ALBUMINS OR PROTEINS. 261 



alanine, leucine, glutamic acid, aspartic acid, lysine, arginine, histidine, 

 etc. Phenylalanine and proline are undoubtedly present in combination, 

 for they are unacted upon by trypsin. They remain unattacked, com- 

 bined with other amino acids in the form of polypeptides. All these 

 cleavage-products are absorbed. The albumin synthesis starts in the 

 intestinal walls, the serum-albumins being first formed. The rapidity of 

 this synthesis is dependent on the presence of certain amino acids, for, as 

 recent investigations have proved, the relative amounts of the amino acids 

 in the serum-albumins are very constant. The albumin synthesis would 

 necessarily be regulated by the amount of that amino acid which is present 

 to the smallest relative extent. There is always the possibility that one 

 amino acid may change into others. This is known to be true only of 

 amino acids of the aliphatic series, among themselves; but in the same 

 manner we can imagine the production of aromatic amino acids from one 

 another. On the other hand, it is hardly probable that relations exist 

 between these two groups or at most only in the sense that an aromatic 

 amino acid might give rise to an aliphatic one, or at least, a fatty acid. 

 Our knowledge of the formation of one food-stuff from another, 1 e.g. the 

 production of fat from the carbohydrates,- forces us to admit the pos- 

 sibility of several amino acids being produced from a single one. 



If such transformations take place then naturally the extent of the 

 albumin synthesis is considerably widened. Those amino acids which are 

 not utilized for the production of new proteins are evidently already 

 broken down in the walls of the intestine; i.e. the amino group is first 

 split off and the residue consumed. The ammonia thus formed prob- 

 ably is utilized in the production of urea. From these conceptions, we 

 should expect to find a priori, that the various albuminous substances 

 behave differently; i.e. under certain conditions an influence upon the 

 extent of the synthesis of albuminous substances in the intestines might 

 affect the entire metabolism. This is well shown in the case of gelatin, 

 which not only lacks whole groups of amino acids, but at the same time 

 possesses combinations of such, which affect unfavorably the breaking 

 down of the molecule and indirectly the formation of new proteins. The 

 other proteins might also, according to the amino acids in their composition, 

 be more or less favorable to the synthesis of protein, and especially for 

 the formation of the serum-albumins. It could even be expected that it 

 would not be possible to maintain the same nitrogen balance with every 

 albuminous substance. Here it is assumed that the amino acids which 

 are already broken down in the intestines are to a certain extent eliminated 

 from the albumin metabolism. 2 We have intentionally dwelt on these 



1 Cf. Lecture XIV. 



2 Cf. Lecture XI, p. 225. 



