417 



Nucleic acid is then a dehydrolyzed product of phosphoric acid, car- 

 bohydrate and four nitrogenous ring compounds. These eight substances 

 are the fundamental groups of nucleic acids (Jones). The carbohydrate 

 group in animal nucleic acid is represented by the decomposition product 

 levulinic acid. This probably indicates a hexose precursor, but the par- 

 ticular hexose has not been discovered. Feulgen, however, claims that the 

 carbohydrate group is not hexose (C 6 H 12 O 6 ), but glucal (C 6 H 10 O 4 ). 

 Plant nucleic acid contains always uracil and pentose, while animal nucleic 

 acid contains always thymin and a carbohydrate .other than pentose. It 

 is generally stated to contain hexose. The pentose in plant nucleic acid 

 was found by Levene to be d-ribose. 



Levene (&) (fr) (c) has shown by his brilliant investigations that nucleic 

 acid belongs to a class of substances to which he has given the name of 

 "nucleotids." They are compounds in which a carbohydrate group links 

 a phosphoric acid group with a purin or pyrimidin group. 



Two mononucleotids have been obtained from animal tissues. These 

 have long been known in physiological chemistry by the names of inosinic 

 acid and guanylic acid. Inosinic acid was obtained from meat extract 

 by Liebig in 1847. Levene and Jacobs found that it was composed of 

 phosphoric acid and hypoxanthin united by d-ribose. It is then hypo- 

 xanthin nucleotid. Guanylic acid was discovered by Hammarsten in 

 1894. It was the non-protein constituent of a substance obtained from the 

 *pancreas and called P-nucleoprotein. Levene and Jacobs in 1909 proved 

 that guanylic acid was composed of guanin united to phosphoric acid by 

 the pentose d-ribose. Guanin nucleotid prepared from yeast by Jones 

 and his co-workers did not differ from guanylic acid in any respect. 



From the structural point of view these two mononucleotids may be 

 regarded as simple nucleic acids. "Their groups are those of plant nucleic 

 acid, and they cannot therefore be directly derived from the nucleic acid 

 of the animal tissues in which they occur" (Jones(<i)). 



Guanylic acid has been prepared from yeast nucleic acid by Jones 

 and Richards. Probably both inosinic acid and guanylic acid found in 

 animal tissues are formed from the nucleic acid of plants taken as food. 

 Levene and Jacobs split the mononucleotids, guanylic acid and inosinic 

 acids, by neutral hydrolysis under pressure into phosphoric acid and a 

 compound, consisting of a pentose joined to a purin base. To this com- 

 pound Levene gave the name "nucleosid." 



HO X 

 0=PO C 5 H 8 3 C 5 H 4 N 5 + H 2 =H 3 P0 4 -f C 5 H 9 O 4 .C 5 H 4 N 6 



jro/ 



guanylic acid guanosin 



Emil Fischer (&) (d} has reported the synthetic production of a nucleo- 

 tid composed of the purin group theophyllin united by a carbohydrate 



