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

Abstract

T. pyriformis strain HSM had monoamine oxidase (MAO) and a catechol-O-methyl transferase-like (COMT) activity. As in mammalian tissues, MAO activity was 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. Substrate preference of Tetrahymena MAO was tryptamine > serotonin > dopamine, and activity increased with increasing pH from pH 6.5-7.8, as did that of mouse liver MAO. The Km of Tetrahymena MAO for tryptamine was .apprx. 4 .mu.M, 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.