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The inhibition of monoamine oxidase by brofaromine
M C Anderson1, P C Waldmeier, K F Tipton
1Department of Biochemistry, Trinity College, Dublin, Ireland.
Abstract:
The inhibition of rat liver mitochondrial monoamine oxidase-A (MAO-A) by brofaromine was time-dependent at low enzyme and inhibitor concentrations. The apparent sensitivity to inhibition decreased when the concentration of the mitochondrial preparation was increased. After preincubation of the enzyme with brofaromine repeated washing of the preparation, by sedimentation and resuspension, resulted in a gradual recovery of activity. This occurred more slowly than was the case when the reversible inhibitor amphetamine was used. After incubation with radioactively-labeled brofaromine the loss of radioactivity also occurred slowly. After incubation with radioactively-labeled pargyline polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphage (SDS-PAGE) showed the radioactivity to be associated with a peptide of approximate Mr 50,000, corresponding to the subunit of MAO. Pretreatment with unlabeled pargyline depressed this labeling by pargyline, indicating the latter compound to bind to the active-site of the enzyme. Labeling experiments with radioactive brofaromine indicated that there was a high degree of non-specific binding but that no significant radioactivity remained associated with the enzyme on SDS-PAGE. Chromatographic techniques and determination of H2O2 liberation indicated that, in liver there was no appreciable metabolism of brofaromine under the conditions used in the inhibition experiments. These data indicate brofaromine to be a tight-binding, but reversible inhibitor of MAO.
Insights
Brofaromine is a tight-binding, reversible inhibitor of monoamine oxidase-A (MAO-A). Its inhibition is time-dependent and shows slow recovery after washing, unlike typical reversible inhibitors.
Area of Science:
- Biochemistry
- Pharmacology
Background:
- Monoamine oxidase-A (MAO-A) is a key enzyme in neurotransmitter metabolism.
- Understanding MAO-A inhibition is crucial for developing treatments for neurological and psychiatric disorders.
Purpose of the Study:
- To investigate the inhibitory mechanism of brofaromine on rat liver MAO-A.
- To characterize the binding kinetics and reversibility of brofaromine as an MAO-A inhibitor.
Main Methods:
- Enzyme kinetics assays with varying enzyme and inhibitor concentrations.
- Time-dependent inhibition studies and recovery experiments after washing.
- Radioactive labeling with brofaromine and pargyline, followed by SDS-PAGE and chromatography.
Main Results:
- Brofaromine exhibited time-dependent inhibition of MAO-A at low concentrations.
- Inhibition sensitivity decreased with increased mitochondrial preparation concentration.
- Recovery of MAO-A activity after brofaromine preincubation was slow, suggesting tight binding.
- Radioactive brofaromine showed non-specific binding with no significant enzyme association post-SDS-PAGE.
- Radioactive pargyline labeled the MAO-A subunit, confirming active-site binding.
Conclusions:
- Brofaromine acts as a tight-binding, yet reversible, inhibitor of MAO-A.
- The slow dissociation rate distinguishes brofaromine from rapidly reversible inhibitors.
- Brofaromine does not appear to be significantly metabolized in rat liver under experimental conditions.