98 PHYSIOLOGY 



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 solutions 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.y. fibrin peptone, gluten peptone. These are all soluble 

 in pure water, diffuse fairly readily through animal membranes, but other- 

 wise 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 phosphorus-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 thje 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 liberation of phosphoric acid, and do not contain purine 

 bases. The phosphorus of the nucleoproteins is not split off by alkali 

 (1 per cent.), and on hydrolysis the nucleic acid constituent gives rise to 

 purine bases. 



(<>) CONJUGATED PROTEINS, Various complex bodies which play an 

 important part in building up cells and in the various processes of the body 

 make up this group of compounds. They resemble one another only in the 

 fact that in each of them a protein radical is combined with some other body, 

 often spoken of as the prosthetic group.* 



* By the Germans the term ' proteid ' is often applied to this group. In English 

 however the term ' proteid ' has been generally used for the simple protein known 



