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170 THE PKOTEIN SUBSTANCES.
Muller obtained 35 per cent glucosamine from mucous-membrane
mucin and 23.5 per cent from the submaxillary mucin.
On boiling mucin with dilute mineral acids, acid albuminate and
bodies similar to proteoses are obtained, besides a reducing substance
which is not free glucosamine (Steudel 1
). By the action of strong
acids upon mucins or mucoids Otori 2
obtained several of the cleavage
products of the proteins, such as leucine, tyrosine, glycocoll, glutamic
acid, oxalic acid, guanidine, arginine, lysine, and humus substances,
and also carbohydrate cleavage products, such as levulinic acid. Cer-
tain mucins, as the submaxillary mucin, are easily changed by very
dilute alkalies, as lime-water, while others, such as tendon-mucin, are
not affected. If a strong caustic-alkali solution, such as a 5-per cent
KOH solution, is allowed to act on submaxillary mucin, we obtain alkali
albuminate, bodies similar to proteoses and peptones and one or more
substances of an acid reaction which have strong reducing powers.
On peptic digestion proteoses and peptone-like bodies, still con-
taining the carbohydrate group, are produced. On tryptic digestion
still simpler cleavage products are formed, namely, leucine, tyrosine,
and tryptophane (Posner and Gies 3
). The glucosamine, so far as we
know, is not split off by proteolytic enzymes, but only after strong
hydrolysis with acids.
In one or another respect the various mucins act somewhat dissimilarly.
For example, the snail and sputum mucins are insoluble in dilute hydro-
chloric acid of 1-2 p. m., while the mucin of the submaxillary gland and
the navel-cord is soluble. The former become flaky with acetic acid,
while the submaxillary mucin is precipitated in more or less fibrous,
tough masses. Still all the mucins have certain reactions in common.
In the dry state mucin forms a white or yellowish-gray powder. When
moist it forms, on the contrary, flakes or yellowish-white tough lumps
or masses. The mucins are acid in reaction. They give the color reac-
tions of the proteins. They are not soluble in water, but may give a
neutral solution with water with the aid of small amounts of alkali. Such
a solution does not coagulate on boiling, but acetic acid gives at the
normal temperature a precipitate which is nearly insoluble in an excess
of the precipitant. If 5-10 per cent NaCl be added to a mucin solution,
it can be carefully acidified with acetic acid without giving a pre-
cipitate. Such acidified solutions are copiously precipitated by tan-
nic acid; with potassium ferrocyanide they give no precipitate, but on
sufficient concentration they become thick or viscous. A neutral solu-
tion of alkali mucin is precipitated by alcohol in the presence of neutral
1
Zeitschr. f. physiol. Chem., 34.
2
Ibid., 42 and 43.
’’
Amer. Journ. of Physiol., 11.
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