250 Biological Chemistry. 



protein molecule is nucleic acid. This product can, as a 

 rule, be isolated in a purer condition than the nucleo- 

 protein from which it is derived. The substance itself is 

 relatively stable towards alkalis, although it is readily 

 decomposed by acids. It is a product of great biological 

 significance, and as it is found invariably combined with 

 proteins its chemical nature is best discussed in this place. 



Various methods for the preparation of nucleic acid 

 have been employed. The method originally suggested by 

 Neumann is that generally adopted, and consists in extract- 

 ing tissues with hot solutions of sodium hydroxide which 

 contains sodium acetate. The extract is then neutralized 

 with acetic acid and thrown into a large amount of alcohol, 

 when the nucleic acid is precipitated in the form of its 

 sodium salt.* 



Nucleic acid is readily hydrolyzed by acids, and yields 

 the following products: (1) The alloxuric bases, guanine, 

 adenine, xanthine, and hypoxanthine ; (2) the pyrimidine 

 substances, cytosine, uracil, and thy mine ; (3) carbo- 

 hydrates, or derivatives produced from those substances by 

 the action of acids, such as formic and Isevulinic acids; 

 (4) phosphoric acid. 



According to the more recent investigations of Levene, 

 Jacobs, and their collaborators, nucleic acid is built up by 

 the condensation of a number of complexes called " nucleo- 

 tides," which are formed by the condensation of phosphoric 

 acid with a carbohydrate and a base such as guanine. 

 Nucleic acid is, accordingly, a " polynucleotide." From 

 the nucleotides, furthermore, phosphoric acid can be split 

 off, leaving a residue consisting of a sugar combined with 

 an alloxuric or pyrimidine group, and known as a nucleo- 



* The plant nucleic acids appear to be much less stable towards alkalis and 

 more stable towards acids than do the animal nucleic acids. A method for 

 their preparation has been recently elaborated by Clarke and Schryver. 



