the digestive tracts of animals; and prevent the digestive action of the
enzymes of the stomach and intestines upon these worms. Probably similar
"antienzymes" are located in the mucous linings of the intestinal tract
itself, and serve to prevent the auto-digestion of these organs by the
active enzymes with which they are almost continually in contact.

The difference between an antienzyme, which protects material which would
otherwise be subject to the attack of an enzyme, and an inhibitor, which
renders the enzyme itself inactive, is apparent.

So far as is known, however, no such substances as antienzymes are present
in plant tissues; although the question as to why the proteoclastic enzymes
which are elaborated by a given mass of protoplasm do not attack the
protoplasm itself, might well be raised.

ZYMOGENS

It is apparent that, since enzymes are produced by protoplasm for the
special needs of any given moment or stage of development, there must be a
preliminary stage, or condition, in which they do not exert their
characteristic catalytic effect. When in this stage, the compound is known
as "proenzyme," or "zymogen." In this stage, it is inactive, but can be
made to exhibit its catalytic effect, usually by bringing it into contact
with a suitable activator. When once so activated, however, it cannot be
returned again to the inactive state.

This phenomenon has been studied in connection with the zymogens of the
digestive proteases, pepsin and trypsin. Trypsinogen may be rendered active
by contact with either calcium salts or with another substance (apparently
itself an enzyme) known as enterokinase, which is secreted in the
intestinal tract.

Similarly, proenzymes have been reported as occurring in numerous plant
tissues. These proenzymes are believed to be present in the plant cells in
the form of definite characteristic granules, which may be observed under
the microscope, and which disappear when the enzyme becomes active. Thus,
"proinulase" has been reported as occurring in artichoke tubers:
"prolipase," in castor beans; "proinvertase," in several species of fungi;
and, probably, "prooxidase," in tobacco leaves. In the case of the
last-named zymogen, it has been observed that after the zymogen has been
once activated, as in response to the need for increased activity due to
the entrance of the germs of certain leaf-diseases, it can once again
produce a second supply of t