Monoamine oxidase A (MAOA) is an enzyme involved in the metabolism of the monoamines, eg 5-HT and noradrenaline. It converts the monoamines into their corresponding carboxylic acid via an aldehyde intermediate. MAOA regulates both the free intraneuronal concentration and the releasable stores of 5-HT and noradrenaline. MAOA inhibitors, such as phenelzine, bind to and inhibit MAOA, preventing monoamine degradation. This results in greater stores of monoamines available for release. MAOA inhibitors are used in the treatment of depression. People with depression have lower than normal levels of the monoamines and MAOA inhibitors restore the levels to within the normal range.