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Monoamine oxidase (MAO) inhibition assay

Understand if your compound is an inhibitor of MAO-A or MAO-B enzyme.

Monoamine oxidase (MAO) inhibition is a non-CYP mediated metabolism assay within our portfolio of in vitro ADME screening services. Cyprotex deliver consistent, high quality data with the flexibility to adapt protocols based on specific customer requirements.

Determining potential inhibition of MAO enzymes

  • Monoamine oxidases (MAO) are membrane-associated enzymes located specifically to the outer mitochondrial membrane. They are the major enzymes participating in the catabolism of monoamine neurotransmitters and related exogenous amines.
  • Two isoforms of MAO exist, MAO-A and MAO-B, which differ in their substrate specificity, inhibitor selectivity and tissue distribution.
  • Selective MAO-A inhibitors are useful in the therapy of depression and anxiety whereas MAO-B inhibitors are often used in the treatment of Parkinson’s and Alzheimer’s diseases
  • Cyprotex’s MAO inhibition assay identifies if your compound is an inhibitor for either MAO-A or MAO-B.
In human brain the predominant form is MAO-B, expressed at highest levels in astrocytes and serotonergic neurons, while MAO-A is expressed at highest levels in catecholaminergic neurons.

1Hotamisligil GS and Breakefield XO (1991) Am J Hum Genet 49(2); 383–392


Monoamine oxidase (MAO) inhibition protocol

Substrate Kynuramine
Metabolite 4-hydroxyquinolinol
Test Article Concentrations 0, 0.4, 1, 4, 10, 40 and 100 µM (different concentrations available)
Enzyme Source hMAO-A and hMAO-B expressed enzymes (Supersomes™)
Test Article Requirements 100 μL of a 40 mM DMSO solution (or equivalent amount in solid)
Positive Control Clorgyline MAO-A
Selegiline MAO-B
Analysis Method LC-MS/MS
Data Delivery IC50
Standard error of IC50
% Control at each concentration


Data from the monoamine oxidases inhibition assay

Figure 1
Inhibitory specificity demonstrated for MAO-A inhibitor clorgyline using MAO-A or MAO-B expressed enzyme and kynuarmine as the substrate.
Figure 2
Inter-assay reproducibility of MAO-B inhibition using specific MAO-B inhibitor selegiline and non-specific inhibitor tranylcypromine. Each inhibitor was investigated on three separate occasions using hMAO-B expressed enzyme and kynuramine as substrate. The error bars represent the standard error of the IC50 determination.


1 Hotamisligil GS and Breakefield XO (1991) Human monoamine oxidase A gene determines levels of enzyme activity. Am J Hum Genet 49(2); 383–392

Learn More

Learn more about drug metabolism in chapter 3 of our popular Everything you need to know about ADME guide. 

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