There are few enzymes whose names have been established by long usage such as ptyalin, pepsin, trypsin, and erepsin. Apart from these, enzymes are usually named by adding the suffixes to the main part of the name of the substrate upon which they act. Thus amylases act upon starch (amylum), lac- tase acts upon lactose, lipases act upon lipids, maltase acts upon maltose and protesses act upon lipids, maltase acts upon maltose and protesses act upon proteins. There are, however, several enzymes which act upon many substances in different ways. These enzymes are named by their functions rather than substrates. Thus, an enzyme which causes deaminations is called a deaminase and oxidising enzyme an oxidase.

Some enzymes work efficiently only if some other specific substance is present in addition to substrate. This other substance is known as an "activator" or a "conenzyme". "Activators" are usually inorganic ions. They increase the activity of a complete enzyme and may take part in the formation of the enzyme-substrate complex. Many of the conenzymes are related to vitamins.

This explains why vitamin deficiencies profoundly alter metabolism. Thus, for instance, thiamine, as thiamine pyrophosphate, functions as a conenzyme in at least 14 enzymes systems.

Conenzymes, like enzymes, are being continuously regenerated in the cells. Enzymes play a decisive role in the digestion of food as they are responsible for the chemical changes which the food undergoes during digestion. The chemical changes comprise the breaking up of the large molecules of carbohydrates, fats and proteins into smaller ones or conversion of complex substances into simple ones which can be absorbed by the intestines.

They also control the numerous reactions by which these simple substances are utilized in the body for building up new tissues and producing energy. The enzymes themselves are not broken down or changed in the process. They remain as powerful at the end of a reaction as they were at the beginning. Moreover, very small amounts can convert large amounts of material. They are thus true catalysts.

The process of digestion begins in the mouth. The saliva in the moth, besides helping to masticate the food, carries an enzyme called ptyalin which begins the chemical action of digestion. It initiates the catabolism (breakdown) of carbohydrates by converting starches into simple sugars. This explains the need for thorough mastication of starchy food in the mouth. If this is not done the ptyalin cannot carry out its functions as it is active in an alkaline, neutral or slightly acid medium and is inactivated by the highly acid gastric juices in the stomach.

Although enzymatic action starts while food is being chewed, digestion moves into high gear only when the chewed food has passed the esophagus and reached the stomach. While the physical action of peristalsis churns and kneads solid food into a semi-solid amorphous mixture called chyme, this mixture undergoes chemical changes initiated by gastric juices secreted by the walls of the stomach. These juices include mucus for lubricating the stomach, hydrochloric acid and gastric juice. The enzyme or active principle of the gastric juice is pepsin. This enzyme in combination with hydrochloric acid starts the breakdown of proteins into absorbable amino acids called polypeptides. An additional enzyme, rennin, plays an important role in the stomach of the infant. It curdles milk and allows the pepsin to work upon it. The gastric juice has no effect upon starches or fats.

When the chyme leaves the stomach and enters the small intestine through the pylorus - the lower escape valve, it still contains much food which is in the form of raw material not yet ready for absorption in the body. Digestion is completed inside the small intestine by several juices.

From liver comes a liquid called bile which converts fat globules into a smooth emulsion. The pancreas contributes various enzymes which continue the breakdown of proteins, help to divide starch into sugars and work with bile in digesting fats. The small intestine itself secretes enzymes from its inner wall to complete the reactions. When all the enzymes have done their work, the food is digested and rendered fit for absorption by the system.