ical 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 CO_{2} 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