enzymes.

Enzymes are specific in their action. Any given enzyme affects only a
single reaction; or at most acts only upon a single group of compounds
which have similar molecular configuration. Usually it is only a single
compound whose decomposition is accelerated by the action of a particular
enzyme; but there are a few enzymes, such as _maltase_ (which acts on all
[alpha]-glucosides) and emulsin (which acts on all [beta]-glucosides) which
act catalytically upon groups of considerable numbers of similar compounds.

Enzymes, like all other catalysts, act more energetically at increased
temperatures; but for each particular enzyme there is an "optimum
temperature," (usually between 40 deg. and 65 deg.) above which the
destructive effect of the temperature upon the enzyme itself more than
offsets the accelerating influence of the increased temperature. At still
higher temperatures (usually 80 deg. to 100 deg.) the enzymes are
"killed," i.e., rendered permanently inactive. All enzymes are "killed" by
boiling the solutions in which they are contained. Dry preparations of
enzyme material can withstand somewhat higher temperatures, for somewhat
longer periods of time, than can the same enzyme in moist condition or in
solution. When an enzyme has once been inactivated by heating, or
"killed," it can never be restored to activity again.

Enzymes are extremely sensitive to acids, bases, or salts, their activity
being often enormously enhanced or, in other cases, entirely inhibited, by
the presence in the reacting medium of very small amounts of free acids, or
bases, or even of certain neutral salts. For example, pepsin, the enzyme of
the stomach will act only in the presence of a slightly acid medium and is
wholly inactive in a mixture which contains even the slightest amount of
free alkaline material; while trypsin, the similar enzyme of the intestine,
acts only under alkaline conditions. Practically all enzymes are rendered
inactive, but not destroyed, by the presence of either acid or alkali in
excess of N/10 strength. Many will act only in the presence of small
quantities of certain specific neutral salts; while, on the other hand,
other salts are powerful inhibitors of enzyme action. Enzymes often differ
from the protoplasm which secretes them in their response to antiseptics,
such as toluene, xylene, etc., which inhibit the activity or growth of the
cell, but have no effect upon the activity of the enzymes which it
con