124 PHYSIOLOGICAL CHEMISTRY 



NUCLEOPROTEIN 



(gastric digestion) 



Protein Nuclein 



(pancreatic digestion) 



Protein Nucleic Acid 



The Nucleic Acids. The nucleic acids of the animal body occur 

 mainly in combination with protein material in the so-called nucleo- 

 proteins of which they form the characteristic radicals (see page 123). 

 The amount and character of the protein with which the nucleic acid 

 molecule is combined varies and the acid may in certain cases be found 

 in cells in a free form. Naturally those tissues are richest in nucleic 

 acid which contain the largest amount of nuclear material and of 

 nucleoprotein. Such are the glandular tissues of the body as the 

 thymus, spleen, pancreas, liver, etc. The heads of the spermatozoa 

 consist almost entirely of nucleic acid in combination with protamine. 



The nucleic acids are a distinct class of substances, characterized 

 by their decomposition products. They are strongly acid in reaction 

 and contain considerable phosphorus. They may be divided into two 

 main groups, the animal and the plant nucleic acids. The two 

 classes differ in certain respects but all of the true animal nucleic acids 

 appear to be practically identical in composition. Animal nucleic 

 acid is most readily prepared from the thymus while plant nucleic 

 acid is most readily obtained from yeast. 



The nucleic acids are difficultly soluble in cold water, more readily 

 in hot water, insoluble in alcohol, but readily soluble in weak alkali 

 with the formation of the alkali salt. If pure they do not give the 

 protein color reactions. They are optically active. They are pre- 

 cipitated from their alkaline solutions by HC1, but only the plant nu- 

 cleic acid is precipitated by acetic acid. In weak acid solution they are 

 precipitated by protein the combination being considered a "nuclein." 

 They form insoluble salts with alkaline earth and heavy metals. 

 The sodium salt of animal nucleic acid in 4 per cent solution is liquid 

 while warm but solidifies to a gelatinous mass on cooling. Plant 

 nucleic acid does not do this. 



The nucleic acids on hydrolysis yield phosphoric acid, purine and 

 pyrimidine bases, and a carbohydrate or carbohydrate derivative. The 

 composition varies slightly with the type of nucleic acid. Plant nucleic 

 acids contain a pentose group while animal nucleic acids contain a 



