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 thymus-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 Z-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 

 d-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 vernine with the guanin- 

 pentoside (see below) prepared by LEVENE and JACOBS. Still we have 

 no explanation why NEUBERG and REWALD 1 obtained only Z-xylose 

 from the pancreas on the hydrolysis of the entire organ, and LEVENE 

 and JACOBS on the contrary only d-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, Monatsh. f. Chem., 31; Schulze and Trier, Zeitschr. f. physiol. Chem., 70; 

 Rewald, Ber. d. d. chem. Gesellsch., 42. 



