106 PHYSIOLOGICAL CHEMISTRY. 



Albuminoids. (Scleroproteins.) 



The albuminoids yield similar hydrolytic products to those ob- 

 tained from the other simple proteins already considered, thus indi- 

 cating that they possess essentially the same chemical structure. 

 They differ from all other proteins, whether simple, conjugated or 

 derived, in that they are insoluble in all neutral solvents. The 

 albuminoids include "the principal organic constituents of the 

 skeletal structure of animals as well as their external covering and 

 its appendages." Some of the principal albuminoids are keratin, 

 elastin, collagen, reticulin, spongin, and fibroin. Gelatin cannot 

 be classed as an albuminoid although it is a transformation product 

 of collagen. The various albuminoids differ from each other in 

 certain fundamental characteristics which will be considered in 

 detail under Epithelial and Connective Tissue (see Chapter XIV, 

 p. 227). 



CONJUGATED PROTEINS. 



Conjugated proteins consist of a protein molecule united to some 

 other molecule or molecules otherwise than as a salt. We have 

 glycoproteins, nude o proteins, hemoglobins (chromoproteins), 

 phospho proteins and lecithoproteins as the five classes of conju- 

 gated proteins. 



Glycoproteins may be considered as compounds of the protein 

 molecule with a substance or substances containing a carbohydrate 

 group other than a nucleic acid. The glycoproteins yield, upon 

 decomposition, protein and carbohydrate derivatives, notably gly- 

 cosamine, CH 2 OH- (CHOH) 3 -CH(NH 2 ) -CHO, and galactosa- 

 mine, OHCH 2 - (CHOH) 3 -CH(NH 2 ) -CHO. The principal gly- 

 coproteins are mucoids, mucins and chondro proteins. By the term 

 mucoid we may designate those glycoproteins which occur in tis- 

 sues, such as tendomucoid from tendinous tissue and osseomucoid 

 from bone. The elementary composition of these typical mucoids 

 is as follows : 



N. S. C. H. O. 



Tendomucoid 11.75 2.33 48.76 6.53 30.60 (Chittenden and Gies) 



Osseomucoid 12.22 2.32 47.43 6.63 31.40 



The term mucins may be said to include those forms of glyco- 

 proteins which occur in the secretions and fluids of the body. 

 Chondroproteins are so named because chondromucoid, the prin- 

 cipal member of the group, is derived from cartilage (chondrigen). 

 Amyloid, which appears pathologically in the spleen, liver and 

 kidneys, is also a chondroprotein. 



