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178 THE PROTEIN SUBSTANCES.
statements as to the occurrence of more than two purine bases in a nucleic
acid are not correct and depend upon the fact that the two purine bases
xanthine and hypoxanthine can be secondarily formed from guanine and
adenine. There is no doubt that the most thoroughly studied nucleic
acids, such as the thvmus-nucleic acids, the closely related or perhaps
identical acids of the salmon sperm (salmo-nucleic acid), of the herring
sperm and burbot sperm, and of the pancreas, do not contain more than
two purine bases, namely, guanine and adenine.
Of the known nucleic acids we have two, the guanylic acid and inosinic
acid, which contain only one purine base, namely, guanine and hypoxan-
thine, respectively. These two acids do not contain any pyrimidine
bases, which are found thus far in all carefully investigated nucleic acids.
The occurrence of pyrimidine bases is somewhat different in the various
nucleic acids. In one group of animal nucleic acids (thymonucleic acids)
thymine, cytosine and uracil are found, the uracil being produced second-
arily from the cytosine. The plant nucleic acids (the triticonucleic acid
and the yeast nucleic acid, which may perhaps be identical with it) do
not contain any thymine and yields as nitrogenous cleavage products
besides the two purine bases only cytosine and uracil.
All nucleic acids, as above stated, contain a carbohydrate group.
In the plant nucleic acids and in two animal ones, the guanylic and inosinic
acids, the carbohydrate is a pentose. In the remaining animal nucleic
acids it is on the contrary a hexose or at least a hexacarbohydrate.
The nature of this hexacarbohydrate has not been determined and
the nature of the pentoses occurring in the nucleic acids is also a disputed
point. Based upon the investigations of Neuberg we have considered
the pentose of guanylic acid and of inosinic acid as /-xylose. The correct-
ness of this view is disputed by others. According to Levene and Jacobs
the pentose of all nucleic acids containing pentose, is d-ribose. Haiser
and Wenzel who for a time considered the pentose of inosinic acid as
’/-xylose are now of the view that it is probably d-ribose. The view of
Levene and .Jacobs, that the pentose of the guanylic acid is d-ribose has
received important support by the investigations of Schulze and Trier
on the identity of the plant guaninpentoside vcrnine with the guanin-
pentoside (see below) prepared by Levene and Jacobs. Still we have
no explanation why Neuberg and Rewald l
obtained only /-xylose
from the pancreas on the hydrolysis of the entire organ, and Levene
and Jacobs on the contrary only r/-ribose.
1
Neuberg and Brahn, Bioch. Zeitschr., 5; see also Ber. d. d. chem. Gesellsch
41 and 42: Levene and Jacobs, Ber. d. d. chem. Gesellsch., 42 and 43; Haiser and
Wenzel, Monatah. f. Chern., 61; Schulze and Trier, Zeitschr. f. physiol. Chem., 70;
lid, Ber. d. d. chem. Gesellsch., 42.
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