454 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



THE SCLEROPROTEINS. 



The sclerqproteins constitute the greater part of the skeletal struc- 

 tures of animal organisms. Just as their origin is very various, so also 

 are their properties, but usually they are proteins insoluble in most 

 reagents. They may be divided into several groups. 



Keratins. 



The keratins form the hard structure of hair, nails, feathers, horn, 

 tortoise-shell, whalebone, etc. 



The finely chopped material is boiled with alcohol, ether and water. 

 Fats, salts and other soluble constituents are thus removed ; other proteins 

 which may be present are removed by digestion with pepsin and trypsin. 

 The insoluble residue is washed with water, alcohol and ether and dried. 



The keratins, when they have been dried, are hygroscopic substances 

 and swell up slightly in water. They are insoluble in all reagents, but when 

 boiled with acids and alkalies they dissolve and are converted into derivatives. 

 Hydrogen sulphide and mercaptan are given off when they are boiled with 

 acids. They are characterised chiefly by their high sulphur content, from 

 2 per cent, in pig's hoof to 1 5 per cent, in human hair. 



They give most of the colour reactions for proteins (p. 365). 



Egg-membranes. 



The materials surrounding the eggs of birds, turtles and fish are 

 grouped together as a special group of scleroproteins, but they are 

 closely related to the keratins. The hard material, koilin, in birds* 

 crops may be included here. 



The membrane is treated for several days with dilute caustic soda 

 (0*1 per cent), washed with water and soaked in dilute hydrochloric acid 

 for several days to remove any inorganic matter or gelatinous matter which 

 is separated mechanically. The material is washed with cold water, boiling 

 water, hot dilute acetic acid and boiled with alcohol and ether. 



They are insoluble substances resembling the keratins ; they give most 

 of the colour reactions of proteins, but generally contain less sulphur than, 

 the keratins. 



Elastin. 



Elastin is the constituent of elastic tissue and is especially abund- 

 ant in the ligamentum nuchae. The skeletal structure of the eggs of 

 several fish and reptiles is said to consist of elastin. 



Ligamentum nuchae is washed with water, treated for several days with: 

 fresh portions of half- saturated lime water to remove mucoids, boiled with 

 10 per, cent, acetic acid, treated with cold 5 per cent, hydrochloric acid, again 

 boiled with acetic acid and treated with hydrochloric acid, washed with 

 water and boiled with alcohol and ether. 



Elastin as thus prepared has a yellowish colour. It can be ground 

 into a powder. The finely powdered substance dissolves slowly in cold "z 

 per cent, hydrochloric acid or i per cent, potash on warming. By stronger 

 acids it is decomposed and forms derivatives* 



It gives most of the colour reactions for proteins, but not the sulphur 

 test. 



