THE PROTEINS 111 



deepens to a violet on addition of more copper (the biuret reaction). 

 Their solutions can be boiled without undergoing coagulation. Many 

 of them may be thrown down from their solutions by absolute alcohol, 

 but are not rendered insoluble even by prolonged standing under the 

 alcohol. The characters of the different members of these groups 

 will be considered at greater length when dealing with the changes 

 undergone by the proteins during the process of digestion. At 

 present we may merely summarise the distinguishing features of these 

 two classes. 



(a) PROTEOSES, e.g. albumose from albumin, caseose from casein, 

 elastose from elastin. All of these are precipitated from their solu- 

 tions on saturation with ammonium sulphate. In the presence of a 

 neutral salt they give a precipitate on the addition of nitric acid. 

 This precipitate is dissolved on heating the solution, but reappears on 

 cooling. All, with the exception of heteroalbumose, are soluble in 

 pure water, and all are soluble in weak salt solutions or dilute acids 

 or alkalies. They are slightly diffusible through animal membranes. 



(6) PEPTONES, e.g. fibrin peptone, gluten peptone. These are all 

 soluble in pure .water, diffuse fairly readily through animal mem- 

 branes, but otherwise give the same reactions as albumoses. From 

 the latter class peptones are distinguished by the fact that they are not 

 precipitated on saturation of their solutions either in acid or alkaline 

 reaction with ammonium sulphate or any other neutral salt. Many of 

 them are soluble in alcohol. 



(8) THE PHOSPHOPROTEINS. In this class may be grouped a 

 number of substances of very diverse properties which, however, 

 resemble one another in containing phosphorus as an integral part 

 of their molecule. When subjected to digestion with pepsin and 

 hydrochloric acid they are dissolved, but a small quantity of a phos- 

 phorus-containing complex may remain behind undissolved. This 

 residue has been called paranuclein or pseudonuclein. It is in reality 

 derived from nucleoprotein, which is present in the phosphoprotein 

 as impurity and should be called simply nuclein. The phosphoproteins 

 have markedly acid characters. They are insoluble in pure water, 

 easily soluble in alkalies and ammonia from which the original body 

 is thrown down again on addition of acid. Their solutions in alkali 

 are not coagulated by heating. To this class belong caseinogen, the 

 chief protein of milk, vitellin, the main protein in the yolk of egg, and 

 the vitellins in the eggs of fishes and frogs. The vitellins are generally 

 associated with a large amount of lecithin. The phosphoproteins differ 

 from the nucleoproteins, which also contain phosphorus, in the facts 

 that they are readily decomposed by caustic alkali with the libera- 

 tion of phosphoric acid, and do not contain purine bases. The phos- 

 phorus of the nucleoproteins is not split off by alkali (1 per cent.), 



