ALBUMINS OR PROTEINS. 163 



proteic reaction indicates the presence of aromatic groups. We are also 

 acquainted with another important color reaction, which does not properly 

 characterize any group as such. This is the so-called " Biuret-reaction." 

 If we freely add caustic soda or potash to an albumin solution, and 

 then carefully, drop by drop, a dilute solution of copper sulphate, a 

 blue to rose-violet coloration appears, which goes over into a blue on the 

 addition of more copper sulphate. The higher decomposition products 

 of albumin, the peptones, give a red coloration. 



The cleavage-products of protein just mentioned, have been obtained 

 by the hydrolytic action of acids and of alkalies. We can easily imagine 

 that in these cases secondary decompositions take place. A number of 

 scientists doubted the occurrence of so many amino acids, and preferred 

 to assume that the proteins contained groups which gave rise to the forma- 

 tion of these various amino acids during the hydrolysis brought about by 

 the reagents. 1 It were conceivable that ornithine, proline, and amino- 

 valeric acid originate from the same atomic grouping; also lysine and 

 leucine, on the one hand, and tyrosine and phenylalanine on the other. 

 Such a conclusion does not harmonize with our present knowledge of the 

 actions of acids and alkalies, because they invariably yield the indi- 

 vidual amino acids in the same quantities. That these amino acids occur 

 in the proteins is very evident from their appearance, as such, in sprouting 

 plants, and even in the animal organism under specific conditions. The 

 most important proof of their original occurrence is indicated by their 

 appearance during digestion. The albumins are broken down by the 

 hydrolytic action of ferments, especially by trypsin, into amino acids. 

 The decomposition by fermentation is the mildest imaginable. It takes 

 place at 37 C. All the known amino acids have been obtained from diges- 

 tion mixtures except phenylalanine and diamino-trihydroxy-dodecylic 

 acid. The latter has never been looked for, while the former does not 

 appear in a state of combination which is accessible to the proteolytic 

 ferments. 



The albumin as it reaches the digestive organs of an animal is subjected 

 to the action of two proteolytic ferments, pepsin and trypsin. Later on 

 we shall go more into detail regarding the behavior of the albumins during 

 the process of natural digestion. We shall, at present, devote our atten- 

 tion to the subject of artificial digestion, i.e., the digestion of the protein 

 outside of the alimentary tract. We must say that the results obtained 

 in these investigations do not harmonize in matters of detail. This is 

 mainly due to the different methods employed in using these ferments. 

 Until recently, the physiological chemist utilized extracts of organs whether 

 of the stomach or of the pancreatic gland, and the extirpated organs 



1 O. Loew: Hofmeister's Beitr. 1, 567 (1900). 



