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PEPSIN DIGESTION. 473
bones, from which last the acid dissolves only the inorganic substances,
is converted into gelatin by digesting with gastric juice. The gelatin is
further changed so that it loses its property of gelatinizing and is con-
verted into gelatoses and peptone (see page 120). True mucin (from the
submaxillary) is dissolved by the gastric juice, yielding substances similar
to peptone, and a reducing substance similar to that obtained by boil-
ing with a mineral acid. • Mucoids from tendons, cartilage, and bones
dissolve, according to Posner and Gies,1
in pepsin-hydrochloric acid,
but leave a residue which amounts to about 10 per cent of the original
material and which, as it seems, consists in great part, if not entirely,
of a combination of proteid with glucothionic acid (Chapters VI and
VII). The solution contains primary and secondary mucoproteoses
and mucopeptones. The former contain glucothionic acid, but the latter
do not. Elastin is dissolved more slowly and yields the previously men-
tioned substances (page 117). Keratin and the epidermal formations
are insoluble. The nucleins are dissolved with difficulty, and the cell
nuclei, therefore, remain in great part undissolved in the gastric juice.
According to London 2
and his collaborates the nucleic acids are not
attacked in the stomach. The animal cell-membrane is, as a rule, more
easily dissolved the nearer it stands to elastin, and it dissolves with
greater difficulty the more closely it is related to keratin. The mem-
brane of the plant-cell is not dissolved. Oxyhemoglobin is changed into
hsematin and protein, the latter undergoing further digestion. It is
for this reason that blood is changed into a dark-brown mass in the
stomach. The gastric juice does not act upon fat, but, on the contrary,
dissolves the cell-membrane of fatty tissue, setting the fat free. Gastric
juice has no action on starch or the simple varieties of sugar. The
statements in regard to the ability of gastric juice to invert cane-sugar
are very contradictory. At least this action of the gastric juice is not
constant, and if it is present at all, it is probably due to the action of the
acid.
Pepsin alone, as above stated, has no action on proteins, and an acid of the
intensity of the gastric juice can only very slowly, if at all, dissolve coagulated
albumin at the temperature of the body. Pepsin and acid together not only
act more quickly, but qualitatively they act otherwise than the acid alone, at
least upon dissolved protein. This has led to the assumption of the presence of
a pepsin-hydrochloric acid whose existence and action are only hypothetical.
As pepsin digestion, it seems, yields finally the same products as the hydrolytic
cleavage with acids, we can say for the present only that this enzyme acts like
other catalysts in very powerfully accelerating a process which would also pro-
ceed without the catalvte.
1
Amer. Journ. of Physiol., 11.
* Zeitschr. f. physiol. Chem. 70, 72.
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