MAO inhibitors of various chemical classes (hydrazine derivatives, tranylcypromine, pargyline, modaline) in general cause competitive, followed by non-competitive, irreversible inhibition of the enzyme. For this purpose, the drugs, except tranylcypromine, have to be metabolized to form the actual inhibitors. Enzymes not related to MAO are affected more markedly by hydrazine than by nonhydrazine MAO inhibitors.
All types of MAO inhibitor cause some typical effects in vivo, such as increase of endogenous and exogenous monoamines, antagonism towards monoamine releasers, or change in the excretion pattern of monoamines and their metabolites. A rise of endogenous monoamines occurs only when MAO has been inhibited to a high degree. Intensity, onset and duration of action depend on the species, the tissue, the amine, the mode of administration, and the chemistry of the drug.
The action of MAO inhibitors bears mainly on those amines which are preferentially metabolized by MAO. Thereby, the intra- as well as the extraneuronal amines seem to be increased. For some clinical effects a relationship with MAO nibibition is probable.