THE SIMPLE PROTEINS 127 



salt solutions but they are soluble in dilute solutions of strong bases 

 or acids. 



Prolamins. The prolamins are soluble in 70 per cent, to 90 per cent, 

 alcohol. They are insoluble, or nearly so, in distilled water, but dis- 

 solve readily in dilute solutions of strong acids or bases. They occur 

 in a variety of grains, typical members of the group being Gliadin, 

 found in the seeds of wheat and rye, Hordein found in the seeds of 

 barley and Zein found in the seeds of maize. They are characterized 

 by the high proportion of Proline which they yield when hydrolyzed. 



Scleroproteins. The scleroproteins, termed albuminoids in Ameri- 

 can and Continental European publications, form a very heterogeneous 

 group of substances. The various proteins which we have hitherto 

 been considering are either constituents of cellular tissues, concerned 

 in the life and maintenance of the protoplasm, or else they form 

 reserve-materials which are sooner or later to be called upon to supply 

 the requirements of protoplasm. Quite other is the function of the 

 scleroproteins, for these are proteins of a primarily structural or 

 architectural rather than nutritional significance. They are binding, 

 cementing and supporting substances which contribute in a mechanical 

 rather than in a chemical fashion to the furtherance and maintenance 

 of life. They occur especially in the various connective tissues, and 

 corresponding with their peculiar function, we find that they display 

 a variety of physical characteristics, distinguishing them from the 

 proteins of cellular origin, and also distinguishing the individual 

 members of the group very sharply from one another. Typical 

 members of this class are Gelatin and its parent-substance Collagen 

 which forms the chief constituent of white fibrous connective tissue, 

 and also the main organic constituent of bones. On boiling, especially 

 in the presence of dilute acid, Collagen yields the cleavage-product 

 Gelatin. Collagen itself is insoluble in water, salt solutions and dilute 

 acids or alkalies, but gelatin swells in cold water and dissolves in 

 warm water, forming jellies on cooling if the solutions are sufficiently 

 concentrated. Reticulin, occurring in the reticular fibrous tissues of 

 glands differs from collagen in several respects, notably in containing 

 phosphorus. 



Keratin is another scleroprotein and forms the chief constituent 

 of the horny epidermal structures, hair, wool, nails, hoofs, horns, 

 feather, tortoise-shell, etc. A form of keratin, Neurokeratin, also 

 occurs in nervous tissues. Keratin is insoluble in water, dilute acids or 

 alkalies and salt solutions; it is soluble with difficulty in strongly 

 alkaline solutions. It is also characterized by the high percentage of 

 sulphur w r hich it contains and which is attributable to the amino-acid 

 radical Cystine. 



Elastin forms the chief constituent of the elastic fibers of connective 

 tissue. It is distinguished by its elasticity and tensile strength and 

 also by its extreme insolubility, being soluble only in strong caustic 

 alkalies or concentrated mineral acids. Fibroin, the substance forming 



