inhibitors was variable. In some preparations, no inhibition was observed. In others clear inhibition was obtained, harmine and clorgyline being among the most potent inhibitors.