obtain a clear liquid,
entirely free from living cells, which was just as active in producing
fermentation as was the yeast itself. This discovery paved the way for a
long series of investigations, which have conclusively demonstrated that
there is no distinction between "organized" and "unorganized" ferments,
that all living organisms perform their characteristic functions by means
of the enzymes which they contain, and that these enzymes can bring about
their characteristic catalytic effects outside the cell, or tissue which
elaborates them, just as well as within it, provided only that the
conditions of temperature, acidity or alkalinity of the medium, etc., are
suitable for the particular enzyme action which is under consideration.

GENERAL PROPERTIES OF ENZYMES

Since enzymes are catalysts, it is plain that an accurate description of
their activity should, in each case, refer to the influence which they
exert upon some definite reaction velocity. But since the phrases necessary
to describe such an effect are cumbersome and inconvenient, and since most
of the reactions which are accelerated by the catalytic action of enzymes
are either simple hydrolyses, changes in oxygen content, or other simple
decompositions or condensations, which will otherwise proceed so slowly as
to be practically negligible, it is customary to speak of the enzyme as
"acting upon" the material in question. It should be understood, however,
that this is a misstatement, as the enzyme cannot actually initiate a
reaction, or "act upon" any substance; it only acts as a catalyzer to
accelerate the action of water, oxygen, etc., upon the material in
question.

Generally speaking, most enzymes are colloidal in form and, hence, do not
diffuse through membranes such as the cell-walls. Some of them perform
their characteristic functions only within the cell, or organ, which
elaborates them, and can be obtained outside these tissues for purposes of
study only by first rupturing the cell-wall or other membrane with which
they are surrounded. Such enzymes are known as "intracellular." Others are
regularly secreted by glands which discharge them onto other organs, as the
stomach or intestines of animals, where they perform their useful
functions; or, as in the case of germinating seeds, they move to other
parts of the organ, and can be extracted from the tissue by simple
treatment with water. These are known as the "extracellular"