Practically all of the numerous and complex biochemical reactions that take place in animals, plants, and microorganisms are regulated by enzymes, and so there are many examples. Among some of the better-known enzymes are the digestive enzymes of animals. The enzyme pepsin, for example, is a critical component of gastric juices, helping to break down food particles in the stomach. Likewise, the enzyme amylase, which is present in saliva, converts starch into sugar, helping to initiate digestion. In medicine, the enzyme thrombin is used to promote wound healing. Other enzymes are used to diagnose certain diseases. The enzyme lysozyme, which destroys cell walls, is used to kill bacteria. The enzyme catalase brings about the reaction by which hydrogen peroxide is decomposed to water and oxygen. Catalase protects cellular organelles and tissues from damage by peroxide, which is continuously produced by metabolic reactions.