256 LECTURE XII. 



We will state once more that according to the experiments at ha^id, 

 para-cresol and phenol are to be regarded solely as products of putre- 

 faction, whereas it is still a question with regard to the other products as 

 to how much is formed in the intestine and how much is caused by the 

 intermediate breaking down of albumin, or of tyrosine, in the cells. It is 

 unquestionably certain that p-hydroxyphenylpropionic acid and p-hydroxy- 

 phenyl-acetic acid may be formed beyond the intestine. This discovery 

 serves to give us an interesting insight into the decomposition of tyrosine 

 in the tissues. We first observe that the amino group is split off and that 

 oxidation then sets in. It is still questionable as to how far these observa- 

 tions may be applied to the decomposition in the metabolism of the cells. 

 Some observations seem to indicate that the elimination of the amino 

 group is the first stage of the decomposition of the amino acids. 



We must refer to another specific property of tyrosine. In discussing 

 the digestion of albuminous substances by trypsin, we called attention to 

 the fact that this amino acid, together with tryptophane, is very quickly 

 split off from the albumin. It is very possible that this fact may cause 

 tyrosine and tryptophane, which we shall soon learn to recognize as the 

 mother-substance of skatole and indole, to fall easy prey to the putrefac- 

 tive bacteria. 



Another question of considerable interest confronts us : What becomes of 

 the other aromatic amino acid, phenylalanine? Investigations on tryptic 

 digestion show that this amino acid shows an entirely different behavior 

 from that of tyrosine. It is not set free by trypsin. In the putrefaction 

 of albumin outside the body, phenylaminopropionic acid breaks down into 

 phenylpropionic acid and phenylacetic acid. The former is not found as 

 such, in urine, but is combined with glycocoll as phenaceturic acid. In 

 this form it has been isolated from normal horse urine. Whether it occurs 

 in human urine, or not, has never been decided. If it were present to a 

 considerable extent, one might, aside from the possibility of its formation 

 from other sources, e.g. from the decomposition products of tyrosine, and 

 its formation during the decomposition processes in the tissues, draw the 

 conclusion that albumin, or a higher complex of amino acids such as a 

 polypeptide, is attacked by the bacteria of putrefaction. The increased 

 appearance of the other two acids would be an indication of intense putre- 

 factive changes taking place in the intestines. 



Tyrosine and phenylalanine have also been supposed to participate in 

 the production of hippuric acid. If so, they would furnish the benzoic 

 acid radical. According to the above, it is evident that phenylalanine can 

 hardly play a part here, at least, as far as it is a question of putrefactive 

 processes in the intestines. Tyrosine alone is to be considered in this con- 

 nection. It is, of course, possible that phenylalanine, and tyrosine as well, 

 are used for the production of hippuric acid in the metabolism of the cell. 



