ALBUMINOIDS 89 



albumin are quite closely related chemically, though differing in their specific 

 rotation of polarized light. The albumins contain, as a rule, more sulphur 

 than do the other classes of proteins. 



Globulins. The globulins are simple proteins which are insoluble in 

 pure (salt-free) water, but which are soluble in neutral solutions of salts of 

 strong bases with strong acids. Most globulins are precipitated from 

 their solutions on slight acidification and on saturation with sodium chloride 

 and magnesium sulphate. They are precipitated also from other solutions 

 on adding equal volume of saturated ammonium sulphate solution; this 

 precipitation is commonly termed " precipitation at half saturation ammon- 

 ium sulphate." Since they are insoluble in pure water, dilution of the 

 weak salt solution containing protein causes precipitation. The globulins 

 are especially predominant in the vegetable kingdom. They occur in rela- 

 tively large amounts as the reserve protein in seeds of various sorts. There 

 are, however, no essential differences as a class between the globulins of 

 animal and of vegetable origin. 



The globulins are precipitated from weak salt solutions on dialysis in 

 pure water. The inorganic salts diffuse through parchment, and with the 

 reduction in salt content the globulins are precipitated. Many of the 

 vegetable globulins can be obtained in crystalline form by precipitating 

 them in this way. 



As a class the globulins are relatively less stable than the albumins. 

 They are converted over into proteans on successive reprecipitation or 

 simply by standing under water. 



Albuminoids. The albuminoids yield similar amino acids on hydrol- 

 ysis to those obtained from the simple proteins. They possess essentially 

 the same general chemical structure. They differ from all other proteins in 

 that they are insoluble in neutral solvents. The classification, then, is based 

 purely on this property, though they are characterized by their occurrence as 

 the principal organic constituents in the structure of the supporting tissues 

 of the body and of the skin and its appendages. The individual albumin- 

 oids differ from each other fundamentally in certain chemical character- 

 istics. The albuminoids also are differentiated along with the morphologi- 

 cal variations of the connected tissues in which they occur. For example, 

 the keratins occur in the skin and its appendages. Collagen is the principal 

 albuminoid of white fibrous tissue, though found also in cartilage and bone. 

 Elastin characterizes the yellow elastin tissue, as, for instance, the nuchal 

 tendon, the elastic tendon so well developed in the neck of the ox. 



Keratins. The epidermis of the skin, the nails, hair and horn, feathers, 

 tortoise shell, silk, and the supporting neuroglia of nervous tissue may be 

 considered to be keratins in relatively pure form. The keratins take the 

 form of the tissue from which they are prepared. On heating they are 

 decomposed with the odor of burnt horn. They are insoluble in water, 



