Figure 2: Serotonin metabolism. Tryptophan is the precursor for serotonin synthesis, with different enzymatic reactions in plant and animals [6]; hydroxylation is the rate limiting step (enzyme mediated by tryptophan hydroxylase in animals or tryptamine hydroxylase in plants), while decarboxylation is a rapid conversion by the aromatic amino acid decarboxylase (tryptophan decarboxylase). The catabolic metabolite of serotonin is 5-hydroxyindoleacetic acid, via 5-hydroxyindole acetaldehyde enzymatically converted by the membrane-bound mitochondrial flavoprotein monoamino oxidase. Modified images from NCBI PubChem Substance Database.