NATIVE AND MODIFIED PROTEINS. 95 



tions in constant proportions, but are rather to be considered as loose 

 adsorption compounds of the proteid with the salt. 1 These reactions 

 are irreversible in so far that dilution with water or removal of the salt 

 by means of dialysis does not restore the unchanged proteid. On the 

 other hand the precipitate, at least in certain cases may be redissolved 

 in an excess of the salt solution or of the proteid solution, and in this 

 sense the process is a reversible one. 



The precipitation of proteids and also other soluble proteins by salts 

 stands in close relation to their colloidal nature, and in this connec- 

 tion we refer to what has been said in Chapter II. The proteids do not 

 as a rule diffuse through animal membranes, or only to a very slight 

 extent, and hence have in most cases a pronounced colloidal nature in 

 GRAHAM'S sense. They belong to the hydrophile colloids; their solutions 

 show properties in common with those of typical colloids and also true 

 solutions. Certain of them, especially the peptones and a few proteoses, 

 which will be discussed later, seem to occupy an intermediate position, 

 as their solutions are characterized by a lesser viscosity and greater 

 diffusibility and nitration ability, are not readily precipitable by alcohol 

 or coagulable by heat, and are only slightly precipitable by salts. 



The solutions (or suspensions) of proteids in water, the proteid hydro- 

 sols, are converted by various means into proteid hydrogels. Of these 

 means we must specially mention the following: nocking out with salts-, 

 precipitation with alcohol, gelatinization of a gelatin solution on cool- 

 ing, and coagulation by the action of enzymes or heat. 



Those proteids which occur, according to the common views, pre- 

 formed in the animal fluids and tissues, and which have been isolated 

 from these by indifferent chemical means without losing their original 

 properties, are called native proteids. New modifications having other 

 properties can be obtained from the native proteids by heating, by the 

 action of various chemical reagents such as acids, alkalies, alcohol, and 

 others, as well as by proteolytic enzymes. These new proteids are called 

 modified (" denaturierte ") proteids, to differentiate them from the 

 native proteids. 



The precipitation with alcohol is a reversible reaction, as the pre- 

 cipitate redissolves on subsequent dilution with water. The proteids 

 are changed by the action of alcohol, some readily and quickly, others 

 with difficulty and very slowly; the proteid then becomes insoluble in 

 water and is modified. 



On heating a solution of a native proteid it is modified at a different 

 temperature for each different proteid. With proper reaction and other 



1 See Galeotti, Zeitschr. f. physiol. Chem., 40, 42, 44, and 48 and Bonamartini and 

 Lombard!, ibid., 58. 



