le sucrose into glucose and fructose, which
are readily fermentable. Invertase is also present in moulds and other
microorganisms; and in the buds, leaves, flowers, and rootlets of those
higher order plants which store their carbohydrate reserves in the form of
sucrose. It appears that sucrose, while easily soluble, is not readily
translocated, or utilized, by plants until after it has been hydrolyzed
into its constituent hexoses.

The optimum temperature for invertase is 50 deg. to 54 deg.; it is killed
if heated, in the moist condition, to 70 deg. Its activity is increased
by the presence of small amounts of free acids; but is inhibited by free
alkalies.

=Zymase= is the active alcoholic fermentation enzyme of yeasts. It
accelerates the well-known reaction for the conversion of hexose sugars
into alcohol and carbon dioxide, namely,

C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.

Because of its scientific interest and industrial importance in the
fermentation industries, its action has been extensively studied. It acts
only in the presence of soluble phosphates and of a coenzyme (see below)
which is dialyzable and not destroyed, which is probably an organic ester
of phosphoric acid. The significance of the molecular configuration of the
hexose sugars in their susceptibility to action by zymase has already been
discussed in detail (see page 56).

The optimum temperature for zymase action is 28 deg. to 30 deg. The enzyme
is killed by heating to 45 deg. to 50 deg. in solution, or to 85 deg. if in
dry preparation.

=Proteases= of the erepsin type, i.e., those which break proteins down to
amino-acids instead of only to the proteose or peptone stage, as is
characteristic of the enzymes of the trypsin type, are widely distributed
in plants. Except in the case of the two which occur in large amounts in
certain special fruits (papain in papaws, and bromelin in pineapples), they
are very difficult to prepare in pure form for study. In general, all
proteolytic actions, even when accelerated by active enzymes, proceed much
more slowly than do the hydrolyses of carbohydrates or fats. It seems that
metabolic changes of the complex protein molecules are much more difficult
to bring about and take place much more slowly than do those of the
energy-producing types of compounds.

The presence of proteolytic enzymes in most vegetative cells, and in seeds,
may be demonstrated, however, by studying the action of extrac