THE INDIVIDUAL GROUPS OF PROTEINS 465 



NUCLEOPROTEINS. 



The nucleoproteins are the constituents of cell nuclei and are conse- 

 quently widely distributed. A nucleoprotein has been prepared from 

 almost every organ, but only a few have been at all well investigated. 

 They are made up of varying amounts of protein and nucleic acid and 

 are best considered as salts of protein with nucleic acid in different pro- 

 portions, in the same way as the tribasic phosphoric acid can form 

 three series of salts with bases. The combination of the nucleic acid 

 and the protein is very unstable, thus favouring the idea that they are 

 salts. The two components are easily separated by the action of 

 alkali and the protein can be precipitated by alkaloidal reagents, leav- 

 ing the nucleic acid in solution. The substance in combination with 

 protein is termed a prosthetic group. The prosthetic group is nucleic 

 acid. 



Two varieties of nucleoprotein can be distinguished : 



a-Nucleoproteins. 



These are obtained when a tissue is treated with cold water, or cold 

 0-95 per cent, salt solution, and the milky solution after filtration 

 acidified with acetic acid. 



^-Nucleoproteins. 



/3-Nucleoproteins are obtained by boiling a tissue with water 

 so as to coagulate most of the proteins and adding acetic acid to the 

 clear filtrate. They contain less protein than the a-nucleoproteins 

 and the prosthetic group is different. The guanylic acid of the 

 pancreas is the best-known example of a y3-nucleoprotein. The 

 prosthetic group is a mononucleotide and contains a pentose in its 

 molecule ; it is consequently a constituent of a plant nucleic acid (p. 300). 



Jones considers that the /3-nucleoproteins are not constituents 

 of the cell nuclei, but that they are present in the contents of the 

 cell having entered the tissue with the food. 



Nucleins. 



Nucleoproteins on digestion with pepsin are not completely hydro- 

 lysed, i.e. the protein portion is not completely separated. A residue 

 remains which is insoluble in the pepsin solution and consists of pro- 

 tein combined with nucleic acid. This residue is termed nuclein. 

 Nucleins are the insoluble residues (excluding remainders of sclero- 

 proteins) which remain after tissues have been digested with pepsin. 

 These residues frequently contain iron. The nuclein remaining when 

 egg-yolk is digested with pepsin has been called haematogen on account 

 of its iron content. It arises from the cell nuclei of the germinal layer. 



Nucleoproteins and nucleins are insoluble in water and very dilute 

 acids, but they dissolve in dilute alkali and stronger acids. One of 

 their features is the presence of phosphorus and frequently the 



