Polystyrene microbridges used in sitting-drop crystallization release 1,4-diphenyl-2-butene, a novel inhibitor of human MAO B

In the course of protein-structure determinations of the membrane-bound enzyme monoamine oxidase B (MAO B) by X-ray crystallography, a compound was found in the active site of the enzyme that consists of two phenyl rings separated by four C atoms. This compound was identified by chromatography and by mass spectrometry to be 1,4-diphenyl-2-butene and found to be a component of the polystyrene microbridges that are used in protein crystallization. This compound is present at a level of ∼0.3 mg (∼1.5 µmol) per microbridge and functions as a competitive inhibitor of MAO B with a K_i of 35 µM. The presence of detergents in the crystallization solutions facilitates the extraction of this compound from the polymer medium.