NITROGENOUS COMPOUNDS 79 



They are homogeneous chemical compounds of true proteins, 

 with a non-protein radicle called the prosthetic group. They 

 are classified as nucleo-proteins, chromo-proteins, and gluco- 

 proteins, according as the prosthetic group is a nucleic acid, a 

 chromogenic substance, or a carbohydrate. 



The nucleic acids, of which there are probably several, 

 contain phosphorus and nitrogen, but no sulphur, in the 

 molecule. The phospho-proteins previously described, e.g. 

 casein, contain no nucleic acid they are simple proteins 

 and should not be confused with the nucleo-proteins. The 

 dissociation products of nucleic acid are very complex. They 

 include phosphoric acid, pyrimidine and purine derivatives, 

 pentoses, and laevulinic acid. The probable composition of 

 the nucleic acids is represented by the empirical formula, 

 C 40^54Ni4O 27 P 4. They are readily soluble in hot water and 

 in alkalis. They are precipitated from solution by dilute 

 acids, but are soluble in excess. Alcohol, salts of heavy 

 metals, and the alkaloid reagents, also precipitate the nucleic 

 acids. 



Nucleic acids are never split off directly from the nucleo- 

 proteins. On dissociation these substances are resolved first 

 into histone and nuclein. The latter is an albumin-nucleate, 

 and splits up into albumin and nucleic acid on further resolu- 

 tion. This may be expressed, graphically, as follows : 



( Simple protein Histone 

 Nucleo-protein < XT , . (Albumin 



( Nuclem |Nucleicacid 



Nucleo-proteins were so called because they enter largely 

 into the composition of the cell nuclei. They are, therefore, 

 obtained from those tissues of plants and animals which have 

 a cellular structure. They are all markedly acid, are soluble 

 in water and salt solution, and more readily in alkalis. They 

 are precipitated by acids, but are soluble in excess. They are 

 coagulated by heat and by various reagents like the simple 

 proteins, but the nucleic acid takes no part in the change, and 

 may be separated from the coagulum. Nucleo-proteins give 

 the same precipitation and colour reactions as the simple 



