En′zymes (often called soluble ferments), substances of unknown but very complex composition produced in plants and animals for digesting insoluble foods or those which cannot be used readily without such change. The changes they produce are of two sorts: (1) the commoner consists in causing the food to take into its composition an additional molecule of water; (2) without any additions it may be split up into two or more compounds. Enzyme itself does not take part in the chemical reaction, but seems to act only by its presence. Consequently a very small amount of an enzyme may digest large quantities of a food. Each enzyme is restricted in its action to a particular substance. Some of the more important enzymes, the substance on which they act and the chief products are shown in the following table: