Pepsin. Polarized light micrograph of crystals of the enzyme pepsin. Pepsin is an enzyme secreted in the stomach of vertebrates, along with hydrochloric acid. In acid solution pepsin splits proteins into peptones (large protein fragments), thereby contributing to the digestive process. The stomach lining itself is protected from the action of the acid and the enzyme by its coating of mucus. Enzymes are extremely efficient catalysts of reactions. About 30 grammes of pure crystalline pepsin would be capable of digesting nearly 2 metric tonnes of egg white in a few hours. Magnification: x100 at 6x7cm size.