DIAMINE OXIDASE (DAO) also known as histaminase or amine oxidase (copper containing), is an enzyme involved in the metabolism, oxidation, and inactivation of histamine in animals. Highest content is observed in the digestive tract and placenta. An imbalance between histamine intake and the capacity for histamine degradation can lead to histamine intolerance (HIT). Measuring DAO activity in serum can be useful in diagnosing HIT. This non-radioactive, fluorimetric DAO assay is based on the oxidation of putrescine to pyrroline plus NH3 and H2O2. The generated H2O2 is then used by HRP to oxidize a dye making it fluorescent. The increase in fluorescence at lambda ex/em = 530/585 nm is directly proportional to the enzyme activity.