PROTEINS AND THEIR CLASSIFICATION. 911 



(Chromoproteins (hemoglobin, hemocyanin, etc). 

 Glucoproteins. 

 Nucleoproteins. 

 f Keratin. 

 III. Albuminoids (pro- J Elastin. 



tein-like bodies) j Collagen (gelatin). 

 [ Reticulin. 



The Albumins. In addition to the albumins found in the cellular tis- 

 sues, the cell albumins, the conspicuous examples of this group are serum- 

 albumin, milk-albumin (lactalbumin) , and egg-albumin (ovalbumin). They 

 are characterized as a class by the fact that they are coagulable by heat in 

 solutions with a neutral or acid reaction, and are soluble in water free from 

 salts. In accordance with the latter part of this definition they are not 

 precipitated by dialysis. They are precipitated from their solutions with 

 more difficulty by saturation with neutral salts, ammonium sulphate, than 

 the globulins with which they are usually associated. Empirically, as re- 

 gards the liquids of the body, it is stated that they require more than half 

 saturation with ammonium sulphate for precipitation (see section on Blood). 

 All three albumins referred to here may be obtained in crystallized form. 

 They are not precipitated by saturation with sodium chlorid or magnesium 

 sulphate unless the solution is made acid. They are rich in sulphur, con- 

 taining from 1.6 to 2.2 per cent. 



The Globulins. Proteins belonging to this group are found in the cell 

 tissues together with albumins. The forms that have been most studied 

 are serum-globulin (paraglobulin) and fibrinogen (blood, lymph, and transu- 

 data), milk-globulin (lactoglobulin), and egg-globulin. As contrasted with 

 the albumins, they are coagulable by heat, but are not soluble in water free 

 from salts. In consequence of this last property they are precipitated by 

 dialysis. This reaction is not distinctive, however, as the precipitation is 

 not complete. Some of the so-called globulin remains in solution after the 

 salts have been removed as completely as possible by dialysis. They are 

 also precipitated partially from their dilute solutions by the addition of 

 weak acids or by a stream of carbon dioxid Practically they are isolated 

 from accompanying albumins by precipitation with neutral salts. In neu- 

 tral solutions the globulins are completely precipitated by saturation with 

 magnesium sulphate or half saturation with ammonium sulphate. In the 

 blood several different forms of globulin are distinguished by the degree of 

 saturation with ammonium sulphate necessary for their precipitation (see 

 Blood). The separations made by this method are not, however, satisfac- 

 tory. Nor, indeed, is the separation between globulins and albumins alto- 

 gether satisfactory. It would seem that these proteins are so closely related 

 that distinctive reactions are difficult to obtain on account of the existence 

 of forms intermediate between the extremes that are used as types. 



Albuminates or Derived Albumins. Since the albumins and globulins 

 occur normally in the tissues and liquids of the body, they are frequently 

 designated as the native proteins. The albuminates, on the contrary, are 

 derived from these native proteins by the action of strong acids or alkalies. 

 We distinguish, therefore, acid and alkali albuminates. They are nearly 

 insoluble in water or in water containing small amounts of neutral salts. 

 They are readily soluble in dilute acid or alkaline solutions ; are not pre- 

 cipitated by heat, but are precipitated readily upon neutralization. 



Nucleo-albumins (Phosphoproteins). This name may be applied 

 to a group of native proteins which are characterized by containing phos- 

 phorus, such as casein of milk, vitellin from the yolk of the egg, and va- 

 rious similar proteins found in the cytoplasm of the cellular elements. They 

 are often classed with or confounded with the nucleoproteins (or nucleo- 

 albumins) found in the nuclei of the cells. These latter belong to the com- 

 pound proteins and on decomposition yield nucleic acid or its split products, 

 the purin bases, etc., whereas the group now under consideration gives no 

 such result, yielding on decomposition so-called pseudonuclein or paranuclein, 

 which is not a nuclein at all, but a phosphorus-containing protein body. 



