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90 THE PROTEIN SUBSTANCES.
certain conclusions as to the structure of the peptides by comparing the hydro-
lytic products before and after deamidation.
A comparison of the artificially prepared polypeptides with the pro-
teins, and especially with the cleavage products of these last, the so-called
preoteoses and peptones, is of great interest in several respects, especially
in connection with certain reactions. For instance there are several
polypeptides which give the biuret reaction which is characteristic of
the proteins in general, and also several (polypeptides containing tyrosine),
which give Millon’s reaction (see further on). The above-mentioned
octadecapeptide is precipitated by phosphotungstic acid, tannin and
ammonium sulphate ; we also know tri- and pentapeptides containing
tyrosine, which are very similar in properties to the proteoses.
The behavior of the polypeptides with proteolytic enzymes is of
great interest. As this interesting question will be thoroughly treated
in other chapters (I and VIII) it is sufficient here to recall that the
possibility that polypeptides as well as proteins are hydrolyzed by the
same enzymes, yielding amino-acids, is a weighty proof of the prob-
ability that in the proteins the amino-acid chains are of the same kind
as in the polypeptides.
A very important support for such a view is found in the occurrence
of polypeptides among the cleavage products of proteins, a find which
to a certain extent forms the reverse of the above-mentioned syntheses.
Such polypeptides are chiefly di- but also tri- and tetrapeptides. They
have been obtained in the hydrolytic products of silk waste, silk fibroin
and elastin (Fischer, Abderhalden), gelatin (Levene, Wallace and
Beatty) and of gliadin (Osborne and Clapp) 1
. Of special interest in
this connection are those polypeptides which like glycyl-d-alanine,
d-alanyl-glycine, glycyl-Z-tyrosine, Z-prolyl-Z-phenylalanine and d-alanyl-
glyeyl-Z-tyrosine, are identical with the corresponding synthetically pre-
pared polypeptides or at least very closely related.
We have therefore conclusive reasons for the assumption that in the
proteins, peptide bindings chiefly occur, i.e., a combination of the a-
amino-acids by means of the imide binding. It is also possible that
other linking may occur, and Fischer has also given expression to such
a possibility. Besides the above-mentioned imide binding another kind
must also without doubt exist in the proteins, namely, the anchoring
of the urea-forming group (the guanidine residue) with the ornithin
(diamino-valeric acid) by the imide binding. This imide linking is
1
Fischer and Abderhalden, Sitz. Ber. d. d. Berl. Akad. d. Wissensch, 30, and Ber.
d. d. Chem. Gesellsch., 89, 40; Abderhalden, Zeitschr. f. physiol. Chern., 02, 03 and 72;
Levene and Wallace, ibid., 47, with Beatty, Ber. d. d. Chem. Gesellsch., 39, and Bioch.
Zcitsrhr., 4; Osborne and Clapp, Amer. Journ. of Physiol., 18.
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