628 PHYSIOLOGICAL CHEMISTRY. 



Serum- or para-globulin of blood, lacto-globulin of milk, and 

 fibrinogen are globulins. 



(c) Glutelins are simple proteins insoluble in all neutral solvents, 

 but readily soluble in very dilute acids and alkalies. They occur in 

 abundance in the seeds of cereals. 



(d) Prolamines or alcohol-soluble proteins. These are soluble 

 in 70 to 80 per cent, alcohol, and in dilute acids and alkalies, but in- 

 soluble in water, absolute alcohol, and other neutral solvents. Similar 

 to glutelins, they are found chiefly in the vegetable kingdom. For 

 instance, zein is found in maize, gliadin in wheat, hordein in barley, 

 etc. 



(e) Albuminoids. Simple proteins which possess essentially the 

 same chemical structure as the other proteins, but are characterized 

 by great insolubility in all neutral solvents. They occur chiefly as 

 constituents of the skeleton, of the skin and its appendages, and 

 exist, as a rule, in an insoluble condition. To the albuminoids 

 belong : Keratins, elastin, collagen, and a few other substances. 



Keratins occur as the principal constituents of the horny por- 

 tion of the skin and its appendages. A special keratin, neurokeratin, 

 is found in the nervous system. The keratins contain proportion- 

 ately more sulphur than other proteins, part of it in very loose com- 

 bination. The darkening of the hair by the use of a lead comb, 

 forming black lead sulphide, is due to the action of this sulphur. 

 The products of deep cleavage of the keratins are the same as those 

 of the proteins, but with relatively greater quantities of the sul- 

 phurized products, mainly in the form of cystine. 



Keratins dissolve slowly in cold caustic alkalies, more rapidly on 

 heating. They are insoluble in water, alcohol, ether, and in gastric 

 and pancreatic juices. They give the xanthoproteic, biuret, and 

 Millon's reactions. 



Elastin occurs in the connective tissue, particularly in yellow 

 elastic fiber; it contains very little sulphur (less than 0.5 per cent.), 

 and yields on deep cleavage the same products as simple proteins, 

 giving, however, glycocoll, little glutamic acid, and no aspartic acid. 



Elastin is insoluble in water and in cold solutions of caustic alka- 

 lies ; it dissolves slowly in alkalies on boiling and in cold sulphuric 

 acid ; it is easily dissolved by warm nitric acid, as also by the action 

 of proteolytic enzymes. It shows the same color-reactions as the 

 keratins. 



Collagen occurs in the fibre of connective tissue. Ossein, the 

 chief organic constituent of bone, is a collagen; and chondrin, a 



