86 PHYSIOLOGICAL CHEMISTRY. 



CLASSIFICATION OF PROTEINS ADOPTED BY 

 THE AMERICAN PHYSIOLOGICAL SOCIETY 

 AND THE AMERICAN SOCIETY OF 

 BIOLOGICAL CHEMISTS. 



I. SIMPLE PROTEINS. 



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

 rivatives on hydrolysis. 



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

 e. g., ovalbumin, serum albumin, lactalbumin, 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, ovoglobulin, edestin, amandin and other vegetable globu- 

 lins. 



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

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



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

 soluble in 70-80 per cent alcohol, insoluble in water, absolute alcohol 

 and other neutral solvents, 4 e. g., zein, gliadin, hordein and bynin. 



(e) Albuminoids. Simple proteins possessing a similar struc- 

 ture to those already mentioned, but characterized by a pronounced 

 insolubility in all neutral solvents, 5 e. g., elastin, collagen, keratin. 



(/) Histones. Soluble in water and insoluble in very dilute 

 ammonia, and, in the absence of ammonium salts, insoluble even in 

 excess of ammonia; yield precipitates with solutions of other pro- 

 teins and a coagulum on heating which is easily soluble in very 

 dilute acids. On hydrolysis 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 pro- 

 teins, e. g.j globin, thymus histone, scombrone. 



'The precipitation limits with ammonium sulphate should not be made a basis 

 for distinguishing 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. 



"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. 



* The subclasses defined (a, b, c, dj) are exemplified by proteins obtained from 

 both plants and animals. The use of appropriate prefixes will suffice to indicate 

 the origin of the compounds, e. g., ovoglobulin, lactalbumin, etc. 



5 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. 





