THE CONJUGATED PROTEINS 129 



residue which still contains protein and is termed Nuclein. All of the 

 nucleoproteins appear to be very closely associated, or possibly com- 

 bined with Iron. 



Glucoproteins. In the glucoproteins the prosthetic group is either, 

 an ammo-carbohydrate, a polysaccharide derived from glucosamin 

 or acetylated derivatives of glucosamin, or else chondroitin-sulphuric 

 acid (see p. 91). The glucoproteins are subdivided into Mucins, 

 Mucoids and Chondroproteins. The true mucins yield extraordinarily 

 glutinous or mucilaginous solutions from which the mucin is precipi- 

 tated by acetic acid. The mucoids are not precipi table by acetic acid 

 and do not, as a rule, yield such highly viscous solutions as the mucins. 

 The chondroproteins are insoluble in water, but are soluble in dilute 

 alkalies, from which solutions the protein is precipitated by neutral- 

 ization with strong acids or by an excess of acetic acid. They yield 

 Chondroitin-sulphuric Acid on hydrolysis, a substituted sulphuric acid 

 formed by the union of a molecule of Chondroitin with a molecule of 

 sulphuric acid. Chondroitin resembles gum-arabic in physical char- 

 acteristics, and is a compound of Glucuronic Acid and Glucosamin. 

 The mucins occur in mucous secretions, as for example the secretions 

 from the skin-gland of snails or slugs. Mucoids are found in connec- 

 tive tissues, in the vitreous humor of the eye and in the white of egg 

 (ovomucoid) . The chondroproteins occur especially in cartilaginous 

 tissues, and in the interstitial substance of connective tissue. Chondro- 

 proteins are also found in the accumulations of colloidal material which 

 characterize the "amyloid degeneration" of certain organs under 

 pathological conditions. 



Phosphoproteins. The phosphoproteins are proteins which yield 

 phosphoric acid when hydrolyzed. The most typical example of this 

 group is Casein, the chief protein constituent of milk, but phospho- 

 proteins also occur in a variety of vegetable tissues, and in the yolk 

 of egg (ovovitellin) . They are predominantly acid in character, as 

 might be expected, not only from their content of phosphoric acid, but 

 also from the fact that they yield a high proportion of dicarboxylic 

 amino-acids on hydrolysis. 



Chromoproteins. The chromoproteins are compound proteins in 

 which the prosthetic group is colored. The most typical examples of 

 the group are Hemoglobin, the red coloring-matter and oxygen-carrier 

 of blood, in which the prosthetic group is a complex iron-containing 

 organic acid Hematin, and Hemocyanin, a blue pigment containing 

 copper which plays a role corresponding to that of hemoglobin in the 

 Arachnidce and Crustacea. The chromoproteins, hemoglobin and hemo- 

 cyanin, are exceptional among proteins in the relative ease with which 

 they are obtainable in crystalline condition. The protein radical in 

 hemoglobin is a predominantly basic protein, known as Globin and 

 related to the histones. 



Lecithoproteins. The lecithoproteins are compound proteins in 

 which the prosthetic group is a phospholipin. This is rather a con- 

 9 



