82 THE VEGETABLE PROTEINS 



II. CONJUGATED PROTEINS. 



(a) Nucleoproteins. 



The nucleoproteins deserve especial attention because they are 

 among the most important constituents in the cells of animals and 

 plants. They were first described as existing in vegetable cells by 

 Hoppe-Seyler (169), who obtained a preparation from yeast which was 

 very similar to those then recently obtained by Miescher from animal 

 sources. Kossel (201, 202) next investigated this substance and 

 found that the phosphorus content of different preparations varied 

 widely, but that most of them contained about 3-5 per cent, of phos- 

 phorus, which fact he regarded as evidence of a more stable combina- 

 tion with this proportion of phosphorus. He also found hypoxanthin 

 among its decomposition products, but as he later obtained adenin 

 from yeast nuclein he considered this to be the mother substance from 

 which the hypoxanthin had originated. Liebermann (220) in 1890 

 obtained reactions with yeast nuclein which, as he thought, showed the 

 presence of metaphosphoric acid in this substance. 



Altmann (587), in 1889, discovered nucleins to be compounds con- 

 taining both nucleic acid and protein. It is difficult to determine just 

 what views are now held concerning the character of the union of the 

 nucleic acid with the protein, but, from what appears in the literature 

 of this subject, most writers evidently consider nuclein to be something 

 other than a protein nucleate. The available evidence in regard to 

 the nature of this union is very scanty and practically all that is de- 

 finite relates to the formation of protein nucleates. It is not impossible 

 that other forms of union may exist, but the conditions under which 

 nucleoproteins and nucleins are at present obtained make it extremely 

 difficult, if not impossible, to prove the existence of an organic com- 

 bination between the nucleic acid and protein. The methods thus far 

 employed in isolating these substances depend on processes which 

 would give a salt of protein and nucleic acid if these two substances 

 were present in the solution. No hydrolytic splitting of the nuclein 

 thus obtained appears to be necessary to set its component parts free, 

 unless hydrolysis is effected with extraordinary ease and with great 

 rapidity. Furthermore, the experiments of Milroy (600) and Lobisch 

 (598) show that artificial mixtures of phosphorus-free protein and free 

 nucleic acid yield products which have the properties usually con- 

 sidered to be characteristic of the nucleins, although from the conditions 

 of their production these artificial compounds could have been nothing 

 other than protein nucleates. 



