THE PROTEINS 113 



that the imclein obtained from this source contained GO'S per cent, 

 nucleic acid and 35-56 protamine, and was in fact a nucleate of prota- 

 inine. The nuclein derived from the spermatozoa of echinoderms 

 has been found to be a compound of nucleic acid and hist one. From 

 organs rich in cells, such as the thymus and the pancreas, and from 

 nucleated red blood corpuscles, nucleoproteins may be obtained 

 which can be broken down into nuclein and protein, the nuclein again 

 being composed of a protein residue with nucleic acid. 



As first extracted from the animal cell the nucleoproteins are associated 

 with a considerable proportion of lecithin, and in this labile compound form the 

 ' tissue fibrinogen ' of Wooldridge. To prepare this substance an organ rich 

 in cells, such as the thymus, is minced and extracted with water or normal 

 salt solution, After separating the cells by means of the centrifuge, the clear 

 fluid is decanted off and acidified with acetic acid. A precipitate is produced 

 consisting of ' tissue fibrinogen.' This substance is soluble in excess of acid 

 and is easily soluble in alkalies. All the tissue fibrinogens are highly unstable 

 bodies and undergo changes in the mere act of precipitation and re-solution. 

 When injected into the blood they cause intravascular clotting. On digestion 

 with gastric juice they yield a precipitate of nuclein, and this precipitate contains 

 a large proportion of the lecithin present in the original substance. In the 

 nucleoproteins nucleic acid is combined with proteins in two degrees, a large 

 portion of the protein being separable by gastric digestion, while the remainder 

 needs stronger reagents for its dissociation. The relation of the two portions 

 of the nucleoprotein may be represented therefore by the following schema : 



Nucleo-protein 

 Protein Nuclein 



Protein Nucleic acid 



(generally histone 

 or protamine) 



Since we have already dealt with the chemical constitution of 

 the proteins, it remains only to discuss the nature of nucleic acid. 

 By various means, all of which involve hydrolysis, the nucleic acid 

 may be broken up into its proximate constituents. These differ 

 according to the source of the nucleic acid. Whatever the source, the 

 disintegration products belong to closely allied groups of substances. 

 These may be grouped as follows . 



(1) Phosphoric Acid. The proportion of phosphorus varies 

 within but narrow limits in the different nucleic acids, the average 

 being about 10 per cent. It is probable that the phosphoric acid 

 represents, so to speak, the combining medium for the groups contained 

 in the nucleic acid molecule, as is the case with the various groups 

 which make up the lecithin molecule. 



(2) The Purine Bases. Among the products of disintegration 

 of nucleic acid we find constantly one of the bases adenine, C 6 H 5 N' 5 , and 



