92 THE CHEMICAL COMPOSITION OF THE BODY 



Conjugated proteins consist of a protein molecule united with some 

 other molecule or molecules otherwise than as a salt. There are nucleo- 

 proteins, glycoproteins, phosphoproteins, hemoglobins or chromoproteins and 

 lecithoproteins five classes of conjugated proteins. 



Nucleoproteins. Nucleoproteins contain phosphorus and in most 

 instances iron. They are combinations of simple proteins with a substance 

 known as nucleic acid. On boiling with strong acids they undergo hydroly- 

 tic cleavage, yielding the ordinary protein cleavage products from the pro- 

 tein in the combination, and purine and pyrimidine bases, carbohydrates 

 and phosphoric acid from the nucleic acid moiety. Nucleoproteins are 

 differentiated from the phosphoproteins in that the latter do not yield purine 

 and pyrimidine bases on hydrolytic cleavage. Nucleoproteins are the es- 

 sential organic constituents of cell nuclei. They go into solution when the 

 tissues are extracted with cold water or dilute salt solution. They are pre- 

 cipitated from these extracts by careful acidification and dissolved if an 

 excess of acid is added. The solutions of nucleoproteins coagulate on 

 heating. They give the color reactions of proteins. 



By boiling nucleoproteins with water or very dilute acetic acid, some 

 of the protein is split off. There result substances which are precipitated 

 by very dilute acids. These bodies are known as /?- nucleoproteins and 

 have a smaller carbon and higher phosphorus content than the original nu- 

 cleoprotein. On digestion of the original nucleoprotein or of the /?-nucleo- 

 protein with pepsin hydrochloric acid, a precipitate of nuclein is obtained. 

 On further digestion with pepsin hydrochloric acid, or with trypsin, or on 

 cleavage with acids and alkalies, there is a further splitting away of the 

 protein, and substances are formed known as nucleic acids. The structure 

 and cleavage of the nucleoprotein is indicated in the following diagram. 



Nucleoprotein 



on boiling with water or on digestion 

 with pepsin hydrochloric acid yields 



Nuclein Protein 



On long boiling with water, i 

 per cent, hydrochloric acid, or 

 dilute alkalies gives 



i i 



Nucleic acid Protein 



Nucleic acids are not merely known as cleavage products of nucleopro- 

 tein, but occur preformed in the cell nuclei. A special group of nucleic 

 acids are bound with protamines to form the principal constituent of the 

 spermatozoa. 



