134 LECTURE VII. 



appearing as an insoluble precipitate. Later on this goes into solution. 

 This complex has been called paranuclein by Kossel, 1 and pseudonuclein 

 by Hammarsten. 2 It is at present entirely arbitrary to include the nucleo- 

 albumins among the simple proteins. Didactic considerations were mainly 

 responsible for placing them in this class. The nucleo-albumins have 

 often been classified with the nucleoproteids on account of their com- 

 mon phosphorus content. The latter, however, are sharply distinguishable 

 from the former by the fact that purine bases, pyrimidine derivatives, and 

 pentoses enter into their composition. O. Cohnheim 3 proposes instead 

 of nucleo-albumin the name of phospho-globulin to prevent such confusion. 

 This group includes casein, vitellin, and a series of cell-neucleo-albumins. It 

 is possible that legumin and the so-called vegetable-casein belongs to this 

 class. The nucleo-albumins are invariably distinct acids. When pure 

 they are insoluble in water. Their salts, on the other hand, are easily 

 soluble in alkalies and ammonia. They are precipitated by acids. Boiling 

 a solution of their salts is not sufficient to coagulate them. 



We shall consider casein and its digestion more in detail later. Here 

 its occurrence in milk will be merely mentioned. Vitellin occurs in the 

 yolk of eggs. It has never been prepared in a pure condition. Nucleo- 

 albumins are supposed to occur in all cells. With the exception of casein, 

 no other member of this group has so far been isolated in a pure condition. 

 This group already shows the total inadequacy of our methods of prepara- 

 tion. As soon as we are compelled to attempt the isolation of a given 

 protein from a mixture of albuminous material by precipitation and 

 certain questionable reactions, we meet with unsurmountable difficulties. 

 The names of the various proteins are here largely derived from their 

 morphological source. The physical properties of the albuminous sub- 

 stances are naturally dependent on the medium in which they are found. 

 That these substances are largely influenced by extraneous conditions, 

 can be easily shown by studying the behavior of the same -protein when 

 dissolved in various solvents. 4 The exceptionally large amounts of 

 admixed salts necessarily have an effect on the other physical properties 

 of any individually precipitated protein. Although we are undoubtedly 

 right in considering the albumins and the globulins as distinct individuals, 

 this is not the case with those others just mentioned. Many are unques- 

 tionably mixtures. 



We shall now consider the proteins which are relatively rich in di-amino 

 acids. As a result of their composition they are of a more or less basic 



1 A. Kossel: Arch. Anat. Physiol. 1891, 181. 



2 O. Hammarsten: Z. physiol. Chem. 19, 19 (1893). 



3 O. Cohnheim: loc. cit. p. 190. 



4 Cf. E. Abderhalden and O. Rostoski: Z. physiol. Chem. 46, 125 (1905); E. 

 Abderhalden: Z. exp. Path. Therap. 2, 642, 1905. 



