NUCLEIC ACIDS AND NUCLEOPROTEINS 125 



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 car- 

 bohydrate group combined with a single base and a single phosphoric 

 acid molecule. Thus the following structural formula has been given 

 to yeast nucleic acid by Levene and Jacobs 1 indicating that it contains 

 four nucleotide radicals and may hence be called a tetranucleotide. 



HO 



/ Guanine group 



o 



\ 



0=POC 5 H S 03-C 5 H 4 N5 



/ Adenine group 



O 

 O = PO-C 5 H 8 O3-C4H 3 N2O2 



/ Uracil group 



O 



\ 



= PO-C 5 H 8 03-C 4 H4N30 



/ Cytosine 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 nucleosides. Many tissues however contain enzymes capable of 

 completing the decomposition with liberation of the carbohydrate 

 and basic radicals. The purine bases may also be deaminized while 

 still in combination as nucleosides and further hydrolysis would then 

 lead to the direct liberation of the oxypurines instead of their precursors, 

 the amino-purines. 



1 Levene and Jacobs: Ber. d. dentsch. Chcm. Ges., 43, 3151, 1910; 44, 1027, 1911 



