Contribution of tryptophan hydroxylase and MAO in serotonin metabolism. Tryptophan hydroxylase catalyzes the monooxygenation of tryptophan to 5-hydroxytryptophan, which is subsequently decarboxylated to form serotonin (5-hydroxytryptamine; 5-HT). Monoamine oxidase (MAO) catalyzes the oxidative deamination of 5-HT to the corresponding aldehyde. This is followed by oxidation by aldehyde dehydrogenase to 5-HIAA, the indole acetic acid derivative.