Monoamine oxidase is a single-molecular enzyme with multiple binding sites, which has low specificity for substrates and can oxidatively deaminate various amines. Monoamine oxidase contains copper ions, which are distributed in the mitochondria of liver, kidney and other tissues, and its content is followed by liver, heart, kidney, brain, lung, skeletal muscle, platelets and placenta. Monoamine oxidase is involved in the production of collagen fibers, so this enzyme tends to be elevated in some fibrotic diseases. Determination of enzyme activity in serum can reflect its activity in connective tissue, so as to understand the degree of tissue fibrosis.