128 THE PROTEINS AND THE AMINO-ACIDS 



the core of silk fibers, is characterized by possibly even greater tensile 

 strength, while it is somewhat more readily dissolved by concentrated 

 acids and alkalies than elastin. Sericin or silk gelatin forms the outer 

 coating of the silk fiber, and is sticky when freshly secreted, so that it 

 enables intersecting and adjacent fibers to adhere. It is soluble in hot 

 water, and the solution resembles a solution of ordinary gelatin in 

 that, if concentrated, it gelatinizes on cooling. Finally, Spongin forms 

 the chief part of the ordinary sponge from which the originally living 

 protoplasm has been extracted. It is insoluble in acids but soluble in 

 concentrated alkalies. Some of the spongins contain iodine as an 

 integral part of the molecule. 



The scleroproteins are for the most part incomplete proteins in the 

 sense that they do not yield when completely hydrolyzed, all of the 

 amino-acids that we are accustomed to obtain from the more typical 

 proteins of cellular tissues. Thus gelatin yields neither tyrosine nor 

 trypto;phane, elastin and fibroin yield neither aspartic nor glutamic 

 acids, and spongin yields neither tyrosine nor phenylalanine. 



The extraordinary variety of physical properties and peculiarities 

 displayed by the various scleroproteins reveals the possibility of sub- 

 stances of very unique physical characteristics being derived from 

 proteins, and would point to the ultimate possibility of very important 

 industrial applications of such derivatives. At the present time, horny 

 derivatives of the protein of milk, casein, are extensively used in the 

 manufacture of substitutes for ivory, celluloid and bone. The animal 

 proteins, being among the most expensive foodstuffs we require, can 

 never be employed very extensively in the industries, except ing when 

 they form by-products of the foodstuffs-industry, as in the manufacture 

 of glue from slaughter-house or fish- wastes, and of casein products from 

 skimmed milk. Certain vegetable proteins might, however, be rendered 

 relatively cheap and abundant and offer an interesting field for the 

 investigation of the special physical characteristics of their derivatives. 



H. THE CONJUGATED PROTEINS. 



Nucleoproteins. The conjugated proteins are complex substances 

 formed by the union of a protein with a non-protein radical, which 

 may be termed the Prosthetic Group. The Nucleoproteins, for example, 

 are compounds of Nucleic Acids, which are substituted phosphoric 

 acids containing carbohydrate and nitrogenous radicals, with a protein 

 which plays the part of a base in the compound. These compounds 

 are the most characteristic constituents of the nuclei of cells. When 

 the protein constituent is a histone, the compound is termed a Nucleo- 

 histone. 



The nucleoproteins are insoluble in distilled water, but soluble in 

 dilute alkalies from which solutions they are precipitated by weak 

 acids, such as acetic acid or carbon dioxide. They are as a rule incom- 

 pletely digestible by the pepsin of gastric juice, leaving an indigestible 



