430 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



7. The Conjugated Proteins. 



These are compounds in which the protein molecule is united to 

 other organic materials, which are as a rule also of complex nature. 

 This second constituent of the compound is usually termed a pros- 

 thetic group. They may be divided into the following sub-classes : 



i. Chromo-proteins. These are compounds of protein with a 

 pigment, which usually contains iron. They are typified by haemo- 

 globin and its allies, which will be fully considered under Blood. 



ii. Gluco-proteins. These are compounds of protein with a 

 carbohydrate group. This class includes the mucins and the mucoids. 



The mucins are widely distributed and may occur in epithelial 

 cells, or be shed out by these cells (mucus, mucous glands, goblet 

 cells). The mucins obtained from different sources are alike in being 

 viscid and tenacious, soluble in dilute alkalis such as lime water, 

 and precipitable from solution by acetic acid. 



The mucoids differ from the mucins in minor details. The term 

 is applied to the mucin-like substances which form the chief con- 

 stituent of the ground substance of connective tissues (tendo-mucoid, 

 chondro-mucoid, etc.). Another (ovo-mucoid) is found in white of 

 egg, and others (pseudo-mucin and para-mucin) are occasionally found 

 in dropsical effusions, and in the fluid of ovarian cysts. 



The differences between the mucins and mucoids are due either 

 to the nature of the carbohydrate group, or more probably to the 

 nature of the protein to which it is united. The carbohydrate 

 substance, however, is not sugar, but a nitrogenous substance 

 which has a similar reducing power to sugar, and which is called 

 glucosamine (C 6 H 11 5 NH 2 ), that is, glucose in which HO is replaced 

 by NH 2 . 



Pavy and others have shown that a small quantity of the same 

 carbohydrate derivative can be split off from various other proteins 

 which we have already placed among the albumins and globulins. 

 It is, however, probable that this must not be considered a prosthetic 

 group, but is more intimately united within the protein molecule. 



iii. Nucleo-proteins. These are compounds of protein with a 

 complex organic acid called nucleic acid, which contains phosphorus. 

 They are found both in the nuclei and cell-protoplasm of cells. In 

 physical character they often simulate mucin. 



Nuclein is the name given to the chief constituent of cell-nuclei. 

 It is identical with the chromatin of histologists (see p. 10). 



On decomposition it yields an organic acid called nucleic acid, 

 together with a variable but usually small amount of protein. It 

 contains a high percentage (10-11) of phosphorus. 



The nuclein obtained from the nuclei or heads of the spermatozoa 

 consists of nucleic acid without any protein admixture. In fishes' 



