Soluble monoamine oxidase; its properties and actions on serotonin.

5-Hydroxyindoleacetic acid (5HIAA) has been shown to be the major metabolite of 5-hydroxytryptamine (serotonin) (1). In man 2 to 8 mg. of the acid are normally excreted in the urine per day (2). This suggests that the major route of metabolism of serotonin proceeds via oxidative deamination, catalyzed, presumably, by monoamine oxidase (MAO). Experiments carried out in vitro (3-6) and in viva demonstrate that MAO is a key substance in the physiology of serotonin and other amines. MAO has been found in most organ tissues in which it is primarily localized in the mitochondria (5). The inability to make soluble the mitochondrial enzyme has prevented studies concerning its cofactor requirements and its mechanism of action. However, although the bulk of MAO activity is in mitochondria, appreciable activity is present in soluble form in some tissues. This *MAO lends itself to standard procedures of enzyme purification. The present report deals with the purification of soluble MAO from guinea pig liver and with preliminary studies on its properties and mode of action . A comparison is also made with mitochondrial MAO.