228 LECTURE XI. 



protected against further oxidation by its combination with benzoic acid, 

 and is excreted as such. 



The question now arises regarding the manner in which urea is formed 

 from ornithine, and the other amino acids. We can say that it has been 

 shown that urea is actually formed from amino acids in the animal 

 organism. 1 This discovery is of great significance. It had always been 

 thought possible that the proteins were broken down, not through the 

 amino acids, but in some other, still unknown, manner, and that the secret 

 of urea formation was hidden somewhere in such decomposition. If we 

 administer glyeocoll, alanine, leucine, glutamic acid, aspartic acid, aspara- 

 gine, or arginine, to a mammal, either by the mouth or subcutaneously, we 

 find an increase of urea in the urine corresponding to the amounts of nitro- 

 gen added in the form of amino acids. We observe the same result when 

 we use the polypeptides instead of the amino acids, e.g. glycyl-glycine, 

 diglycyl-glycine, alanyl-alanine, and leucyl-leucine. 2 Even the anhydrides 

 so far investigated, glycine anhydride and alanine anhydride, are disinte- 

 grated by the organism of the dog into urea. An especially interesting 

 case is that of arginine, already referred to, which is composed of two parts, 

 guanidine and diaminovaleric acid (ornithine). We have already called 

 attention to the ease with which urea is formed from the first sub- 

 stance. W. H. Thompson has shown 3 that when arginine is administered 

 to a dog, the nitrogen of the first component, guanidine, quickly reappears 

 in the urine as urea. Ornithine itself is more slowly converted into urea. 

 Even 100 per cent of the nitrogen of arginine can be found in the urine of 

 dogs as urea. Very appreciable differences sometimes occur in different 

 individuals. S. Salaskin 4 has shown that the liver probably plays an 

 important part in the production of urea from the amino acids. He passed 

 glyeocoll, leucine and aspartic acid through a dog's liver and found that 

 the blood used as a circulating medium showed an increase in its content 

 of urea. His method of proof is not very satisfactory, owing to the fact 

 that his method of estimating urea is not free from criticism. 



Before discussing the statements which give us an idea of the chemical 

 processes participating in the formation of urea, we wish to mention those 

 hypotheses, which, being based upon experimental results, will give us the 

 best conception of the formation of urea from albumin and its cleavage- 

 products. We find it necessary to add that it is extremely difficult, from 



1 O. Schultzen and M. Nencki: Ber. 2, 566 (1869), and Z. Biol. 8, 124 (1872). M. 

 Nencki: Ber. 5, 890 (1872). W. Knieriem: Z. Biol. 10,264 (1874). E. Salkowski: ibid. 

 4, 54 and 100 (1880). 



3 E. Abderhalden and Y. Teruuchi: loc. cit. E. Abderhalden and Franz Samuely: 

 loc. cit. E. Abderhalden and B. Babkin: Z. physiol. Chem. 47 (1906). 



3 J. Physiol. 32, 137 (1905), and 33, 106 (1905). 



4 Z. physiol. Chem. 25, 128 (1898). 



