ALBUMINS OR PROTEINS. 215 



The reason that many albuminous substances, like the keratins, are not 

 looked upon as food material, is due to the fact that their amino acid con- 

 stituents are in such combinations that they are attacked with difficulty, 

 or not at all, by trypsin. Among the albuminous substances so far con- 

 sidered, we mentioned one which was characterized by the absence of 

 almost all of the members of the aromatic series. This is gelatin. Tyrosine 

 and tryptophane are entirely absent, while phenylalanine is present in only 

 very small amount. We are justified in concluding, from the idea pre- 

 viously suggested, that gelatin is not a satisfactory substitute for albumin 

 in the ordinary sense. The animal organism seems unable to synthesize 

 tyrosine and tryptophane. It would have to produce aromatic compounds 

 from substances of the fatty acid series. It is, in fact, impossible to main- 

 tain the nitrogen balance by feeding gelatin exclusively. It is, of course, 

 possible to substitute gelatin for a part of the nutrient albumin, in the 

 same manner as when we do this with a large supply of carbohydrates or 

 fats. Taking the above-described metabolic processes into consideration, 

 we may conclude that gelatin is a much better "albumin-sparer" than 

 the nitrogen-free foodstuffs mentioned, for the reason that it delivers to 

 the organism a whole series of albumin constituents which it is capable of 

 uniting with the other albumin degradation products to form its serum- 

 albumins. We must expect that it can act as a substitute for more nutrient 

 albumin, in proportion to the richness of the latter in aromatic groups. 

 Experiments to confirm this have not yet been undertaken, although 

 efforts have been made to increase the " nutritive index " of gelatin by 

 the addition of the missing ingredients, tyrosine and tryptophane. It has 

 in fact been found possible to increase greatly the amount of gelatin used 

 as substitute by adding simultaneously these amino acids. 1 Cystine was 

 used in these experiments. The reason that we have so far been unable 

 to replace completely albumin with gelatin, and the addition of the missing 

 amino acids, may be due to the fact that we are not yet acquainted with 

 all of the albumin components. It seems far more probable, however, 

 that gelatin contains numerous combinations, which are very resistant to 

 the action of the proteolytic ferments, and, possibly, it also restricts the 

 rapid decomposition and reconstruction in cell-metabolism. 



Effort has also been made to obtain values, as albumin-sparers of sub- 

 stances closely related to albumin, especially of asparagine, which occurs 

 so abundantly in germinating seeds. 2 The interesting discovery was 



1 M. Kauffmann: Pfliiger's Arch. 109, 1 (1905). Cf. also the earlier investigations 

 of Eschle: Vierteljahresschrift naturforsch. Ges. Zurich, 1876, 36. K. H. Lehmann: 

 Sitzber. Miinchener morphol.-physiol. Ges. March 10, 1885. 



2 O. Kellner: Z. Biol. 39, 313 (1900). Politis: ibid. 28, 492 (1891). S. Gabriel: ibid. 

 29, 115 (1892). C. Voit: ibid. 29, 125 (1892). Mauthner: ibid. 28, 507 (1891). I. 

 Munk: Virchow's Arch. 94, 441 (1883). Weiske: Z. Biol. 17, 415 (1881); ibid. 30, 

 254 (1894). W. Voltz: Pfluger's Arch. 107, 360 (1905); 107, 415 (1905). 



