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CLASSIFICATION. 91
not, like the «-amino-acids, broken by trypsin, but rather by an enzyme
arginase, discovered by K08SEL and Dakin. 1
If the proteins are considered as consisting chiefly of peptide-like
complexes consisting of amino-acids united and containing also several
NH2 groups at the ends, it is readily understood that the proteins are
amphoteric electrolytes, like the amino-acids, which form salts with bases
as well as with acids and undergo hydrolytic dissociation. As we also
accept the theory that the protein molecule contains a large number of
( ( M >II as well as XHL
» groups, it follows that the proteins may be poly-
basic acids as well as polvacidic bases. The different proteins act in
this regard somewhat differently, thus the protamines are strongly basic
while casein behaves strikingly acid, and others take a certain mean
position. It is unfortunately impossible to base a classification of the
proteins upon this behavior, as well as upon chemical constitution.
The general properties, such as solubility and precipitation properties,
are too uncertain to aid us, and especially as in the investigations of
proteins we, as a rule, cannot decide whether we are dealing with a pure
or with a contaminated substance, namely, with mixtures. Experience
has shown that the solubility and precipitation properties of the proteins
are strongly influenced by the presence of other bodies, and under such
circumstances a proper classification, as demanded by science, is impos-
sible. On the other hand, a classification is important, and as the
ones used up to the present time" were based upon the solubility and
precipitation properties, we give the following schematic summary of the
chief groups of protein bodies:
I. Simple Proteins.
A. True Albuminous Bodies or Proteids.
Albumins,
Globulins
Seralbumin,
Lactalbumin, and others.
Fibrinogen,
Serglobulins, and others.
Phosphoproteins (Nucleoalbu- I Ovovitellin,
mins) [ Casein, and others.
(Coagulated proteins.)
Histones.
(Protamines?)
1
Zeitschr. f. physiol. Chem., 41.
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