I2 4 



PHYSIOLOGICAL CHEMISTRY 



tain a hexose group. Both types contain the purine bases, guanine and 

 adenine and the pyrimidine base cytosine. Plant nucleic acid contains 

 also the pyrimidine base uracil, which in the animal nucleic acid is sub- 

 stituted by the base thymine. The nucleic acids are not, however, 

 simple substances whose molecules contain a single phosphoric acid or 

 carbohydrate group. They are apparently combinations of several 

 radicals known as nucleotides each of which contains one carbohydrate 

 group combined with a single base and a single phosphoric acid 

 molecule. Thus the following structural formula has been sug- 

 gested for yeast nucleic acid by Jones and Read 1 indicating that it 

 contains four nucleotide radicals linked through the carbohydrate 

 groups. Levene 2 does not consider this linkage through carbohydrate 

 as established. All agree that the compound is a tetranucleotide. 



HO 



\ 



HO 



Adenine group 



O 



HO 



\ 



/ 



HO 



Uracil group 



O 



HO 



\ 



= P - 0- C 5 H 6 O- C 4 H 4 N 3 O 



Cytosine group 



HO 





 HO 



= P- 0- C 5 H 7 2 - C 5 H 4 N 6 



/ Guanine group 



HO 



Yeast nucleic acid (tetranucleotide) 



The cleavage of the nucleic acid molecule into its corresponding 

 nucleotides is brought about during digestion by enzymes present in the 

 intestinal juice and intestinal mucosa. Enzymes of similar origin 

 act further on the nucleotides thus formed and split off the phosphoric 

 acid radicals together with carbohydrate-base compounds which are 

 called nucleosides. The decomposition prior to absorption does not 

 probably proceed further than to the formation of nucleotides and 



1 Jones and Read: Jour. BioL Chem., 31, in, 1917. 



2 Levene: Jour. BioL Chem., 31, 591, 1917. 



