The word "enzyme" (from the Greek "in yeast") was introduced by Kuhne as a general designation for the substances formed in plants or animals which had previously been called "soluble" or "unorganized" ferments to distinguish them from "organized" ferments (fermentation organisms). As more and more of the activities previously regarded as characteristic of organisms have been found to be due to enzymes, the conception of enzyme action has broadened until now the term enzyme is applied by most writers to all organic catalysts formed in plant or animal cells. Those which are ordinarily secreted from the cell and exert their activities outside of it (as in the case of the digestive ferments) are sometimes called extracellular enzymes, and those which normally perform their functions within the cells in which they are formed (as in yeast or in muscle cells) may be called intracellular enzymes even though it be possible by artificial means to cause them to act independently of living matter. Although each enzyme is generally supposed to be a definite chemical substance, the identification and classification of enzymes are based upon the changes which they bring about. Some of the better-known groups of enzymes are as follows:

* Recently Falk has suggested that the lipolytically active grouping is the tautomeric enol-lactim form of the peptide linking which becomes inactive on rearrangement to the keto form. Experiments testing this view resulted in the production of lipolytically active substances by the action of alkali on castor bean globulin, casein, and gelatin. Further confirming evidence was obtained on studying the ester-hydrolyzing action of glycine, glycyl-glycine, and hippuric acid at different hydrogen ion concentrations. Falk holds that "given a definite chemical grouping, the nature of which has been indicated, and which may be present in different classes of substances, certain definite lipolytic actions will result."

1. The hydrolytic enzymes.

a. Proteolytic or protein-splitting enzymes.

b. Lipolytic or fat-splitting enzymes.

c. Amylolytic or starch-splitting enzymes.

d. Sugar-splitting enzymes.

2. The coagulating enzymes, such as thrombin or thrombase (the fibrin ferment), and rennin, which causes the clotting of milk.

3. The oxidizing enzymes, or "oxidases" (which, if the oxidation be accompanied by a splitting off of amino groups, may be called "deamidizing" or "deaminizing" enzymes).

4. The reducing enzymes or "reductases."

5. Those which, like the zymase of yeast, produce carbon dioxide without using free oxygen.

6. Enzymes causing a breaking down of a larger into a smaller molecule of the same composition, as in the production of lactic acid from glucose.

7. Enzymes causing chemical rearrangement without breaking down of larger into smaller molecules, "mutases."