Amine Oxidase, Copper Containing 3 (AOC3) is a copper amine oxidase with a topaquinone cofactor. AOC3 is a Type II integral membrane protein, but a soluble form of the enzyme is present in human serum, and its level increases in diabetes and some inflammatory liver diseases. AOC3 catalyzes the oxidative deamination of small primary amines such as methylamine, benzylamine, and aminoacetone in a reaction that produces an aldehyde, ammonia, and H2O2. The enzyme is sensitive to inhibition by semicarbazide. AOC3 expression is highest in the endothelium of lung, heart, and intestine, but low in tissues such as brain, spleen, kidney, and liver. The activity of recombinant human AOC3 was measured by its ability to produce hydrogen peroxide during the oxidation of benzylamine. The reaction was performed in 50 mM HEPES, pH 7.5 (assay buffer), initiated by addition 50 μL of various concentrations of AOC3 (diluted by assay buffer) to 50 μl substrate mixture of 2 mM Benzylamine, 2 units/mL HRP and 100 μM AUR. Read at excitation and emission wavelengths of 544 nm and 590 nm (top read), respectively in kinetic mode for 5 minutes.