THE BIO-CHEMISTRY OF ANIMALS AND PLANTS 6S7 



The Vegetable Proteins 



The preponderance of carbohydrates in plants explains the 

 fact, shown in the history of plant physiology, that the general 

 occurrence of nitrogen in plants has been recognised relatively 

 late, and that the presence of proteins especially has been 

 considered as an exceptional occurrence. 



History. — From an historical point of view it is interesting 

 to note that the earliest known vegetable protein was wheat 

 gluten ; this was isolated by Beccari in the eighteenth century. 

 Berzelius examined it at the beginning of last century and 

 called it " vegetable glue " or " vegetable casein " on account of 

 its similarity to the corresponding animal substances. Braconnot 

 isolated about the same time the proteins of peas and beans, 

 which he called " legumin." Gay-Lussac subjected vegetable 

 proteins to elementary analysis, and was the first to point 

 out the general occurrence of proteins in seeds. But Liebig 

 and his pupils were the real founders of the system which led 

 to our modern knowledge of protein chemistry. He distin- 

 guished between plant albumin (coagulated by heat, not 

 precipitated by acids), plant casein (not coagulated by heat, 

 not precipitated by acids), and plant fibrin. The view also 

 originated from his laboratory that elementary analyses of 

 proteins teach us very little about the nature of these substances 

 and that more knowledge is obtained by the investigation of 

 their hydrolytic cleavage products. The classical work of 

 Ritthausen, published in 1872, was the outcome of the applica- 

 tion of these methods to vegetable proteins. » Ritthausen's work 

 still remains of importance, although his methods have been 

 superseded by the introduction of neutral salts for the pur- 

 pose of isolation and separation of proteins. The names of 

 Hoppe-Se3der, Weyl, Schmiedeberg, must be mentioned in 

 this connection. It was mainly Kuhne's;"' pupil, Chittenden, 

 and later Osborne, two American workers who applied the 

 method of fractional salt precipitation to vegetable proteins. 

 This method is still a valuable one, and it *is to be hoped 

 that together with the latest processes for the isolation of 

 hydrolytic cleavage products, which we owe to the genius 

 of E. Fischer, we shall be able in the near future to form 

 clearer ideas of the constitution of proteins. 



Occurrence and Properties. — With regard to most of the 



