A protein substance which will catalyze a biochemical reaction. It may act independently of the living cell by which it is secreted, but is an essential constituent of it. In addition to the general properties of catalysis, the enzyme, possesses certain characteristics of its own: it is a colloid, highly specific in its action, heat-labile, sensitive to PH and to particular substances which may act as activators, coenzymes or toxins. The enzyme is classified according to the type of reaction it catalyzes, and it commonly named by adding the suffix -ase to the particular substrate upon which it acts. The most important groups of enzymes are the lyases, which bring about hydrolysis, the oxidoreductase and the desmolases. Each main group is divided into various sub-groups which particularize the relations still further, and of these the following are of chief importance carbohydrases, esterases and proteases, which catalyze the decomposition of carbohydrates, esters (including fats) and proteins, respectively and belong to the hydrolase group, and dehydrogenases, oxidases and catalases,which are concerned chiefly in the oxidative processes of cell respiration and belong to the oxidoreductose group.