In order to test whether a drug is an inhibitor of monoamine oxidase in the human subject, three main measures are available:
1. We can measure an increased excretion of the normal substrates of the enzyme, e.g. tryptamine (Sjoerdsma, et al., 1959b).
2. We can measure a decreased excretion of the metabolites resulting from the enzyme's action, e.g. 5-hydroxyindoleacetic acid (5-HIAA).
3. We can administer a loading dose of an enzyme substrate and observe how the subject deals with it before and after the drug, e.g. 5-hydroxytryptamine (Sjoerdsma et al., 1958), and epinephrine (N-methyl-C14) (Resnick et al., 1958).