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HYDROLYSES OF PROTEINS. 81
form in CONH; so according to Treat B. Johnson l
two analogous forms
of sulphur bondage exist in the proteins, namely the mercaptan form
SH as in cystine and the form NH.CH.CS.NH corresponding to the oxygen
binding in the polypeptids (see page 86). He has in fact also prepared
thio-polypeptides from glycocoll and these were analogous to the corre-
sponding glycin polypeptids (see page 88) and like certain proteins
gave HoS on acid hydrolysis.
The constitution of the protein bodies is still unknown, although
the great advances made in the last few years have brought us very much
closer to the elucidation of the question. In studying the constitution
of the protein bodies they have been broken up in various ways into
simpler portions, and the methods used for this purpose have been of
different kinds. In such decompositions, for which only purified proteins
are to be used, first large atomic complexes—proteoses and peptones
—
are obtained which still have protein characteristics, and these then
suffer further decomposition until finally we obtain simpler, generally
crystalline, or at least well-characterized, end products.
As to the products obtained by hydrolytic cleavage with mineral acids,
important investigations have been carried out by numerous older and
more recent experimenters.2
Besides certain acids, which will be men-
tioned later and which occur in few cases only, we obtain the following:
monamino-acids such as glycocoll, alanine, aminovaleric acid, leu-
cine, isoleucine, serine, aspartic and glutamic acids, cysteine and its disul-
phide cystine, phenylalanine, tyrosine, pyrollidine—and oxypyrollidine
carboxylic acid, tryptophane and also the three hexone bases, histidine,
arginine and lysine, the two latter being diamino-acids. Besides these
also ammonia, sulphureted hydrogen, ethyl sulphide and melanoidins,
which latter seem to be secondary products, have been obtained.
On the hydrolysis with alkalies we obtain, after a preliminary forma-
tion of intermediary steps which will be discussed later, chiefly the same
cleavage products as in acid hydrolysis but with the exception that in
the alkali hydrolysis a considerable part of the amino-acids become
racemerized and therefore appear in optically inactive form while in the
acid hydrolysis chiefly optically active acids are obtained. Because
of the action of the alkali a part may suffer further decomposition which
loads to the formation of simpler cleavage products and ammonia.
On fusing proteins with caustic alkali, ammonia, methyl mercaptan and other
volatile products are evolved and other products are produced such as leucine,
1
Journ. of biol. Chem., 9.
2
In regard to the literature see O. Cohnheim, Chemie der Eiweisskorper, Braun-
schweig, 1911, and F. Hofmeister, Ergebnisse der Physiologie, Jahrg. I, Abt. 1, 759,
1902; E. Fischer, Untersuchungen uber Aminosauren, Polypeptide und Proteine (1899-
1906), Berlin, 1906. See also special references.
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