Purification and characterization of tribulin, and endogenous inhibitor of monoamine oxidase and of benzodiazepine receptor binding.

A low molecular weight fraction of human urine (less than 500 daltons) which both inhibits monoamine oxidase and benzodiazepine binding to central and peripheral receptors has been purified by ethyl acetate extractions, HPLC and thin layer chromatography. This material extracted equally well at acid and basic pH and was insoluble in heptane. It competitively inhibited binding of 3H-clonazepam, a central benzodiazepine receptor agonist and, in addition, displaced 3H-Ro 5-4864, a specific peripheral benzodiazepine receptor ligand, from its binding sites. It showed no GABA shift with the benzodiazepine receptor antagonist, Ro-15 1788. MAO A and B were inhibited approximately equipotently and the material competitively inhibited tyramine oxidation by rat liver. It was stable on boiling and is unlikely to be a peptide.