Pepsin cleaves the 44 amino acids from pepsinogen to create more pepsin. Gastric juice is an acidic fluid containing hydrochloric acid and the digestive enzyme pepsin. Gastric juice mainly contains hydrochloric acid and pepsin. Fab fragments are generated by cleavage of IgG with papain instead of pepsin. Hydrochloric acid activates this inactive form of enzyme into the active form, pepsin. Examples of cleaving agents used are cyanogen bromide, pepsin, and trypsin. Humans have other proteins to digest milk, such as pepsin and lipase. Methylxanthines induce acid and pepsin secretions in the gastrointestinal tract. In addition to chymosin, rennet contains other enzymes, such as pepsin and a lipase. In the stomach, assorted acids and the enzyme pepsin break down food. Pepsin breaks down proteins into polypeptides. Pepsin or another enzyme is then added to enhance the protein foaming abilities. The embryonic and basement membranes are removed by the host's digestive enzymes (particularly pepsin). The two key components of natural rennet are chymosin and bovine pepsin. Gastric chief cells produce pepsinogen, which is activated by the acid to form pepsin.