222 SCIENCE PROGRESS 



know that the highly complex proteins are built up from rela- 

 tively few kinds of simple " bricks " and it is from these that 

 physiologically active substances may be derived by compara- 

 tively slight modifications. In the present article an account 

 is given of the discoveries made in this field during recent years 

 as a result of the great extension of our knowledge of protein 

 chemistry within the last two decades. 



Amines formed from Amino-Acids by withdrawal of Carbon 



Dioxide 



As almost all the amino-acids obtainable from proteins 

 are well known chemically and have been synthesised, the 

 nature of the bases derived from them is easily inferred. 

 Various laboratory methods are available for the removal of 

 carbon dioxide from the acids. In some cases, as in that of 

 leucine and of tyrosine, it is possible to convert small quantities 

 of the amino-acid into the corresponding amine simply by 

 destructive distillation. Another method, which consists in 

 heating the amino-acid with a strong mineral acid, has been 

 applied to histidine. A third method, which is more com- 

 plicated but affords better yields, involves the displacement of 

 the amino-group first by halogen and then by hydrogen ; sub- 

 sequently the amide or hydrazide of the acid is broken down 

 either by Hofmann's or by Curtius's method. /3-Iminazolylethyl- 

 amine was first obtained from histidine by the latter process. 



Such laboratory methods, however, are much less important 

 than the biological process of removing carbon dioxide by means 

 of putrefactive bacteria. The resolution of proteins to the amino- 

 acid stage but not beyond is effected by " digestive " enzymes, 

 both in the small intestine of animals and in the germinating 

 seeds of plants. The lower plants (yeasts, fungi and especially 

 bacteria) not only bring about the same change but are able to 

 carry the process of degradation still further. Usually nitrogen 

 is first eliminated ; thus in putrefaction tyrosine is for the most 

 part converted into^-hydroxyphenylpropionic acid, 



OH.C 6 H,.CH 2 .CH(NHo)COOH -> OH . C.H 4 . CH 2 . CH 2 . COOH 



Yeast converts leucine into the optically inactive and iso- 

 leucine into the optically active amylic alcohol present in fusel 

 oil (F. Ehrlich, Ber. 1907, 40, 2538) : 



