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48 GENERAL AND PHYSICO-CHEMICAL.
Of the above mentioned enzymotic reactions the hydrolytic cleavage
processes have been best studied, and the general properties of the
enzymes which will be given apply chiefly to the hydrolytically splitting
enzymes. Among these the following are to be mentioned especially:
1. Enzymes which split fats and other esters with the formation of
the corresponding alcohol and acid. They are called lipases or esterases.
2. Enzymes which split complex carbohydrates with the formation
of simpler ones. To these belong:
a. Disaccharide splitting enzymes for instance saccharase (invertase,
invertin), maltase, lactase which act upon the corresponding disaccharide
saccharose (cane-sugar) maltose and lactose (milk sugar);
b. Polysaccharide splitting enzymes such as amylase, ptyalin. The
name diastase is often used to designate all the enzymes of this group.
In close relation to these enzymes stand the glucoside splitting enzymes
which occur especially in higher plants and the best known of which is
amygdalase (emulsin) occurring in the almond.
3. Enzymes which act upon the proteins or their related cleavage
products. Of these we have
:
a. Peptidases and erepsin which split polypeptides or peptones;
b. Proteases which act upon proteins as substrate (pepsin, trypsin,
autolytic enzymes).
Among the hydrolytic enzymes of the animal kingdom we also
include the arginase, which splits arginine into urea and ornithin and the
histozym, which splits hippuric acid. The two following groups also
belong here, namely, the nucleases which split nucleic acids and which
will be discussed in Chapter II, and the coagulating enzymes, rennin
and thrombin, which are probably active as proteases. The deamidizing
enzymes which split off the NH2 group from amino combinations are,
at least in certain cases, to be classed as hydrolytic enzymes. This is
for example the case with the adenase and guanase which splits off ammonia
from the two bodies adenine and guanine converting them into hypoxan-
thine and xanthine respectively. The urease which splits urea also belongs
to this group.
General Properties of the Enzymes. When possible we make use
of watery solutions of enzymes in experimentation. In case they are
insoluble in water (certain lipases) we use them in the form of more or
less purified powders or together with the tissue where they are formed.
We have no general method for preparing enzyme solutions. In certain
cases they are contained in secretions (gastric and pancreatic enzymes);
in others they are prepared from the cells by crushing and pressing out
the cell juice (zymase, organ enzymes), and finally, most enzymes can
be extracted from the cells with water or glycerin, and as this last gives
permanent solutions it has found great use as an extraction medium.
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