THE COMPONENTS OF PLANTS AND ANIMALS 35 



which seem always to contain both purin and pyrimidin nucleo- 

 sids. 



54. Glycoproteins. The glycoproteins are denned as " Com- 

 pounds of the protein molecule with a substance or substances 

 containing a carbohydrate group other than a nucleic acid. 

 The principal compounds of this group are the mucins and the 

 mucoids." 



55. Phosphoproteins. These are denned as compounds of 

 the protein molecule with some, as yet undefined, phosphorus- 

 containing substance other than a nucleic acid or lecithin. 

 The casein, or rather caseinogen, of milk is one of the most 

 familiar and important of this group. 



56. Haemoglobins. The haemoglobins are compounds of 

 the protein molecule with hsematin or some similar substance, 

 and constitute the red coloring matter of the blood. 



57. Lecithoproteins. Compounds of the protein molecule 

 with lecithins. 



The derived proteins 



58. Primary protein derivatives. Derivatives of protein ap- 

 parently formed through hydrolytic changes which involve only slight 

 alterations of the molecule. 



Proteans. Insoluble products which apparently result from the 

 incipient action of water, very dilute acids or enzyms. 



Metaproteins. Products of the further action of acids and alkalies 

 whereby the molecule is so far altered as to form products soluble in 

 very weak acids and alkalies but insoluble in neutral fluids. This 

 group will thus include the familiar "acid proteins" and " alkali pro- 

 teins," not the salts of proteins with acids. 



Coagulated proteins. Insoluble products which result from (i) the 

 action of heat on their solutions, or (2) the action of alcohols on the 

 protein. 



59. Secondary protein derivatives. Products of the further 

 hydrolytic cleavage of the protein molecule. 



Proteases. Soluble in water, uncoagulated by heat, and pre- 

 cipitated by saturating their solutions with ammonium or zinc 

 sulphate. 



Peptones. Soluble in water, uncoagulated by heat but not pre- 

 cipitated by saturating their solutions with ammonium sulphate. 



Peptids. Definitely characterized combinations of two or more 

 amino acids, the carboxyl group of one being united with the ammo 

 group of the other with the elimination of a molecule of water (48). 



