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102 THE PROTEIN SUBSTANCES.
mine the quantity of acetic acid necessary- to completely precipitate
the proteids in small measured portions of the neutralized liquid which
have previously been heated on the water-bath, so that the nitrate does
nut respond to Heller’t test. Now warm a larger weighed or meas-
ured quantity of the liquid on the water-bath, and add gradually the
required quantity of acetic acid, with constant stirring, and continue
heating for some time. Filter, wash with water, extract with alcohol
and then with ether, dry, weigh, incinerate, and weigh again. With
proper work the filtrate should not give Heller’s test. This method
serves in most cases, and especially so in cases where other bodies are
to be quantitatively estimated in the filtrate.
In many cases good results may be obtained by precipitating all the
proteid with tannic acid and determining the nitrogen in the washed
precipitate by means of Kjeldahl’s method. On multiplying the quan-
tity of nitrogen found by 6.25 we obtain the quantity of proteid. Many
other methods for the quantitative estimation of proteins have been
suggested.
The removal of proteids from a solution may in most cases be per-
formed by boiling with acetic acid. Small amounts of proteid which
remain in the filtrates may be separated by boiling with freshly pre-
cipitated lead carbonate or with ferric acetate, as described by Hof-
meister.1
. If the liquid cannot be boiled, the proteid may be precipi-
tated by the very careful addition of lead acetate, or by the addition
of alcohol. If the liquid contains substances which are precipitated
by alcohol, such as glycogen, then the proteid may be removed by tri-
chloracetic acid as suggested by Obermayer and Frankel.2 Recently
Michaelis and Rona have suggested a method for the removal of proteids
by using kaolin, colloidal ferric hydrate or a mastic emulsion. The
principle of these methods has already been given on page 97 and in
regard to the practical execution of the method we refer to the works
there cited.
In the precipitation of proteid as well as the quantitative estimation by means
of heat, it must be borne in mind, as shown by Spiro, 3
that several nitrogenous
substances, such as piperidine, pyridine, urea, etc., disturb the coagulation of the
proteids.
Synopsis of the Most Important Properties of the Different Groups of
Albuminous Bodies.
As it is not possible to base the classification of the different proteid
groups according to their constitution, we are obliged to make use of
their different solubilities and precipitation properties in their general
characterization. As there exist no sharp differences between the various
groups in this regard it is impossible to draw a sharp line between them.
Albumins. These bodies are soluble in water in neutral reaction and
are not precipitated by the addition of a little acid or alkali. They are
1
Zeitschr. f. physiol. Chem., 2 and 4.
2
Oberrnayer, Wien. med. Jahrb., 1888; Frankel, Pfluger’s Arch., 52 and 55.
1
Zeitschr. f . physiol. Chem., 30.
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