Since nothing is known of the chemical composition of enzymes, they can only be studied by considering the effects which they produce. This is reflected in the systems which have been adopted for their nomenclature and classification.
As they were first supposed to be proteins, the earlier representatives of the group were given characteristic names ending with the suffix in, similar to that of the proteins. Since this idea has been found to be incorrect, however, a system of nomenclature has been adopted which assigns to each enzyme the name of the material upon which it acts, followed by the suffix ase. Thus, cellulase is the enzyme which accelerates the hydrolysis of cellulose; glucase, that acting upon glucose; amylase, that acting upon starch (amylum), etc.
The substance upon which the enzyme acts (or, strictly speaking, the substance whose hydrolysis, oxidation, or other chemical change, is catalytically affected by the enzyme) is called the substrate.
Most enzymes are catalysts for hydrolysis reactions and are, hence, classed as hydrolytic in their action, and may be spoken of as "hydrolases." Those which accelerate oxidation are called "oxidases"; while those that stimulate reduction reactions are "reductases"; those that aid in the splitting off of ammonia, or amino-acid groups, are "deaminases"; and those that aid in the splitting off of CO2 from COOH groups are "carboxylases," etc.
The hydrolytic enzymes are further subdivided into the sucroclastic (sugar-splitting), or sucrases; the lipoclastic (fat-splitting), or lipases; the esterases (ester-splitting); proteoclastic (protein-splitting), or proteases; etc.
OCCURRENCE AND PREPARATION FOR STUDY
Enzymes are present in all living matter. In animal tissues, they occur in the largest amounts in those glands or organs where active vital processes take place, as in the brain, the digestive tract, blood, etc. In plants, they may be found in all living cells, and are especially abundant in the seeds, where they serve to render soluble and available to the young plant the stored food materials. The enzymes of moulds, and other parasitic plants, are usually extracellular in type, being secreted for the purpose of making the material of the host plant available to the parasite. Extracellular enzymes are also developed in seeds during germination, in order that the stored food material of the endosperm may be rendered soluble and translocated into the tissues of the growing seedling. But most other plant enzymes are intracellular in type. Hence, in all preparations of plant enzymes for study, or for commercial use, the first step in the process is, necessarily, a thorough rupturing of the cell-walls of the plant material.
The rupturing of the cells may be accomplished in a variety of ways, as follows: (1) mechanical disintegration, as by grinding in a mortar with sharp sand; (2) freezing the material, by treatment with liquid air, then grinding; (3) killing the cells by drying, by treatment with alcohol or acetone, then grinding the mass in a paint mill with toluene; (4) killing the cells by chemicals (sulfuric acid, 0.5 to 1.0 per cent, or other suitable agents) followed by extraction with water; (5) autolysis, or self-digestion, in which the cells are mixed with toluene or some other antiseptic which kills the cells without injuring the enzymes, then the material is minced or ground up and suspended in water containing the antiseptic, until the enzymes dissolve the cell-walls and so escape into the liquid—this process being especially adapted to the preparation of active extracts from yeasts, which contain the necessary cell-wall dissolving enzymes to facilitate autolysis.
Enzymes may be separated out of the aqueous extract obtained from cells ruptured by any of the above methods, by precipitation with alcohol, acetone, or ether, in which they are insoluble; but if this is done, the precipitate must be at once filtered off and rapidly washed and dried, as prolonged contact with these precipitating agents greatly diminishes the activity of most enzymes. Or, they may be adsorbed out of solution on gelatinous, or colloidal, materials, like aluminium hydroxide, or various hydrated clays. If the dry preparations obtained in any of these ways are contaminated by carbohydrates, proteins, etc., these may be removed by treatment with suitable digesting enzymes obtained from the saliva, gastric, and pancreatic juices, and the digested impurities washed out with 60 to 80 per cent alcohol, leaving the enzyme preparation in a purified but still active form.
In any study of the "strength," or possible catalytic effects, of an enzyme preparation, it is necessary, first, to determine what particular reaction it affects, by qualitative tests with various substrate materials, such as starch, sugars, glucosides, proteins, etc., and then to determine quantitatively its accelerating effect upon the reaction in question. The latter may be done by measuring either the time required to carry a unit quantity of the substrate material through any determined stage of chemical change, or the quantity of the substrate which is changed in a unit period of time. It would not be profitable to go into a detailed discussion here of the methods of making these quantitative measurements of enzyme activity. Such discussions must necessarily be left to special treatises on methods of study of enzyme action. It may be said, however, that generally both the qualitative tests for, and the quantitative measurements of, the accelerating influence of enzymes depend upon the observation of some change in the physical properties of the substrate material, such as the optical activity, electrical conductivity, or viscosity, of its solution. In some cases, it is convenient to make an actual quantitative determination of the amount of end-products produced in a given time, as in the inversion of cane sugar, the hydrolysis of maltose, etc., but such determinations necessarily involve the removal of some of the reaction mixture for the purposes of the determinations, and are not, therefore, suitable for the study of the progressive development of the reaction which is being studied.