Monoamine oxidase A-inhibiting components of urinary tribulin: purification and identification

The endogenous monoamine oxidase (MAO) inhibitory activity, termed tribulin, contains several components. We have previously identified one of them, isatin, which is a selective inhibitor of MAO B. In the present study we have purified several further components of human urinary tribulin which act as selective inhibitors of MAO A. They have been identified by gas chromatography-mass spectrometry (GC-MS) as ethyl indole-3-acetate (and/or methyl indole-3-propionate), methyl indole-3-acetate and ethyl 4-hydroxyphenylacetate. IC50 values for MAO A were found to be 44 μM (105 μM for methyl indole-3-propionate), 88 μM and 120 μM, respectively, whilst those for MAO B were each greater than 1 mM. The artificial formation of these esters was excluded by carrying the parent acids, from which they are presumably synthesized, through the purification procedure.