Human amine oxidases (AO) catalyze the oxidative deamination of biogenic amines (neurotransmitters such as serotonin, noradrenaline, the hormone adrenaline and polyamines such as the spermines) and xenobiotic amines (exogenous dietary tyramine and phenylethylamine). The basic reaction is the oxidative cleavage of the alpha-H to form an imine product with the concomitant reduction of a FAD cofactor. The imine product then hydrolyses to an aldehyde and ammonia (or amine for secondary and tertiary amine substrates). Reduced FAD is reoxidized to form hydrogen peroxide to complete the catalytic cycle.

The reaction can be summarized as

RCH2NH2 + H2O + O2 = RCHO + NH3 + H2O2

The resultant hydrogen peroxide is the source of the most toxic free radical, the hydroxyl radical (.OH). This free radical is produced in the Fenton reaction with the use of ferrous (Fe2+) iron.