PROTEINS 93 



CLASSIFICATION OF PROTEINS ADOPTED BY THE AMERI- 

 CAN PHYSIOLOGICAL SOCIETY AND THE AMERICAN 

 SOCIETY OF BIOLOGICAL CHEMISTS 



I. SIMPLE PROTEINS 



Protein substances which yield only a-amino acids or their deriva- 

 tives on hydrolysis. 



(a) Albumins. Soluble in pure water and coagulable by heat, 

 e.g., ovalbumin from egg white, serum albumin from blood serum, 

 lactalbumin from milk, vegetable albumins. 



(b) Globulins. Insoluble in pure water but soluble in neutral 

 solutions of salts of strong bases with strong acids, 1 e.g., serum globulin, 

 ow globulin from egg yolk, edestin from hemp seed, amandin from almond 

 and peach kernel, and other vegetable globulins. 



(c) Glutelins. Simple proteins insoluble in all neutral solvents, but 

 readily soluble in very dilute acids and alkalis, 2 e.g., glutenin from wheat. 



(d) Alcohol-soluble Proteins (Prolamins). 3 Simple proteins solu- 

 ble in 70-80 per cent alcohol, insoluble in water, absolute alcohol, and 

 other neutral solvents,* e.g., zein from corn, gliadin from wheat and 

 rye, hordein from barley, and bynin from malt. 



(e) Albuminoids. Simple proteins possessing a similar structure to 

 those already mentioned, but characterized by a pronounced insolubility 

 in all neutral solvents, 5 e.g., elastin from ligament, collagen from tendon, 

 keratin from horn and hoof. 



(f) Histones. Soluble in water and insoluble in very dilute ammo- 

 nia, and, in the absence of ammonium salts, insoluble even in excess of 

 ammonia; yield precipitates with solutions of other proteins andacoagu- 

 lum on heating which is easily soluble in very dilute acids. On hydroly- 

 sis they yield a large number of amino acids among which the basic 

 ones predominate. In short, histones are basic proteins which stand 

 between protamines and true proteins, e.g., globin from hemoglobin, 

 scombrone from mackerel sperm, thymus his tone. 



(g) Protamines. Simpler polypep tides than the proteins included 

 in the preceding groups. They are soluble in water, uncoagulable by 



1 The precipitation limits with ammonium sulphate should not be made a basis for dis- 

 tinguishing the albumins from the globulins. 



2 Such substances occur in abundance in the seeds of cereals and doubtless represent a 

 well-defined natural group of simple proteins. 



3 The name prolamins has been suggested for these alcohol-soluble proteins by Dr. 

 Thomas B. Osborne (Science, 1908, xxviii, p. 417). It is a very fitting term inasmuch as 

 upon hydrolysis they yield particularly large amounts of proline and^ammonia. 



4 The subclasses defined (a, 6, c, d y } are exemplified by proteins obtained from both 

 plants and animals. The use of appropriate prefixes will sufi&ce to indicate the origin of 

 the compounds, e.g., owglobulin, /acJalbumin, etc. 



6 These form the principal organic constituents of the skeletal structure of animals and 

 also their external covering and its appendages. This definition does not provide for 

 gelatin which is, however, an artificial derivative of collagen. 



