CLASSIFICATION OF THE PROTEINS 87 



All of the above are coagulable by heat. 



d. Prolamins or Alcohol- soluble Proteins. Soluble in 70 to 80 per cent, 

 alcohol; insoluble in water, absolute alcohol, and other neutral solvents; e.g., 

 zein from corn, gliadin from wheat, and hordein from barley. 



e. Albuminoids. Simple proteins characterized by a pronounced in- 

 solubility in all neutral solvents. These form the principal organic con- 

 stituents of the connective tissues of animals including their external covering 

 and its appendages. Examples: elastin, collagen, and keratin. 



The above sub-classes are characterized by physical rather than by 

 chemical differences. When the protein, for instance, is termed a globulin 

 it means that it is a typical simple protein with certain characteristic solu- 

 bilities. Proteins intermediate in character between albumins and globulins 

 are met with, and the use of these terms as a hard-and-fast classification 

 has led to considerable confusion. 



/. Histones. On hydrolysis these yield a large number of amino acids, 

 among which the basic ones predominate. The histones stand chemically 

 between the typical simple proteins and the following group of protamins. 

 Examples are: globin, thymus histone, scombrone. 



g. Protamines. Simpler polypeptids than the proteins included in the 

 preceding groups. They yield comparatively few amino acids, among which 

 the basic ones predominate. They are the simplest natural proteins. 

 Examples are: salmin, sturine, clupeine, and scombrine. 



II. Conjugated Proteins. Substances which contain the protein mole- 

 cule united to some other molecule or molecules otherwise than as a salt. 



a. Nucleoproteins. Compounds of one or more protein molecules with 

 nucleic acid; e.g., nucleohistone. 



b. Glycoproteins. Compounds of the protein molecule with a substance 

 or substances containing a carbohydrate group other than a nucleic acid; 

 e.g., mucins and mucoids. 



c. Phosphoproteins. Compounds of the protein molecule with phos- 

 phorous containing substances other than a nucleic acid or lecithin; e.g., 

 casein, ovovitellin. 



d. Hemoglobins. Compounds of a protein molecule with hematin or 

 some similar substance. These include the respiratory pigments; e.g., 

 hemoglobin and hemocyanin. 



e. Lecithoproteins. Compounds of the protein molecule with the lipoid 

 lecithin; e.g., lecithans, phosphatides. 



III. Derived Proteins. Class i. Primary Protein Derivatives. 

 Derivatives of the protein molecule apparently formed by hydrolytic changes 

 which involve only slight alteration of the protein molecule. 



/. Proteans. Insoluble products which apparently result from the in- 

 cipient action of water, from dilute acids or enzymes; e.g., myosan, edestan. 

 g. Metaproteins. Products of the further action of acids and alkalies 



