Time-dependent inhibition of monoamine oxidase by beta-phenethylamine.

Several reports have suggested that monoamine oxidase activity towards beta-phenethylamine is inhibited by high concentrations of that substrate. This inhibition is not found if initial velocities are measured, but there is a slower time-dependent inhibition at higher beta-phenethylamine concentrations. Such time-dependent inhibition is not found with tyramine as substrate or upon incubation of the enzyme with the reversible inhibitor amphetamine. The inhibition is not due to the accumulation of phenacetaldehyde, phenylethanol or phenacetic acid, or to a reaction of any of these three products either with each other or with beta-phenethylamine. Although the inhibition is time-dependent, the inactivated enzyme slowly regains activity upon removal of the beta-phenethylamine. A model is proposed to explain the observed inhibition.